Distance effects in human scotopic retinal interaction

1. Using the threshold for a small, brief test flash as a measure, the effect of light stimulation of small adjacent retinal regions of the human retina was investigated during adaptation to a medium scotopic illuminance.2. Within about (1/2) degrees of the tested area, light causes a desensitization gradually decreasing with distance. Beyond that there is a sensitizing zone which in turn gives way to an outer desensitizing zone. The sign of these effects is reversed if the peripheral stimulus is a patch of darkness.3. Evidence for non-linearity of summation of individual patches was found. Occlusion of each other's sensitizing influence is observed if two regions are closely adjacent to each other.     

S-potentials from luminosity units in the retina of fish (Cyprinidae)

1. S-potentials were recorded in fish from units which never responded by depolarization. These hyperpolarizing units are the L-units of Svaetichin & MacNichol (1958).2. Figure 5 shows some sets of action spectra from a single unit. For each curve the criterion of action was hyperpolarization to a fixed level, by lights of various wave-lengths. When these lights fell upon zero background (circles) the curves show that two kinds of cone contribute to the action spectrum, one with the 620 nm pigment of Marks and one with the 680 nm pigment of Naka & Rushton (1966a).3. When the lights fell upon (i) a fixed green background (triangles, Fig. 5), or (ii) a fixed red one (squares), the action spectra changed in a way that indicated greater prominence of (i) the 680 nm system (ii) the 540 nm green system that was not conspicuous without adaptation to red.4. These observations (on the tench Tinca) are contrary to the conclusions of Svaetichin & McNichol (on Gerridae) that the action spectrum is unaltered in shape by adaptation to coloured lights. The contribution of the green cones, for example, was actually absolutely greater under deep red adaptation.5. It is concluded that L-units receive signals from 680, 620, 540 nm and possibly also the blue cones, that the quantum catch in all these contribute to the hyperpolarization produced, but their interaction is more complicated than the simple addition of independent cone effects.     

Are there two types of deuteranopes?

1. The colour vision of a Type I deuteranope who fulfils both of Willmer's criteria (normal foveal luminosity curve, two cone mechanisms in the central fovea revealed by a small 10′ test flash) has been studied.

2. Spectral sensitivity curves (at threshold) on bright red or green backgrounds are identical in the red-green range.

3. Heterochromatic brightness-match luminosity curves measured after bright red or green bleaches are identical in the red-green range.

4. Study of prereceptor light losses show normal colour of the ocular media; spectral reflexion coefficient measurements reveal no evidence of macular pigment.

5. Luminosity curves measured through a filter which artificially replaces the missing macular pigment is identical to the deuteranopic (Type II) curve. Lack of macular pigment explains the `normal' luminosity curve.

6. Red and violet backgrounds raise the thresholds for 10′ red and violet tests by different amounts because two cone (the red and the blue) mechanisms are concerned.

7. Reducing the size of the test to 4′ eliminates the contribution of the blue cone mechanism to threshold. Now only the red mechanism determines the threshold.

8. It is concluded that this subject has only a single red-green cone pigment, normal erythrolabe, like other (Type II) deuteranopes.

     

Cone signals in the cat''s retina

1. The discharges of ganglion cells in the cat's retina were recorded under conditions intended to isolate the cone system.

2. Stiles' two-colour threshold technique permitted the photopic system to be studied when at its highest sensitivity. The absolute sensitivity of a ganglion cell, expressed in equivalent photons of λ_max at the cornea per impulse discharged, was about 2500 times less when driven by cones than when driven by rods. This ratio improves to around 200 when allowance is made for the much smaller fraction absorbed by cones of photons incident on the cornea.

3. The number of extra impulses discharged in response to a brief flash was approximately proportional to the number of photons in the flash, up to a limit.

4. There was a region in the middle of the receptive field within which the area of a test spot and its illumination for threshold varied inversely. A flash extending over the peripheral part of the receptive field raised threshold above its minimum, presumably as a result of surround antagonism. Assessed from area—threshold curves, the balance of centre-surround antagonism in the photopic receptive field did not seem to depend upon background illumination.

5. The threshold for a small (0·2°) flash confined to the middle of the receptive field was independent of background illumination until the background exceeded a particular level, the `dark light' (I_o). In different units this ranged about a mean of 7·89 log photons (560 nm equivalent) deg^-2 sec^-1. For backgrounds that exceeded I_o, threshold followed approximately Weber's law up to the highest illuminations that could be produced.

6. With test flashes that filled the centre of the receptive field, the Weber fraction (test flash illumination/background illumination) in some units fell below 1%.

7. Changes in the time course and latency of response accompanied the changes in sensitivity caused by alterations in background illumination. Responses of both X- and Y-cells became more transient and faster.

8. The loss of sensitivity to a test flash brought about by a steady background light depended upon the size of that light. Sensitivity varied inversely with background area within a central region that matched closely the summing area for test flashes.

     

The separation of cone mechanisms in dark adaptation

1. In dark adaptation the threshold is raised as though the bleaching of visual pigment generated an equivalent background. Now Stiles has shown that real coloured backgrounds act selectively upon the various colour mechanisms, so we ask: ;Do equivalent backgrounds from coloured bleachings also act selectively?'2. When dark adaptation was plotted using a blue test flash (Fig. 1b) following bleaching by orange light a kinked curve was obtained. The upper branch was shown to have the same dark adapted threshold as Stiles blue (pi(1)) mechanism and the lower branch as his green (pi(4)) mechanism. The pi(4) dark adaptation curve alone (unkinked) was obtained using a white instead of an orange bleach.3. Dark adaptation curves were obtained in which the test flash was presented upon various steady backgrounds. In conditions where only pi(4) was involved (Fig. 3) the experimental results fitted the curves calculated on the assumption that the equivalent background of bleaching simply adds to the real background in raising the threshold-a condition already established for rods.4. In conditions where pi(4) and pi(1) were both present (blue test, yellow-green background and white bleach) kinked dark adaptation curves were obtained (Fig. 2) where the upper branch (pi(4)) coincided with those of Fig. 3 and the lower were due to pi(1).5. The blue mechanism recovers in dark adaptation at about the same rate as red and green, or slightly slower.6. Dark adaptation curves with red (pi(5)) and green (pi(4)) limbs can be obtained after a deep red bleach (Fig. 4) using a red test flash and a green background. The red and the green limbs were also plotted alone in their entirety by slightly changing the conditions.7. We are led to the idea of three colour mechanisms that adapt as independently one of another after bleaching as they do with backgrounds.8. Though this simple independence accounts for the wide and conspicuous range of adaptive phenomena, we have encountered some special conditions (not here described) that seem to imply a measure of interaction between the different colour mechanisms.     

Colour vision in blue-cone `monochromacy'

1. Atypical (blue cone) monochromats show two action spectra when tested by the increment threshold method of Stiles with ;central' fixation. One spectrum peaks near 450 nm and has the spectral characteristics of normal blue cones. The other resembles rhodopsin (pi(0)) modified slightly by photostable macular pigment.2. Under some circumstances such observers are dichromats. There is a neutral point (matched to Illuminant ;C') in the neighbourhood of 460-470 nm.3. The spectral colour matching functions using two primaries have been measured on three such subjects. They can be fit reasonably well, although imperfectly, by linear combinations of pi(0) and pi(1). The chromaticity co-ordinates have been calculated according to the convention of W. D. Wright and compared to the results predicted from pi(0) and pi(1). The comparison suggests that part of the imperfections of the colour matching function fit is due to prereceptor differences (e.g. macular pigment) between the blue-cone monochromats and the hypothetical pi(0), pi(1) observer.4. Colour matches made at high light levels continue to hold when the intensity of the field is reduced below the cone threshold.5. Therefore one of the visual pigments participating in the colour matches has an action spectrum which is not measurably different from that of the rod pigment rhodopsin.6. Increment threshold measurements show that the mechanism which has the rhodopsin action spectrum has the directional sensitivity of cones, not rods.7. Blue test threshold during dark adaptation after a full bleach follow a bipartite exponential recovery curve. The first exponential has a time constant of 1 min, the second 2 min. By comparing these curves to the increment thresholds it is possible to relate the first to the pi(1), the second to the pi(0) cones.8. Using a broad band blue gelatin filter in the measuring light of the retinal densitometer and studying the same retinal region tested in 7 it is possible to follow the regeneration of a pigment after a full bleach which has an exponential recovery with a time constant of 1.0 min. With a yellow green filter in the measuring light the exponential recovery observed after a full bleach has a time constant of 2.0 min.9. One of the two visual pigments participating in the colour matches resides in receptors which have the action spectrum, the directional sensitivity and probably the dark adaptation curve of normal blue cones.10. The other resides in receptors which have the action spectrum of normal rods but the directional sensitivity and the dark adaptation curve of normal red and green cones.     

Ionic mechanism of a quasi-stable depolarization in barnacle photoreceptor following red light.

1. The membrane mechanism of a quasi-stable membrane depolarization (latch-up) that persists in darkness following red light was examined in barnacle photoreceptor with micro-electrode techniques including voltage-clamp and Na+-sensitive micro-electrodes. 2. Current-voltage (I-V) relations of the membrane in darkness following red light (latch-up) and in darkness following termination of latch-up with green light, indicate that latch-up is associated with an increase of membrane conductance. 3. The latch-current (membrane current in darkness following red light minus membrane current in darkness following a gree flash that terminates latch-up) was inward at the resting potential, reversed sign at about +26mV (mean of six cells), and became outward at more positive membrance potentials. 4. Current-voltage relations of the membrane during green light (no latch-up) closely resembled those during latch-up. The light-induced current (LIC) elicited by green ligh (membrane current during the light flash minus membrane current in darkness following the light flash) was inward from the resting potential to +26mV (mean of six cells), then reversed sign and became outward. 5. The latch-current and LIC were both augmented in reduced Ca2+ solutions and decreased as Na-+ was reduced at a fixed Ca2+ concentration. 6. Both LIC and latch-current reversed sign at a more negative membrane potential (increment V equals 14mV) in solutions containing one quarter the normal amount of Na+. 7. The internal Na-+ activity (a-iNa) of a photoreceptor increased from about 10-18 mM upon illumination with long steps of intense red or white illumination. Five minutes in darkness after white light, a-iNa had recovered significantly, whereas a-iNa remained elecated following red illumination. 8. Latch-up seems to be a persistence in darkness of the same membrane mechanism that normally occurs during illumination; i.e. a conductance increase to Na+ ions. Ca2+ ions act primarily to suppress this current. There is evidence for a net Na+ influx during illumination that is sustained in darkness during latch-up.     

Rod-cone interaction in light adaptation

1. The increment-threshold for a small test spot in the peripheral visual field was measured against backgrounds that were red or blue.

2. When the background was a large uniform field, threshold over most of the scotopic range depended exactly upon the background's effect upon rods. This confirms Flamant & Stiles (1948). But when the background was small, threshold was elevated more by a long wave-length than a short wave-length background equated for its effect on rods.

3. The influence of cones was explored in a further experiment. The scotopic increment-threshold was established for a short wave-length test spot on a large, short wave-length background. Then a steady red circular patch, conspicuous to cones, but below the increment-threshold for rod vision, was added to the background. When it was small, but not when it was large, this patch substantially raised the threshold for the test.

4. When a similar experiment was made using, instead of a red patch, a short wave-length one that was conspicuous in rod vision, threshold varied similarly with patch size. These results support the notion that the influence of small backgrounds arises in some size-selective mechanism that is indifferent to the receptor system in which visual signals originate. Two corollaries of this hypothesis were tested in further experiments.

5. A small patch was chosen so as to lift scotopic threshold substantially above its level on a uniform field. This threshold elevation persisted for minutes after extinction of the patch, but only when the patch was small. A large patch made bright enough to elevate threshold by as much as the small one gave rise to no corresponding after-effect.

6. Increment-thresholds for a small red test spot, detected through cones, followed the same course whether a large uniform background was long- or short wave-length. When the background was small, threshold upon the short wave-length one began to rise for much lower levels of background illumination, suggesting the influence of rods. This was confirmed by repeating the experiment after a strong bleach when the cones, but not rods, had fully recovered their sensitivity. Increment-thresholds upon small backgrounds of long or short wave-lengths then followed the same course.

     

Spatial interaction in human cone vision

1. The adaptation state of a uniformly illuminated patch of human cone retina was determined by finding the threshold for a small, brief light spot seen flashing in its centre.2. When the illuminated patch of the retina is increased in diameter, the adaptation state is first raised, and beyond a critical background diameter, lowered.3. This is interpreted as a manifestation of excitatory and inhibitory interaction of adaptation stimuli: illumination of retinal regions in the immediate neighbourhood of the area tested acts to raise the adaptation level, and of those further removed acts to lower it.4. The critical area beyond which adapting light produces inhibition is about 5 min of arc in diameter in the eye's object space for foveal observation. For peripheral cone vision it increases much as the minimum angle of resolution.5. The inhibiting action of outlying areas is substantially reduced, or perhaps even eliminated, by lowering the background luminance.6. Surrounding the retinal patch with a pair of juxtaposed narrow concentric black and white rings superimposed on a uniform field, simulating a border, irrespective of diameter, does not influence the threshold of the probing spot. This argues against a possible threshold raising effect of the border of the background.7. The inhibiting action on a patch of cone retina of a surrounding annulus occurs only when the annulus is seen by the same eye and not when it is seen by the other eye: the site of inhibitory interaction is, therefore, retinal.     

Mixing of color signals by turtle cone photoreceptors

The direct mixing of red and green cone signals in the outer plexiform layer of the turtle retina was studied by using intracellular recordings from red cone photoreceptors. Cone photoresponses were a function of the wavelength of the photons that stimulated them, even when small-diameter stimuli were used. The intensity response curves measured with red and green test flashes had different shapes. The kinetics of approximately equal amplitude red and green responses also differed. To quantify the short wavelength input onto red cones, differential chromatic adaptation was used. The relative sensitivity of the red cone to red and green test flashes was a function of the color and intensity of the background illumination; red backgrounds decreased relative red sensitivity, and green backgrounds increased relative red sensitivity. The spectral sensitivity of the additional short wavelength input onto red cones was determined by using differential chromatic adaptation, and was found to peak approximately 550 nm. We conclude that red cones receive an additional excitatory input from green cones (and possibly blue cones). A model of the cone mosaic suggests that approximately 50% of the red cone response (linear range) to a dim green test flash arises from neighboring green cones.     

Spectral correlates of a quasi-stable depolarization in barnacle photoreceptor following red light.

1. Illumination of B. eburneus photoreceptors with intense red light produces a membrane depolarization that persists in darkness. This quasistable depolarization (latch-up) can be terminated with green light. The phenomenon was investigated with electrophysiological, spectrochemical, and microspectrophotometric techniques. 2. Latch-up was associated with a stable inward current in cells with the membrane potential voltage-clamped at the resting potential in darkness. The stable current could only be elicited at wave-lengths greater than 580 nm. 3. Light-induced current (LIC) was measured at various wave-lengths in dark-adapted photoreceptors with the membrane voltage-clamped to the resting potential. The minimum number of photons required to elicit a fixed amount of LIC occurred at 540 nm, indicating that the photoreceptor is maximally sensitive to this wave-length of light. The photoreceptor was also sensitive to wave-lengths in the near-U.V. region of the spectrum (380-420 nm). 4. Steady red adapting light reduced the magnitude of the LIC uniformly at all wave-lengths except in the near-U.V. region of the spectrum; sensitivity was reduced less in this region. 5. The spectrum for termination of the stable inward current following or during red light was shifted to the blue (peak about 510 nm) compared to the peak for LIC (peak about 540 nm). 6. Absorbance of single cells prepared under bright, red light decreased maximally at 480 nm following exposure to wave-lengths of light longer than 540 nm. 7. A pigment extract of 1000 barnacle ocelli prepared under dim, red light had a maximum absorbance change at 480 nm when bleached with blue-gree light. 8. There was no evidence in the latter two experiments of photointerconversion of pigments with absorbance maxima at 480 and 540 nm. Rather, the maximum absorption of the bleaching products seemed to occur at wave-lengths shorter than 420 nm. 9. Since latch-up induction occurs at wave-lengths longer than 580 nm, it may depend on the 540 pigment or on an undetected red absorbing pigment. 10. A photolabile pigment at 480 nm correlated most closely with termination of the stable inward current associated with latch-up.     

Signals from cones

1. We have studied red and green cones by contrast flash inhibition and found them both to be very similar to rods in their response to flash energy, except that all light quantities must be some 100-fold greater in cones for the same effect.2. Using the methods of a previous paper (A.R.T. a) where no backgrounds were employed, we plotted log test flash lambda against log surround flash varphi with criterion that lambda should just be detected. The experiment was repeated with a ;windmill stop' interposed in the varphi flash which reduced its area symmetrically to (1/8). From these two curves it is possible to extract the relation between N, the inhibitory signal, and varphi, the test flash. It isN = varphi/(varphi+sigma),where sigma, the semi-saturation constant, is about 4.5 log td sec.3. Using the methods of (A.R.T. b) where backgrounds were studied, we measured the increment threshold for the surround flash varphi against its background theta using as criterion not that varphi should just be seen but that it should generate a fixed inhibitory signal N so that the fixed test flash lambda could just be seen.4. This increment threshold curve resembled the Aguilar & Stiles (1954) curve for rods, showed saturation and a complete symmetry about the 45 degrees line through the point with co-ordinates (log theta(D), log sigma).5. These results imply that the cone signal N is related to flash varphi and steady background theta by [Formula: see text], where theta(D) is receptor noise (= eigengrau), and varphi and theta are expressed in units of quantum catch.6. The ordinary increment threshold for cones does not show saturation because a steady saturating background bleaches all the pigment away. When the background is presented for only 100 msec with dark pauses between, no great bleaching occurs and saturation is seen.     

Impulse functions for human rod vision

1. This paper presents accurate increment threshold data for human rod vision for a small number of experimental parameters. The test is small and brief and the large background is either steady or transient.

2. The linear threshold disturbance due to an impulse background consists of an input dependent exponential growth phase and an exponential recovery phase of more or less fixed time constant (ca. 0·08 sec).

3. The data are treated by applying signal/noise decision theory to a hypothetical filter with two shot noise inputs, viz. the testing signal and the background. The gain and time course of the impulse function of the filter are slightly affected by the magnitude of the input.

4. A linear approach is useful since the impulse functions for dark or light-adapted rod vision yield independent information about quantities which have previously only been used to describe the increment thresholds for small tests on steady backgrounds, viz. the integration time and dark light of the fully dark-adapted eye and the gain changes (or changes in the signal/background ratio) which occur on progressive light adaptation.

     

The Purkinje shift in cat: extent of the mesopic range

1. The mesopic range, defined for cats with fully dilated pupils, extends from -1.0 to +1.4 log cd/m(2). The technique employed involves measurement of the relative shift in threshold with adaptation level, between two wave-lengths respectively selective for cones and rods at threshold. The magnitude of this shift is compared with that predicted from the Dartnall nomogram for visual pigments 556 and 507 (the in situ cone and rod absorption maxima).2. The result is based on monochromatic increment threshold determinations for 149 on-centre or off-centre fibres, isolated in the optic tract posterior to the chiasma, under a variety of adaptation levels. All units included receive dual input - from 500 nm rods and from a single class of cones with maximum sensitivity at 556 nm. No units receive input from 450 nm cones; six units with input mediated purely by rods are excluded from the sample.     

The size of rod signals

1. This investigation is based upon Alpern's (1965) contrast flash observations. The threshold for the test flash λ (Fig. 2a) is raised if a second flash ϕ falls on the annular surround. Moreover, if λ excites rods at threshold, it is only the rods in the surround that contribute to the threshold rise.

2. The possibility that the rise in λ threshold might be due to light physically scattered from surround to centre we exclude by several different experiments. We conclude (Fig. 1b) that the ϕ flash sets up a nerve signal N which is conducted to some place C where it inhibits the signal from the centre.

3. If the luminous surround, instead of being a full circle (Fig. 2a) consists only of the sectors shown black in Fig. 2b, that occupy 1/m of the surround area, it is found (in the physiological range) that the light/area on those sectors must be m times as great to produce the same threshold rise at centre, i.e. the total surround illumination must remain the same.

4. This result would obviously follow if N, the inhibitory nerve signal, were proportional to the total surround illumination. We have established the converse; the signal must be proportional to the quantum catch.

5. Light can be increased indefinitely, nerve signals cannot. When ϕ increases sufficiently, N saturates in the same way that S-potentials and receptor potentials saturate, namely according to N = ϕ/(ϕ + σ) where σ, the semi-saturation constant is about 200 td sec, or 800 quanta absorbed per rod per flash.

6. Thus the nerve signal N is proportional to the quantum catch over 4 log units in the physiological range, namely from 1 quantum per 100 rods to 100 quanta per rod per flash. Above this for another 2 log units N continues to increase, but now more slowly, after the manner of S-potentials and receptor potentials.

     

Wave-length discrimination at the foveal chromatic threshold

1. Wave-length discrimination has been measured at the chromatic threshold to test the assumption that when a monochromatic stimulus is reduced to its chromatic threshold, the activity of a single cone mechanism may be isolated.

2. It has been shown that, at the chromatic threshold, all wave-lengths between 559 and at least 665 mμ are significantly confused. Similar ranges of confusion extend from 457 to 570 mμ and from 485 to at least 419 mμ.

3. These results have been shown to be consistent with previous measures of the spectral sensitivities of the cone pigments. They support the view that the hue of a monochromatic stimulus at its chromatic threshold may be dependent on light absorbed by only one cone pigment.

     

Responses of cat retinal ganglion cells to brief flashes of light

1. The responses of cat retinal ganglion cells to brief flashes of light have been illustrated and described with a view to providing material for comparison with psychophysical experiments in the scotopic (rod-dominated) range of performance.2. There is a minimum response duration of 50-70 msec no matter how brief the flash is made. This duration is reached with stimuli lasting 32 msec or shorter.3. Reducing the background illumination obviously increases the latency of responses to stimuli at 4 times threshold intensity (about 10 msec increment per log. unit decrement) but has no obvious effect on the minimum response duration.4. The relation between intensity and duration of a flash for threshold responses closely resembles that in human psychophysical experiments. The Bunsen-Roscoe law is applicable for flash durations up to 64 msec.5. If equal amounts of energy are delivered in the form of a pair of flashes of varying separation rather than by rectangular pulses, the shape of the response changes more abruptly with the temporal factor.6. Non-linear performance is apparent for stimuli as weak as 4 times threshold.7. A method is developed for quantitative analysis of individual responses. It is based upon cross-correlation of the train of impulses with a Gaussian smoothing function and represents local impulse frequency as a smooth function of time. The method also improves the signal-to-noise ratio of post-stimulus time-histograms of the sum of many responses.8. The measurement of variability of individual responses is the main result of the method; its magnitude indicates that it is a significant new factor limiting temporal resolution with suprathreshold stimuli.     

Sensitivity of toad rods: Dependence on wave-length and background illumination.

1. There are five morphological types of photoreceptors in the retina of the toad, Bufo marinus: red and green rods, single cones, and the principal and accessory members of double cones. The largest and most abundant of these is the red rod. 2. Intracellular recordings were used to investigate the dependence of the sensitivity of red rod responses on wave-length and background light. 3. The spectral sensitivity of dark-adapted and moderately light-adapted red rods can be satisfactorily fitted with the absorbance spectrum of the red rod photopigment. There are no significant contributions to red rod responses from cones or green rods. 4. In contrast, L-type horizontal cells, whose responses are dominated by input from the red rods near threshold, can be shown also to receive input from cones. 5. Steady background light produces a response in the red rods consisting of an initial hyperpolarization, followed by a decay of potential to a steady-state plateau level. The slow decay of response amplitude is accompanied by an increase in sensitivity to increment test flashes. 6. The increment sensitivity at steady-state decreases with increasing background intensity according to a modified Weber-Fechner relation. The dependence of increment sensitivity on the wave-length of the background light can be predicted by the red rod spectral sensitivity, showing that cones do not influence the light adaptation of rods. 7. At a backgound intensity of 11-5 log equivalent quanta cm-2sec-1, sensitivity begins to deviate from the Weber-Fechner relation. In background light one log unit brighter, the rods are completely saturated. 8. Small responses having the spectral sensitivity of cones can be recorded from saturated rods. These potentials have a prominent off response whose wave form resembles the d-wave of the e.r.g. 9. A comparison of the increment--sensitivity curves of single receptors shows that rods are light-adapted by backgrounds one thousand times dimmer than those which affect cones. The increment--sensitivity curves of rods and cones cross, so that single cones become more sensitive than single rods even before the rods begin to saturate.     

An attempt to analyse colour reception by electrophysiology

1. The problem of colour reception is that we do not know the action spectra of the visual pigments involved, the nature of the signals generated nor the interaction between these signals. We only know the incident light and the electric results of interaction.2. In Part 1 we show that S-potentials from red/green (R/G) units saturated with deep red light show this property: added green light pulls down the ceiling of depolarization, but more added red had no power to raise it again. Thus lights that depress the deep red ceiling equally stimulate the green pigment equally. From this the action spectrum of the green pigment can be obtained.3. If we assume that only two visual pigments are involved in the R/G unit, and that lights which do not pull down the deep red ceiling are below the threshold for green cones, then in this range only the red pigment is excited and we may obtain its action spectrum. Its maximum is at 680 nm where no visual pigment so far has been found.4. In Part 2 we consider the following mathematical problem: ;Is it possible that two pigments of given action spectra could combine their outputs in such a way that the resultant would be identical with the output of a third pigment of given action spectrum, for every intensity of every monochromatic light?' The solution shows that this is always mathematically possible, and the necessary interaction function is deduced.5. It is shown further that if the log action spectra are the ;visual parabolas' that resemble Dartnall's nomogram, then the interaction function is simply a linear transform such as Hartline & Ratliff (1957) have found with lateral inhibition in Limulus and Donner & Rushton (1959) with silent substitution in the frog.6. An interaction that matches a single pigment to perfection for all monochromatic lights will not match it for certain mixtures. By this criterion the 680 nm excitability is a pigment and not the resultant of two other pigments, i.e. pigments more excitable in other spectral regions.7. In Part 3 monochromatic lights are matched by red+green mixtures that give identical responses. From this the action spectrum of the red pigment may be obtained without involving nerve organization (except as a null detector). The result, which has one arbitrary constant, is given by the curves of Fig. 10, the continuous curve R or one of the dotted curves. Of these only curve R is acceptable.8. Knowing the action spectra for red and green cones we may consider what signals are generated and how they interact to give the records. Figure 11 suggests a model that will account for the size and sign of S-potentials as function of the quantum catch by the two pigments. It does not embrace the time or space parameters which can be very complex.     

The nature of rise in threshold produced by contrast-flashes

1. The rod threshold for seeing a flash on a 2(1/2) degrees square is raised by a nearly simultaneous flash that falls on the surround. When this ;contrast-flash' is held fixed in intensity, it raises the log test threshold by a fixed amount no matter how far that threshold has already been raised by light adaptation owing to background or bleaching.2. This is surprising since fixed backgrounds and bleachings raise the log test threshold much more when the eye is dark than when light adapted.3. When the test flash is held at some fixed supra-threshold value, the contrast flash exhibits a ;critical level', above which the test will no longer be seen. If the surround region upon which the contrast-flash falls is adapted by background or bleaching, its efficacy is reduced so that the ;critical level' is raised.4. Surround adaptation raises the log ;critical level' by the same amount that it raises the log threshold for seeing the contrast-flash itself.5. The way that contrast flashes raise the test threshold is thus entirely different from the way that adaptations by bleachings or backgrounds do. Contrast-flash signals appear to inhibit test-flash signals by interaction at some point central to the site where adaptation occurs.6. This permits the effect of adaptation on signals to be measured. A given state of adaptation attenuates all flash signals in the same proportion. And in any state of adaptation a single flash will reach threshold when the attenuated signal has a fixed size.