Color Perception

Pigeons were exposed to multiple and concurrent second-order schedules of token reinforcement, with stimulus lights serving as token reinforcers. Tokens were produced and exchanged for food according to various fixed-ratio schedules, yielding equal and unequal unit prices (responses per unit food delivery). On one schedule (termed the standard schedule), the unit price was held constant across conditions. On a second schedule (the alternative schedule), the unit price was either the same or different from the standard. Under conditions with unequal unit prices, near-exclusive preference for the lower unit price was obtained. Under conditions with equal unit prices, the direction and degree of preference depended on ratio size (number of responses per exchange period). When this ratio differed, strong preferences for the smaller ratio were observed. When this ratio was equal, preferences were nearer indifference. Response rates on the multiple schedule were generally consistent with the preference data in showing sensitivity to ratio size. Results are discussed in terms of a unit-price model that includes handling and reinforcer immediacy as additional costs. On the whole, results show that preferences were determined primarily by delay to the exchange period.

La couleur n’est pas un élément anodin de la Recherche. Au contraire, elle est l’une des voies d’accès au style de l’écrivain. De la science à la peinture, de la critique d’art à la réflexion philosophique, le rendu chromatique de l’œuvre est questionné à partir des connexions entre Proust et les savoirs sur le chromatisme de son époque. Le but du livre est de montrer que l’ une des solutions données par l’auteur sera la disparition de toute teinte au sein des éléments qui composent la cathédrale du temps. La Recherche est mise en couleur par le biais d’un style délibérément hétéroclite.

Patients suffering from AIDS develop ocular complications, the most frequent being HIV retinopathy. It is however not clear, if functional visual impairments can be observed as early indicators of ocular complications, before clinical diagnosis of HIV retinopathy is made at fundus examination. To address this issue, we measured colour vision in a group of 49 AIDS subjects with normal clinical fundi using the 'two equation method'. This method, combining red-green Rayleigh and the blue-green Moreland metameric matches, enables more complete and quantitative assessments of colour vision than those based on pigmentary tests. Data were collected on our computer controlled colorimeter and compared to those of normal subjects. While most AIDS subjects without HIV retinopathy demonstrated normal colour vision, a significant portion of them had wider matches than normal subjects (11% for the Rayleigh equation and 16% for the Moreland equation). Furthermore, matching ranges of the Moreland equation were significantly correlated with CD4 lymphocyte counts. Patients with low CD4 values tended to produce larger matching ranges than the patients with high CD4 values. A within subject study on 17 patients confirmed this trend and showed that the patients who increased/decreased their CD4 blood counts generally improved/impaired their colour discrimination in the Moreland match. No such correlation was found between the matching ranges of the Rayleigh equation and the CD4 counts. These results show that colour discrimination is slightly reduced in some AIDS subjects, although there are no detectable ocular complications. They also suggest two different types of colour vision impairments in AIDS patients without retinopathy: one reversible process affecting colour discrimination in the blue-green range; and another irreversible process affecting colour discrimination in the red-green range.

When two isoluminant colors alternate at frequencies >10 Hz, we perceive only one fused color with a minimal sensation of brightness flicker. In spite of the perception of color fusion, color opponent (CO) cells at early stages of the visual pathway are known to respond to chromatic flicker at frequencies far exceeding the perceptual fusion frequency. To explain color fusion, several groups have predicted that CO cells in Vi-unlike the retina and lateral geniculate nucleus-should not follow high-frequency flicker. To test this prediction we recorded from 12 CC)cells in various V1 layers. We found, contrary to expectations, that these neurons follow high frequency flicker well above heterochromatic fusion frequencies. All followed 15 Hz flicker and 10/12 followed 30 Hz flicker. For three cells, we tested 60 Hz luminance flicker and found clear responses. We thus present evidence of cortical activity in alert, trained monkeys that is clearly representing visual stimulation, yet is not perceived. Our data call into question explanations of perceptual phenomena that invoke a low temporal frequency cutoff of CO cells in VI to account for the failure to perceive fast temporal changes in the chromatic domain.

The green peach aphid, Myzus persicae (Hemiptera: Aphididae) is an important phytophagous pest of greenhouse and field crops. In the host finding process visual cues are of paramount importance. In order to contribute to the understanding of the perception of visual stimuli in this species, we measured the electroretinogram of alate female summer migrants of M. persicae. The spectral sensitivity was measured in 10 nm steps under both dark and light adaptation from 320 to 640 nm. The dark adapted spectral sensitivity curve showed one maximum in the green region around 530 nm and a distinct shoulder between 500 and 510 nm. In presence of adapting light, a secondary blue-green peak (490 nm) and a third peak in the near UV (330-340 nm) were observed. From these results we conclude that M. persicae has three spectral types of photoreceptors. r

In a valence induction task, one color acquired positive valence by indicating the chance to win money (in the case of fast and correct responses), and a different color acquired negative valence by indicating the danger to lose money (in the case of slow or incorrect responses). In the additional-singleton trials of a visual search task, the taskirrelevant singleton color was either the positive one, the negative one, or one of two neutral colors. We found an additional-singleton effect (i.e., longer RTs with a singleton color than in the no-singleton control condition). This effect was significantly increased for the two valent colors (with no differences between them) relative to the two neutral colors (with no differences between them, either). This result favors the hypothesis that the general relevance of stimuli elicits attentional capture, rather than the negativity bias hypothesis.

Subjects watched either an emotional, neutral, or unusual sequence of slides containing 1 critical slide in the middle. Experiments 1 and 2 allowed only a single eye fixation on the critical slide by presenting it for 180 ms (Experiment 1) or 150 ms (Experiment 2). Despite this constraint, memory for a central detail was better for the emotional condition. In Experiment 3, subjects were allowed 2.70 s to view the critical slide while their eye movements were monitored. When subjects who had devoted the same number of fixations were compared, memory for the central detail of the emotional slide was again better. The results suggest that enhanced memory for detail information of an emotional event does not occur solely because more attention is devoted to the emotional information.

Three experiments investigated whether it is possible to orient selective spatial attention to internal representations held in working memory in a similar fashion to orienting to perceptual stimuli. In the first experiment, subjects were either cued to orient to a spatial location before a stimulus array was presented (pre-cue), cued to orient to a spatial location in working memory after the array was presented (retro-cue), or given no cueing information (neutral cue). The stimulus array consisted of four differently colored crosses, one in each quadrant. At the end of a trial, a colored cross (probe) was presented centrally, and subjects responded according to whether it had occurred in the array. There were equivalent patterns of behavioral costs and benefits of cueing for both pre-cues and retro-cues. A follow-up experiment used a peripheral probe stimulus requiring a decision about whether its color matched that of the item presented at the same location in the array. Replicat...

Schizophrenia patients with obsessive-compulsive disorder (OCD) may be a subgroup of schizophrenia, and OCD patients with poor insight may show psychotic-like symptoms. The aim of this work is to compare the neuropsychological performance of those patients with ...

In this article, the authors respond to J. D. Cohen, K. O. Dunbar, D. M. Barch and T. S. Braver's (1997) comment on their target article. The present article (a) takes issue with the characterization given by Cohen et al. of the authors' approach as a classical speed-of-processing account of Stroop effects, (b) discusses the value and relevance of other theoretical concepts, such as traditional accounts of working memory and parallel distributed processing (PDP) approaches to Stroop effects, (c) further examines the possibility that the differences the authors observed in Experiment 2 between normal participants' performance and that of participants with schizophrenia may have been due to distractor stimulus degradation, particularly for participants with schizophrenia, and (d) argues for the relevance of the prefrontal cortex, as well as other brain areas, in accounting for their results. The authors conclude with a final theoretical question.

The developmental trend from overall similarity to dimensional identity classifications is explained by a quantitative model. I begin with the assumption that objects are represented in terms of constit uent dimensions and that the representation of objects changes little with development. Given this assumption, the model has three major parts. First, the similarity between objects is a function of the combination of the constituent dimensional differences. 1 propose developmental change in the likelihood that dimensions are differentially weighted in the calculation of similarity. Second, the perceived similarities between objects are valued for the purpose of constructing classifications. I propose that similarities are valued more dichotomously with age, such that identity becomes in creasingly special. Third, the valued similarities are used to choose the best classification of those possible. The model provides good qualitative fits to the extant data. Three experiments examining classifications in 2 to 8 year olds and in adults support specific new claims of the model. The data and the model provide new insights about development, classification, and similarity.

Using an uncertainty paradigm and functional magnetic resonance imaging (fMRI) we studied the effect of nonspatial selective and divided visual attention on the activity of specific areas of human extrastriate visual cortex. The stimuli were single ovals that differed from an implicit standard oval in either colour or width. The subjects' task was to classify the current stimulus as one of two possible alternatives per stimulus dimension.

SUMMARY Male glow-worms Lampyris noctiluca find their bioluminescent mates at night by phototaxis. There is good evidence that location of mates by lampyrid beetles is achieved by a single spectral class of photoreceptor,whose spectral sensitivity is tuned to the bioluminescent spectrum emitted by conspecifics, and is achromatic. We ask whether glow-worm phototaxis involves interactions between two spectral classes of photoreceptor. Binary choice experiments were conducted in which males were presented with artificial light stimuli that differ in spectral composition. The normal preference for a green stimulus (λmax=555 nm), corresponding to the bioluminescence wavelength produced by signalling females, was significantly reduced by adding a blue (λmax=485 nm) component to the signal. This implies an antagonistic interaction between long- and short-wavelength sensitive photoreceptors, suggesting colour vision based on chromatic opponency. Cryosections showed a band of yellow filter p...

Introduction: The Ishihara Test (IT) is arguably the most sensitive and commonly used color vision test within aviation and other occupational environments, but when no errors are allowed ~20% of normal trichromats fail the test. The number of allowed errors varies in different occupations and sometimes within the same environment (such as aviation) in order to reflect the difficulties of the color-related tasks. The implicit assumption is that the plates can be ranked in order of difficulty. The principal aim of this study was to investigate whether appropriate 'weights' can be attached to each IT plate to reflect the likelihood of producing a correct response. A second aim was to justify the use of color thresholds for quantifying the loss of red-green (RG) and yellow-blue (YB) chromatic sensitivity. Methods: We investigated 742 subjects (236 normals, 340 deutans and 166 protans) using the first 25 plates of the 38-plate IT and measured RG chromatic sensitivity using the Color Assessment and Diagnosis (CAD) test. The IT error scores provided plate-specific 'weights' which were used to calculate a Severity Index (SI) of color vision loss for each subject. Results: Error scores, SI values and CAD thresholds were measured and compared in each of the three subject groups. Conclusions: Color thresholds can provide a good measure of the severity of both RG and YB color vision loss. Neither the number of IT plates failed nor the SI value computed in this way can be used to determine reliably the severity of color vision loss.

The relationship between food flavors and postingestive feedback enables mammalian herbivores to procure nutrients and avoid toxins within ever-changing environments. We conducted four experiments with red-winged blackbirds (Agelaius phoeniceus) in captivity to determine the relative roles of color and flavor cues paired with negative postingestive feedback. We first conducted baseline tests to assess preferences for colors and flavors. All blackbirds preferred red- to blue-colored food, and they preferred umami- (l-alanine) flavored to bitter/astringent food (tannic acid). We observed no difference in consumption of salty (NaCl) vs sour (citric acid) foods during baseline tests (i.e., neutral flavors). We then conditioned experimentally naïve blackbirds with intraperitoneal injections of lithium chloride (LiCl) to avoid food treated with red and l-alanine, or red and NaCl (n=30 birds per conditioning group). Subsequent to conditioning with LiCl, three test groups were established f...

Novel binocular depth ilhrsions obtained from two-dimensional colour images are presented. It is demonstrated that the magnitude of these illusions is based on transverse chromatic aberation (TCA), however, the depth obtained caNlot be observed unless specific astir are met even if the TCA is present, Some form of perceptual organization occurring at and/or beyond tbe binocular fusion site of the brain, is required for some of these effects to occur. An example of a paradoxical finding leading to this conclusion is the observation that uader some conditious the same colour can be perceived on separate depth planes while spatially adjacent colours from opposing ends of the visible spectrum (i.e. red and blue or green) can be perceived on the same depth plane ~~ta~~ly within the same image. Further, results show that some form of reference pibe is required by the brain to use the colour induced disparity, without which, depth connot be perceived even if the disparity information is preseut. This phenomenon is spatially tuned for medium to high frequency components aud is still detectable under isoluminant conditions which would support the notion that it requires information from the parvocelhdar pathway. Binocular hrstre and rivaldepth are ruled out as being significant factors in the effect. It is argued that this phenomenon represents an instance of global interactive processes induced by TCA whife previous studies on c~o~ter~~ have cou~en~ated on locai aspects. Results of the present study may explain wby under certain situations depth can be perceived in coloured images and not under other circumstances where TCA is still present. Depth perception Colour Chromostereopsis Binocular Stereopsis Chromatic aberration The illusion

Color coding and directional selectivity (DS) of retinal neurons were studied in the Pseudemys turtle by using similar intracellular recording and staining techniques as in the preceding paper (J. Ammermuller and H. Kolb, 1995, J. Comp. Neurol. 358:l-34). Color-coded responses were elicited by red (621 or 694 nm), green (525 or 514 nm), and blue (455 nm) light flashes. In addition to redigreen and yellow/blue types of chromaticity horizontal cells, in our sample of 305 identified cells we found that 17% of bipolar cells, 6.5% of amacrine cells, and 18% of ganglion cells exhibit color-coded responses. DS responses were found in 37% of the tested ganglion cells and 41% of the tested amacrine cells.

In this work it is suggested that color induces phenomenal wholeness, part-whole organization and fragmentation. The phenomenal wholeness subsumes the set of its main attributes: homogeneity, continuity, univocality, belongingness, and oneness. If color induces wholeness, it can also induce fragmentation. Therefore, in order to understand the role played by color within the process of partwhole organization, color is used both as a wholeness and as a fragmentation tool, thus operating synergistically or antagonistically with other wholeness processes. Therefore, color is expected to influence figure-ground segregation, grouping, shape formation and other visual processes that are related to the phenomenal wholeness. The purpose of this study is to rate the influence of color in inducing whole and part-whole organization and, consequently, in determining the perception of figure-ground segregation, grouping, shape formation, numerousness evaluation and time reading. We manipulated experimental conditions by using equiluminant colors to favor or break (parcel-out) the wholeness of objects like geometrical composite figures and words. The results demonstrated that color is aimed, among other psychological and biological purposes, at: (1) relating each chromatic component of an object, thus favoring the emergence of the whole object; (2) eliciting a part-whole organization, whose components are interdependent; (3) eliciting fragments and then breaking up the whole and favoring the emergence of single components. Wholeness, part-whole organization and fragmentation can be considered as three further purposes of color.

Vasarely's 'nested-squares' illusion is the perception of a glowing ''X" along the diagonals of concentric squares with a luminance gradient. We present here the chromatic Vasarely effect, where the concentric angles have a chromatic gradient, under iso-brightness conditions. The strength of the effect was tested psychophysically by two measures, the length and the color of the illusory folds. The color of the illusory fold is perceived as the complementary color of the color of the nested-squares (or angles). The experimental results show that a large repertoire of stimuli with different colors and angles yielded significantly perceived colors. The results show that the strength of the perceived illusory fold (of both the length and the chroma) is significantly larger at sharper angles of the stimuli. The chromatic first-order adaptation computational model predicts most of the above results.

On the contingent capture account, top-down attentional control settings restrict involuntary attentional capture to items that match the features of the search target. Attention capture is involuntary, but contingent on goals and intentions. The observation that only target-similar items can capture attention has usually been taken to show that the content of the attentional control settings consists of specific feature values. In contrast, the present study demonstrates that the top-down target template can include information about the relationship between the target and nontarget features (e.g., redder, darker, larger). Several spatial cuing experiments show that a singleton cue that is less similar to the target but that shares the same relational property that distinguishes targets from nontargets can capture attention to the same extent as cues that are similar to the target. Moreover, less similar cues can even capture attention more than cues that are identical to the target when they are relationally better than identical cues. The implications for current theories of attentional capture and attentional guidance are discussed.

Behavioral tests of vision in squirrel monkeys (Suimirisciureus) reveal that there are widespread individual variations in color vision in this species. The variation has a sex-related component: whereas both trichromatic and dichromatic color vision occurs among female monkeys, males appear exclusively dichromatic. This finding suggests that, unlike humans, squirrel monkeys have only a single photopigment locus on the X chromosome. Squirrel monkeys (Soimiri sciureus)

BACKGROUND Alzheimer disease (AD) primarily affects cognition. A variety of visual disorders was established in AD. Fechner illusory colors are produced by a rotating disk with a black and white pattern. The purpose of our research was to explore the perception of illusory colors in AD. MATERIAL AND METHODS W recruited 40 AD patients (MMSE ≥14) and 40 normal controls (CG group) matched by age, education, gender in this prospective, cross-sectional, case-control study. An achromatic Benham's disk attached to a device to control the speed and direction of rotation was used to produce illusory colors. Primary, secondary, and tertiary RGB system colors were used for matching of illusory and physical colors. RESULTS Subjects in the AD group perceived less illusory colors in 5 arcs (p<0.05) of the 8 arcs assessed. The biggest difference was found between AD and CG groups for pure blue (χ²=26.87, p<0.001 clockwise, χ²=22.75, p<0.001 counter-clockwise). Groups did not differ in...

When attempting to draw a ball of a specified color either from an urn containing 50 red balls and 50 black balls or from an urn containing an unknown ratio of 100 red and black balls, a majority of decision makers prefer the known-risk urn, and this ambiguity aversion effect violates expected utility theory. In an experimental investigation of the effect of urn size on ambiguity aversion, 149 participants showed similar levels of aversion when choosing from urns containing 2, 10, or 100 balls. The occurrence of a substantial and significant ambiguity aversion effect even in the smallest urn suggests that influential theoretical interpretations of ambiguity aversion may need to be reconsidered.

Based on the measurements of 1063 flower reflection spectra, we show that flower colours fall into distinct clusters in the colour space of a bee. It is demonstrated that this clustering is caused by a limited variability in the floral spectral reflectance curves. There are as few as 10 distinct types of such curves, five of which constitute 85% of all measurements. UV reflections are less frequent and always lower in intensity than reflections in other parts of the spectrum. A further cluster of colour loci is formed in the centre of the colour space. It contains the colour loci of green leaves, several other background materials and only very few flowers. We propose a system to classify the reflection functions of flowers, and a set of colour names for bee colours.

This study investigated subjects with eating disorders' selective attention to linguistic and pictorial representations of food stimuli in a version of the Stroop color-naming task. If subjects with eating disorders' attention really are biased by food stimuli, one would expect equally delayed color-naming latencies to food pictures as previous studies have found to food words.

Research on incidental sequence learning typically is concerned with the characteristics of implicit or nonconscious learning. In this article, the authors aim to elucidate the cognitive mechanisms that contribute to the generation of explicit, reportable sequence knowledge. According to the unexpectedevent hypothesis (P. A. Frensch, H. Haider, D. Rünger, U. Neugebauer, S. Voigt, & J. Werg, 2003), individuals acquire reportable knowledge when they search for the cause of an experienced deviation from the expected task performance. The authors experimentally induced unexpected events by disrupting the sequence learning process with a modified serial reaction time task and found that, unlike random transfer sequences, a systematic transfer sequence increased the availability of reportable sequence knowledge. The lack of a facilitative effect of random sequences is explained by the detrimental effect of random events on the presumed search process that generates reportable knowledge. This view is corroborated in a final experiment in which the facilitative effect of systematic transfer blocks is offset by a concurrent secondary task that was introduced to interfere with the search process during transfer.

White's effect is a phenomenon in which grey bars replacing segments of the white phase of a square-wave grating appear darker than those replacing segments of the black phase. The direction of the brightness difference is consistent with brightness assimilation rather than with brightness contrast. We present data from two experiments which measure the degree of the brightness difference in stimuli consisting of just three inducing bars and a single grey test bar, as a function of various spatial manipulations of the inducing and test bars. The spatial manipulations were chosen to maxim& the opportunity for assimilation effects to manifest themselves. The results do not support the view that assimilation is an important component of the effect. The data are shown to be consistent with our model of brightness induction in which both a local and a more spatially extensive contrast mechanism operate to produce White's effect.

Objective. To study the ability of colour naming, colour discrimination and colour preference in Alzheimer's disease (AD). Design. Descriptive, consecutive sample. Participants. Fifty subjects 465 years with AD. Interventions. Testing colour discrimination, colour naming and colour preferences. Main outcome measures. Ability to detect colour dierences in the yellow, red, blue and green areas, ability to assign a name to 22 colour samples, ability to rank seven colours in order of preference. Main results. Discrimination ability was signi®cantly better in the yellow and red area and for lightness variations. Cognitive decline had a signi®cant impact on naming mixed colours and using elaborate colour names. Severity of dementia did not aect the preference rank order of colours. Conclusions. Ability to discriminate is aected in AD, with most errors in the blue and green area. Naming colours shows a cognitive decline. Preferences for colour are stable despite the disease.

The ability to see colors is not universal in the animal kingdom. Those animals that can detect differences in the wavelengths of the electromagnetic spectrum glean valuable sensory information about their environment. They use color vision to forage, avoid predators, and find highquality mates. In the past, the colors that humans could see clouded scientists' study of animals' color perception. Leaving that bias behind has led to new insights about how and why the color vision of animals evolved. This paper provides a brief introduction to color vision, the genetics of color vision in humans, what colors other animals see, and how scientists study color vision. We examine the consequences of having color vision, including speciation, loss of olfactory capabilities, and sexual selection.

In order to ascertain whether the Bezold-BrUcke hue shift is of neural or pigment origin, five observers matched monochromatic spots of light, each with the same luminance, but one darkened by the operation of simultaneous brightness contrast. The occurrence of the hue shift in the contrast-darkened spot provides evidence for a neural locus of the effect. In addition, an extended set of measurements with one observer provides additional support for this hypothesis. INDEX HEADINGS: Color vision; Vision.

Animal color pattern phenotypes evolve rapidly. What influences their evolution? Because color patterns are used in communication, selection for signal efficacy, relative to the intended receiver's visual system, may explain and predict the direction of evolution. We investigated this in bowerbirds, whose color patterns consist of plumage, bower structure, and ornaments and whose visual displays are presented under predictable visual conditions. We used data on avian vision, environmental conditions, color pattern properties, and an estimate of the bowerbird phylogeny to test hypotheses about evolutionary effects of visual processing. Different components of the color pattern evolve differently. Plumage sexual dimorphism increased and then decreased, while overall (plumage plus bower) visual contrast increased. The use of bowers allows relative crypsis of the bird but increased efficacy of the signal as a whole. Ornaments do not elaborate existing plumage features but instead are innovations (new color schemes) that increase signal efficacy. Isolation between species could be facilitated by plumage but not ornaments, because we observed character displacement only in plumage. Bowerbird color pattern evolution is at least partially predictable from the function of the visual system and from knowledge of different functions of different components of the color patterns. This provides clues to how more constrained visual signaling systems may evolve.

To assess the effects of a single 100-mg dose of sildenafil citrate on visual function in men with early-stage age-related macular degeneration. Randomized double-blind placebo-controlled clinical trial. Nine men (mean age 71 years, range 59-85 years) with early-stage (minimal visual impairment and large drusen in the macula) age-related macular degeneration and 20/40 or better-corrected visual acuity in at least one eye were prospectively randomized to receive either placebo or sildenafil citrate (Viagra; Pfizer Inc, New York, New York) 100 mg as a single oral dose. After 7-14 days, they received the alternate treatment. Subjects underwent visual acuity, Amsler grid, color discrimination (D15), traffic light, Humphrey perimetry, and photo-stress testing in each eye before and at specific intervals within 8 hours after dosing. Compared with placebo, no pattern of errors were evident in any visual function test following sildenafil administration. No statistically or clinically relevant changes from baseline were observed in visual acuity, Humphrey perimetry (corrected pattern standard deviation), D15 color discrimination, or photo-stress tests. No clinically relevant changes were observed in the Amsler grid or traffic light tests. Sildenafil treatment was associated with transient mild or moderate headache, flushing, and rhinitis. There were no visual adverse events spontaneously reported to the investigator. A single 100-mg dose of sildenafil was well tolerated and produced no acute visual effects or exacerbation of preexisting visual impairment in nine men with early-stage age-related macular degeneration.

In this paper, I argue that perceptual media like air or water are imperceptible, in the sense that they are not directly discriminated by our senses. I show that, despite their lack of phenomenological features, perceptual media crucially affect what we see by selecting what is perceptually available to the perceiver.
In the second part of the paper, I argue that mirror is a visual medium like air, water and glass. According to this account, mirrors are transparent and invisible and cannot therefore have a distinctive look or appearance. In the last part of the paper, I extend the general account of perceptual media to the sense organs themselves by showing that perceptual media not only include external entities causally involved in the perceptual process but also comprise the perceptual system itself.