Test illuminant location with respect to the Planckian locus affects chromaticity shifts of real Munsell chips.

The asymmetric sequential color-matching technique was used to determine the shifts in chromaticity of real Munsell chips induced by four test illuminants. The reference illuminant was C. Illuminants green (G) and purple (P) induced shifts orthogonal to the Planckian locus, while illuminants S and A induced shifts along the Planckian locus. Vectors describing the shifts induced by A and S were quantitatively and qualitatively different from those due to G and P. The data suggest that physiological factors, influenced by the proximity of the Planckian locus, affect chromatic constancy under nonsimulated viewing conditions.

Lightness constancy and its link with cone contrast.

The link between chromatic constancy (compensation for hue and saturation shifts) and lightness constancy (compensation for a change in surface reflectance) was tested theoretically by computing cone contrasts and by asymmetric matching experiments. The effect of a thin achromatic line (a frame) around the test sample was tested empirically. When the samples were outlined by the frame, lightness constancy was increased and chromatic constancy reduced (p<0.001). Changes in luminance are more likely to be compensated when the luminance contrast edge around the test stimulus is disturbed as with the addition of an achromatic frame.

Naming versus matching and the stability of unique hues.

It is known that there is a distortion of hue and saturation in the peripheral visual field. In a previous study, when an asymmetric matching paradigm was used, four hues in the blue, red, yellow and green regions of colour space were unchanged and these were referred to as peripherally invariant (Parry et al., J Opt Soc Am A, 23, 2006, 1586). Three of these invariant hues were similar to unique blue, red and yellow. However, for most observers there was a marked difference between unique and invariant green. To investigate this apparent paradox, we have measured unique hues using a range of eccentricities and colourimetric purities. An asymmetric matching and a 4-AFC paradigm were used to establish peripherally invariant and unique hues, respectively. In the asymmetric matching task the observer matched a peripheral spot with a para-foveal spot, for 24 different hues at 18° eccentricity. In the 4-AFC paradigm, 41 hues were presented 20 times at three purities (0.5, 0.75 and 1.0) and three eccentricities (18°, 10° and 1°). The observer had to name the hues as red, blue, green or yellow. Unique hues were found to be constant with eccentricity and purity. The unique green, established with 4-AFC, was found to differ from the invariant green, determined using the matching task. However, red, blue and yellow invariant hues correspond well with unique hues. The data suggest that different mechanisms mediate the matching of green compared with the identification of unique hues. This is similar to the difference between detection and discrimination of spectral stimuli: the detection process is dominated by the cone opponent mechanisms and is most sensitive, whereas more central processes, serving unique hues, influence discrimination.

Nasal-temporal differences in cone-opponency in the near peripheral retina.

The purpose of this study is to establish whether nasal-temporal differences in cone photoreceptor distributions are linked to differences in colour matching performance in the two hemi-fields. Perceived shifts in chromaticity were measured using an asymmetric matching paradigm. They were expressed in terms of hue rotations and relative saturation changes and also in terms of activation levels of L-M or S-(L+M) cone-opponent channels. Up to 19 degrees eccentricity there was little difference in chromaticity shifts between nasal and temporal retina for either channel. For matches beyond 19 degrees L-M activation is significantly lower in the nasal field and the S-(L+M) channel was equally activated in both fields. The data are consistent with the asymmetric distribution of L- and M-cones in the nasal and temporal retinae.

Almost complete colour constancy achieved with full-field adaptation.

A successive asymmetric colour-matching task was used to study the changes in colour appearance of simulated Munsell samples. Colour shifts were induced with two Planckian illuminants, standard illuminant A (u'=0.256, v'=0.524) and illuminant S (u'=0.174, v'=0.392). Measurements were conducted with a 20 degrees field and a 120 degrees field. Adaptation period varied from 1 to 30s with the smaller field and from 1 to 60s with the larger field. Colour shifts were specified in terms of a modified Brunswik ratio (BR). Higher values of BR were associated with longer adaptation periods but only when the larger background was used. Supplementary experiments showed that the changes in colour appearance were related to a slight shift in the perceived colour of the background. The timing of the colour shifts are modelled in terms of cone opponent responses. High values of BR correspond to almost complete von Kries adaptation in all three cone types.

Neurotoxic effects of acrylamide on rat retinogeniculate fibres.

The common laboratory chemical, acrylamide, causes various clinical effects including dysfunction of the visual system. Previous electrophysiological recordings from the rat lateral geniculate nucleus (LGN) following exposure to acrylamide suggested that X-like cells were selectively disrupted by the neurotoxin. This study examines the possibility that the diameter of the optic tract fibres may underlie their differential susceptibility, but shows that fibres of all sizes are affected by acrylamide intoxication and the results suggest that the selective effect of acrylamide is not due simply to disruption of optic tract fibres.

The effects of V4 lesions on the visual abilities of macaques: shape discrimination.

Monkeys with bilateral ablation of cortical visual area V4 were compared with unoperated controls for their ability to relearn postoperatively a series of preoperatively acquired two-choice visual discrimination problems. The animals with V4 lesions were impaired on relearning to discriminate between different shapes, and discrimination between identical shapes presented at different orientations was also impaired. Some of the deficits were consistent with disrupting the input to inferotemporal cortex, but discrimination of a subset of the stimuli is known to be unaffected by inferotemporal cortex lesions and could not be explained in the same way. To clarify the nature of the deficit, and to test the hypothesis that shapes differing in orientation are analyzed in the occipitoparietal processing pathway, animals with V4 lesions were also compared to normals on their ability to acquire a version of the landmark task. The V4 animals performed as well as the control animals on this task. The results suggest that V4 is important for the shape discrimination abilities that survive inferotemporal cortex lesions. The role of V4 in shape analysis is discussed in the light of recent evidence that V4 neurones are modulated by visual attention.

The effects of lesions of area V4 on the visual abilities of macaques: colour categorization.

Monkeys with V4 lesions and unoperated controls were tested behaviourally for their perception of the colour categories red, green, blue and yellow. As expected, the monkeys with V4 lesions took longer to acquire the colour discriminations. However, the pattern of learning was the same in the two groups of animals. Both the lesioned and the control animals made more errors when the stimuli to be discriminated belonged to the same colour category than when they belonged to different categories and thus showed normal colour categorization. The results suggest that monkeys with V4 lesions perceive the colour spectrum according to the same fundamental perceptual categories as normal monkeys and humans, and thus support the view that, in the colour domain, the four basic perceptual categories are constructed by chromatic mechanisms operating earlier than visual area V4.

The effects of V4 lesions on the visual abilities of macaques: hue discrimination and colour constancy.

Monkeys with lesions of cortical visual area V4 were compared with unoperated monkeys in three experiments. In Expt. 1 they were tested for the reacquisition of a pre-operatively learned hue discrimination task. In Expt. 2, as a test of colour constancy, the monkeys were required to perform previously overlearned colour discrimination tasks when the amounts of red, green and blue light in the illuminant were changed. In Expt. 3 the animals were compared on the post-operative acquisition of hue, greyness and saturation discrimination tasks. The results of Expt. 1 showed that monkeys with V4 lesions can regain their pre-operative levels of performance on hue discrimination tasks. Expt. 2 showed that monkeys with V4 lesions have a colour constancy deficit. Expt. 3 reinforced the finding that the animals with V4 lesions could learn to perform fine hue discriminations but that their final, asymptotic performance was not as reliable as that of normal animals. The wavelength discrimination data are discussed within the context of recent electrophysiological findings that V4 is involved in selective attention to visual stimuli and the constancy data are interpreted as evidence that V4 is important for defining colour constancy thresholds but not for constructing the perceptual categories underlying constant colour perception.

The electrophysiological basis of colour processing in macaques with V4 lesions.

Monkeys with lesions of visual area V4 have deficits in colour constancy, but are able to discriminate hues and segment the spectrum in a categorical manner. To investigate the nature of the processing mechanisms subserving the spared functions we recorded occipital visual evoked potentials (VEPs) of normal monkeys and monkeys with bilateral lesions of area V4. The stimuli used to elicit the potentials were chromatic and achromatic gratings of low spatial frequency. The waveforms from the two groups of animals were similar in all respects. VEPs for the onset of a chromatic grating were negative-going, indicative of the activity of sustained units, as opposed to those elicited by offset or reversal of the grating which were positive-going. The amplitude of the chromatic, 12.5 Hz reversal VEP went through a minimum at isoluminance, in accord with low temporal resolution of colour processing. The VEP waveforms were identical in character from three weeks to approximately four years post-operatively. These data indicate that chromatic processing in areas V1 and V2 is normal after V4 lesions and, together with the behavioural evidence, that these areas are sufficient for some basic aspects of colour perception.

Stereotaxic head restraint for chronic and semi-chronic marmoset and macaque monkeys.

A device allowing head holding and stereotaxic manipulations in both small-sized Marmoset and in relatively large Macaca monkeys is described. Its design permits free access to most targets in the visual system and optimizes presentation of visual stimuli in awake-chronic and anaesthetized semi-chronic preparations. Extra-aural attachments to temporal bone are implanted stereotaxically, under anaesthesia, on both sides of the skull. This provides means for stereotaxic head-holding which minimizes discomfort to the animal. Maintenance of stereotaxic precision over long periods of time, possibility of realignment and use of one device for various monkey species are special features of the method.

Silent periodic cells in the cat striate cortex.

A class of neurons called silent periodic cells, having properties intermediate between those in the simple and complex families, has been discriminated in the cat striate cortex. Silent periodic cells have relatively small receptive fields, a low spontaneous activity (i.e. relatively silent) and a preference for relatively slow stimulus velocities (less than 3 degrees/sec). In addition they give a mixed on/off response to a stationary flashing bar over virtually the whole of the receptive field with usually somewhat stronger on responses in some locations and stronger off responses in others (partial phase sensitivity). The most characteristic properties of these cells are, however, found in their responses to gratings, namely the nonlinearity manifested by the absence of a null point to a stationary flashing grating, a spatial periodicity revealed by a clearly modulated discharge to drifting gratings of medium and high spatial frequency and, finally, a very sharp spatial frequency tuning curve with a half-sensitivity bandwidth between 0.5 and 0.95 octave, i.e. narrower than that of simple cells whose bandwidths are usually above 1 octave. Silent periodic cells resemble B-cells that have been described in other studies.

Objective specification of tritanopic confusion lines using visual evoked potentials.

The minimum distinct border (MDB) between different isoluminant hues of a bipartite field was set subjectively by adjustment and then according to a 3-point rating allowing determination of the S-cone specific (tritanopic) axis. This rating was compared with visual evoked potentials (VEPs) generated by coarse, isoluminant gratings, modulated along the same chromatic axes. Psychophysically determined tritanopic gratings elicited monophasic onset VEPs with the longest latency for each subject. Departure from this axis produced additional earlier negative components (reflecting additional response mechanisms). Colour-selective VEPs can, therefore, objectively specify tritanopic confusion lines with a comparable degree of accuracy to MDB judgements.

Apparent fineness of stationary compound gratings.

Patterns consisting of the sum of a sinusoidal grating and its second spatial harmonic have an apparent spatial fineness, or periodicity, that is about halfway between the two component spatial frequencies. There are also phase dependent modulations of the apparent fineness about the mean fineness shift. Covariance between individuals' phase dependent fineness shifts indicates the presence of four spatial phase channels. The apparent fineness effects, and the putative phase channels, may both be a product of a local, linear, analysis of spatial frequency content. Illusory second harmonics, as generated in the spatial frequency doubling illusion, also change apparent fineness.

Contribution of two movement detecting mechanisms to central and peripheral vision.

Two mechanisms, one for the detection of fast, and the other for slow movement of a sinusoidal grating are identified, and investigated under central, parafoveal, and peripheral viewing conditions. The fast movement data is considered in terms of the Reichardt model, in which signals from two adjacent inputs are cross-correlated leading to halving of the spatial resolving power for movement detection, compared with that for pattern detection. The mechanism underlying slow movement detection is regarded as being closely related to pattern detection, probably at the single unit level. The characteristics of this mechanism are discussed in the light of recent electrophysiological experiments describing clusters of simple cells in the visual cortex with "directional preference" properties.

Specificity and selectivity of chromatic visual evoked potentials.

A paper by Rabin et al. (1994) Vision Research, 34, 2657-2671, claimed that spatially extensive grating stimuli could be used to generate chromatic-specific visual evoked potentials from subjects assumed to have standard spectral sensitivity and tritanopic confusion lines. Here we demonstrate that such spatially extensive stimuli may generate responses which are contaminated by luminance-contrast intrusions. Such intrusions are mainly due to chromatic aberrations and are compounded by the abovementioned assumptions. Claims regarding the chromatic selectivity of VEPs must, therefore, be substantiated by establishing correlations with the known properties of the chromatic system.

Red--green and blue--yellow mechanisms are matched in sensitivity for temporal and spatial modulation.

The spatial and temporal properties of human colour vision are examined using isoluminant, red--green and blue--yellow tritanopic gratings. Chromatic sensitivity is found to be low-pass as a function of both spatial and temporal frequency along all the chromatic axes investigated, including the tritanopic confusion lines employed to examine the properties of the S-cone driven mechanism. Comparison of sensitivity to on-off and contrast reversing stimuli indicates that transient mechanisms contribute to the detection of red--green patterns but that the detection of S-cone specific patterns is governed by sustained mechanisms. By compensating for transient contributions to red--green sensitivity, it is shown that sensitivity of chromatic mechanisms dominated by L- and M-cone input are closely matched to those with S-cone input.

Macular pigment and the colour-specificity of visual evoked potentials.

A modified Moreland anomaloscope was used to examine two subjects, one with dense macular pigment, the other with relatively light pigmentation. Chromatic visual evoked potentials (VEPs) were elicited from these two subjects using coarse, isoluminant gratings of different sizes. Colour-specificity was verified by comparing chromatic onset VEPs (reflecting sustained activity) with chromatic reversal VEPs (reflecting mainly transient activity) and also by Fourier analysis (colour-specific responses are dominated by the fundamental, transient activity by the second harmonic). Achromatic (transient-type) intrusions, produced by large blue-green gratings could be related to the extent of subject-specific macular pigmentation.

Colour constancy as a function of hue.

The effects of colour categories on colour constancy were studied under two illuminants and two neutral grey backgrounds using Munsell chips and colour matching. It was found that the categorical colours: red, yellow, green and blue, which are processed by basic colour-opponent mechanisms, show relatively better colour constancy than intermediate colours. The dominant wavelength of these categorical colours are closely related to the typical hues obtained in experiments with narrow-band spectral hues.