A. SYMMETRICAL FLICKER

With the apparatus thus arranged various binocular combinations can be investigated and compared either one with another or with uniocular images.

As shown above, the apparatus allows of similar images being thrown on strictly and fully conjugate points of the two retinae, either synchronously right and left or alternately right and left, with a time accuracy not less than .0006″ for the slowest rates of intermission, and not less than .0001″ for the highest. The first comparison made (Experiment 1) may be to observe if there is any difference between the rates of intermittence for just perceptible flicker in two binocular images, one made with synchronous right and left illuminations, the other with alternate right and left illuminations. This arrangement is expressed graphically by the accompanying diagram (Fig. 78).

The diagram makes the lower composite image the “synchronous” one, but in the series of observations the “synchronous” is sometimes the lower, sometimes the upper, and the observer is not informed which it may be. The observations may be made on the transition from flickering to unflickering sensation, or conversely on the transition from unflickering to flickering sensation; the observer in the latter has a more neutral approach to the critical observation. Compared under rates of intermittence giving marked flicker in both images, observers find the flicker “less” in the “alternate” than in the “synchronous” combination. This difference at the lower speeds inclines the observer to expect that complete extinction of the flicker will disappear the more readily in the image which at slow intermissions seems to flicker the less. Judgment is therefore best asked under conditions in which both images start perfectly free from flicker, the rate of intermittence being from the outset high enough to exclude flicker.

The judgment then given is almost uniformly that there does exist a very small difference between the frequency of intermittence required for extinction of flicker in the “synchronous” and “alternate” combinations respectively. In the “alternate” combination flicker disappears at a slightly lower frequency of intermission than in the “synchronous.” All observers agree that directly the frequency of intermission extinguishes flicker in both the discs the appearance of both is indistinguishably similar, and that there is then nothing to choose between the brightness of the two.

For almost all persons I have examined, a spot intermittently illuminated at a frequency of intermission just sufficient to extinguish flicker in it, when looked at with one eye only, still flickers slightly when looked at with both eyes. A like phenomenon is noticed by most observers when examined by the arrangement (Experiment 2) represented by Figure 79.

The binocular arrangement, then, is said by them to require a slightly higher frequency for extinction of flicker than does the uniocular. Again, if under a frequency of intermission just securing extinction of flicker in either of the component uniocular images separately, one of these images, previously screened off, is readmitted, so that the pair act together with a synchronous arrangement of phase, a trace of flicker appears at once in the binocular image. It may be urged that this is due to the fresh retinal area being more sensitive to flicker, and it is true that the flicker so introduced tends soon to become less, but a residuum of the phenomenon seems to remain.

Experiment 3. Conversely, under the arrangement indicated in Figure 80, a number of the persons examined, but not all, decide that the binocular image requires for extinction of flicker a slightly lower frequency of intermittent illumination than does the uniocular. Also, a number of these persons, though not all, find that when the “alternate right and left” combination is observed under a frequency of intermission of illumination just sufficient to extinguish its flicker, the screening out of one of the component uniocular images brings with it a slight appearance of flicker.

From these observations it appears that similar phases of flickering illumination if timed to fall coincidently on conjugate retinal areas do very slightly reinforce each other in sensation, and if timed exactly alternately do very slightly mutually reduce. But the broad outcome of the observations is that so far from bright phases at one eye effacing dark phases at the corresponding spot of the other eye, there is hardly a trace of any such interference. To judge from its absence of influence on the flicker rate, the dark phase incident at retinal point A′ does not, as regards sensual result, modify the bright phase synchronously incident at the conjugal retinal point A, and conversely. If the brightness of the bright phase or the darkness of the dark phase were lessened at A by A′, the rate of frequency of stimulus for extinction of flicker must fall. But except in minute and perhaps equivocal degree it does not alter.

As far as sensual effect goes, the light phases at the one eye practically do not, therefore, interfere or combine with the coincident dark phases at the other; and conversely. Nor do they, in the alternate left and right arrangement, add themselves as a series of additional stimuli to the like series of stimuli applied at the other eye. If they did the revolution rate of the cylindrical shutter required for extinction of flicker in the upper binocular range LR, Fig. 80, would fall far below that required for extinction in the uniocular. This it does not do. It does not fall at all, apart from the minute difference noted by some persons as mentioned above. A similar result is obtained under the, in some ways more decisive, conditions (Experiment 4) represented in Fig. 81.

With this arrangement no observer in my experiments has ever with certainty detected difference between the uniocular and binocular images in regard to either the apparent rate of the flicker when moderately coarse or the rate of intermission required for flicker extinction. This arrangement (Fig. 81) seems the most crucial for deciding the point. In the “alternate right and left” arrangement (Fig. 78, LR, upper combination) the instants of change of phase falling together right or left, it might be that it did not matter as regards flicker sensation whether the direction of change was from light to dark or dark to light; the rates of intermission being the same right and left, and the instants of their incidence being synchronous, it might then be that as regards flicker the arrangement was only tantamount to the “synchronous right and left” arrangement (Fig. 78, lower combination) or to the uniocular intermittence of the same rate. The arrangement (Fig. 81) avoids this dilemma. Moreover it avoids both the minute reinforcement and the minute reduction of flicker inherent, according to the above experience, in the exactly “synchronous” and “alternate” arrangements. It may be termed for convenience of reference the “intermediate” arrangement. The physiological stimulation it delivers to the conjugate retina is by any mode of count delivered at twice the rate of delivery for either retina considered apart from its fellow. Yet the rate of revolution of the cylindrical lantern required to extinguish flicker in this experiment remains for the binocular image the same as for the uniocular.

There arises the question whether we may regard the dark field covering the area correspondent with that to which in the other retina a bright image is presented, as non-existent visually. That assumption has been made above, and is indicated in the diagrams (Figs. 79, 80, 81). In them, where one image is represented as uniocular, the conjugate area of the other retina is left out of the diagram altogether, as though the latter retina were non-existent, or for the time being blind. This seems permissible, when care is taken to insure absence of all detail or contour from the dark field presented to the other retina, except for the one component of the compared binocular image. When that field is perfectly void of other contours, and unchanging and borderless, it is found to matter little what depth of darkness it has; it may be a shade of gray or even a fair white, without perceptibly influencing the sensual vibrations given by the flickering image before the other eye. The condition seems comparable with the familiar disability to see the dark field presented to one closed eye, when with the other eye the observer regards a detailed image.¹⁶⁶ For these reasons the visual image resulting from the presentation of the bright disc to one eye only, as in the arrangements shown by Figs. 79, 80, 81, was regarded as being a truly uniocular product, uncomplicated by any component from the other retina. The corresponding area of this latter was considered as for the time being out of action as regards sense, not so much by darkness as by virtue of borderless void homogeneity of field, — as when eye-closure affords visual rest. Under this blankness the “retinal points” become unhitched from the running machinery of consciousness, if — and this is essential — the “corresponding” retinal area be concurrently under stimulation by a defined image. McDougall’s²³² principle of competition for energy between associate neurones seems at work here, for with both eyes shut the dark blankness of eye-closure does become visible. Even with one eye open, if its field be undetailed and homogeneous, glimpses of the “Eigenschwarz” of a closed eye become obtainable (Purkinje, Volkmann, E. Hering).

The accurately converse stimulation of the twin retinal areas might be expected to give some interference of the flicker sensations so generated. But the experimental evidence indicates absence (practically entire) of any interference between the flicker processes so initiated. The right and left “corresponding retino-cerebral points” do not when tested by flicker reactions behave as though combined or conjugate to a single mechanism. Their sensual reactions retain individuality as regards time-relations even when completely confluent as judged by reference to visual space.

B. ASYMMETRICAL FLICKER

In the foregoing experiments the flicker sensations of “corresponding” areas of the two retinae appear (almost entirely) without influence one upon another. But in other experiments the flicker test reveals very considerable mutual influence between reactions initiated at the corresponding areas.

Suppose (Experiment 5) two binocular images LR and λρ similarly combined from similar uniocular components, all individually equal in brightness and in intermission frequency. Suppose that of the components of one pair (λρ) one (ρ ) be replaced (Fig. 82) by an intermittent uniocular image (ρ′), of the same physical brightness as that giving the visual image ρ′ but of considerably higher intermission frequency. In ρ′ all flicker will disappear at slower speeds of revolution of the lantern than those required to extinguish flicker in L or R or λ. Fig. 82 represents the arrangement.

The frequency of intermission required to extinguish flicker in λρ is then found to be much lower than the frequency required for extinction of flicker in LR, or in L or R or λ separately. Thus the frequency for extinction of flicker in λρ was found (observer H. H.) to average 52.2 phases per second as against 61.9 phases per second for LR, or for L, R, or λ separately.

Screening image ρ′ out of the binocular combination λρ′, when the frequency of intermission was just high enough to free the λρ′ image from flicker, at once brought flicker into it; this disappeared immediately image ρ′ was readmitted to the combination.

In this instance the intensity chosen for the steady illumination of the conjugate area was equal to that employed for the uniocular flickering image. The duration of the light phases and the dark per revolution of the lantern was equal, and the light and dark phases of the same intensity in both. But the phenomenon obtains also when the steady uniocular image is less bright (Experiment 6, Fig. 83) or more bright (Experiment 7, Fig. 84) than the flickering uniocular with which it is combined. The following example illustrates this.

Experiment 6. In the balanced pair of binocular images LR and λρ made of carefully equalized intermittent uniocular images L, R, λ, and ρ the uniocular image ρ was replaced by one ρ′ of five times greater rapidity of intermission and giving a steady image of only ¹/₉ the brightness of the images L, R, λ, and ρ when steady. The frequency of intermission required to extinguish flicker in the binocular image λρ ′ (Fig. 83) was then found to be 72.1 phases per second, whereas in LR, and in λ, L and R separately, it was 75.5 phases per second as it had been in the previous. The steady sensation from image ρ′ therefore damped the vibration of the flickering sensation from the conjugate spot under image λ by an amount represented by 3.4 phases per second.

Experiment 7 (Fig. 84) illustrates an observation in which for the image ρ in the binocular combination λρ an image ρ′ was substituted of three times higher frequency of intermission and giving a steady image of one fifth greater brightness than the image L, R, and λ when steady. It was then found that the frequency of intermission required to extinguish flicker in the binocular image λρ ′ (Fig. 84) was 57.8 alternate equal phases (of λ) per second. Whereas in LR, and in λ, L, and R, taken separately, the number of such phases required was 63.6 per second.

The image λρ′ was distinctly brighter visually than was LR, or any of the uniocular images λ, L, and R.

These observations show, as do the observations represented by Fig. 82, that it is not merely the reduction of brightness in the combined image λρ in the arrangement shown by Fig. 83 that lessens the flicker in the latter. In fact, the observations on the plan illustrated by Fig. 84 we have the, for flicker photometry, interesting case of a brighter intermittently illuminated surface flickering less than a duller one.

Here the conditions of experiment suggest that the addition of the steady brightness at one eye to the dark phase of the intermittent at the “corresponding spot” lighten the latter, and its addition to the phase of equal brightness with it leave that practically unaltered.

That might be evidence of mutual interference between purely physiological processes initiated at the corresponding spots of the right and left retinae. But, on the other hand, the result at once suggests that the binocular product from a uniocular flickering and a uniocular unflickering image arises by a synthetic process akin to that which produces from a pair of individual uniocular brightnesses a binocular brightness near the arithmetic mean of the brightness of the two components. The rule of combination exemplified by these latter finds little solution by appeal to summation or interference of retinal and purely physiological processes.

Moreover, the supposition that the sensual reaction caused by a steady image acting at one of the pair of “corresponding” areas, is interfering with or combining with the individual phases of reaction to the intermittent image at the fellow area, is exactly the supposition that the observations dealing with symmetrical flicker show to be untenable.

UNIOCULAR AND BINOCULAR COMPARISONS

With intermittent lights throughout a wide range of ordinary intensities Talbot’s¹⁴ law is unimpeachable for the single eye; and also for the two eyes if employed together under, as is usual, arrangements practically equivalent to the “simultaneous” rightleft method for “symmetrical flicker.” It is interesting to discover how far the double retina will still observe Talbot’s law when subjected to treatment such that, if the retina did then observe the law, would indicate its integration to a functionally single retina. In other words, under a rapidly repeated stimulus, when one incidence of that stimulus has acted on a retinal point the question is: how far is it the same thing for visual brightness, whether the next incidence be upon the same retinal point or upon the twin point in the other retina? How far can the double retina, when functioning for singleness of perception in binocular vision, be considered as functionally combined to a single retina, and how far does it then react as does a single retina, if examined for Talbot’s law?

The “alternate left-right arrangement” (Experiment 1, LR) supplies the required method of stimulation. With speeds of revolution of the lantern too high to allow flickering, the binocular image LR (Fig. 79) is seen to appear of brightness equal to λρ′ and with the uniocular images λ or ρ taken singly. Therefore in the above sense, Talbot’s law not only does not hold for the double retina considered as functionally single, but no trace of observance of the law is detectable. The two corresponding points are therefore in this respect not integrated to a single retinal surface.

It was often noted that with all four lantern images of equal luminosity, using intermission frequencies too rapid to allow flicker, the brightness of the binocular combination of any two did not distinctly exceed that of the uniocular. In certain instances the binocular combination did appear just distinctly the brighter. This was for instance the case when of the four lantern images the two on the same horizontal level were combined by simple convergence. This excess of brightness is the well-known phenomenon examined by Jurin,⁷ Harris,⁸ Fechner,²⁶ Aubert,³⁷ Valerius,⁵² and others. But there occurred frequent instances in which no excess was observed in the brightness of binocular combinations over that of their carefully balanced uniocular components. In these observations the brightness of the physical images is however always much above the threshold of the light-adapted eye; and I have not made systematic observations with the eye dark-adapted. To obtain good conditions for comparison of the brightness of the binocular and uniocular images the following arrangement can be employed.

Experiment 8. Two images LR and λρ½ are placed in the visual field for mutual comparison. LR is composed of left-eye and right-eye equal and corresponding disc-shaped images as in previous experiments. λρ½ is composed of a left-eye image similar to L and R except that it lies just above or below them in the visual field. With λ’s right half is combined the image of the right half of a lantern image similar again to the others, except that its left half is screened absolutely off into the blank undetailed darkness of the general field. When this is done the two opposite visual images LR and λρ½ regarded under perfectly steady ocular fixation are stable, and no difference of brightness is discernible between them. Moreover no join is seen between the halves of λρ½ and no difference of brightness between the halves. After prolonged inspection of them rivalry becomes troublesome; but a judgment can be clearly arrived at before that happens.

In this experiment it might possibly be that equality of brightness between the halves of λρ½ is due to image ρ½ not really being in consciousness at all during the comparison. The image might possibly lapse under competition with the partly dissimilar correspondingly placed left-eye image λ. Experiments carried out by W. McDougall²²³ give validity to such possible objection. The perceptibility of the horizontal bar in the right half of the image λρ½ is guarantee however that at least part of the uniocular image ρ½ is present. But to ascertain more surely whether image ρ½ is really during the visual equation co-operating in consciousness with λ the following further arrangement can be employed.

Experiment 9 (Fig. 85). With the revolving lantern so arranged that images L, R, λ, and ρ½ are all of equal brightness when steady and unflickering, ρ½ is given at a lesser frequency of intermission, so as to flicker while the others do not. A speed of revolution of lantern is then used at which just a trace of flicker is perceptible in ρ½ when binocularly combined with λ. The equation LR = λρ½ is then found to hold while flicker is still just traceable in the right half of λρ½. There is then no join seen between the halves of λρ½ nor any difference between the brightness of the halves. So long as ocular fixation is steady no rivalry disturbs the observation.

In this case there can, I think, be no question but that the one half of λρ½ is truly binocular, for the trace of flicker is perceptible during the actual performance of the comparison. Yet no difference of brightness is perceived between LR and λρ½, and the two lateral halves of λρ½ compared together seem of like brightness.

Even when the binocular image does show the well-known slight excess of brightness over its uniocular components it, under some conditions (v. supra, “alternate” arrangement), flickers no more or even less than they.

It is doubtful therefore to me whether the slight excess in brightness of the binocular image over its two equal uniocular components is really explicable as summation of the intensities of the reactions at the corresponding spots of the two retinae. Valerius⁵² measured the increase to be one fifteenth of the brightness of the uniocular image. Aubert’s³⁷ diagram gives it as less than one thirtieth. Aubert says it is not perceptible with brightness greater than that of white paper in diffuse daylight indoors.^65a

In certain modes of experiment a uniocular image used as a standard for comparison might itself be suffering some reduction in brightness owing to slight combination with the dark field presented concurrently at the corresponding retinal area. But “rivalry” should reveal such influence. A better definition and greater vividness of detail assured by better accommodation and convergence under binocular regard, might possibly give an appearance of greater brilliance and intensity. But these are only suggestions.

I conclude that, with ordinary intensities of illumination, although a binocular image does sometimes appear of slightly greatervisual brightness than either of two similar uniocular images composing it, more often it has a visual brightness not perceptibly different from that of either of its two co-equal uniocular components. The case then falls within a general rule regarding binocular brightness attested by all observations I have on that subject. A binocular brightness compared with its uniocular components is of value not greater than the greater of those, nor less than the lesser of them; when free from oscillations of rivalry its value is somewhat, but not far, above the arithmetic mean of the values of the two uniocular components as expressed by the measures of the physical stimuli yielding them.

The various combinations cited in Experiments 2, 3, 4, 5, 6, 7, and 8 have all, when steady and unflickering, given brightnesses illustrating the above rule. Other illustrations are

But I have not worked with combinations where the physical luminosity of one uniocular component has been less than ¹/₁₃th the physical luminosity of the other. It was near this limit that Aubert, and just beyond it that Fechner, noted decline of the darkening effect of the darker component. In my own few observations beyond that point the oscillations of rivalry have made judgment difficult. The more manageable examples are but demonstrations of “Fechner’s paradox,” and fall under the above general rule. Hering³⁴ has suggested that rivalry is really occurring even with similar and right and left uniocular images; he says these react according to a law of “complemental shares,” and offers a theory, such as the name he gives implies, in explanation of the phenomenon. My Experiment 9 seems to offer difficulty to such a view.

Binocular combination of a less bright image with a more bright gives a visual image of less brightness than the latter (as stated in the rule above). But the application of the less bright physical image to the same uniocular area as the more bright gives a visual image of brightness greater than either.

As above described, a steady image presented on an area of one retina “damps” the flicker of a flickering image concurrently presented at the corresponding area of the other. A steady image actually physically superposed on the same retinal area as a flickering one also reduces the latter’s flicker: this latter is of course in accordance with Weber’s law. The modes of interference seem incomparably different in the two cases; and experiment shows that the two interferences are often quite of different value.

Finally, to touch on “predominance of contours” Its facts, established by so many workers, are among the most significant concerning the difference between binocular and uniocular fusion of visual reactions. I will merely give one illustration which seems specially instructive for the point before us.

Experiment 11. A steady unflickering disc-shaped image L is present to the left eye: across the disc is a narrow dark line. An image R of similar size and shape but without the dark line is presented to the corresponding area of the right eye. If the luminosity of L is progressively diminished, a value of luminosity is reached at which its cross line, though visible when L alone is observed (e. g. right eye closed) is lost or uncertain in the binocular image RL. This reduction of the luminosity of L much exceeds the reduction at which its cross-line is lost when image R is concurrently thrown on the same area of the same retina, i. e. left retina. Thus, in one experiment the diminution of luminosity of L required for loss of the cross-line under the physical superposition of R and L on the same retina was 84 per cent, while the diminution of luminosity of L required for loss (or great uncertainty) of the line in the binocular image was 96 per cent.

Not only the ease, but the mode of disappearance of the cross-line, is significantly different in the two cases. In the “physical superposition” the dark line became gradually thinner and fainter, and finally imperceptible, as the image L is lessened in luminosity. In the case of binocular fusion the dark line oscillates out of and back into sensation more and more, the disappearances predominating more and more as the darkening of L proceeds. At a reduction of 84 per cent of the luminosity of L the cross-line was steady, dark, and sharp in the binocular image.

Our aim has been information as to the nature of the conjunction between the uniocular components in certain simple binocular sensations. The question concerns the nature of the tie between “corresponding retinal points,” meaning by “retinal point” the retino-cerebral apparatus engaged in elaborating a sensation in response to excitation of a unit area of retinal surface.

That a sensation initiated from corresponding retinal points is commonly referred without ambiguity to a single locus in visual space has often been regarded (Newton,⁵ Wollaston,¹² Rohault,³ Joh. Müller²⁰) as evidence of community of the nerve apparatus belonging to the paired retinal points. Their visual image appears single. Wollaston supposed the twin points attached to one and the same nerve-fibre, which bifurcated at the chiasma. Rohault and Muller supposed the points to be served by twin fibres “from one and the same ganglion-cell in the cerebral substance.” Later (cf. Aubert³⁷), the visual singleness, spatial fusion of right and left impressions to a single perception, was taken to mean confluence of the nerve-processes, started in right and left retinae respectively, to “a single common centre or point of the sensorium.” The discovery later still that the fibre-tracts from corresponding halves of the retinae both go to the occipital region of one and the same hemisphere has also been inferred to mean a spatially conjoint visual sensorium common to both retinae (e. g. Ramon-y-Cajal, Schäfer). But in such questions the inferences obtainable from mere anatomical features are equivocal and often remote in bearing. Were there to exist such a common mechanism situate as a unit at conjunction of the two convergent systems and were phases of excitement timed so to arrive from one retina as exactly to fill pauses between excitations transmitted from the other, then there should be evidence of this in the time-relations of the phenomena induced. The state of excitement should tend to be maintained across periods that would otherwise checker it as pauses.

The retino-cerebral apparatus may be regarded as a structure of linked branching nerve-elements forming a system which expands as traced centrally from the retinal surface. It may be figured as a tree, with its stem at the retina and an arborization spreading into the brain, its ramifications there penetrating a vast cerebral field, interlacing with others in a cerebral forest composed of nervous arborizations. The simile fails, because in the nervous forest the arborizations make functional union one with another. Is the fusion of the perceptions adjunct to paired “corresponding points” the outcome of a close concrescence of their neuronic or neuro-fibrillar arborizations, making of them practically a single upgrowth common to twin (right and left) stems rooted in the corresponding retinal units? If so, how low down, how close to their origin, are the twin systems grafted together, giving structural community to all the superstructure?

In the chain of nerve-elements attached to a sense-organ we infer in general that to the activities of the most peripheral links per se psychical events are not adjunct. Psychical processes, beginning with least complex and ascending toward development through many grades, attach to the chain in such a way that for the simplest only the more peripheral portions of the chain need be connected with the sense-organ, while for the more complex the central portions in addition become more extensively involved. But in the higher reactions of definite physical aspect, e. g. sense-perceptions, the lower apsychic and less definitely psychic activities are also implicate. Where from two sense-organs, e. g. two units of retinal surface, the two nerve-chain arborizations are mutually connected, so that the lower activities of the one affect by low-level side connections the elements forming the other, there analysis must fail to distinguish in the full reaction what higher components may be separately referable to one only of the two individual chains. The processes apsychic, or so indefinitely psychic as to baffle introspection, at root of those amenable to introspection, must by their coalescence defeat attempt to trace the final psychical product to either of its two possible sources, so long as both sources are open for its origination.

Were the nervous reactions initiated at twin points of the retinae early in its path along the retino-cerebral nerve-chain, to enter mechanisms common to both, there must, under “alternate” or “synchronous” right-left arrangement of stimuli (Fig. 78), be interference, algebraic summation, etc., a coalescence of events which, though apsychical in itself, would involve subsequent confusion together of the sense-reactions of the two eyes. A state of things wholly different from this is revealed in the results of our experiments. And it would amount to the same thing whether two quickly successive flashes of a light fell both on one and the same member of a pair of “corresponding points,” or whether the first fell upon one member, the second upon the other. But the experiments show that the effect in the two cases is widely different. Talbot’s law is not applicable to the double retina, that is, to the two retinae functioning together in binocular vision. The experimental results go to disapprove the existence of any such fusion or interference between the apsychical or even the subperceptual events arising from corresponding retinal points. At most they indicate hardly discernible traces of such interference (Experiment 1). They indicate, on the contrary, that such simple forms of binocular perception as have been dealt with here are themselves fusions of elaborated uniocular sensations. Since left and right end-results emerge pure, “hybridization” has not mixed the early stages in their evolution.

But the difference between the modes of stimulation left and right is a difference that, although it should be potent if the left and right physiological machinery were conjoined to unity, should constitute no difference when left stimulation is compared with right stimulation by the perceptual product which each yields. The left eye and right eye flickering visual images, each viewed singly, do NOT (apart from the faint cross-line for recognition) differ to introspection. If the sensations derived from the left eye and right eye respectively appear under introspection indistinguishably alike, what ground is there for mutual interference between them? It is much as though, of the left and right lantern images each were seen by one of two observers, with similar vision, and as though the minds of the two observers were combined to a single mind. It may be recalled that binocular unification of images, as we possess it, seems a comparatively late achievement of phylogenetic evolution.

When the visual product of the two retinae is thus regarded it is not surprising that Talbot’s law fails for the binocular cyclopean retina. It fails because the binocular sensation is a fusion of uniocular sensation and from no two similar sensations can a resultant sensation be compounded different from its components. Were Talbot’s law to hold in the above sense for the binocular retina there would, under the “alternate left-right arrangement” (symmetrical flicker), at rates of intermission too high for flicker, result from an image L of brightness x, and an image R of similar brightness x, a combined image LR of brightness x + x, the value of the summed brightness being in accord with the Weber-Fechner rule of summation of sensual intensities. But, as shown, not only does this summation not occur, but nothing like it occurs. The binocular result most often does not perceptibly differ from either of its two co-equal components.

But the experiments with uniocular components dissimilarly flickering, and with flickering components concurrent with steady components, do evidence (unlike the other experiments) interference between the two eyes. This result might be interpreted as the outcome of community of the physiological mechanisms attaching to the paired “corresponding retinal points.” But the other experiments negative the existence of this community. And the explanation just offered for the absence of interference in the other experiments will account for the presence of interference in these. From two components perceptibly differing between themselves in regard to some quality (e.g. flicker) a single combined sensual quality is obtained, intermediate between that of the two components taken singly. If the perceptible difference, e. g. in flicker, between the components is wide, the fusion is liable to phasic oscillations of predominance of one or other component. Where the difference in flicker is wide, such “rivalry” between the right and left components is in fact not unfrequently seen. One component may at the height of its phase be alone perceptible at the focus of attention, the other component being inhibited out of focal attention or even out of conscious vision altogether. The inference is that only after the sensations initiated from right and left “corresponding points” have been elaborated, and have reached a dignity and definiteness well amenable to introspection, does interference between the reactions of the two (left and right) eye-systems occur. The binocular sensation attained seems combined from right and left uniocular sensations elaborated independently.

And in harmony with this view stands the evidence adduced for the rule formulated regarding the relation of binocular to uniocular brightness. Further, the difference between the sensual result of superposition of two similar images upon one and the same area of a single retina, and upon twin areas of the two retinae, could hardly be so great as it is, did apsychical or subsensual reactions underlying “brightness” combine or interfere in the two retinal systems. The binocular combination must be a synthesis of a left-eye with a right-eye sensation. Similarly, the “prevalence of contours” in binocular vision, and the phenomena of “retinal rivalry,” are explicable if each member of a pair of corresponding points yields a sensual entity which, when not widely dissimilar from that yielded by its twin point, fuses with that to a binocular sensation. In “retinal rivalry” we have an involuntarily performed analysis of this sensual bicompound. The binocular perception in that case breaks down, leaving phasic periods of one or other of the simpler component sensations bare to inspection.

W. McDougall,²⁹⁷ in applying to “retinal rivalry” and “prevalence of contours” his principle of competition of interrelated nerve-elements for energy, also argues a “separateness of the visual cortical areas for the two eyes.” He brings forward striking experiments in evidence of this. In one of these he²²³ shows that an after-image, left from excitation of one retina, is more strongly revived by subsequent weak diffuse excitation of that same retina than of its fellow. More recently, in experiments proving reinforcement of visual sensations by the activity of the ocular muscles, as evidenced by after-image observations, he^261a shows that activity of the intrinsic muscles of an eye sends up to the brain an influence, reinforcing the activity of the cerebro-retinal tract of that eye, while it exerts no such effect upon the corresponding tract of the other eye, or exerts it in a minor degree only. With this separateness of the mechanisms, wherein are produced the sensations generated in the two retinae, our results by a different line of experimentation accord well.

The compounding together of right and left images really nonidentical but not widely dissimilar, is (Panum, Hering) the basis of visual “relative depth-perception.” The compounding of visual images partly dissimilar — flickering with unflickering — seems a simpler case in the same category of synthetic actions. In our flicker experiments the visual components do not differ as to space-attributes, and their combination has therefore no resultant differential space-attribute. But the synthesis gives in each case a compromise between the components in regard to the attribute wherein they do differ; in the flicker experiments, that is in regard to the sensual steadiness of the brightness. This amounts to the same as the rule formulated above for binocular combination of brightness of different intensities, but steady.

Our experiments show, therefore, that during binocular regard of an objective image each uniocular mechanism develops independently a sensual image of considerable completeness. The singleness of the binocular perception results from union of these elaborated uniocular sensations. The singleness is therefore the product of a synthesis that works with already elaborated sensations contemporaneously proceeding.

The cerebral seats of right-eye and left-eye visual images are thus shown to be separate. Conductive paths no doubt interconnect them, but are shown to be unnecessary for visual unification of the two images. The unification of a sensation of composite source is evidently associated with a neurone arrangement different from that which obtains in the synthesis of a reflex movement by the convergence of the reflexes of allied arcs upon its final common paths.

Here we seem to have therefore contemporaneity of itself sufficing for sensual synthesis, without necessarily any spatial fusion of the neural processes or mechanisms involved, i. e. without spatial confluence to a unit apparatus. As mentioned above, W. McDougall’s experiments on after-images lead to a like conclusion. The foundation of new correspondences between retinal points in cases of squint (Tschermak) strengthen the same view. McDougall has recently well summarized the position. But it is one not generally admitted by physiologists or psychologists. Ziehen^242a writes: “Schon physiologisch ist die Verschmeltzung der beiden Netzhautbilder dadurch vorbereitet dass die Erregungen welche auf den linken Hälften beider Netzhaute auftreten, vermoge der eigenthumlichen partiellen Sehnerven-kreuzung susammen in die rechte Grosshirn hemisphere gelangen, and umgekehrt.” And this was the view of Joh. Müller and of Aubert, and is advanced on histological grounds by Ramon-y-Cajal.

The results bear also on the production of sensual reactions and states more complex than those of the examples taken. Hartmann, as quoted by McDougall, writes: “Only because one part of my brain has a direct communication with the other is the consciousness of the two parts unified. Could we unite the brains of two human beings by a path of communication equivalent to cerebral fibres both would no longer have two but one consciousness.” There is no denying the extreme importance and the vast actual extent of the spatial conjunction of cerebral elements by conductive channels in sensual and perceptual reactions. Yet I cannot but think that its limitless postulation leads not so much to explanation of the high degree of unity of the individual mind as to an ultimate fallacy which Professor James has trenchantly termed that of “the pontifical cell.” Pure conjunction in time without necessarily cerebral conjunction in space lies at the root of the solution of the problem of the unity of mind.

Since convergence of the conductors from corresponding halves of the retinae to the same field of brain-cortex does not signify physiological conjunction of right and left sense-impressions, can we decipher at all what it does mean? To do so does not seem difficult, and displays strikingly the different value and directness of spatial union of conducting-paths for motor synthesis and for psychical respectively. In animals with overlapping visual fields the lateral movements of the eyeball have a mutual relation different from the ordinary relations of the movements of a unilaterally placed organ, e. g. a limb. Especially is this the case where the overlap of the visual fields is extensive, e.g. where the ocular axes are parallel. The horizontal movements of each eyeball are balanced about the primary line of vision of the globus in its habitual resting attitude, that is, in man, straight forward. That line sensually, as shown by introspective experiment, lies in the median sagittal plane of the head (Hering). Hence the term ‘Cyclopean’ has been applied to the biunial eye of human binocular vision. Finding the median vertical plane of sight of the resting eyeball to correspond with the median sagittal plane of the body, we may assume that the motor reflexes deal with the eyeball conformably with that; otherwise there would be disaccord between the reflexes and the sensations. Therefore we must in any general consideration of the taxis of the lateral movements of the two eyeballs transfer in thought each eyeball from its own actual sagittal plane to the median sagittal plane of the head; and this latter corresponds in the resting position with the sagittal median plane of the animal as a whole.

Each lateral muscle of the eyeball comes therefore to bear to the median plane of the body the same relation as does a limb on one side of the body. Thus, the external rectus muscle of the right eyeball bears the same lateral relation to the median sagittal plane of the body as does the right arm. And the internal rectus muscle of the left eyeball bears the same lateral relation to the median sagittal plane of the body as does the right arm. Now a general arrangement evident in the cerebral cortex is that the taxis of muscles lying to right of the median sagittal plane is entrusted to the left hemisphere, and vice versa. It is in accord with this that in animals with overlapping visual fields the horizontal movement of the eyes to one side should for both eyeballs be represented in one and the same hemisphere. And as a fact the conjugate movement is found represented for both eyeballs together in each hemisphere. If we regard, and it was shown above that we may do so, the median sagittal planes of both eyeballs as identical with the median sagittal plane of the head, they are identical with each other, and the scheme of cortical representation may be expressed thus: a point in the right retina and its twin point in the left demand each of them identical movement of the two eyeballs when, apart from convergence, those points excite their own replacement by the fovea (i. e. when initiating a gaze). It is obvious that the paths from the visual cortex of each side to the eyeball muscles — experiment shows such a path to exist — is connected therefore with both the right and left twin points. That is, it is a common path. The confluence of conductors from the two retinae to the same cortical field, though not uniting their retinal impressions, gives them access to a common efferent path which both must use.^* At entrance to every common path lies, as shown before, a co-ordinating mechanism. A co-ordinative mechanism is thus obtained. This “common path” with its bilateral twin origin impinges in its turn, directly or indirectly, on the motor neurones for the lateral eye-muscles, the final common paths. We have therefore to alter such a scheme as that furnished by Cajal by attaching his convergent paths to efferent paths, and by divesting their supposed nodal cortical point of its hypothetical powers as a sensual Deus ex machina. And we thus meet another instance of convergence of afferent paths leading to motor synthesis, but not, or only remotely, to sensual. Seen in this light the gulf between sensation and movement looms even wider than was allowed for in the tentative suppositions which prompted the above experiments on flicker.

We are thus warned against any hasty conclusion that the neural mechanisms which synthesize reflex movements illustrate in their arrangement also those concerned where sensual fusion is the phenomenon. But that does not invalidate a broad practical inference which study of the nervous system in regard to motor reaction allows. This inference is that toward the solution of the problems of motor taxis help is obtainable by appeal to characters evident in sensual reaction. This practical inference need not in the least involve any doctrinal attitude whatever toward the hypothesis of psycho-physical parallelism. It may proceed quite apart from that. It simply insists on the likeness of nervous reactions expressed by muscular and other effector-organs to reactions whose evidence is sensual. It insists on this likeness being close and fundamental enough to make each of the two classes of phenomena of use to the student of the other. A number of excellent investigators hold, on the opposite hand, that the study of the two should proceed apart even more rigidly than they do at present. Confusion has, it is true, been caused in both by the loose application of the terms of the one set of phenomena to the other. But to disregard the many significant similarities which exist between the two sets is, it seems to me, to throw away one of the best instruments for discovery in both. We saw how suggestive psychological data prove for classifying the various species of receptors considered as initiators of reflexes. The after-discharge of a nervous arc finds expression not only in reflex movement but in, for instance, a visual after-image. Centripetal impulses from eye-muscles reinforce visual (i. e. extero-ceptor) sensations (Macdougall) just as centripetal impulses from the leg muscles reinforce reflex movement induced from the skin (extero-ceptor) of the foot. The “immediate spinal induction” exemplified by reflexes has a counterpart in visual irradiation. Visual contrast, if translated into terms of reflex contraction, bears close resemblance to “successive spinal induction.” The features of fatigue repeat themselves in both sets of phenomena. Receptors which initiate reflex movements adapted in regard to objects at a distance initiate as sense-organs sensations projected into circumambient sensual space. Receptors which initiate reflex movements advantageous in regard to some locus of the surface of the body itself, e. g. removal of irritation thence, initiate as sense-organs sensations referred to that same locus. Instances might be multiplied, but they have risen prominently in several of the foregoing lectures, and are sufficiently before our minds now. A practical inference from them is that physiology and psychology, instead of prosecuting their studies, as some now recommend, more strictly apart one from another than at present, will find it serviceable for each to give to the results achieved by the other even closer heed than has been customary hitherto.

Besides this similarity of time-relation and other features between the physiological and the psychical signs of neural activity, another link connects the psychological and the physiological for the biologist. To the physiology of pure reflexes, that is, reflexes devoid of psychical accompaniment so far as introspection can discover, psychological interest nevertheless attaches, and on a very distinct ground. This ground of connection is seen if inquiry is followed along the animal scale in the direction from higher forms to lower rather than by the usually more favorable reverse approach. This is partly because we directly observe psychical phenonema by introspection only, that is, only in ourselves; and the facts discovered by introspection are applicable to other beings the more readily the more those beings resemble ourselves, namely, are animals ranking near to man.

Pure reflexes are admirably adapted to certain ends. They are reactions which have long proved advantageous in the phylum, of which the existent individual is a representative embodiment. Perfected during the course of ages, they have during that course attained a stability, a certainty, and an ease of performance beside which the stability and facility of the most ingrained habit acquired during an individual life is presumably small. But theirs is of itself a machine-like fatality. Their character in this stands revealed when the neural arcs which execute them are separated, e. g. by transection of the spinal cord, from the higher centres of the nervous system. They can be checked, it is true, as we have seen, by collision with other reflexes as ancestral and as fatally operative as themselves (Lectures V and VI). To these ancient invariable reflexes, consciousness, in the ordinary meaning of the term, is not adjunct. The subject as active agent does not direct them and cannot introspect them.

Yet it is clear, in higher animals especially so, that reflexes are under control. Their intrinsic fatality lies under control by higher centres unless their nervous arcs are sundered from ties existing with those higher centres. In other words, the reactions of reflex-arcs are controllable by mechanisms to whose activity consciousness is adjunct. By these higher centres, this or that reflex can be checked, or released, or modified in its reaction with such variety and seeming independence of external stimuli that the existence of a spontaneous internal process expressed as “will” is the naïve inference drawn. Its spring of action is not now our question; its seat in the nervous system seems to correspond with that of processes of perceptual level. It is urgently necessary for physiology to know how this control — volitional control — is operative upon reflexes, that is, how it intrudes and makes its influence felt upon the running of the reflex machinery. How is the cough, or eye-closure, or the impulse to smile suppressed? How is the convergence of the eyeballs, innately associate to visual fixation of a near object, initiated voluntarily without recourse to fixation on an object? Or how is the innate respiratory rhythm voluntarily modified to meet the passing requirements of vocal utterance? No exposition of the integrative action of the nervous system is complete, even in outline, if this control is left without consideration. Reflexes ordinarily outside its pale can by training be brought within it. The actor, it is asserted, can shed tears at will, or blush or blanch. Occasional instances are recorded of power to slow the rhythm of the heart at will; others, of power to suppress the reflex of swallowing when it has entered on its pharyngeal stage. Volitional movement can certainly become involuntary, and, conversely, involuntary movements can sometimes be brought under subjection to the will. From this subjection it is but a short step to acquisition of coördinations which express themselves as movements newly acquired by the individual. The controlling centres can pick out from an ancestrally given motor reaction some one part of it, so as to isolate that as a new separate movement, and by enhancement this can become a skilled adapted act added to the powers of the individual. In regard to the ring finger, the motor co-ordination ancestrally provided gives extension of that finger only in company with the fingers on each side of it. We can soon train ourselves to lift the ring-finger alone without the others. The “will” dissociates the ancestral co-ordination. Similarly we can acquire the power to move a part which neither reflexly nor otherwise would seem to come within the scope of our voluntary innervation, although of course there must be motor nerve and muscle upon which our innervation can operate. Thus, we can learn to retract the pinna of the ear; the movement is at first accompanied by other facial movement, but later with practice it becomes executable without other facial movement. A new reaction and co-ordination has been gained by the individual.

The transition from reflex action to volitional is not abrupt and sharp. Familiar instances of individual acquisition of motor co-ordination are furnished by the cases in which short, simple movements, whether reflex or not, are by practice under volition combined into new sequences and become in time habitual in the sense that though able to be directed they no longer require concentration of attention upon them for their execution. As I write, my mind is not preoccupied with how my fingers form the letters; my attention is fixed simply on the thought the words express. But there was a time when the formation of the letters, as each one was written, would have occupied my whole attention.

Volitional control of reflexes is a question of co-ordination not explicitly before us previously in these lectures. Its analysis has not indeed proceeded far. We may premise that some extension of the same processes outlined in Lectures V and VI, as operative in simultaneous combination and in successive combination of reflexes, must be operative in this control. There we saw reflexes modifying each other, and the more complex reactions being built up from simpler and more restricted ones. Some extension of the same process should, in view of our inferences regarding the nature of the dominance of the brain (Lecture IX), apply here also.

It is significant that, although the reflexes controlled are so often unconscious, consciousness is adjunct to the centres which exert the control. A biologist, Professor Lloyd Morgan, has urged that “the primary aim, object, and purpose of consciousness is control. Consciousness in a mere automaton is a useless and unnecessary epiphenomenon.”^* A somewhat similar thought rose incidentally to our lips in a previous lecture (Lecture IX). The pleasure-pain accompaniment of reflexes has often been interpreted as carrying that meaning. Certain it is that if we study the process by which in ourselves this control over reflex action is acquired by an individual, psychical factors loom large, and more is known of them than of the purely physiological modus operandi involved in the attainment of the control. Hence, psychological studies have been more numerous than physiological in this field. It is found that kinæsthetic sensations of the movement to be acquired or controlled, though helpful, are less important than the resident sensations from the part in its “resting” state. These latter, with the power to focus attention upon them, appear, in a number of instances, to be a most necessary condition for the acquirement of the control. And in the monkey, voluntary control of a limb is largely lost when the limb has been rendered apæsthetic.¹⁵⁷

A biological inference arises at this point. We have admitted that the organs to which psychosis is adjunct, namely, the brain, and especially in higher vertebrates the cerebral hemispheres, supply the surest touchstone to rank in the scale of animal creation. That is to admit, in other words, that development of these organs constitutes, on the whole, the best criterion to the success of an animal form in the competition which lies at the root of animal evolution. These organs, we have just seen, are the organs of nervous control; and that control is exercised mainly in the perfecting and readjusting of manœuvres of ancient heritage. The way in which we ourselves acquire a new skilled movement, the means by which we get more precission and speed in the use of a tool, the handling of an instrument, or marksmanship with a weapon, is by a process of learning in which nervous organs of control modify the activities of reflex centres, themselves already perfected for other though kindred actions. Our process of learning is accompanied by conscious effort. These nervous organs of control form, therefore, a special instrument of adaptation and of readjustment of reaction to better suit requirements which may be new. New adaptations whence the individual may reap benefit are thus attained. The more complex an organism, the more points of contact it has with the environment, and the more frequently will it need readjustment amid an environment of shifting relationships. These nervous organs of control being organs of adjustment will be more prominent the further the animal scale is followed upward to its crowning species, man. And these organs which give adjustability to the running of the reflex machinery, as such, seem themselves — perhaps, by reason of their constant relative newness — to be among the most plastic in the body. In man and the species near him, these organs are most developed, and their mechanisms are cerebral. These cerebral mechanisms constitute the clearest criterion of evolutional success. In these types it is cerebral function which best compasses that modification of old and that development of new reaction, which perfects the adaptation of the individual to the environment. The relatively high development in man of this organ for individual adjustment of reactions makes him the most successful animal on earth’s surface at the present epoch. No doubt the greater part of all this adjustment of reaction will, in his case, as he stands now, come under intellectual activity. In him reason enables the individual profitably to forecast the future, and to act the more suitably to meet it, from memory of the past. Mere experience can, however, apart from reason, mould nervous reactions in so far as they are plastic. The “bahnung” of a reflex exhibits this faculty in germ. In the humble spheres of nervous activity, such as alone fall within the scope of these lectures, simple sensori-motor experience seems to count for more than reason in the actual process of acquiring new motor co-ordinations. Of course reason, directing effort, counts in the selection of the field of operation of motor experience. But the inefficacy, as a means to arrive at a new motor correlation, of instruction merely verbal, or of ideas constructed without motor experience, is common knowledge. To learn skating or racquets by simple cogitation or visual observation is, of course, impossible. Here mere sensori-motor experience is more valuable than any course of reasoning can be. Hence the training for a new skilled motor manœuvre must be simply ad hoc, and is of itself no training for another motor co-ordination, — apart from the well-known mutual influence of training on symmetrical parts of the body. Yet, in high animal types, the connection between skilled movements and the so-called “motor” region of the cortex cerebri, and the defect in these which injury of that region entails, countenances the belief that the “experience” involved in this training, though not rational, is cerebral. The compensation of co-ordinative defects which the cerebrum accomplishes after cerebellar or labyrinthine lesions, points to a similar conclusion. And we must remember that though the mere sensori-motor experience counts for so much in the mastering of a new movement, they are perceptual, and in man rational, processes which initiate, maintain, and guide effort toward acquirement of an act which is new.

We thus, from the biological standpoint, see the cerebrum, and especially the cerebral cortex, as the latest and highest expression of a nervous mechanism which may be described as the organ of, and for, the adaptation of nervous reactions. The cerebrum, built upon the distance-receptors and entrusted with reactions which fall in an anticipatory interval so as to be precurrent (Lect. IX), comes, with its projicience of sensation and the psychical powers unfolded from that germ of advantage, to be the organ par excėllence for the readjustment and the perfecting of the nervous reactions of the animal as a whole, so as to improve and extend their suitability to, and advantage over, the environment. These adjustments, though not transmitted to the offspring, yet in higher animals form the most potent internal condition for enabling the species to maintain and increase in sum its dominance over the environment in which it is immersed. A certain measure of such dominance is its ancestral heritage; on this is based its innate right to success in the competition for existence. But the factors and elements of that competition change in detail as the history of the earth proceeds. The creature has to be partially readjusted if it is to hold its own in the struggle. Only by continual modification of its ancestral powers to suit the present can it fulfil that which its destiny, if it is to succeed, requires from it as its life’s purpose, namely, the extension of its dominance over its environment. For this conquest its cerebrum is its best weapon. It is then around the cerebrum, its physiological and psychological attributes, that the main interest of biology must ultimately turn.

Bibliographical References