Translucence perception is not dependent on cortical areas critical for processing colour or texture.

Translucence is an important property of natural materials, and human observers are adept at perceiving changes in translucence. Perceptions of different material properties appear to arise from different cortical regions, and it is therefore plausible that the perception of translucence is dependent on specialised regions, separate from those important for colour and texture processing. To test for anatomical independence between areas necessary for colour, texture and translucence perception we assessed translucency perception in a cortically colour blind observer, who performs at chance on tasks of colour and texture discrimination. Firstly, in order to establish that MS has shown no significant recovery, we assessed his colour perception performance on the Farnsworth-Munsell 100 Hue Test. Secondly, we tested him with two translucence ranking tasks. In one task, stimuli were images of glasses of tea varying in tea strength. In the other, stimuli were glasses of tea varying only in milkiness. MS was able to systematically rank both strength and milkiness, although less consistently than controls, and for tea strength his rankings were in the opposite order. An additional group of controls tested with greyscale versions of the images succeeded at the tasks, albeit slightly less consistently on the milkiness task, showing that the performance of normal observers cannot be transformed into the performance of MS simply by removing colour information from the stimuli. The systematic performance of MS suggests that some aspects of translucence perception do not depend on regions critical for colour and texture processing.

Context-dependent judgments of color that might allow color constancy in scenes with multiple regions of illumination.

For a color-constant observer, a change in the spectral composition of the illumination is accompanied by a corresponding change in the chromaticity associated with an achromatic percept. However, maintaining color constancy for different regions of illumination within a scene implies the maintenance of multiple perceptual references. We investigated the features of a scene that enable the maintenance of separate perceptual references for two displaced but overlapping chromaticity distributions. The time-averaged, retinotopically localized stimulus was the primary determinant of color appearance judgments. However, spatial separation of test samples additionally served as a symbolic cue that allowed observers to maintain two separate perceptual references.

Latency characteristics of the short-wavelength-sensitive cones and their associated pathways.

There are many distinct types of retinal ganglion and LGN cells that have opponent cone inputs and which may carry chromatic information. Of interest are the asymmetries in those LGN cells that carry S-cone signals: in S-ON cells, S+ signals are opposed by (L + M) whereas, in many S-OFF cells, L+ signals are opposed by (S + M), giving -S + L - M (C. Tailby, S. G. Solomon, & P. Lennie, 2008). However, the S-opponent pathway is traditionally modeled as +/-[S - (L + M)]. A phase lag of the S-cone signal has been inferred from psychophysical thresholds for discriminating combinations of simultaneous sinusoidal modulations along +/-[L - M] and +/-[S - (L + M)] directions (C. F. Stromeyer, R. T. Eskew, R. E. Kronauer, & L. Spillmann, 1991). We extend this experiment, measuring discrimination thresholds as a function of the phase delay between pairs of orthogonal component modulations. When one of the components isolates the tritan axis, there are phase delays at which discrimination is impossible; when neither component is aligned with the tritan axis, discrimination is possible at all delays. The data imply that the S-cone signal is delayed by approximately 12 ms relative to (L - M) responses. Given that post-receptoral mechanisms show diverse tuning around the tritan axis, we suggest that the delay arises before the S-opponent channels are constructed, possibly in the S-cones themselves.

Sensory, computational and cognitive components of human colour constancy.

When the illumination on a scene changes, so do the visual signals elicited by that scene. In spite of these changes, the objects within a scene tend to remain constant in their apparent colour. We start this review by discussing the psychophysical procedures that have been used to quantify colour constancy. The transformation imposed on the visual signals by a change in illumination dictates what the visual system must 'undo' to achieve constancy. The problem is mathematically underdetermined, and can be solved only by exploiting regularities of the visual world. The last decade has seen a substantial increase in our knowledge of such regularities as technical advances have made it possible to make empirical measurements of large numbers of environmental scenes and illuminants. This review provides a taxonomy of models of human colour constancy based first on the assumptions they make about how the inverse transformation might be simplified, and second, on how the parameters of the inverse transformation might be set by elements of a complex scene. Candidate algorithms for human colour constancy are represented graphically and pictorially, and the availability and utility of an accurate estimate of the illuminant is discussed. Throughout this review, we consider both the information that is, in principle, available and empirical assessments of what information the visual system actually uses. In the final section we discuss where in our visual systems these computations might be implemented.

Is the S-opponent chromatic sub-system sluggish?

The S-opponent pathway has a reputation for being sluggish relative to the L/M-opponent pathway. Cottaris and De Valois [Nature 395 (1998) 896] claim that S-opponent signals are available in Macaque V1 only after 96-135 ms whereas L/M-opponent signals are available after 68-95 ms. Our experiments tested whether this large latency difference could be observed psychophysically. We measured reaction times to S/(L + M) and L/(L + M) increments. Both the equiluminant plane and the tritan line were empirically determined and we used spatio-temporal luminance noise to mask luminance cues. An adaptive staircase progressed according to observers' performance on a 'go, no-go' task and provided concomitant estimates of threshold and of reaction time. When brief stimuli are confined to chromatic channels and presented at equivalent (threshold) levels and when latency is estimated from visually triggered reaction times, we find that the difference between the L/M-opponent and S-opponent sub-systems is, at most, 20-30 ms.