Summary The optical quality and properties of over 200 telescopes residing in museums and private collections have been measured and tested with the goal of obtaining new information about the early development of the achromatic lens (1757–1770). Quantitative measurements of the chromatic and spherical aberration of telescope objective lenses were made and are discussed within the context of John and Peter Dollond's description of their efforts to overcome these two optical defects inherent in any single lens. Their work was chronicled (...) in the Philosophical Transactions of 1758 and 1765. An important finding of this investigation is that a particular form of the achromatic lens that has been suggested as being a rare and early form by some investigators, and presumably introduced by Chester Moor Hall, was in fact found to be numerous and present in most museums and many private collections. It is shown that the colour correction of an early eighteenth-century doublet telescope objective is very sensitive to the ratio of the flint to crown glass focal lengths, as well as the variations of the indices of refraction with colour. This sensitivity would preclude any simple recipe for constructing an achromatic lens. The third order theory of spherical aberration of Nevil Maskelyne is reexamined, in which he pointed out that John Dollond developed a theoretical formula equivalent to what today is called geometric third order spherical aberration theory. (shrink)

There is no evidence to suggest that even as late as January 1672, when Newton was elected a Fellow of the Royal Society, anyone had any inkling of his new theory of colours. His name exploded on the scientific scene as the inventor and constructor of a new kind of telescope—what later became known as the reflector . Had the erudition of the London virtuosi been a little broader, they would have known that in fact he was not the inventor (...) of the telescope, even though the precise form he gave it was his. Not only was the idea a hundred years old, during which period it was repeatedly suggested by various writers, but also Newton himself took the idea straight from the most recent of these suggestions, namely that included in James Gregory's Optica promota of 1663. The situation becomes even more ironic when we realize that the new instrument was admired for wrong reasons and on merits that were far from Newton's intentions. Nevertheless, admired it was, and there was a good reason for this: Newton's instrument was in fact the first reflector actually to be constructed and, moreover, for a few weeks it performed quite well. Several astonomers became interested because of the high magnifying power relative to its dimensions. (shrink)

What ecological advantages do animals gain by being able to detect, extract and exploit wavelength information? What are the advantages of representing that information as hue qualities? The benefits of adding chromatic to achromatic vision, marginal in object detection, become apparent in object recognition and receiving biological signals. It is argued that this improved performance is a direct consequence of the fact that many animals' visual systems reduce wavelength information to combinations of four basic hues. This engenders a simple categorical (...) scheme that permits a rich amount of sensory information to be rapidly and efficiently employed by cognitive machinery of limited capacity. (shrink)

One sometimes believes a proposition without grasping it. For example, a complete achromat might believe that ripe tomatoes are red without grasping this proposition. My aim in this paper is to shed light on the difference between merely believing a proposition and grasping it. I focus on two possible theories of grasping: the inferential theory, which explains grasping in terms of inferential role, and the phenomenal theory, which explains grasping in terms of phenomenal consciousness. I argue that the phenomenal theory (...) is more plausible than the inferential theory. (shrink)

Where are we to look for the unique hues? Out in the world? In the eye? In more central processing? 1. There are difficulties looking for the structure of the unique hues in simple combinations of cone-response functions like ( L − M ) and ( S − ( L + M )): such functions may fit pretty well the early physiological processing, but they don’t correspond to the structure of unique hues. It may seem more promising to look to, (...) e.g., Hurvich & Jameson’s ‘chromatic response functions’; but these report on psychophysical behaviour, not on underlying physiology. So ‘opponent processing’ isn’t any particular help on the unique hues—and even physiology in general seems not to have come up with any good correlate or explanation. 2. Wright ( Review of Philosophy and Psychology 2: 1–17, 2011 ) looks in a different place: to (a) magnitude of total visual response that a stimulus light provokes, the maxima and minima of which, he thinks, give us the boundaries of the main hue categories (Wright connects these with Thornton’s ‘prime’ and ‘antiprime’ colours in an illuminant: Journal of the Optical Society of America 61 ( 1971 ): 1155–1163); and (b) the ratio of chromatic to achromatic response, the maxima and minima of which, he suggests, give us the focal points of the unique hues. The suggestions are extremely interesting; but the desired correspondences have some counterexamples; and where they hold, one could wonder how much they depend upon the particular choice of functions to measure (a) and (b); and one could hope for more of an explanatory linkage between the sets of items in question. 3. Could the unique hues come from, so to speak, the external world? White and black can easily be defined as particular kinds of reflectance. What of the standard four unique hues? Variation in kinds of sunlight and skylight coincides well with variation along a line from unique yellow to unique blue (cf. Shepard 1992 , Mollon 2006 ). If we wanted something to calibrate our standards for unique yellow and blue as the lens of the eye changes with age, and despite interpersonal cone differences, this would be a good basis—and there are several ways this can be extended to surface colours. But is there any essential connection between these things: is there any rationale why the light of the sun and the sky should be counted as unique hued ? An answer may be: because in our environment, these illuminants are as close to white (or the natural illuminant colour) as you can get—to see things tinged with sunlight or skylight should be to see them minimally tinged with any alien colour. Whereas other hues in an illuminant would be treated as tinging with a more alien colour the thing seen. (shrink)

We have used fMRI to measure responses to chromatic and achromatic contrast in retinotopically defined regions of macaque and human visual cortex. We make four observations. Firstly, the relative amplitudes of responses to color and luminance stimuli in macaque area V1 are similar to those previously observed in human fMRI experiments. Secondly, the dorsal and ventral subdivisions of macaque area V4 respond in a similar way to opponent (L j M)-cone chromatic contrast suggesting that they are part of a single (...) functional area. Thirdly, we find that macaque area V4, like area V1, responds preferentially to chromatic contrast compared to luminance contrast and the degree of preference is strongly influenced by the temporal frequency of the stimulus. Finally, we observe that while macaque V4d is a region on the dorsal surface of the macaque visual cortex that responds robustly to chromatic stimuli, human chromatic responses to identical stimuli are largely confined to the ventral surface suggesting a fundamental difference in the topographical organization of higher visual areas between humans and macaques. (shrink)

Recent research on grapheme-colour synesthesia has focused on whether visual attention is necessary to induce a synesthetic percept. The current study investigated the influence of synesthesia on overt visual attention during an oculomotor target selection task. Chromatic and achromatic stimuli were presented with one target among distractors among multiple ‘5’s ). Participants executed an eye movement to the target. Synesthetes and controls showed a comparable target selection performance across conditions and a ‘pop-out effect’ was only seen in the chromatic condition. (...) As a pop-out effect was absent for the synesthetes in the achromatic condition, a synesthetic element appears not to elicit a synesthetic colour, even when it is the target. The synesthetic percepts are not pre-attentively available to distinguish the synesthetic target from synesthetic distractors when elements are presented in the periphery. Synesthesia appears to require full recognition to bind form and colour. (shrink)

This study examines the cross‐cultural generality of Hering's (1878/1964) color‐opponent theory of color appearance. English‐speaking and Somali‐speaking observers performed variants of two paradigms classically used to study color‐opponency. First, both groups identified similar red, green, blue, and yellow unique hues. Second, 25 English‐speaking and 34 Somali‐speaking observers decomposed the colors present in 135 Munsell color samples into their component Hering elemental sensations—red,green,blue, yellow, white, and black—or else responded “no term.” Both groups responded no term for many samples, notably purples. Somali (...) terms for yellow were often used to name colors all around the color circle, including colors that are bluish according to Hering's theory. Four Somali Grue speakers named both green and blue elicitation samples by their term for green. However, that term did not name the union of all samples called blue or green by English speakers. A similar pattern was found among three Somali Achromatic speakers, who called the blue elicitation sample black or white. Thus, color decomposition by these Somali‐speaking observers suggests a lexically influenced re‐dimensionalization of color appearance space, rather than a simple reduction of the one proposed by Hering. Even some Somali Green‐Blue speakers, whose data were otherwise similar to English, showed similar trends in yellow and blue usage. World Color Survey data mirror these results. These within‐ and cross‐cultural violations of Hering's theory do not challenge the long‐standing view that universal sensory processes mediate color appearance. However, they do demonstrate an important contribution of language in the human understanding of color. (shrink)

The thresholds of human observers detecting line targets improve significantly when the targets are presented in a spatial context of collinear inducing stimuli. This phenomenon is referred to as spatial facilitation, and may reflect the output of long-range interactions between cortical feature detectors. Spatial facilitation has thus far been observed with luminance-defined, achromatic stimuli on achromatic backgrounds. This study compares spatial facilitation with line targets and collinear, edge-like inducers defined by luminance contrast to spatial facilitation with targets and inducers defined (...) by color contrast. The results of a first experiment show that achromatic inducers facilitate the detection of achromatic targets on gray and colored backgrounds, but tend to suppress the detection of chromatic targets. Chromatic inducers facilitate the detection of chromatic targets on gray and colored backgrounds, but tend to suppress the detection of achromatic targets. Chromatic spatial facilitation appears to be strongest when inducers and background are isoluminant. The results of a second experiment show that spatial facilitation with chromatic targets and inducers requires a longer exposure duration of the inducers than spatial facilitation with achromatic targets and inducers, which is already fully effective at an inducer exposure of 30 ms only. The findings point towards two separate mechanisms for spatial facilitation with collinear form stimuli: one that operates in the domain of luminance, and one that operates in the domain of color contrast. These results are consistent with neural models of boundary and surface formation which suggest that achromatic and chromatic visual cues are represented on different cortical surface representations that are capable of selectively attracting attention. Multiple copies of these achromatic and chromatic surface representations exist corresponding to different ranges of perceived depth from an observer, and each can attract attention to itself. Color and contrast differences between inducing and test stimuli, and transient responses to inducing stimuli, can cause attention to shift across these surface representations in ways that sometimes enhance and sometimes interfere with target detection. (shrink)

This paper argues that the distinctiveness of the Hering primary hues—red, green, blue, and yellow—is already evident at the retina. Basic features of spectral sensitivity provide a foundation for the development of unique hue perceptions and the hue categories of which they are focal examples. Of particular importance are locations in color space at which points of minimal and maximal spectral sensitivity and extreme ratios of chromatic to achromatic response occur. This account builds on Jameson and D’Andrade’s (1997) insight about (...) the relationship between the Hering primaries and chromatic/achromatic ratios, Romney and Chiao’s (2009) color appearance model, and Thornton’s (1971, 1999) research on artificial lighting. (shrink)

It remains unclear what is being processed in blindsight in response to faces, colours, shapes, and patterns. This was investigated in two hemianopes with chromatic and achromatic stimuli with sharp or shallow luminance or chromatic contrast boundaries or temporal onsets. Performance was excellent only when stimuli had sharp spatial boundaries. When discrimination between isoluminant coloured Gaussians was good it declined to chance levels if stimulus onset was slow. The ability to discriminate between instantaneously presented colours in the hemianopic field depended (...) on their luminance, indicating that wavelength discrimination totally independent of other stimulus qualities is absent. When presented with narrow-band colours the hemianopes detected a stimulus maximally effective for S-cones but invisible to M- and L-cones, indicating that blindsight is mediated not just by the mid-brain, which receives no S-cone input, or that the rods contribute to blindsight. The results show that only simple stimulus features are processed in blindsight. (shrink)

Summary The twofold objective of this study is (1) to identify and give a brief review of the historical development of the various designs of early (pre-1850) telescope eyepieces, and (2) to determine by measurements and calculations the axial and lateral chromatic aberrations of a number of extant eyepieces from that period in order to provide basic data on which to judge the relative quality of different eyepiece forms. Eight distinct types of eyepieces containing one to five lens elements are (...) discussed and illustrated. The second objective was addressed by measuring the focal lengths of the individual lens elements and the lens spacings in each eyepiece. The data were processed by a ray-tracing program that yields the chromatic aberration (CA) and focal length of each of the eyepieces. Twenty-one telescopes from that period were studied in this way. Similar calculations were also made using data available in the literature on several additional historic telescopes. The conclusions drawn from the data are: (1) Telescopes with Galilean eyepieces have a smaller CA than those with Keplerian or Huygens eyepieces. (2) The two-lens Keplerian terrestrial eyepieces are demonstrated to have larger axial and lateral CA than any of the other eyepiece types. (3) Calculations from examples of telescopes by Divini and Campani for which data were available indicate that while the axial CAs of the two were the same, Campani's instrument had a much smaller lateral CA. (4) Five element eyepieces were found to be better corrected for CA than the early four-lens systems that replaced them. (5) This study tends to confirm the speculation that the reason makers of early achromatic telescopes overcorrected the objective lenses was to compensate for the CA of the eyepieces. (shrink)

We show that true colors as defined by Chevreul (1839) produce unsuspected simultaneous brightness induction effects on their immediate grey backgrounds when these are placed on a darker (black) general background surrounding two spatially separated configurations. Assimilation and apparent contrast may occur in one and the same stimulus display. We examined the possible link between these effects and the perceived depth of the color patterns which induce them as a function of their luminance contrast. Patterns of square-shaped inducers of a (...) single color (red, green, blue, yellow, or grey) were placed on background fields of a lighter and a darker grey, presented on a darker screen. Inducers were always darker on one side of the display and brighter on the other in a given trial. The intensity of the grey backgrounds varied between trials only. This permitted generating four inducer luminance contrasts, presented in random order, for each color. Background fields were either spatially separated or consisted of a single grey field on the black screen. Experiments were run under three environmental conditions: dark-adaptation, daylight, and rod-saturation after exposure to bright light. In a first task, we measured probabilities of contrast, assimilation, and no effect in a three-alternative forced-choice procedure (background appears brighter on the ‘left’, on the ‘right’ or the ‘same’). Visual adaptation and inducer contrast had no significant influence on the induction effects produced by colored inducers. Achromatic inducers produced significantly stronger contrast effects after dark-adaptation, and significantly stronger assimilation in daylight conditions. Grouping two backgrounds into a single one was found to significantly decrease probabilities of apparent contrast. Under the same conditions, we measured probabilities of the inducers to be perceived as nearer to the observer inducers. These, as predicted by Chevreul’s law of contrast, were determined by the luminance contrast of the inducers only, with significantly higher probabilities of brighter inducers to be seen as nearer, and a marked asymmetry between effects produced by inducers of opposite sign. Implications of these findings for theories which attempt to link simultaneous induction effects to the relative depth of object surfaces in the visual field are discussed. (shrink)

Brown highlights cases of “chromatic layering”—scenarios in which one perceives an opaque object through a transparent volume/film/filter with a chromatic or achromatic content of its own—as a way of reining in the argument from perceptual variation sometimes used to motivate a relationalist account of color properties. Brown urges that the argument in question does not generalize smoothly to all types of perceptual variation—in particular, that it fits poorly in layering cases in which there is either experiential fusion or scission. While (...) he doesn’t want to reject the relationalist description of variation in all cases, he does suggest that there is an alternative, and preferable, description available for layering cases, and that the availability of this alternative puts limits on the scope of both relationalist arguments and relationalist conclusions. I claim that the obstacles for the argument from perceptual variation posed by cases of layering are merely apparent. I argue that relationalism can be extended smoothly to cases of layering, and indeed that denying this extension, as Brown proposes, contravenes ordinary and more or less universally accepted canons of rational inquiry. (shrink)

Summary The twofold objective of this study is (1) to identify and give a brief review of the historical development of the various designs of early (pre-1850) telescope eyepieces, and (2) to determine by measurements and calculations the axial and lateral chromatic aberrations of a number of extant eyepieces from that period in order to provide basic data on which to judge the relative quality of different eyepiece forms. Eight distinct types of eyepieces containing one to five lens elements are (...) discussed and illustrated. The second objective was addressed by measuring the focal lengths of the individual lens elements and the lens spacings in each eyepiece. The data were processed by a ray-tracing program that yields the chromatic aberration (CA) and focal length of each of the eyepieces. Twenty-one telescopes from that period were studied in this way. Similar calculations were also made using data available in the literature on several additional historic telescopes. The conclusions drawn from the data are: (1) Telescopes with Galilean eyepieces have a smaller CA than those with Keplerian or Huygens eyepieces. (2) The two-lens Keplerian terrestrial eyepieces are demonstrated to have larger axial and lateral CA than any of the other eyepiece types. (3) Calculations from examples of telescopes by Divini and Campani for which data were available indicate that while the axial CAs of the two were the same, Campani's instrument had a much smaller lateral CA. (4) Five element eyepieces were found to be better corrected for CA than the early four-lens systems that replaced them. (5) This study tends to confirm the speculation that the reason makers of early achromatic telescopes overcorrected the objective lenses was to compensate for the CA of the eyepieces. (shrink)

People with grapheme-color synesthesia perceive specific colors when viewing different letters or numbers. Previous studies have suggested that synesthetic color experience can be bistable when induced by an ambiguous character. However, the exact relationship between processes underlying the identity of an alphanumeric character and the experience of the induced synesthetic color has not been examined. In the present study, we explored this by focusing on the temporal relation of inducer identification and color emergence using inducers whose identity could be rendered (...) ambiguous upon rotation of the characters. Specifically, achromatic alphabetic letters and digits were presented at varying angles to 9 grapheme-color synesthetes. Results showed that grapheme identification and synesthetically perceived grapheme color covary with the orientation of the test stimulus and that synesthetes were slower naming the experienced color than identifying the character, particularly at intermediate angles where ambiguity was greatest. (shrink)

Canada balsam is secreted by cells in the bark of the balsam fir Abies balsamea, a conifer widely distributed across Canada and northern U.S.A. Collected by an extraordinarily labour-intensive process, it was initially exported to Europe for use in pharmacy. However, it found a much wider application after Brewster showed that the refractive index of 1·54–1·55 characterizing the vitreous mass remaining after heating the natural product exceeded that of most resins, and therefore closely matched the indices of ‘crown’ and ‘light (...) flint’ optical glasses. ‘Cooked’ Canada balsam was therefore very suitable for cementing achromatic lenses for telescopes and microscopes, as well as for mounting specimens for microscopical examination. Canada balsam remained pre-eminent for over a century, but the rise of synthetic adhesives and other substitutes has now relegated the natural balsam to a few specialized niches. (shrink)

Two experiments compared “Red-Green” dichromats’ empirical and metacognized capacities to discriminate basic color categories and to use the corresponding basic color terms. A first experiment used a 102-related-colors set for a pointing task to identify all the stimuli that could be named with each BCT by each R-G dichromat type. In a second experiment, a group of R-G dichromats estimated their difficulty discriminating BCCs-BCTs in a verbal task. The strong coincidences between the results derived from the pointing and the verbal (...) tasks indicated that R-G dichromats have very accurate metacognition about their capacities and limitations in the use of BCTs. Multidimensional scaling solutions derived from both tasks were very similar: BCTs in R-G dichromats were properly represented in 2D MDS solutions that clearly show one chromatic dimension and one achromatic dimension. Important concordances were found between protanopes and deuteranopes. None of these dichromats showed substantial difficulty discriminating the Red-Green pair. So, to name them “R-G” dichromats is misleading considering their empirical capacities and their metacognition. Further reasons to propose the use of the alternative denomination “Brown-Green” dichromats are also discussed. We found some relevant differences between the “Brown-Green” dichromats’ empirical and self-reported difficulties using BCTs. Their metacognition can be considered a “caricature” of their practical difficulties. This caricature omits some difficulties including their problems differentiating “white” and “black” from other BCTs, while they overestimate their limitations in differentiating the most difficult pairs. Individual differences scaling analyses indicated that the metacognition regarding the use of BCTs in “Brown-Green” dichromats, especially deuteranopes, is driven slightly more by the chromatic dimension and driven slightly less by the achromatic dimension, than their practical use of BCTs. We discuss the relevance of our results in the framework of the debate between the linguistic relativity hypothesis and the universal evolution theories. (shrink)

Euler ’s wave theory of light developed from a mere description of this notion based on an analogy between sound and light to a more and more mathematical elaboration on that notion. He was very successful in predicting the shape of achromatic lenses based on a new dispersion law that we now know is wrong. Most of his mathematical arguments were, however, guesswork without any solid physical reasoning. Guesswork is not always a bad thing in physics if it leads to (...) new experiments or makes the theory coherent with other theories. And Euler tried to find such experiments. He saw the construction of achromatic lenses, the explanation of colors of thin plates and of the opaque bodies as proof of his theory. When it came to the fundamental issues, the correctness of his dispersion law and the prediction of frequencies of light he was not at all successful. His wave theory degenerated, and it was not until Augustin Fresnel introduced transverse waves and an elaborate notion of interference that the wave theory again progressed. (shrink)

��Grapheme-color synaesthesia is a neurological phenomenon in which particular graphemes, such as the numeral 9, automatically induce the simultaneous perception of a particular color, such as the color red. To test whether the concurrent color sensations in graphemecolor synaesthesia are treated as meaningful stimuli, we recorded event-related brain potentials as 8 synaesthetes and 8 matched control subjects read sentences such as ‘‘Looking very clear, the lake was the most beautiful hue of 7.’’ In synaesthetes, but not control subjects, congruous graphemes, (...) compared with incongruous graphemes, elicited a more negative N1 component, a less positive P2 component, and a less negative N400 component. Thus, contextual congruity of synaesthetically induced colors altered the brain response to achromatic graphemes beginning 100 ms postonset, affecting pattern-recognition, perceptual, and meaning-integration processes. The results suggest that grapheme-color synaesthesia is automatic and perceptual in nature and also suggest that the connections between colors and numbers are bidirectional. (shrink)

Central to Wilfrid Sellars ' philosophical system is his belief that science's current ontology is inadequate as it fails to provide for an acceptable account of perceptual experience. Unfortunately, this remains the most puzzling plank in his philosophy. Sellars himself argues for this position via his wellknown example of a pink ice cube and its homogeneous colour. This homogeneity, says Sellars, bars the acceptance of science's present ontology of achromatic particles, and requires the introduction of items which are truly coloured. (...) Only with such a revised and expanded ontology, with all that entails, can science adequately meet its explanatory demands. I aim here to remedy at least some of the confusions and misunderstandings this position has engendered. But I mean to take a different route from Sellars. In short, given the problems with Sellars ' views on homogeneity, I will argue for the Sellarsian conclusion as to the inadequacy of present scientific ontologies, yet without reliance on the puzzling doctrine of homogeneity. I begin then with a detailed examination of Sellars official position, indicate the trouble spots, and begin an alternative route. As I conceive things, however, the position I will sketch is still thoroughly Sellarsian, for it proceeds from premises Sellars himself has endorsed. (shrink)

What are cells? How are they related to each other and to the organism as a whole? These questions have exercised biology since Schleiden and Schwann (1838–1839) first proposed cells as the key units of structure and function of all living things. But how do we try to understand them? Through new technologies like the achromatic microscope and the electron microscope. But just as importantly, through the metaphors our culture has made available to biologists in different periods and places. These (...) two new volumes provide interesting history and philosophy of the development of cell biology. Reynolds surveys the field's changing conceptual structure by examining the varied panoply of changing metaphors used to conceptualize and explain cells – from cells as empty boxes, as building blocks, to individual organisms, to chemical factories, and through many succeeding metaphors up to one with great currency today: cells as social creatures in communication with others in their community. There is some of this approach in theVisionsedited collection as well. But this collection also includes rich material on the technologies used to visualize cells and their dialectical relationship with the epistemology of the emerging distinct discipline of cell biology. This volume centres on, but is not limited to, ‘reflections inspired by [E.V.] Cowdry's [1924 volume]General Cytology’; it benefits from a conference on the Cowdry volume as well as a 2011 Marine Biological Lab/Arizona State University workshop on the history of cell biology. (shrink)

The concept of a technical frontier in branches of experimental measurement, such as the resolution of the microscope, angular measure and time telling, has been around for more than 60 years. The purpose of this brief paper is to identify the technical frontier operating on the achromatic astronomical telescope, where a limiting factor of the resolution of fine detail was the quality of the optical glass available. The achromatically corrected objective is formed from two kinds of glass, the common crown (...) glass and the heavy clear flint glass or lead glass. This last was difficult to make homogeneous, that is without regions of different density, and therefore different refraction and dispersion. Unusually, optical glass had to pass a second frontier, this time placed on the whole glass industry by the English Government in the form of excise duty, administered with a bureaucratic efficiency that effectively stopped, in around 1800, the making of optical glass suitable for the serious astronomical telescopes. The result of the tax imposition was to delay the English production of improved optical glass for more than 80 years. (shrink)

This paper argues that the distinctiveness of the Hering primary hues – red, green, blue, and yellow – is already evident at the retina. Basic features of spectral sensitivity provide a foundation for the development of unique hue perceptions and the hue categories of which they are focal examples. Of particular importance are locations in color space at which points of minimal and maximal spectral sensitivity and extreme ratios of chromatic to achromatic response occur. This account builds on Jameson & (...) D’Andrade’s (1997) insight about the relationship between the Hering primaries and chromatic/achromatic ratios, Romney & Chiao’s (2009) color appearance model, and Thornton’s (1971; 1999) research on artificial lighting. (shrink)

What is colour vision for? In the popular imagination colour vision is for “seeing the colours” — adding hue to the achromatic world of shape, depth and motion. On this view colour vision plays little more than an ornamental role, lending glamour to an otherwise monochrome world. This idea has guided much theorising about colour within vision science and philosophy. However, we argue that a broader approach is needed. Recent research in the psychology of colour demonstrates that colour vision is (...) integral to a variety of visual processes, helping us to perform many types of visual tasks. We discuss some of this research and consider its implications for philosophical theories of colour. (shrink)

Luminance and color are strong and self-sufficient cues to pictorial depth in visual scenes and images. The present study investigates the conditions Under which luminance or color either strengthens or overrides geometric depth cues. We investigated how luminance contrasts associated with color contrast interact with relative height in the visual field, partial occlusion, and interposition in determining the probability that a given figure is perceived as ‘‘nearer’’ than another. Latencies of ‘‘near’’ responses were analyzed to test for effects of attentional (...) selection. Figures in a pair were supported by luminance contrast or isoluminant color contrast and combined with one of the three geometric cues. The results of Experiment 1 show that luminance contrasts associated with hue, when it does not interact with other hues, produces the same effects as achromatic luminance contrasts: The probability of‘‘near’’ increases with luminance contrast while the latencies for ‘‘near’’ responses decrease. Partial occlusion is found to be a strong enough pictorial cue to support a weaker red luminance contrast. Interposition cues lose out against cues of spatial position and partial occlusion. The results of Experiment 2, with isoluminant displays of varying color contrast, reveal that red color contrast on a light background supported by any of the three geometric cues wins over green or white supported by any of the three geometric cues. On a dark background, red color contrast supported by the interposition cue loses out against green or white color contrast supported by partial occlusion. These findings reveal that color is not an independent depth cue, but is strongly influenced by luminance contrast and stimulus geometry. Systematically shorter response latencies for stronger ‘‘near’’ percepts demonstrate that selective visual attention reliably detects the most likely depth cue combination in a given configuration. (shrink)

Colour synaesthesia is the mental experience involving a strong association between specific colours and specific auditory stimuli, such as words, or achromatic visual stimuli, such as numerals or letters. In the contemporary literature on colour synaesthesia, the majority view treats the phenomenon as one arising from some of the same neural events mediating colour perception triggered by genuinely coloured objects; this view that synaesthesia is perceptually based, however, is not universally endorsed. What strategies have been utilized to evaluate the perceptual (...) reality of colour synaesthesia, and what is the evidence produced by those strategies? This chapter tackles those questions within the context of colour graphemic synaesthesia, the most widely studied form of synaesthesia. We divide the research strategies into those employing behavioral measures to assess whether synaesthetia influences performance on tasks known to be sensitive stimulus colour and those employing indirect measures that use physiological responses as proxies for colour perception. Our chapter concludes that there is sufficient justification for the belief in the perceptual reality of colour grapheme synaesthesia. At the same time, we applaud those who remain disbelievers, for their skepticism has underscored the logical issues surrounding research on this question. (shrink)

Erickson's conclusion that if basic tastes are not appropriate at one level, reference to labeled lines is inappropriate at any level, depends on matters of definition and scope. His population model mirrors Young's theory of color perception. However, there is evidence for distinct pathways to the cortex for two cone-opponent and one achromatic channel. Depending on the use made of key terms, sensory systems may display both across-fiber and labeled-line features.

Spatial facilitation has been observed with luminance-defined, achromatic stimuli on achromatic backgrounds as well as with targets and inducers defined by colour contrast. This paper reviews psychophysical results from detection experiments with human observers showing the conditions under which spatially separated contour inducers facilitate the detection of simultaneously presented target stimuli. The findings point towards two types of spatial mechanisms: (i) Short-range mechanisms that are sensitive to narrowly spaced stimuli of small size and, at distinct target locations, selective to the (...) contrast polarity of targets and inducers. (ii) Long-range mechanisms that are triggered by longer stimuli, generate facilitation across wider spatial gaps between targets and inducers, and are insensitive to their contrast polarity. Spatial facilitation with chromatic stimuli requires a longer inducer exposure than spatial facilitation with achromatic stimuli, which is already fully effective at inducer exposures of 30 ms. This difference in temporal dynamics indicates some functional segregation between mechanisms for colour and luminance contrast in the perception of space. (shrink)

In psychophysical experiments, the use of a psychophysical procedure of brightness/darkness cancellation shed light on interactions between spatial arrangement and figure–ground contrast in the perceptual filling in of achromatic and colored surfaces.Achromatic and chromatic Kanizsa squares with varying contrast, contrast polarity, and inducer spacingwere used to test how these factors interact in the perceptual filling in of surface brightness or darkness. The results suggest that the neuronal processing of surfaces with apparent contrast, leading to figure–ground segregation (i.e., perceptual organization), is (...) governed by mechanisms that integrate both luminance contrast and spatial information carried by the inducing stimuli, while discarding information on contrast polarity or color. The findings are discussed in relation to earlier observations on brightness assimilation and contrast. They support theories of nonantagonistic neural mechanisms suppressing local contrast or color signs in brightnessbased figure–ground percepts. Such mechanismsmight be necessary to cancel potentially conflicting polarities in geometricallycomplex visual stimuli so that perceptual filling in resulting in the most plausible representation of figure and ground can be achieved. (shrink)