Perception

The role of eye-movement control in dyslexia is still unclear. Recent studies, however, confirmed that dyslexics show poor saccadic control in single and sequential target tasks. In the present study we investigated whether dyslexic subjects are impaired on an antisaccade task requiring saccades against the direction of a stimulus. Altogether, 620 subjects between the ages of 7 and 17 years were classified as dyslexics (N = 506) or control subjects (N = 114) on the grounds of the discrepancy between their intellectual abilities and reading/spelling achievements. All subjects performed an overlap prosaccade and a gap antisaccade task with 100 trials to each side of stimulation in random order. Variables analysed were the overall saccadic reaction time of both tasks; and from the antisaccade task the number of errors (prosaccades), the number of corrected errors, and the number of trials in which the subjects still failed to reach the side opposite the stimulus even after two saccades. An analysis of variance was carried out taking into account the development of saccadic behaviour with age and the differences between the groups. The results confirm development of saccade control with age, especially in the voluntary component (a frontal-lobe function) for both groups, but indicate that the antisaccade task performance, as measured by the error and the correction rate, is significantly worse in the dyslexic group at ages above 8 years. Up to 50% of the dyslexics performed the antisaccade task 1.5 standard deviations below the mean of the controls.

This study describes the capacity of children to detect fast changes of a small visual pattern. Three visual detection tasks for a group of normally reading (N = 140) and another group of dyslexic children (N = 366) in the age range of 7 to 16 years have been used. All three tasks require the detection of the fast changing orientation of a small pattern before it disappears. In one task, stationary fixation was required, because the orientation changes took place always at the same location. In the saccade condition, the pattern was displaced suddenly to one or the other side and a saccade was required to detect the orientation. In a third condition, a distractor was presented at one side shortly before the oriented pattern appeared at the opposite side. In this case, an antisaccade with respect to the distractor was required. In all three conditions, the dyslexic group as a whole performed significantly below the level of the control group. The performance improved with age in both groups. The differences between the test and control group were largest in the distractor condition. When compared with eye-movement performance in an antisaccade task, a parallel development of the performance of both tasks was observed in both groups. The study shows that a certain percentage of dyslexic children has difficulties in the perception of fast changing stimuli, a task presumably challenging the magnocellular system.

This study reports the effects of daily practice of three visual tasks on the saccadic performance of 85 dyslexic children in the age range of 8 to 15 years. The children were selected from among other dyslexics because they showed deficits in their eye-movement control, especially in fixation stability and/or voluntary saccade control. Their eye movements were measured in an overlap prosaccade and a gap antisaccade task before and after the training. The three tasks used for the training included a fixation, a saccade, and a distractor condition. In any of these tasks, the subject had to detect the last orientation of a small pattern which rapidly changed its orientation between up, down, right, and left, before it disappeared after some time. The task was to press one of four keys corresponding to the last orientation. The visual pattern was presented on an LCD display of a small hand-held instrument given to the children for daily use at home. The results indicate that daily practice improved not only the perceptual capacity, but also the voluntary saccade control, within 3 to 8 weeks. After the training, the group of dyslexics was no longer statistically different from the control group.

An ellipse rotating rigidly about its center may appear to rotate rigidly or to deform nonrigidly so that it appears gelatinous. We use this ambiguous stimulus to study how motion information is propagated across space. We find that features that are quite far from the contour of the ellipse may have a strong influence on the percept of the ellipse, provided they move in a way consistent with the motion of the ellipse. We show that the percept cannot be accounted for by computational models that pool constraints over a local area only, or by models that propagate information along contours, or by models that indiscriminately propagate information across space. However, the percept can be accounted for by a class of models that assume smoothness in a layered representation.

Color-from-motion displays consist of a sparse array of dots which never move but change color according to various algorithms. Yet such displays can trigger human vision to construct apparent motion of a subjective surface which is uniformly colored and bounded by a subjective contour. We show that the perceptual strength of this construction depends on the density and regularity of dot placement. We studied three objective measures of density and regularity: nearest-neighbor distance, mean of maximal disks, and variance of maximal disks. We found that nearest-neighbor mechanisms alone are inadequate to account for the perceptual strength of the subjective surfaces and contours. Mechanisms sensitive to areal gaps provide a more adequate account.

We compared the role of the red–green, blue–yellow, and luminance post-receptoral mechanisms in the perception of density. The task requires the comparison of densities between two stimuli composed of oriented bandpass elements, pseudo-randomly scattered across an area of constant size. The perception of density differences was measured by a temporal 2AFC procedure for all pairs of mechanisms and for four possible densities. We found that stimuli of identical physical densities are not perceived equally: there is a consistent bias in favour of blue-yellow stimuli which are perceived as significantly more dense than red–green and achromatic stimuli. We considered three factors that could have differentially affected the density perception of blue – yellow stimuli: an increase in the perceived size of the individual blue – yellow elements, a perceived contrast difference, and the presence of local orientation cues. We found that the increased perceived density of the blue–yellow stimuli occurred despite the fact that there was no increase in perceived size of the individual elements, and remained despite corrections for the two other factors. We conclude that the significant increase in perceived density for the blue–yellow mechanism is a global effect, associated with a perceived colour ‘melting’ of the elements in the array. Our data were fitted with the occupancy model of Allik and Tuulmets (1991, Perception & Psychophysics 49 303–314) and we found that blue–yellow stimuli have a greater ‘occupancy’ than red–green or achromatic stimuli.

The task of human vision is to reliably infer useful information about the external environment from images formed on the retinae. In general, the inference of scene properties from retinal images is not deductive; it requires knowledge about the external environment. Further, it has been suggested that the environment must be regular in some way in order for any scene properties to be reliably inferred. In particular, Knill and Kersten [1991, in Pattern Recognition by Man and Machine Ed. R J Watt (London: Macmillan)] and Jepson et al [1996, in Bayesian Approaches to Perception Eds D Knill, W Richards (Cambridge: Cambridge University Press)] claim that, given an ‘unbiased’ prior probability distribution for the scenes being observed, the generic viewpoint assumption is not probabilistically valid. However, this claim depends upon the use of representation spaces that may not be appropriate for the problems they consider. In fact, it is problematic to define a rigorous criterion for a probability distribution to be considered ‘random’ or ‘regularity-free’ in many natural domains of interest. This problem is closely related to Bertrand's paradox. I propose that, in the case of ‘unbiased’ priors, the reliability of inferences based on the generic viewpoint assumption depends partly on whether or not an observed coincidence in the image involves features known to be on the same object. This proposal is based on important differences between the distributions associated with: (i) a ‘random’ placement of features in 3-D, and (ii) the positions of features on a ‘randomly shaped’ and ‘randomly posed’ 3-D object. Similar considerations arise in the case of inferring 3-D motion from image motion.

Enns and Shore (1997 Perception & Psychophysics 59 23 – 31) found additive effects of test orientation (upright or inverted) and direction of lighting (brow or chin lit) when they studied the inversion effect on face identification. A two-stage model was inferred in which inversion was processed by an orientation-sensitive component after which chin-lighting was processed by a lighting-sensitive component. Face identification is also strongly influenced by contrast reversal. A study is reported which aimed to (i) determine if contrast reversal interacts with lighting direction or orientation, findings that would support Enns and Shore's model; and (ii) to test their assumption that holistic encoding is prerequisite for their model by inducing featural encoding through training names to inverted faces. Names for unfamiliar brow-lit positive-contrast faces were trained with the faces upright or inverted. Identification accuracy was measured with combinations of orientation, lighting, and contrast. Consistent with their model, test orientation and direction of lighting were additive after training on upright faces and lighting and contrast reversal interacted. When holistic encoding was prevented following training on inverted faces, test orientation and lighting direction interacted for positive-contrast faces. Negative faces showed only an effect of direction of lighting. These results support Enns and Shore's two-stage model and their interpretation that orientation and direction of lighting interact following featural encoding of faces.

In this study we examine the strategies used by blindfolded subjects asked to freely explore raised line drawings and identify what is depicted in them. We were particularly interested in how often a single finger is spontaneously used because in several studies subjects are forced to use only one fingertip and the extent to which this restriction may depress haptic perception is unclear. The results suggest that despite a variety of strategies, people ‘naturally’ use single fingertips sufficiently often to allow confidence in conclusions that are based on studies imposing this restriction.