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Virus and Haapasalo report that the Blakemore and Sutton apparent spatial-frequency shift is not color specific. Also supposedly demonstrated is a McCollough effect that is not possible to spatial frequency. It is suggested that inadequate test procedures and a confusion between retinal and cortical adaptation negate these results.
In a paper recently published in this journal, Meyer criticised our study on relationships between channels for colour and spatial frequence for not being able to demonstrate a size aftereffect not specific to colour, a McCollough effect not specific to size, or the functions of cortical colour mechanisms. In fact, our study attempted none of these demonstrations in the sense suggested by Meyer because the first would have been impossible for empirical reasons, the second for conceptual reasons, and the third for methodological reasons. Instead, our study yielded evidence that at least three different types of perceptual channel underlie our capacity to perceive the size and colour of objects.
For long periods observers fixated low spatial frequency coloured gratings. Black and white test gratings of the same spatial frequency and orientation as the adapting gratings appeared coloured with the hue complementary to the adapting patterns when the dark test stripes fell on retinal areas previously occupied by the dark adapting stripes; no colour or very weak colour was seen when the test gratings were reversed in phase (contrast reversed). No colour aftereffects were produced with coloured gratings that lacked luminance contrast. The selectivity to the polarity of local luminance contrast can be explained by mechanisms that respond conjointly to colour and luminance contrast. The aftereffects are selective to spatial phase.
The visual persistence of sinusoidal gratings of varying spatial frequency and contrast was measured. It was found that the persistence of low-contrast gratings was longer than that of high-contrast stimuli for all spatial frequencies investigated. At higher contrast levels of 1 and 4 cycles deg-1 gratings, a tendency for persistence to be independent of contrast was observed. For 12 cycles deg-1 gratings, however, persistence continued to decrease with increasing contrast. These results are compared with recently published data on other temporal responses, and are discussed in terms of the different properties of sustained and transient channels.
Some tests are reported on a young woman who was believed to have unusual powers of eidetic imagery and voluntary control of vivid hallucinations. No evidence was found of abnormal eidetic ability. Measurements of detection thresholds for light of different wavelengths, while the subject tried to hallucinate various colours, suggested that the supposed hallucinations acted like coloured filters held over the eye.
In a recent application of an algorithm developed in computer and optical pattern recognition, Cavanagh has suggested that a composite of spatial frequency mapping and complex logarithmic mapping would provide translationally, rotationally, and size-invariant mechanism for human vision. In this work, Cavanagh has not made explicit the fact that this transformation is composite, that is, that the first step (global Fourier analysis) perceptually, anatomically, and physiologically inconsistent with primate vision, but that the second step (complex logarithmic mapping) is actually embodied in the anatomy of the primate retinostriate projection. Moreover, it is the complex logarithmic remapping step which is entirely responsible for the computational simplification of the symmetries of size and rotation invariance. These facts, which have been extensively discussed in a recent series of papers, are briefly reviewed and illustrated. Furthermore, it is shown that the architecture of the retinostriate map may provide an example of computational anatomy in vision, such that the spatial representation of a stimulus in the brain may be of direct functional significance to perception, and to the nature of certain visual illusions.
Van Tuijl's neon color effect arises in the Ehrenstein figure if a colored cross is added such as to connect the black arms across the central gap. The effect consists of a circular veil of color in the illusory area and has the same hue as the inducing cross. The neon-like coloration is uniform, or when elicited by two color bipartite; it is strongest on backgrounds resembling the color of the cross. The effect cannot be attributed to chromatic aberration or eye movements. In foveal vision (and for red crosses) neon spreading is limited to gap sizes between 4 and 35 min of arc. Extrafoveally, gap sizes may be larger by a factor of two. Neon perception is enhanced by flicker and weakened if stimuli are oriented obliquely. It does not occur with dichoptic presentation. A maximum illusion requires that the Ehrenstein figure and cross are laterally and angularly aligned for good perceptual continuation. A neuronal origin by spreading and summation, together with cognitive processes, is proposed.
Recent studies have provided evidence that dyslexic children tend to show longer visual persistence than control children when presented with low-spatial-frequency grating stimuli. The possibility that this phenomenon might reflect an impairment of inhibitory Y-cell activity in the visual system of dyslexics has been investigated. A flicker masking technique was used to mask Y-cell activity selectively in a group of dyslexic boys and a group of age-matched controls. There were no overall differences in reaction times to the offsets of grating patterns of various spatial frequencies between the groups, and no differences between subgroups defined by age, degree of reading impairment, or any other criterion. The results show no evidence of abnormal Y-cell function in developmental dyslexia.
The visual motion aftereffect (MAE) was initially described after observation of movements in the natural environment, like those seen in rivers and waterfalls: stationary objects appeared to move briefly in the opposite direction. In the second half of the nineteenth century the MAE was displaced into the laboratory for experimental enquiry with the aid of Plateau's spiral. Such was the interest in the phenomenon that a major review of empirical and theoretical research was written in 1911. In the latter half of the present century novel stimuli (like drifting gratings, isoluminance patterns, spatial and luminance ramps, random-dot kinematograms, and first-order and second-order motions), introduced to study space and motion perception generally, have been applied to examine MAEs. Developing theories of cortical visual processing have drawn upon MAEs to provide a link between psychophysics and physiology; this has been most pronounced in the context of monocular and binocular channels in the visual system, the combination of colour and contour information, and in the cortical sites most associated with motion processing. The relatively unchanging characteristic of the study of MAEs has been the mode of measurement: duration continues to be used as an index of its strength, although measures of threshold elevation and nulling with computer-generated motions are becoming more prevalent. The MAE is a part of the armoury of motion phenomena employed to uncover the mysteries of vision. Over the last 150 years it has proved itself immensely adaptable to the shifts of fashion in visual science, and it is likely to continue in this vein.
The stimuli in these experiments are square-wave luminance gratings with an array of small random dots covering the high-luminance regions. Owing to the texture, the direction of these gratings, when seen through a circular aperture, is disambiguated because the visual system is provided with an unambiguous motion energy. Thus, the direction of textured gratings can be varied independently of grating orientation. When subjects are required to judge the direction of textured gratings moving obliquely relative to their orientation, they can do so accurately (experiment 1). This is of interest because most studies of one-dimensional motion perception have involved (textureless) luminance-defined since-wave or square-wave gratings, and the perceived direction of these gratings is constrained by the aperture problem to be orthogonal to their orientation. Thus, direction and orientation have often been confounded. Interestingly, when subjects are required to judge the direction of an obliquely moving textured grating during a period of adaptation and then the direction of the motion aftereffect (MAE) immediately following adaptation (experiments 2 and 3), these directions are not directly opposite each other. MAE directions were always more orthogonal to the orientation of the adapting grating than the corresponding direction judgments during adaptation (by as much as 25 degrees). These results are not readily explained by conventional MAE models and possible accounts are considered.
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When two visual patterns are presented in rapid succession, their contours may be combined into a single unified percept. This temporal integration is known to be influenced by such low-level visual factors as stimulus intensity, contour proximity, and stimulus duration. In this study we asked whether temporal integration is modulated by an attentional-blink procedure. The results from a localisation task in experiment 1 and a detection task in experiment 2 pointed to two separate effects. First, greater attentional availability increased the accuracy of spatial localisation. Second, it increased the duration over which successive stimuli could be integrated. These results imply that theories of visible persistence and visual masking must account for attentional influences in addition to lower-level effects. They also have practical implications for use of the temporal-integration task in the assessment of group and individual differences.
Ambiguous figures are a special class of images that can give rise to multiple interpretations. Traditionally, switching between the possible interpretations of an ambiguous figure, or reversing one's interpretation, has been attributed either to top-down or to bottom-up processes (e.g. attributed to having knowledge of the nature of the ambiguity, or to a form of neuronal fatigue). Here we present evidence that is incompatible with both forms of explanations. Observers aged 5-9 years can reverse ambiguous figures when uninformed about the ambiguity, negating purely top-down explanations. Further, those children who make these 'spontaneous' reversals are more likely to succeed on a high-order theory-of-mind task, negating purely bottom-up explanations.
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Using a quasistatic method, it has been found previously that one-third of normal subjects retained pupillary constriction under empty-field conditions for a substantial period following a near-vision task. This pupil after-effect was dissociated from adaptation of tonic accommodation. In this paper, essentially the same method was used to investigate further the effects of near vision on pupil response in VDU (Visual Display Unit) workers with complaint of general fatigue and/or asthenopia. In some cases, large and slow pupillary oscillation accompanied by miosis was noted after near vision. Pupillary oscillation is a well-established phenomenon that has a broadly spread frequency spectrum with an occasional component 'hippus' at approximately 0.2 Hz. A proportion of subjects (30 out of 100) showed hippus, a consistent oscillation of 0.22 Hz (SD = 0.03 Hz) in VDU workers, while only one out of 24 normal subjects showed this. Occurrence of pupillary hippus following near vision increased in VDU workers. It was suggested that this hippus is concerned with cumulative asthenopia and/or general fatigue although the origin of hippus is still unknown.
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