'Exner and Charpentier have pointed out that the peripheral retina is more sensitive to flicker than the central' (33)

'Hering [note: 'Beiträge z. Physiologie, Heft v. S. 310, Leipzig, 1864'] suggests that rivalry is really occurring even with similar right and left uniocular image; 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.' (50)

'It is doubtful... 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. Valerius [note: 'Poggendorff's Annalen, Bd. CL. S. 117, 1873.'] measured the increase to be one-fifteenth of the brightness of the uniocular image. Aubert's [note: 'Physiologie d. Netzhaut, S. 286, Breslau, 1865.'] diagram gives it as less than one-thirtieth.' (49)

That a perception initiated from corresponding retinal points is commonly referred without ambiguity to a single locus in visual space has often been regarded... as evidence of community of the nerve apparatus belonging to the paired retinal points... Later (cf. Aubert), 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 [note: Physiol, d. Netzhaut, 1865.']."' (53)

'It is doubtful... 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. Valerius [note: 'Poggendorff's Annalen, Bd. CL. S. 117, 1873.'] measured the increase to be one-fifteenth of the brightness of the uniocular image. Aubert's [note: 'Physiologie d. Netzhaut, S. 286, Breslau, 1865.'] 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 [note: 'Physiologische Optik S. 500, Leipzig, 1876']' (49)

'the binocular brightness of two component uniocular brightnesses, of such order of intensity as used in these experiments (and with the eye not adapted for dark [note: 'Piper, Zeits. f. Psych. u. Physiol. d. Sinnesorgan., XXXII. 161, 1903.']), seems to lie near the arithmetic mean of the two components' (43)

'The chief evidence for some slight low-level communication between the right and left eye-systems, elicited by my work, has been the slightly lower frequency of stimulation found necessary for flicker-extinction under 'alternate left-right ' excitation than under 'simultaneous.' The slight excess of brightness of the haploscopic over the uniocular image, recorded by so many observers, and shown by Piper [note: 'Zeits. f. Psychol. u. Physiologie. d. Sinnesorgane., XXXII. 161, July, 1903.'] to be much more considerable for the dark-adapted eye, may be taken as evidence in the same direction.' (58-59)

'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 [note: 'Smith-Kästner, Lehrbegriff der Optik, 1755.'], Harris [note: 'Optiks, 1775.'] Fechner [note: 'Abhandlung d. Akad. Wiss. Leipzig, VII. 423, 1860.'], Aubert [note: 'Physiologie d. Netzhaut, S. 287, Breslau, 1865.'], Valerius [note: Poggendorff's Annal. Bd. 150, S. 17, 1873.'] and others.' (47)

'It is doubtful... 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. Valerius [note: 'Poggendorff's Annalen, Bd. CL. S. 117, 1873.'] measured the increase to be one-fifteenth of the brightness of the uniocular image.' (49)

'That a perception initiated from corresponding retinal points is commonly referred without ambiguity to a single locus in visual space has often been regarded (Newton [note: 'Opticks, Quer. 112.'] Wollaston [note: Philosoph. Trans. London, 1824.'], Rohault [note: 'Physique, I. 31.'], Joh. Müller [note: 'Elements of Physiol. Vol. II. p. 1199, Baly's edit. 1843.'] as evidence of community of the nerve apparatus belonging to the paired retinal points.' (53)

'That a perception initiated from corresponding retinal points is commonly referred without ambiguity to a single locus in visual space has often been regarded (Newton [note: 'Opticks, Quer. 112.'] Wollaston [note: Philosoph. Trans. London, 1824.'], Rohault [note: 'Physique, I. 31.'], Joh. Müller [note: 'Elements of Physiol. Vol. II. p. 1199, Baly's edit. 1843.'] as evidence of community of the nerve apparatus belonging to the paired retinal points. Rohault and Miiller supposed the points to be served by twin fibres "from one and the same ganglion-cell in the cerebral substance."' (53)

'Helmholtz in opposition to Panum [note: 'Physiologische Untersuchung über das Sehen mit zwei Augen, Keil, 1858.'] argued in favour of a purely psychical origin for 'prevalence of contours.' He invoked an explanatory 'direction of attention.' An inference he drew at the time regarding retinal rivalry accords with the inference drawn above from the flicker observations dealt with here, viz.[note: 'Physiologische Optik, 2 Auf., Leipzig, 1896, S. 921.']: "dass der Inhalt jedes einzelnen Sehfeldes, ohne durch organische Einrichtungen mit dem des anderen verschmolzen zu sein, zum Bewusstsein gelangt, und dass die Verschmelzung beider Sehfelder in ein gemeinsames Bild, wo sie vorkommt, also ein psychisches Act ist." A finely illustrative experiment on contours given by E. Hering [note: 'Hermann's Handbuch d. Physiologie, Bd. III. S. 921.'] is applicable in the same sense.' (58)

'Of binocular colour mixture E. Hering writes [note: Hermann's Handbuch der Physiologie, Bd. III. Tl. I. S. 596, 1879.']: "Hat man durch haploskopische Betrachtung zweier farbigen Flächen eine Mischfarbe erhalten und lässt dann genau dieselbe farbigen Lichtmengen auf eine und dieselbe Netzhautstelle fallen, so ergibt sich eine ungleich hellere oder weisslichere Mischfarbe." "Mischt man aber die beiden Farben binocular, so ist die resultirende Mischfarbe nur ungefähr gleich hell, wie die Einzelfarbe. Diese Thatsachen genügen schon, um selbst in den Fällen, wo die binoculare Mischung vollkommen gelingt, dieselbe der unocularen nicht gleichzustellen." With this conclusion my rule given above (page 50) is in complete agreement.' (59)

'Helmholtz in opposition to Panum [note: 'Physiologische Untersuchung über das Sehen mit zwei Augen, Keil, 1858.'] argued in favour of a purely psychical origin for 'prevalence of contours.'' (58)

'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 [note: 'Smith-Kästner, Lehrbegriff der Optik, 1755.'], Harris [note: 'Optiks, 1775.'] Fechner [note: 'Abhandlung d. Akad. Wiss. Leipzig, VII. 423, 1860.'], Aubert [note: 'Physiologie d. Netzhaut, S. 287, Breslau, 1865.'], Valerius [note: Poggendorff's Annal. Bd. 150, S. 17, 1873.'] and others.' (47)

'Exner and Charpentier have pointed out that the peripheral retina is more sensitive to flicker than the central, and Exner has shown that an area only 1330µ from the fovea is more sensitive in this respect than the fovea itself. In the observations now in consideration the flicjker sensitivity of points only 750µ from the foveal centre was found perceptibly greater than that of the foveal centre itself.' (33)

'McDougall' [note: '"The Principle underlying Fechner's 'Paradoxical Experiment' and the Predominance of Contours in the struggle of two Visual Fields." This Journ. p. 114.'], in applying to 'retinal rivalry' and 'prevalence of contours' his principle of competition of inter-related 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 he [note: Mind, 1901, N.S. X. p. 56.'] 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 [note: Mind, 1903, N.S. XII. p. 473.'] 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.' (58)

'Experiment 8. Two images LR and λp½ were placed in the visual field for mutual comparison. LR was composed of left-eye and right-eye equal and corresponding disc-shaped images as in previous experiments. λp½ was composed of a left-eye image similar to L and R except that it lay just above or below them in the visual field. With λ's right half was combined the image of the right half of a lantern image similar again to the others, except that its left half was screened absolutely off into the blank undetailed darkness of the general field. When this was done the two opposite visual images LR and λp½ regarded under perfectly steady ocular fixation, were stable, and no difference of brightness was discernible between them. Moreover no join was seen between the halves of λp½ and no difference of brightness between the halves. After prolonged inspection of them rivalry became troublesome; but a judgment could be clearly arrived at before that happened.

In this experiment it might possibly be that equality of brightness between the halves of λp½ was due to image p½ 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. Some of the experiments carried out by McDougall [note: 'Mind, 1901, N.S. x. p. 63 .'] give validity to such possible objection. The perceptibility of the horizontal bar in the right half of the image λp½ was guarantee however that at least part of the uniocular image p½ was present. But to ascertain more surely whether image p½ was really during the visual equation co-operating in consciousness with λ the following further arrangement was employed.

[Diagram 11: image showing altered exposure patterns of LR and λp½]

Experiment 9, (Diagram 11). With the revolving lantern so arranged that images L, R, λ and p½ were all of equal brightness when steady and unflickering, p½ was given a lesser frequency of intermission, so as to flicker while the others did not. A speed of revolution of lantern was then used at which just a trace of flicker was perceptible in p½ when binocularly combined with λ. The equation LR—λp½ was then found to hold while flicker was still just traceable in the right half of λp½. There was then no join seen hetween the halves of λp½ nor any difference between the brightness of the halves. So long as ocular fixation was steady no rivalry disturbed the observation.

In this case there could, I venture to think, be no question but that the one half of λp½ was truly binocular, for the trace of flicker was perceptible during the actual performance of the comparison. Yet no difference of brightness was perceived between LR and λp½, and the two Literal halves of compared together were of like brightness.' (48-49)

'McDougall' [note: '"The Principle underlying Fechner's 'Paradoxical Experiment' and the Predominance of Contours in the struggle of two Visual Fields." This Journ. p. 114.'], in applying to 'retinal rivalry' and 'prevalence of contours' his principle of competition of inter-related 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 he [note: Mind, 1901, N.S. X. p. 56.'] 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 [note: Mind, 1903, N.S. XII. p. 473.'] 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.' (58)

'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... in all the experiments in which a binocular image was compared with one assumed to be purely uniocular, great care was exercised to ensure absence of all trace of detail or contour from the homogeneous darkness present at the time over the whole of the other retina... When all detail and contour were absent from the field containing this correspondent area, when in fact that field was perfectly void of contours, and homogeneous, unchanging and borderless, it was found that it mattered little what depth of darkness it might have; it might be a shade of grey or even a fair white, without perceptibly influencing the sensual vibrations given by the flickering image before the other eye. The absolute blankness of the field seemed to unhitch the region of retina which it covered from higher cerebral connexions, at least to prevent its reactions from contributing to consciousness... 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's [note: 'Mind, 1902, N.S. XI. p. 316'] 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.' (39-40)

McDougall's article introduces what Sherrington calls his 'principle of competition for energy' as follows:

'The constituent neurones of the nervous system with all their branches are regarded, 'in primitivster Weise' as Münsterberg says, as a vast system of channels in all parts of which potential chemical energy is constantly being transformed, in virtue of the normal activity o fthe neurones, into a peculiar form of active energy. This energy, which in the present state of our ignorance, can be most profitably regarded as a fluid, tends always to flow, like heat, electricity or water, from places of higher to places of lower potential, following the paths of least resistance, and for convenience of description it may be called 'neurin'... neurin flows perpetually through the intricate labyrinth of paths that constitutes the central nervous system from the afferent towards the efferent neurones. But the channels along which it has to find its way are not completely or equally open; while each neurone presents throughout its length, dendrites, cell-body and axone, an open channel offering no resistance, each is separated from all others with which it is functionally connected at synapses by an intercellular substance which presents a certain resistance to the passage of neurin from one neurone to another. In the resting state, as during deep sleep, neurin flows slowly and equally through all parts, maintaining in some degree the tonus of the nervous and muscular systems, and escapes across the resistant synapses by a sort of leakage. But, when a definite supraliminal stimulus, is applied to a sense-organ, the sensory neurones affected by it produce neurin much more rapidly than it can escape by leakage across their efferent synapses, so that the potential of their charge very rapidly reaches what may be called the level of the threshold of the synapses, i.e., it reaches such a degree that a rapid discharge of neurin takes place through the intercellular substance of the synapses into efferent neurones, [note: 'It is convenient to speak of each neurone in any chain of neurones forming a conduction-path from sense-organ to muscle as afferent to its successor and ae efferent to its predecessor in the chain.'] i.e., into neurones of the second of those several layers in which the neurones leading from sensory to motor organs are arranged.' (329-330)

'McDougall' [note: '"The Principle underlying Fechner's 'Paradoxical Experiment' and the Predominance of Contours in the struggle of two Visual Fields." This Journ. p. 114.'], in applying to 'retinal rivalry' and 'prevalence of contours' his principle of competition of inter-related 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 he [note: Mind, 1901, N.S. X. p. 56.'] 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 [note: Mind, 1903, N.S. XII. p. 473.'] 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.' (58)

'That a perception initiated from corresponding retinal points is commonly referred without ambiguity to a single locus in visual space has often been regarded (Newton [note: 'Opticks, Quer. 112.'] Wollaston [note: Philosoph. Trans. London, 1824.'], Rohault [note: 'Physique, I. 31.'], Joh. Müller [note: 'Elements of Physiol. Vol. II. p. 1199, Baly's edit. 1843.'] 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.' (53)