Retinal illuminance and the dissociation of letter and grating acuity in age-related macular degeneration.

PURPOSE: To report on the differential effect of retinal illuminance on letter and grating acuity. METHODS: For 13 subjects with age-related macular degeneration (ARMD) and four subjects with normal vision, standard distance and near letter acuity, as well as grating acuity data were obtained at mesopic to high-photopic light levels. RESULTS: In general, both acuity forms improved with increasing light level, but not in proportion with one another. The ratio of letter/grating acuity separated the ARMD subjects into two subgroups, one with relatively low letter acuity scores for which the dissociation of the two acuity forms increased with retinal illuminance, and another with higher letter acuity values for which the ratio converged toward unity. CONCLUSIONS: These findings demonstrate that both letter and grating acuity increase with retinal illuminance and that equating grating acuity with optotype acuity is untenable in subjects with ARMD, irrespective of light levels. The latter conclusion is of importance whenever acuity is used as a criterion to classify the visually impaired with regard to their legal and social status.

Unique hues: an old problem for a new generation.

The practical success of the classical theories of colour vision, such as that of Young-Helmholtz when applied to the measurement and reproduction of colour stimuli, and that of Hering's in art and architecture, has overshadowed the fact that neither theory achieved its main goal, namely to explain colour qualities. Neither the three types of cone, nor the first opponent stages of neural processing in the retina and the lateral geniculate nucleus can serve as direct correlates to the perception of elementary, or unique colours, such as red, green, yellow and blue. While our subjective experiences of these qualities do not submit to measurement, physiological conditions that are required to perceive colours of a constant hue can be identified. For instance, a constant ratio of responses of different types of opponent cells in the retina and the lateral geniculate nucleus of primates may serve as a neurophysiological correlate of a constant hue. This is, however, not the correlate for seeing a particular hue quality, say unique red. This latter correlate, if it exists as a separable entity, must be associated with yet unidentified, higher-level neural activities. The fundamental problems encountered in relating colour qualities to neural activities are discussed and references are made to the current debate about phenomenal consciousness.

Visual evoked potentials and magnocellular and parvocellular segregation.

We have measured visual evoked potentials (VEPs) to luminance-modulated, square-wave alternating, 3-deg homogeneous disks for stimulus frequencies ranging from 1 Hz to 16.7 Hz. The aim of the study was to determine the range of frequencies at which we could reproduce the two-branched contrast-response (C-R) curves we had seen at 1 Hz (Valberg & Rudvin, 1997) and which we interpreted as magnocellular (MC) and parvocellular (PC) segregation. Low-contrast stimuli elicited relatively simple responses to luminance increments resulting in waveforms that may be the signatures of inputs from magnocellular channels to the visual cortex. At all frequencies, the C-R curves of the main waveforms were characterized by a steep slope at low contrasts and a leveling off at 10%-20% Michelson contrast. This was typically followed by an abrupt increase in slope at higher contrasts, giving a distinctive two-branched C-R curve. On the assumption that the low-contrast, high-gain branch reflects the responsivity of magnocellular-pathway inputs to the cortex, the high-contrast branch may be attributed to additional parvocellular activation. While a two-branched curve was maintained for frequencies up to 8 Hz, the high-contrast response was significantly compromised at 16.7 Hz, revealing a differential low-pass filtering. A model decomposing the measured VEP response into two separate C-R curves yielded a difference in sensitivity of the putative MC- and PC-mediated response that, when plotted as a function of frequency, followed a trend similar to that found for single cells. Due to temporal overlap of responses, the MC and PC contributions to the waveforms were hard to distinguish in the transient VEP. However, curves of time-to-peak (delay) as a function of contrast often went through a minimum before the high-contrast gain increase of the corresponding C-R curve, supporting the notion of a recruitment of new cell ensembles in the transition from low to high contrasts.

Possible contributions of magnocellular- and parvocellular-pathway cells to transient VEPs.

We have measured transient visual evoked potentials (VEPs) to low-contrast luminance stimuli favoring responses of magnocellular pathway cells and to low-contrast red-green stimuli favoring parvocellular cells. Stimuli were square-wave alternating, 3-deg homogeneous disks. Low-contrast stimuli modulated in luminance elicited relatively simple responses. For some observers, a negativity was present that saturated at low contrast. This may be the signature of inputs from magnocellular channels to the visual cortex. The slope of the contrast-response curve for low-contrast stimuli was about the same for all subjects. For medium contrasts, these contrast-response curves displayed an abrupt increase of slope. The shallower slope may reflect the responsivity of magnocellular-pathway inputs to the cortex, whereas the steeper slope may be caused by additional parvocellular activation. Contrast-response curves for the most sensitive waveforms of the isoluminant green-red modulation also showed two branches, although not as clearly as for luminance. This may indicate parvocellular-mediated activity for small chromatic differences, and a combination of parvocellular and magnocellular inputs for larger contrasts. Curves of time-to-peak response as a function of contrast often changed their monotonous behavior near the kink of the corresponding contrast-response curve, thus supporting the notion of a contribution from several mechanisms to the main waveforms.

Physiological mechanisms underlying psychophysical sensitivity to combined luminance and chromatic modulation.

If psychophysical detection thresholds are plotted in a middle-wavelength-sensitive (M) and long-wavelength-sensitive (L) cone coordinate system, the shape of the contour can be used to infer underlying detection mechanisms. We measured responses of macaque ganglion cells to combine chromatic and luminance modulation and expressed our results in such an M,L-cone space. Our aim was to test whether, with the use of this space, readily separable luminance and chromatic psychophysical mechanisms might be expected from physiological data. For parvocellular pathway cells, detection contours approximated elongated ellipses with maximum responsivity to chromatic modulation. The degree of elongation decreased as temporal frequency increased. Responses could be well described by linear subtraction of M- and L-cone signals, with a phase delay of 1-3 deg/Hz. For cells of the magnocellular pathway, detection contours were more complex. Orientation was variable between cells and temporal frequency dependent, and a frequency-doubled component was evoked by chromatic modulation. In relation to psychophysical detection thresholds plotted in such a space, the properties of parvocellular-pathway cells were sufficiently linear and homogeneous to make it plausible that this pathway might form the substrate for a linear chromatic mechanism. The properties of magnocellular-pathway cells, however, indicate that, insofar as a psychophysical luminance mechanism is based on their activity, its signature in the M,L-cone contrast space would be more difficult to identify.

Responses of macaque ganglion cells to the relative phase of heterochromatically modulated lights.

1. We measured the response of macaque ganglion cells to sinusoidally modulated red and green lights as the relative phase, theta, of the lights was varied. 2. At low frequencies, red-green ganglion cells of the parvocellular (PC-) pathway with opponent inputs from middle-wavelength sensitive (M-) and long-wavelength sensitive (L-) cones were minimally sensitive to luminance modulation (theta = 0 deg) and maximally sensitive to chromatic modulation (theta = 180 deg). With increasing frequency, the phase, theta, of minimal amplitude gradually changed, in opposite directions for cells with M- and L-cone centres. 3. At high frequencies (at and above 20 Hz), phasic cells of the magnocellular (MC-) pathway were maximally responsive when theta approximately 0 deg and minimally responsive when theta approximately 180 deg, as expected from an achromatic mechanism. At lower frequencies, the phase of minimal response shifted, for both on- and off-centre cells, to values of theta intermediate between 0 and 180 deg. This phase asymmetry was absent if the centre alone was stimulated with a small field. 4. For PC-pathway cells, it was possible to provide an account of response phase as a function of theta, using a model involving three parameters; phases of the L- and M-cone mechanisms and a L/M cone weighting term. For red-green cells, the phase parameters were monotonically related to temporal frequency and revealed a centre-surround phase difference. The phase difference was linear with a slope of 1-3 deg Hz-1. If this represents a latency difference, it would be 3-8 ms. Otherwise, temporal properties of the M- and L-cones appeared similar if not identical. By addition of a scaling term, the model could be extended to give an adequate account of the amplitude of responses. 5. We were able to activate selectively the surrounds of cells with short-wavelength (S-) cone input to their centres, and so were able to assess L/M cone weighting to the surround. M- and L-cone inputs added linearly for most cells. On average, the weighting corresponded to the Judd modification of the luminosity function although there was considerable inter-cell variability. 6. To account for results from MC-pathway cells, it was necessary to postulate a cone-opponent, chromatic input to their surrounds. We developed a receptive field model with linear summation of M- and L-cones to centre and surround, and with an additional M,L-cone opponent input to the surround.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of macaque ganglion cells to movement of chromatic borders.

1. We have measured responses of macaque ganglion cells to moving borders under conditions designed to simulate the minimally distinct border (MDB) task. 2. Extending previous results, we show that minimization of responses of phasic ganglion cells of the magnocellular (MC)-pathway obey the photometric laws of transitivity and additivity. 3. To equal luminance borders, a residual response was present in MC-pathway cells analogous to the second harmonic response seen in these neurones with temporal chromatic modulation. It was proportional to the tritanopic purity difference (magnitude of delta Pt, the rectified middle- to long-wavelength cone opponent signal) between the two colours on either side of the border. For a delta Pt of one, the mean residual response was equivalent to the response evoked by achromatic borders of about 14% luminance contrast. Both these properties of the MC-pathway closely resemble psychophysical estimates as to the distinctness of equal luminance borders. 4. We show how MC-pathway cell responses could be used centrally to support the MDB task. It was difficult to generate a model from responses of tonic ganglion cells of the parvocellular (PC)-pathway which would support the task. 5. The MDB task is still possible psychophysically after blurring the retinal image. Although blurring the border spatially smeared the responses of MC-pathway ganglion cells and reduced their amplitude, responses still went through a minimum close to equal luminance. Thus, blurring the image did not affect the ability of MC-pathway cells to support the task. Blurring the retinal image decreased the 'sharpness' of the border response of tonic, PC-pathway ganglion cells, but response amplitude was unaffected. Response features indicative of centre-surround organization were attenuated. A central mechanism reliant on centre-surround field structure of PC-pathway cells would thus not be able to support the task after blurring. 6. Taken together, these results strongly suggest that the MC-pathway forms the sole physiological substrate of the MDB task, and any contribution of the PC-pathway is, indeed, minimal.

The neurophysiological correlates of colour and brightness contrast in lateral geniculate neurons. II. Adaptation and surround effects.

We report on experiments which were undertaken in an attempt to clarify mechanisms underlying the contrast effects of chromatic surround illumination on spectral responsiveness of cells in the parvocellular layers of the LGN (P-LGN-cells), that had been demonstrated under standard conditions in the preceding companion paper. The experiments were done in anesthetized macaques (Macaca fascicularis). In some neurons, S-potentials were recorded together with the post-synaptic action potentials, and all effects seen in P-LGN-cells were present already in their retinal afferents indicating their retinal origin. The responsiveness of the cells for center stimuli of different wavelengths and during illumination of the receptive field center or the outer surround was determined. Continuous outer surround illumination altered maintained discharge rate (MDR), sensitivity and gain of P-LGN and retinal ganglion cells in the same way and empirically not distinguishable from direct illumination of the receptive field. Responses to surround flashes showed the same dependence on spectral composition as those to center flashes. Adaptation and excitation caused by outer surround illumination (inner diameter 5 degrees, outer diameter 20 degrees) were, in the average, ten times weaker than those exerted by light of the same spectral composition shone directly into the receptive field. Surround effects decreased proportional to r-2. Excitation by outer surround flashes was reduced by adaptation of the receptive field center in the same manner as responses to center flashes. The findings indicate that outer surround light has a direct excitatory and adaptive effect on the excitatory or inhibitory cones feeding into the receptive field. This indicates that straylight from the surround into the center could be responsible for the adaptive and excitatory effects of surround illumination. The straylight fraction from the remote surround into the receptive field must be higher, however, than that estimated from the psychophysically determined point spread function. It comes closer to earlier direct straylight measurements in excised eyes, but may be enhanced by chromatic aberration. If a surround of excitatory colour is flashed simultaneously with an excitatory center stimulus, additivity of center and surround excitation is observed only at low center intensities, while at higher center intensities the gain for center excitation is reduced similar to adaptive gain control. This could be explained by lateral interaction through horizontal connections in the retina, which decays within seconds, while adaptation of the cones feeding into the receptive field center is fully effective only after about 3 s.(ABSTRACT TRUNCATED AT 400 WORDS)

Colour changes as a function of luminance contrast.

When spectral light increases in luminance, the hues change. Normally, long-wavelength light becomes increasingly yellow, and short-wavelength light turns blue or blue-green. This is known as the Bezold-Brücke hue shift. Less notice has been paid to the change in relative chromatic content (saturation or chromatic strength) that accompanies these shifts in hue. As luminance contrast increases from zero, chromatic strength increases to reach a maximum at a luminance that is wavelength dependent. Short-wavelength blueish light reaches this maximum at low relative luminances, whereas midspectral yellowish stimuli need several log units higher luminance. Red and green are somewhere in between. For luminances above this maximum, the chromatic content usually diminishes, and most light becomes more whitish in appearance. In this study it is demonstrated how the combined chromatic appearance of hue and chromatic strength change with intensity. Both phenomena find a common physiological interpretation in the nonlinear and nonmonotonic responses of colour-opponent P cells in the retina and lateral geniculate nucleus of the primate. A model that combines the outputs of six P-cell types accounts for observers' estimates of hue and chromatic strength.

Luminance and chromatic modulation sensitivity of macaque ganglion cells and human observers.

We measured the sensitivity of macaque ganglion cells to luminance and chromatic sinusoidal modulation. Phasic ganglion cells of the magnocellular pathway (M-pathway) were the more sensitive to luminance modulation, and tonic ganglion cells of the parvocellular pathway (P-pathway) were more sensitive to chromatic modulation. With decreasing retinal illuminance, phasic ganglion cells' temporal sensitivity to luminance modulation changed in a manner that paralleled psychophysical data. The same was true for tonic cells and chromatic modulation. Taken together, the data suggest strongly that the cells of the M-pathway form the physiological substrate for detection of luminance modulation and the cells of the P-pathway the substrate for detection of chromatic modulation. However, at high light levels, intrusion of a so-called luminance mechanism near 10 Hz in psychophysical detection of chromatic modulation is probably due to responses in the M-pathway, arising primarily from a nonlinearity of cone summation. Both phasic and tonic ganglion cells responded to frequencies higher than can be psychophysically detected. This suggests that central mechanisms, acting as low-pass filters, modify these cells' signals, though the corner frequency is lower for the P-pathway than for the M-pathway. For both cell types, the response phase at different frequencies was consistent with the cells' description as linear filters with a fixed time delay.

The physiological basis of the minimally distinct border demonstrated in the ganglion cells of the macaque retina.

1. The minimally distinct border method involves setting the relative radiances of two adjacent, differently coloured fields until the border between them is minimally distinct. At these radiance settings, the two fields are found to be of equal luminance. The task shares with flicker photometry all the requirements of a photometric method. 2. We have recorded responses of macaque ganglion cells to such borders moved back and forth across the receptive field; the size of the luminance step across the border was systematically varied. 3. Phasic ganglion cells gave transient responses to such borders, consisting of an increase or decrease in firing rate depending on direction of luminance contrast and cell type (on- or off-centre). Tonic ganglion cells gave sustained responses dependent on chromatic contrast across the border. 4. An analysis of phasic cell responses showed a minimum near equal luminance, suggesting their signal could readily support the minimally distinct border task. We could not devise a scheme whereby tonic cells could support the task. 5. Spectral sensitivity of phasic cells, determined from their minima, closely resembled the 10 deg luminous efficiency function, as required of a mechanism underlying the psychophysical performance. 6. For phasic cells, the minimum was independent of movement speed, and hence of eye movement velocity under natural viewing conditions. 7. Proportionality, additivity and transitivity are found psychophysically with the minimally distinct border method. All these properties were also exhibited by phasic cell responses. 8. Residual responses were present in individual phasic cells to equal-luminance borders, probably due to a non-linearity of M- and L-cone summation. The amplitude of residual response depended on the wavelengths on either side of the border, and was zero for pairs of lights lying along a tritanopic confusion line. These residual responses could be correlated with residual border distinctness at equal luminance as reported psychophysically. 9. There was some variability in spectral sensitivity among phasic cells, and this could be described in terms of variability in weighting of the middle- and long-wavelength cone inputs to each cell. With equal-luminance borders, the residual response of the phasic cell population will thus be made up of the residual responses from individual cells and a contribution due to variation in spectral sensitivity among cells. 10. The responses of phasic ganglion cells thus form the physiological substrate of psychophysical performance on the minimally distinct border task. These cells also provide a residual signal to equal-luminance borders which correlates with residual distinctness.(ABSTRACT TRUNCATED AT 400 WORDS)

"Colour constancy" in Mondrian patterns: a partial cancellation of physical chromaticity shifts by simultaneous contrast.

Edwin Land's Mondrian demonstrations (Land 1977, 1983, 1986a) are striking examples that the perceived colours of objects are largely independent of the chromaticity of the light incident upon them. Attempts to implement this independence in artificial vision systems have renewed interest in colour constancy and contrast, and the explanation of these phenomena in the Retinex theory. We use colour matches to demonstrate that departures from "colour constancy" are large and that it is possible to obtain the same colour shifts when the complex Mondrian pattern is replaced by a homogeneous grey field surrounding a test patch. A given patch has the same colour when surrounded by the Mondrian as when set in a grey background, provided that the grey represents the spatially weighted average of the Mondrian. Neither the colour shifts nor the equivalence of this neutral surround are correctly predicted by the Retinex theory. The phenomenon of partial cancellation of physical chromaticity shifts with changes of illuminant thus reduces to one of simultaneous contrast and adaptation where a spatio-chromatic and luminance average over a Mondrian pattern is the same as for a grey surround. Experiments with simultaneous contrast demonstrate that spatial weighting factors need to be applied in computations of the effect of the separate areas of a complex Mondrian pattern.

Sensitivity of macaque retinal ganglion cells to chromatic and luminance flicker.

1. We have studied the sensitivity of macaque retinal ganglion cells to sinusoidal flicker. Contrast thresholds were compared for stimuli which alternated only in luminance ('luminance flicker') or chromaticity ('chromatic flicker'), or which modulated only the middle- or long-wavelength-sensitive cones ('silent substitution'). 2. For luminance flicker, the lowest thresholds were those of phasic, non-opponent ganglion cells. Sensitivity was maximal near 10 Hz. 3. Tonic, cone-opponent ganglion cells were relatively insensitive to luminance flicker, especially at low temporal frequencies, but were sensitive to chromatic flicker, thresholds changing little from 1 to 20 Hz. Those with antagonistic input from middle- and long-wavelength-sensitive (M- and L-) cones had a low threshold to chromatic flicker between red and green lights. Those with input from short-wavelength-sensitive (S-) cones had a low threshold to chromatic flicker between blue and green. Expressed in terms of cone contrast, the S-cone inputs to blue on-centre cells had higher thresholds than M- and L-cone inputs to other cell types. 4. Phasic, non-opponent cells responded to high-contrast red-green chromatic flicker at twice the flicker frequency. This frequency-doubled response is due to a non-linearity of summation of M- and L-cone mechanisms. It was only apparent at cone contrasts which were above threshold for most tonic cells. 5. M- or L-cones were stimulated selectively using silent substitution. Thresholds of M- and L-cone inputs to both red and green on-centre cells were similar. This implies that these cells' sensitivity to chromatic flicker is derived in equal measure from centre and surround. Thresholds of the isolated cone inputs could be used to predict sensitivity to chromatic flicker. The high threshold of these cells to achromatic contrast is thus, at least in part, due to mutual cancellation by opponent inputs rather than intrinsically low sensitivity. 6. Thresholds of M- and L-cone inputs to phasic cells were similar at 10 Hz, and were comparable to those of tonic cells, suggesting that at 1400 td cone inputs to both cell groups are of similar strength. 7. The modulation transfer function of phasic cells to luminance flicker was similar to the detection sensitivity curve of human observers who viewed the same stimulus. For chromatic flicker, at low temporal frequencies thresholds of tonic cells (red or green on-centre cells in the case of red-green flicker or blue on-centre cells in the case of blue-green flicker) approached that of human observers. We propose the different cell types are the substrate of different channels which have been postulated on the basis of psychophysical experiments. 8. At frequencies of chromatic flicker above 2 Hz, human sensitivity falls off steeply whereas tonic cell sensitivity remained the same or increased. This implies that high-frequency signals in the chromatic, tonic cell pathway are not available to the central pathway respons

Amplitude and phase of responses of macaque retinal ganglion cells to flickering stimuli.

1. We have measured responses of macaque retinal ganglion cells to a uniform flickering field, with variation in luminance, chromaticity or both (heterochromatic flicker). 2. With heterochromatic flicker, as the luminance ratio of the flicker components was varied, phasic ganglion cell activity went through a minimum and an abrupt phase change close to equal luminance. Tonic ganglion cell responses underwent a gradual phase change without any minimum close to equal luminance. For red on-centre cells, when wavelengths above 570 nm were altered with white, a progressive phase advance occurred as luminance ratio (L lambda/LW) was increased. With wavelengths below 570 nm a progressive phase lag occurred. For green on-centre cells, the opposite pattern was found. For all tonic cells, the higher the temporal frequency, the more rapidly did such phase changes occur. A simple model incorporating a centre-surround delay of 3-8 ms could quantitatively account for these changes. 3. With luminance flicker of different dominant wavelengths, amplitudes and phase of responses of phasic ganglion cells were independent of wavelength at all frequencies. The amplitude and phase of the responses of tonic ganglion cells was very dependent on wavelength, as well as on flicker frequency. Their characteristics hardly ever resembled results from phasic cells. 4. For achromatic flicker, response phase of tonic cells at or above 10 Hz was variable, probably due to the centre-surround delay. Such variability was not seen among phasic cells. 5. An interesting implication of these results is that the ability of tonic ganglion cells to unambiguously signal rapid chromatic or spatial change is limited.

Nonlinear summation of M- and L-cone inputs to phasic retinal ganglion cells of the macaque.

We have studied the responses of ganglion cells in the macaque retina to stimuli that alternate in color. With most color combinations, the phasic retinal ganglion cells, which sum input from M- and L-cones in both center and surround, showed a response with twice the alternation frequency at equal luminance. This frequency doubling was directly related to the degree to which the M- and L-cones were stimulated out-of-phase with one another, and thus varied with the wavelength combinations used. It was absent with wavelength combinations that lay along tritanopic confusion lines, when at equal luminance the M- and L-cones are not modulated. Such a frequency-doubled response is evidence for a nonlinearity at or before M- and L-cone summation. The effect became much smaller or was abolished when the receptive field center alone was stimulated, indicating that its mechanism lies in the surround or in a center-surround interaction. Also, it was much more marked at high luminance levels, being almost absent at retinal illuminances below 100 td. Its origin is not clear, but it seems to derive more from the L- than the M-cone. The results imply that phasic cells, through this nonlinearity, could respond to the red-green equal luminance borders used in some psychophysical experiments.

The physiological basis of heterochromatic flicker photometry demonstrated in the ganglion cells of the macaque retina.

1. Heterochromatic flicker photometry is a way of measuring the spectral sensitivity of the human eye. Two lights of different colour are sinusoidally alternated at, typically, 10-20 Hz, and their relative intensities adjusted by the observer until the sensation of flicker is minimized. This technique has been used to define the human photopic luminosity, or V lambda, function on which photometry is based. 2. We have studied the responses of macaque retinal ganglion cells using this stimulus paradigm. The responses of the phasic ganglion cells go through a minimum at relative radiances very similar to that predicted from the V lambda function. At this point, defined as equal luminance, an abrupt change in response phase was observed. A small residual response at twice the flicker frequency was apparent under some conditions. 3. The spectral sensitivity of parafoveal phasic cells measured in this way corresponded very closely to that of human observers minimizing flicker on the same apparatus. 4. Minima in phasic cell activity were independent of flicker frequency, as is the case in the psychophysical task. 5. The response minima of phasic cells obey the laws of additivity and transitivity which are important characteristics of heterochromatic flicker photometry. 6. As the relative intensities of the lights were altered responses of tonic, spectrally opponent cells usually underwent a gradual phase change with vigorous responses at equal luminance. The responses of tonic cells treated individually or as a population could not be related to the V lambda function in any meaningful way. 7. We conclude that the phasic, magnocellular cell system of the primate visual pathway underlies performance in the psychophysical task of heterochromatic flicker photometry. It is likely that other tasks in which spectral sensitivity conforms to the V lambda function also rely on this cell system.

Visual resolution of macaque retinal ganglion cells.

1. The visual resolving ability of different types of macaque retinal ganglion cells was estimated at different retinal eccentricities, by measuring the amplitude of modulated responses to black-white gratings of spatial frequencies near the resolution limit for each cell. 2. The resolving ability of tonic, spectrally opponent ganglion cells was usually similar to that of phasic, non-opponent ganglion cells at similar eccentricities, except that at eccentricities greater than 10 deg some tonic ganglion cells with remarkably high resolution (up to ca. 15 cycles/deg) were found. Our cell sample was limited within the central 2 deg of the visual field, however. 3. Only a small proportion of phasic ganglion cells showed an increase of mean firing level to gratings near the resolution limit. The maintained firing of tonic ganglion cells was higher than that of phasic ganglion cells. 4. With red-black or green-black gratings, the resolution of phasic ganglion cells was unaffected. For red or green on-centre ganglion cells, a marked deterioration of resolving ability occurred when the grating was of a colour to which a cell responded poorly (green-black gratings for red on-centre cells, and red-black gratings for green on-centre cells). A slight improvement in resolving ability occurred when the grating was of an excitatory colour. 5. For a sub-sample of cells, we compared resolution limit with centre size as determined from area-threshold curves. For both phasic and tonic ganglion cells, resolution limit (the period length just resolved) was about half the centre diameter, as is the case for cat ganglion cells. This implies that the centre sizes of phasic and tonic monkey ganglion cells are similar at most eccentricities. 6. We attempt to relate these results to primate retinal anatomy and visual resolution, determined behaviourally.

Thresholds to chromatic spots of cells in the macaque geniculate nucleus as compared to detection sensitivity in man.

1. The relation between wavelength and psychophysical threshold for chromatic spots on a white background provides evidence for the existence of chromatic channels in the primate visual system. To find the physiological substrate of this task, we compared increment thresholds of different cell types in the macaque lateral geniculate nucleus with human psychophysical thresholds to the same stimuli, using two spot sizes, 4 and 0.4 deg. 2. At different wavelengths, different opponent cell classes in the parvocellular layers of the nucleus were most sensitive, so that at long wavelengths (greater than 600 nm) red on-centre cells were most sensitive, while at short wavelengths (less than 500 nm) S-cone, blue on-centre cells were most sensitive, from 500 to about 550 nm green on-centre cells being most sensitive. A rare cell type with inhibition from S-cones was most sensitive at about 570 nm, although its maximum contrast increment sensitivity was poor compared with that of other cell types. Variation in strength of cone opponency caused a considerable range in threshold in each of the opponent cell classes of the parvocellular layers. 3. On- and off-centre cells from the magnocellular layers were more sensitive than opponent cells to white and yellow spots (as is the case with achromatic gratings). 4. With different wavelengths and spot sizes, the most sensitive cells found approached (to within 0.1-0.3 log units) human psychophysical sensitivity, suggesting that the most sensitive cells available may underlie detection. 5. Measurements of psychophysical chromatic discrimination thresholds, both with nearly monochromatic spots and with spots of differing saturation (purity), support this hypothesis. When magnocellular cell sensitivity corresponded to psychophysical threshold, a suprathreshold stimulus, capable of activating opponent cells, was required for chromatic discrimination.

An account of responses of spectrally opponent neurons in macaque lateral geniculate nucleus to successive contrast.

Coloured surfaces in the normal environment may be brighter or dimmer than the mean adaptation level. Changes in the firing rate of cells of the parvocellular layers of macaque lateral geniculate nucleus were studied with such stimuli; chromatic mixtures briefly replaced a white adaptation field. This paradigm is therefore one of successive contrast. Families of intensity-response curves for different wavelengths were measured. When taking sections at different luminance ratios through these families of curves, strongly opponent cells displayed spectrally selective responses at low luminance ratios, while weakly opponent cells had higher chromatic thresholds and responded well to stimuli at higher luminance ratios, brighter than the adaptation field. Strength of cone opponency, defined as the weight of the inhibitory cone mechanism relative to the excitatory one, was thus related to the range of intensity in which cells appeared to operate most effectively. S-cone inputs, as tested with lights lying along tritanopic confusion lines, could either be excitatory or inhibitory. Families of curves for different wavelengths can be simulated mathematically for a given cell by a simple model by using known cone absorption spectra. Hyperbolic response functions relate cone absorption to the output signals of the three cone mechanisms, which are assumed to interact linearly. Parameters from the simulation provided estimates of strength of cone opponency and cone sensitivity which were shown to be continuously distributed. Cell activity can be related to cone excitation in a trichromatic colour space with the help of the model, to give an indication of suprathreshold coding of colour and lightness.

Simulation of responses of spectrally-opponent neurones in the macaque lateral geniculate nucleus to chromatic and achromatic light stimuli.

The responses of spectrally opponent, parvocellular-layer cells in the lateral geniculate nucleus of the macaque vary with stimulation along all three dimensions of colour space (luminance, dominant wavelength and purity). Here, the responses of different cell types to light stimuli successively replacing a white adaptation field are simulated mathematically by a simple model using known cone spectral sensitivities. Hyperbolic response functions with two adjustable parameters relate cone excitation with the output signals of each cone mechanism. For the majority of cells, differences of outputs of two cone mechanisms, receiving inputs from L- and M-cones, describe responses to chromatic and achromatic stimuli. For each opponent combination of cone inputs, either L-M or M-L, two cell types were found. Off-centre cells had lower maximum firing rates of their on-component, but were the more sensitive of the two groups, preferring dark colours, whereas on-centre cells showed better responsiveness to brighter stimuli. For all four cell types, the predictions of the equations replicated well the responses measured to stimuli varying along all three dimensions of colour space. The quantification of cell responses presented here provides possibilities for studying suprathreshold coding of colour and lightness, and the model can help to link physiology with the psychophysics of colour vision.