Transient covert attention and the perceived rate of flicker

Transient covert attention affects basic visual dimensions such as contrast sensitivity, spatial resolution, and temporal resolution. Two recent studies provide evidence of corresponding phenomenological changes: The increase in contrast sensitivity and spatial resolution at the attended location is associated with increased apparent contrast (M. Carrasco, S. Ling, & S. Read, 2004) and apparent spatial frequency (J. Gobell & M. Carrasco, 2005). Here, we assessed a phenomenological correlate of attention for temporal vision, asking whether and how transient attention affects perceived flicker rate. We employed a psychophysical method developed to assess changes in appearance by manipulating transient attention via uninformative spatial cues. In each trial, two suprathreshold Gabor stimuli, appearing briefly to the left and right of fixation, were counterphase modulated at either the same or different temporal frequencies. To assess appearance, we asked observers to perform an orientation discrimination task contingent on perceived flicker rate: "What was the orientation of the Gabor that flickered faster?" Results indicated that perceived flicker rate increased at the cued location. A control experiment, in which observers reported the orientation of the Gabor that flickered slower, ruled out a cue bias explanation. We conclude that transient attention increases perceived flicker rate.     

Attention makes moving objects be perceived to move faster

Although it is well established that attention affects visual performance in many ways, by using a novel paradigm [Carrasco, M., Ling, S., & Read. S. (2004). Attention alters appearance. Nature Neuroscience, 7, 308-313.] it has recently been shown that attention can alter the perception of different properties of stationary stimuli (e.g., contrast, spatial frequency, gap size). However, it is not clear whether attention can also change the phenomenological appearance of moving stimuli, as to date psychophysical and neuro-imaging studies have specifically shown that attention affects the adaptability of the visual motion system. Here, in five experiments we demonstrated that attention effectively alters the perceived speed of moving stimuli, so that attended stimuli were judged as moving faster than less attended stimuli. However, our results suggest that this change in visual performance was not accompanied by a corresponding change in the phenomenological appearance of the speed of the moving stimulus.     

Cue contrast modulates the effects of exogenous attention on appearance

Exogenous spatial attention can be automatically engaged by a cue presented in the visual periphery. To investigate the effects of exogenous attention, previous studies have generally used highly salient cues that reliably trigger attention. However, the cueing threshold of exogenous attention has been unexamined. We investigated whether the attentional effect varies with cue salience. We examined the magnitude of the attentional effect on apparent contrast [Carrasco, M., Ling, S., & Read, S. (2004). Attention alters appearance. Nature Neuroscience, 7(3), 308-313.] elicited by cues with negative Weber contrast between 6% and 100%. Cue contrast modulated the attentional effect, even at cue contrasts above the level at which observers can perfectly localize the cue; hence, the result is not due to an increase in cue visibility. No attentional effect is observed when the 100% contrast cue is presented after the stimuli, ruling out cue bias or sensory interaction between cues and stimuli as alternative explanations. A second experiment, using the same paradigm with high contrast motion stimuli gave similar results, providing further evidence against a sensory interaction explanation, as the stimuli and task were defined on a visual dimension independent from cue contrast. Although exogenous attention is triggered automatically and involuntarily, the attentional effect is gradual.     

Evaluating comparative and equality judgments in contrast perception: attention alters appearance

Covert attention not only improves performance in many visual tasks but also modulates the appearance of several visual features. Studies on attention and appearance have assessed subjective appearance using a task contingent upon a comparative judgment (e.g., M. Carrasco, S. Ling, & S. Read, 2004). Recently, K. A. Schneider and M. Komlos (2008) questioned the validity of those results because they did not find a significant effect of attention on contrast appearance using an equality task. They claim that such equality judgments are bias-free whereas comparative judgments are bias-prone and propose an alternative interpretation of the previous findings based on a decision bias. However, to date there is no empirical support for the superiority of the equality procedure. Here, we compare biases and sensitivity to shifts in perceived contrast of both paradigms. We measured contrast appearance using both a comparative and an equality judgment. Observers judged the contrasts of two simultaneously presented stimuli, while either the contrast of one stimulus was physically incremented (Experiments 1 and 2) or exogenous attention was drawn to it (Experiments 3 and 4). We demonstrate several methodological limitations of the equality paradigm. Nevertheless, both paradigms capture shifts in PSE due to physical and perceived changes in contrast and show that attention enhances apparent contrast.     

Large effects of peripheral cues on appearance correlate with low precision

In a previous study (M. Carrasco, S. Ling, & S. Read, 2004), observers selected one of two Gabors that appeared to have higher contrast (comparative judgment). A peripheral cue preceded one of the Gabors by 120 ms. Results showed that the cue increased the perceived contrast of the adjacent Gabor. We replicated the experiment and found correlations between the precision of judgments and perceptual cueing effects. Larger cueing effects occurred in conditions with less precise judgments and in observers who saw less difference between the stimuli. Further, we asked observers to judge whether the two Gabors were equal or different (equality judgment). This method avoids decision biases but turned out to be less efficient. Cueing effects were absent for equality judgments and trained observers but present for untrained observers with poor ability to discriminate the stimuli. In another experiment, we showed that when observers' responses became more rapid over the course of the experiment, a cueing effect in brightness perception emerged that is unlikely to reflect perceptual changes. Overall, the results show that large effects of peripheral cues on appearance correlate with poor ability to discriminate the stimuli. We suggest that the cue biased observers' decision but did not affect their perception.     

Apparent contrast differs across the vertical meridian: visual and attentional factors

It is known that visual performance is better on the horizontal than the vertical meridian, and in the lower than the upper region of the vertical meridian (Vertical Meridian Asymmetry, "VMA"), and that exogenous spatial attention increases the apparent contrast of a stimulus. Here we investigate whether the VMA also leads to differences in the subjective appearance of contrast between the upper and lower vertical meridian, and how the effects of exogenous spatial attention on appearance interact with the VMA. Two Gabor stimuli were presented North and South of fixation at 4 degrees eccentricity along the vertical meridian. Observers were asked to report the orientation of the Gabor that was higher in contrast. By assessing which stimulus observers perceived to be higher in contrast, we obtained psychometric functions and their concomitant points of subjective equality (PSE). These functions were measured both when a neutral cue was presented in the middle of the display and transient attention was deployed via a peripheral cue to the location of one of the stimuli. Observers were told that the cues were uninformative as to the stimulus contrast or its orientation. We report two novel findings. First, apparent contrast is higher on the lower vertical meridian than on the upper. Second, the attentional enhancement of apparent contrast is asymmetrical with both low and high contrast stimuli; the effect of exogenous spatial attention is greater on the lower than the upper vertical meridian. As in prior studies, we find no corresponding asymmetry in orientation discrimination. Signal detection-based models explain the asymmetrical appearance effects as a function of differential multiplicative gain factors for the North and South locations, and predict a similar but much smaller asymmetry for orientation discrimination.     

Infant color vision: prediction of infants' spontaneous color preferences

Infants show spontaneous looking preferences among isoluminant chromatic stimuli [Adams, R. J. (1987). An evaluation of color preferences in early infancy. Infant Behavior and Development, 10, 143-150; Bornstein, M. H. (1975). Qualities of color vision in infancy. Journal of Experimental Social Psychology, 19 (3), 401-419.]. These differences in preference have often been called "hue" or "color" preferences, and attributed to differences in hue, but there are alternative explanations. Spontaneous preference variations remain after stimuli are equated for adult brightness, and thus cannot be attributed to adult-like brightness differences [Teller, D. Y., Civan, A., & Bronson-Castain, K. (2004). Infants' spontaneous color preferences are not due to adult-like brightness variations. Visual Neuroscience, 21 (3), 397-401]. In the present paper, we address three more alternative explanations: colorimetric purity; infant detection thresholds; and adult-like variations in saturation. Three experiments were conducted. In Experiment 1 we measured infants' spontaneous preferences for each of 22 different chromatic stimuli of varying dominant wavelength and colorimetric purity, each paired against the same white standard. In Experiment 2, we measured infants' chromatic detection thresholds. In Experiment 3, adult subjects made saturation matches between a blue-green standard and each of five other chromatic stimuli. Infant detection thresholds accounted for 34% of the variance in infant "hue" preferences, much more than colorimetric purity (2.4%) or adult saturation judgments (3%), but none of the three variables accounted for the majority of the variance. In our view, the most likely remaining option is that infants' spontaneous "hue" preferences indeed arise from preferences for the hues of stimuli that adults see as blue, purple and red.     

Are there separate first-order and second-order mechanisms for orientation discrimination?

In a series of experiments we compared orientation discrimination performance for Gabor stimuli in which the stimulus profile was either matched to the receptive field profile of single V1 simple cells ('simple'), or in which the carrier and envelope orientations were different ('tigertails'). In the first Experiment, using small, high spatial frequency, peripheral stimuli to minimise the number of detectors involved, we found that simple stimuli were more detectable than tigertails of the same contrast energy, and that orientation discrimination thresholds for simple stimuli were lower than for tigertails of equal detectability. In later experiments with larger stimuli we measured thresholds for detecting tilts of the envelope with the carrier fixed in orientation. Envelope thresholds were similar for different carrier orientations, but carrier orientation had a strong biasing effect upon perceived envelope orientation. When the orientation difference between envelope and carrier was small, the carrier orientation was attracted to that of the envelope; when the difference was large (>10 degrees ) repulsion was found. The biases were reduced by half-wave rectifying the stimuli, putatively making the envelope visible to a first-order filter (Experiment 2). Discrimination thresholds for envelope orientation were higher than those for carrier orientation, and this difference was greater for briefly-presented parafoveal stimuli than for long duration foveal stimuli (Experiments 3 and 4). We conclude from these results that there are separate mechanisms for envelope and carrier orientation discriminations for large stimuli, but that first- and second-order mechanisms are not independent in the discrimination of orientation.     

Spatial covert attention increases contrast sensitivity across the CSF: support for signal enhancement

This study is the first to report the benefits of spatial covert attention on contrast sensitivity in a wide range of spatial frequencies when a target alone was presented in the absence of a local post-mask. We used a peripheral precue (a small circle indicating the target location) to explore the effects of covert spatial attention on contrast sensitivity as assessed by orientation discrimination (Experiments 1-4), detection (Experiments 2 and 3) and localization (Experiment 3) tasks. In all four experiments the target (a Gabor patch ranging in spatial frequency from 0.5 to 10 cpd) was presented alone in one of eight possible locations equidistant from fixation. Contrast sensitivity was consistently higher for peripherally- than for neutrally-cued trials, even though we eliminated variables (distracters, global masks, local masks, and location uncertainty) that are known to contribute to an external noise reduction explanation of attention. When observers were presented with vertical and horizontal Gabor patches an external noise reduction signal detection model accounted for the cueing benefit in a discrimination task (Experiment 1). However, such a model could not account for this benefit when location uncertainty was reduced, either by: (a) Increasing overall performance level (Experiment 2); (b) increasing stimulus contrast to enable fine discriminations of slightly tilted suprathreshold stimuli (Experiment 3); and (c) presenting a local post-mask (Experiment 4). Given that attentional benefits occurred under conditions that exclude all variables predicted by the external noise reduction model, these results support the signal enhancement model of attention.     

Foveal contour interactions and crowding effects at the resolution limit of the visual system

We describe several experiments on contour interactions and crowding effects at the resolution limit of the visual system. As test stimuli we used characters that are often employed in optometric practice for testing visual acuity: Landolt C's, Snellen E's, and rectangular gratings. We tested several hypotheses that have been put forward to explain contour interaction and crowding effects. In Experiment 1 and Experiment 2, Landolt C's were the test stimuli, and bars, or Landolt C's, or gratings served as distractors. In Experiment 1, we showed that neither scale invariance nor spatial frequency selectivity is a characteristic of foveal crowding effects. These results allowed us to conclude that mechanisms other than lateral masking contribute to observers' performance in 'crowded' tasks. R. F. Hess, S. C. Dakin, and N. Kappor (2000) suggested that the spatial frequency band most appropriate for target recognition is shifted by the surrounding bars to higher spatial frequencies that cannot be resolved by observers. Our Experiment 2 rejects this hypothesis as the experimental data do not follow theoretical predictions. In Experiment 3, we employed Snellen E's, both as test stimuli and as distractors. The masking functions were similar to those measured in Experiment 1 when the test Landolt C was surrounded by Landolt C's. In Experiment 4, we extended the range of test stimuli to rectangular gratings; same-frequency or high-frequency gratings were distractors. In this case, if the distracting gratings had random orientation from trial to trial, the critical spacing was twice larger than in the first three experiments. If the orientation of the distractors was fixed during the whole experiment, the critical spacing was similar to that measured in the first three experiments. We suggest that the visual system can use different mechanisms for the discrimination of different test stimuli in the presence of particular surround. Different receptive fields with different spatial characteristics can be employed. To explain why crowding effects at the resolution limit of the visual system are not scale invariant, we suggest that a range of stimuli, slightly varying in size, may all be processed by the same neural channel--the channel with the smallest receptive fields of the visual system.     

Action modulates object-based selection

Cueing attention to one part of an object can facilitate discrimination in another part (Experiment 1 [Duncan, J. (1984). Selective attention and the organization of visual information. Journal of Experimental Psychology: General, 113, 501-517]; [Egly, R., Driver, J., & Rafal, R. D. (1994). Shifting visual attention between objects and locations: evidence from normal and parietal lesion subjects. Journal of Experimental Psychology: Human Perception and Performance, 123, 161-177]). We show that this object-based mediation of attention is disrupted when a pointing movement is prepared to the cued part; when a pointing response is prepared to a part of an object, discrimination does not differ between (i) stimuli at locations in the same object but distant to the part where the pointing movement is programmed and (ii) stimuli at locations equidistant from the movement but outside the object (Experiment 2). This remains true even when the pointing movement cannot be performed without first coding the whole object (Experiment 3). Our results indicate that pointing either (i) emphasizes spatial selection at the expense of object-based selection, or (ii) changes the nature of the representation(s) mediating perceptual selection. In addition, the results indicate that there can be a distinct effect on attention of movement to a specific location, separate from the top-down cueing of attention to another position (Experiment 3). Our data highlight the interactivity between perception and action.     

The effects of aging on orientation discrimination

The current experiments measured orientation discrimination thresholds in younger (mean age approximately 23 years) and older (mean age approximately 66 years) subjects. In Experiment 1, the contrast needed to discriminate Gabor patterns (0.75, 1.5, and 3c/deg) that differed in orientation by 12deg was measured for different levels of external noise. At all three spatial frequencies, discrimination thresholds were significantly higher in older than younger subjects when external noise was low, but not when external noise was high. In Experiment 2, discrimination thresholds were measured as a function of stimulus contrast by varying orientation while contrast was fixed. The resulting threshold-vs-contrast curves had very similar shapes in the two age groups, although the curve obtained from older subjects was shifted to slightly higher contrasts. At contrasts greater than 0.05, thresholds in both older and younger subjects were approximately constant at 0.5deg. The results from Experiments 1 and 2 suggest that age differences in orientation discrimination are due solely to differences in equivalent input noise. Using the same methods as Experiment 1, Experiment 3 measured thresholds in 6 younger observers as a function of external noise and retinal illuminance. Although reducing retinal illumination increased equivalent input noise, the effect was much smaller than the age difference found in Experiment 1. Therefore, it is unlikely that differences in orientation discrimination were due solely to differences in retinal illumination. Our findings are consistent with recent physiological experiments that have found elevated spontaneous activity and reduced orientation tuning on visual cortical neurons in senescent cats (Hua, T., Li, X., He, L., Zhou, Y., Wang, Y., Leventhal, A. G. (206). Functional degradation of visual cortical cells in old cats. Neurobiology Aging, 27(1), 155-162) and monkeys (Yu, S., Wang, Y., Li, X., Zhou, Y. & Leventhal, A. G. (2006). Functional degradation of visual cortex in senescent rhesus monkeys. Neuroscience, 140(3), 1023-1029; Leventhal, A. G., Wang, Y., Pu, M., Zhou, Y. & Ma. Y. (2003). GABA and its agonists improved visual cortical function in senescent monkeys. Science,300 (5620), 812-815).     

Latencies for chromatic and achromatic visual mechanisms

Relative visual onset latency and offset latency (visual persistence) were measured (using perceived simultaneity methodsl for pulsed chromatic stimuli presented under two conditions: as luminance increments of 0.2 or 0.3 log unit above a prevailing achromatic background field, or in hue substitution, matched in luminance to the achromatic field so as to eliminate spatio-temporal luminance transients. In the first experiment, for the hue substitution condition, both onset and offset latencies were found to be slow relative to latencies for the increment condition, and varied with wavelength according to a function resembling trichromatic saturation discrimination (slowest latency at 570 nm). Latencies to increment pulses of equal luminance did not vary with wavelength. In a second experiment, offset latency was measured as a function of pulse duration and pulse luminance level. The observed decrease in offset latency with pulse duration was markedly steeper at short pulse durations for chromatic increments than for hue substitution stimuli. Offset latency decreased with increasing pulse luminance for incremental stimuli but not for hue substitution stimuli. These results are interpreted as reflecting the temporal response properties of chromatic neural mechanisms (the hue substitution data) and the achromatic mechanism (the increment data).     

Orientation discrimination and tilt aftereffects with luminance and illusory contours.

Orientation discrimination and tilt aftereffects (TAEs) were measured to determine if the orientation of luminance and illusory contours are processed by separate mechanisms. The assumption was made that if a single mechanism supports the perception of both types of contours, then illusory and luminance contours that support the same level of orientation discrimination will be equally effective adapting patterns. Experiment I found that orientation discrimination psychometric functions for illusory and luminance contours are similar, confirming that performance could be matched. Experiment II measured orientation discrimination for a range of intensities for both contours. Experiment III measured TAEs following adaptation to illusory and luminance contours that supported a similar range of orientation discrimination. Similar TAEs were not observed, thus rejecting the single mechanism hypothesis. Experiments IV and V sought to validate the assumption that equivalent orientation discrimination predicts equivalent TAEs by using stimuli that seemed likely to be represented by the same visual mechanism. Luminance contours masked by randomly placed dots and unmasked luminance contours were used with the same procedures as experiments II and III. Equal TAEs were not observed for masked and unmasked contours matched on orientation discrimination, suggesting the assumption relating discriminability to adaptation was incorrect.     

The retention and disruption of color information in human short-term visual memory

Previous studies have demonstrated that the retention of information in short-term visual perceptual memory can be disrupted by the presentation of masking stimuli during interstimulus intervals (ISIs) in delayed discrimination tasks (S. Magnussen & W. W. Greenlee, 1999). We have exploited this effect in order to determine to what extent short-term perceptual memory is selective for stimulus color. We employed a delayed hue discrimination paradigm to measure the fidelity with which color information was retained in short-term memory. The task required 5 color normal observers to discriminate between spatially non-overlapping colored reference and test stimuli that were temporally separated by an ISI of 5 s. The points of subjective equality (PSEs) on the resultant psychometric matching functions provided an index of performance. Measurements were made in the presence and absence of mask stimuli presented during the ISI, which varied in hue around the equiluminant plane in DKL color space. For all reference stimuli, we found a consistent mask-induced, hue-dependent shift in PSE compared to the "no mask" conditions. These shifts were found to be tuned in color space, only occurring for a range of mask hues that fell within bandwidths of 29-37 deg. Outside this range, masking stimuli had little or no effect on measured PSEs. The results demonstrate that memory masking for color exhibits selectivity similar to that which has already been demonstrated for other visual attributes. The relatively narrow tuning of these interference effects suggests that short-term perceptual memory for color is based on higher order, non-linear color coding.     

Changes in orientation discrimination at the time of saccadic eye movements

Perceptual performance has been known to change around the time of saccadic eye movement. In the current study, we measured the accuracy and sensitivity of orientation discrimination of bar stimuli presented during fixation and before saccadic eye movements. Human participants compared the orientations of the test and reference bar stimuli with the head erect in a two-interval forced choice task. For the targets presented during steady fixation, the accuracy and sensitivity of orientation discrimination were better near the cardinal than oblique axes, a perceptual anisotropy known as the oblique effect. For the targets presented during the 100 ms interval immediately before a saccade was executed, the anisotropy decreased mainly due to reduction in sensitivity for cardinal orientations. Directing attention to the goal location of the impending saccade emulated the saccadic effects on orientation discrimination for the targets at saccadic goal, suggesting that the saccadic effects on orientation discrimination are partly mediated by the shift of spatial attention that accompanies the saccade. These results were in line with the anti-oblique effect that perceptual judgment of motion direction along the oblique angle becomes relatively accurate for motion targets presented before saccadic eye movements [Lee, J., & Lee, C. (2005). Changes in visual motion perception before saccadic eye movements. Vision Research, 45(11), 1447-1457].     

Infants' spontaneous color preferences are not due to adult-like brightness variations

In the present work, we explore the perceptual bases of infants' spontaneous looking preferences among isoluminant chromatic stimuli (Bornstein, 1975). Three experiments were conducted. In Experiment 1, adult subjects made brightness matches between a white standard and each of six isoluminant chromatic stimuli. The classic variations of brightness with chromaticity were found. In Experiment 2, 12-week-old infants' spontaneous looking preferences were measured for white lights of different luminances. Preference increased with increasing luminance, suggesting that brightness differences are sufficient to create looking preferences among isochromatic stimuli. In Experiment 3, infants' preferences were tested for each of the six chromatic stimuli paired against white, at both isoluminance and (adult) isobrightness. All chromatic stimuli were preferred to white, and the pattern of preferences was similar for both isoluminance and isobrightness conditions. It is concluded that hue and/or saturation, rather than brightness, control infants' spontaneous looking preferences among chromatic stimuli.     

Surface properties and the perception of color

We examined whether perception of color saturation and lightness depends on the three-dimensional (3D) shape and surface gloss of surfaces rendered to have different hues. In Experiment 1, we parametrically varied specular roughness of predominantly planar surfaces with different mesoscopic relief heights. The orientation of surfaces was varied relative to the light source and observer. Observers matched perceived lightness and chroma (effectively saturation) using spherical objects rendered using CIE LCH color space. We observed strong interactions between perceived saturation and lightness with changes in surface orientation and surface properties (specular roughness and 3D relief height). Declines in saturation and increases in lightness were observed with increasing specular roughness. Changes in relief height had greater effects on perceived saturation and lightness for blue hues compared with reddish and greenish hues. Experiment 2 found inverse correlations between perceived gloss and specular roughness across conditions. Experiment 3 estimated perceived specular coverage and found that a weighted combination of perceived gloss and specular coverage could account for perceived color saturation and lightness, with different coefficients accounting for the perceptual experience for each of the three hue conditions. These findings suggest that perceived color saturation and lightness depend on the separation of specular highlights from diffuse shading informative of chromatic surface reflectance.     

Orientation discrimination in foveal and extra-foveal vision: effects of stimulus bandwidth and contrast

The parameter E2 is used in many spatial scaling studies to characterize the rate at which stimulus size must increase with eccentricity to achieve foveal levels of performance in detection and discrimination tasks. We examined whether the E2 for an orientation discrimination task was dependent on the spatial frequency bandwidth of the stimulus used. Two methods were employed. In Experiments 1 and 2 stimuli were presented at a fixed high level of contrast across viewing conditions. In both experiments the E2s recovered for narrowband stimuli were larger than those recovered for broadband stimuli. In Experiment 3 we controlled for the potentially confounding effects of perceptual contrast by measuring orientation thresholds over a range of stimulus contrast levels. Only thresholds which had reached an asymptotic level, such that increases in stimulus contrast led to no further changes to thresholds, were included in the calculation of E2. We observed that E2s recovered in the latter condition were in the range of 1.29 degrees -1.83 degrees and similar for narrowband and broadband stimuli. We conclude that a failure to consider the role of perceptual contrast may result in inflated estimates of E2.     

A behavioural investigation of human visual short term memory for colour

We examined visual short term memory (VSTM) for colour using a delayed‐match‐to‐sample paradigm. In these experiments we measured the effects of increasing inter‐stimulus interval (ISI), varying between 0 and 10 s, on the ability of five colour normal human observers to make colour matches between a reference and subsequently presented test stimuli. The coloured stimuli used were defined by different chromatic axes on the isoluminant plane of DKL colour space. In preliminary experiments we used a hue scaling procedure to identify a total of 12 colour stimuli which served as reference hues in the colour memory experiments: four stimuli were exemplars of red, green, blue and yellow colour appearance categories, four were located between these categories and a further four were located on the cardinal axes that isolated the activity of the cone‐opponent mechanisms. Our results demonstrate that there is a reduction in the ability of observers to make accurate colour matches with increasing ISIs and that this reduced performance was similar for all colour stimuli. However, the shifts in hue that were measured between the reference and matched test stimuli were significantly greater for the cardinal stimuli compared to those measured for the stimuli defined by the hue scaling procedure. This deterioration in the retention of hue in VSTM for stimuli that isolate cone‐opponent mechanisms may be a reflection of the reorganisation of colour processing that occurs in the cortex where colour appearance mechanisms become more prominent.