Perception

Morgan and Moulden have shown that the café wall illusion can be explained by the effects of processing with a difference-of-Gaussians filter which reveals Fraser twisted cords in the figure. It is deduced that their account leads to the novel prediction of the reverse tilt illusion if the width of the mortar lines is suitably increased. This prediction is confirmed by demonstration.

In the work reported in the literature the reduction or decrement in the magnitude of the Müller-Lyer illusion with continued inspection has been typically investigated with the use of the composite illusion form. Three experiments are reported in which the illusion decrement was separately examined in the underestimated (wings-in) and the overestimated (wings-out) forms of the Müller-Lyer illusion, with particular attention paid to the transfer of illusion decrement between the two forms. Decrement occurred in both forms of the Müller-Lyer illusion, although there was considerable intersubject variability in decrement effects, and nonuniform rates of decrement across the inspection period. In none of the experiments did transfer of illusion decrement between the two forms occur. It is argued that the attentional/differentiation hypothesis of illusion decrement provides a plausible account of the present findings as well as of those found with the composite Müller-Lyer figure.

Prolonged inspection of a surface slanted in the third dimension of visual space typically results in a negative aftereffect such that, after adaptation, a surface in the fronto-parallel plane will appear slanted in the opposite direction. Binocular disparity is not necessary to generate such effects, since they can be obtained monocularly, presumably via adaptation to texture gradient. Six experiments demonstrated durable stereoscopic depth aftereffects in the absence of a texture gradient—by using discrete disparate objects rather than slanted surfaces— and demonstrated that adaptation was to the interobject disparity gradient rather than to the relative disparity of the objects per se. The disparity required to null the obtained aftereffects was inversely proportional to the horizontal separation of elements, for a constant disparity, and directly proportional to the separation of subsequently presented probes. When elements differed in depth (disparity), but were not laterally separated, nulling disparity was significant but invariant with changes in the horizontal separation of probe elements. In that case, adaptation was (i) either to the disparity gradient generated by the vertical separation of probe elements (of which the relative disparity component was tapped); or (ii) to relative disparity per se.

Evidence was reported earlier from a single case that chromatic–lexical (CL) synaesthesia was a genuine phenomenon. A study is presented in which nine subjects were tested who also reported having coloured hearing. The following questions were addressed: (a) were these cases also genuine (ie consistent over time), (b) were they truly lexical, or rather variants of this condition, such as chromatic–graphemic (CG) or chromatic–phonemic (CP) synaesthesia, (c) did the experimental subjects show any commonalities between them, and (d) were they able to give information on a standard questionnaire about the phenomenology and ontogenesis of the condition? Subjects were asked to describe the colour sensation experienced on hearing items from a list of 130 words, phrases, and letters. The experimental group were not informed of any retest, but were retested more than one year later. A control group (n = 9), matched for IQ, memory, age, and gender, were read the same list and asked to associate a colour with each list item. They were informed at the time of testing that they would be retested on a sample of items from the list a week later. 92.3% of the responses of the experimental group when retested one year later were identical to those given in the original test, compared with only 37.6% of the control subjects' responses (retested one week later). This confirmed the genuineness of these nine cases. All nine experimental subjects showed CG synaesthesia, none showing either CL or CP synaesthesia. Among the experimental group, some consistency was found in the colours evoked by hearing specific letters, suggesting the condition has a neurological basis. There was also evidence of a genetic sex-linked familial pattern underlying the condition. The importance of these distinct forms of synaesthesia for our understanding of the modularity of speech perception and colour vision is discussed.

Centre – surround stimuli evoke colour appearances (resembling surface colours) which cannot be produced by a single homogeneous spot of light alone (eg brown or grey). Although this seems of great impact to a general theory of colour (including ‘colour constancy’), the psychophysics of these ‘minimal relational stimuli’ is still less well understood than often assumed. On the basis of empirical as well as theoretical observations concerning centre– surround-type stimuli we introduce a relational model of colour coding. At the core of this model is the concept of a three-dimensional linear incremental colour code which behaves differently for increments and decrements. This model takes into account results on ‘discounting the background’ mechanisms and it is closely related to ratio-based relational concepts and to certain opponent-colour theories. In addition, it provides an analogue to the classical distinction between light and object colours, and covers colour appearances related to object colours as well. Within the conceptual framework offered, problems of complex colour perception (eg ‘colour constancy’) and judgmental modes are discussed. Conclusions regarding general limitations of three-dimensional modelling in colour perception are derived and corresponding refinements of the relational perspective are briefly outlined.

The paired-preference procedure was used in a series of experiments to explore the abilities of infants aged 3 and 4 months to categorize photographic exemplars from natural (adult-defined) basic-level categories. The question of whether the categorical representations that were evidenced excluded members of a related, perceptually similar category was also investigated. Experiments 1–3 revealed that infants could form categorical representations for dogs and cats that excluded birds. Experiment 4 showed that the representation for cats also excluded dogs, but that the representation for dogs did not exclude cats. However, a supplementary experiment showed that the representation for dogs did exclude cats when the variability of the dog exemplars was reduced to match that of the cat exemplars. The results are discussed in terms of abilities necessary for the formation of more complex categorical representations.

Theoretical considerations of the physiology of eye tracking have long implied that the kinematics of smooth pursuit need not parallel those of the pursued target, with pursuit in the absence of a physically moving target as an extreme case. Yet these theoretical implications have been largely ignored, with the consequence that observed kinematic discrepancies between target and pursuit have generally been presented as somewhat surprising, and this view has maintained unnecessary diversity among reported smooth-pursuit phenomena. The case of yet another stimulus-discrepant smooth-pursuit phenomenon is presented and is used to suggest a role which recognized eye-tracking theory might advantageously play in classifying smooth-pursuit phenomena. The new phenomenon, smooth pursuit along a stationary solid straight line under continuous illumination, was demonstrated in a study where eye movements were documented by continuous recordings of the electrooculogram. All twelve subjects were able to initiate smooth pursuit along the line and to maintain it in the absence of any other source of stimulation, and all reported experiencing apparent movement during smooth pursuit. It is suggested that smooth-pursuit phenomena can best be classified in terms of degree of efference-copy dependency.

Two competing models of the effects of pattern element proximity, masking, and perceptual integration on the discriminability of spatiotemporal vibrotactile patterns are compared. Kirman's ‘integration hypothesis' predicts that pattern perception is facilitated by a process of perceptual integration which requires that pattern elements be presented in close spatial and temporal proximity. Conversely, the ‘isolation hypothesis' predicts that the strong masking effects which occur when pattern elements are presented in close proximity impede the perception of patterns. Traditional masking studies do not provide a fair test of these two hypotheses because they rely on methods that measure the subject's ability to identify the target when the target is presented in conjunction with the mask, rather than the discriminability of the complex percept resulting from the integration of the target and mask. To account for this, a new procedure was devised where the amount of interelement masking and the discriminability of the pattern as a whole were measured independently as the spatial and temporal separation of the pattern elements were varied. As expected under both hypotheses, masking between pattern elements increased as either the spatial or the temporal separation between them was decreased. The pattern discrimination data also support the isolation hypothesis in that the patterns were discriminated less well with increasing temporal element separation with a similar but nonsignificant trend in the case of spatial separation. It is concluded that this new methodology should be applied to a wider range of tactile pattern processing situations in order to assess the generality of the results obtained.