Crossmodal temporal order and processing acuity in developmentally dyslexic young adults

We investigated crossmodal temporal performance in processing rapid sequential nonlinguistic events in developmentally dyslexic young adults (ages 20-36 years) and an age- and IQ-matched control group in audiotactile, visuotactile, and audiovisual combinations. Two methods were used for estimating 84% correct temporal acuity thresholds: temporal order judgment (TOJ) and temporal processing acuity (TPA). TPA requires phase difference detection: the judgment of simultaneity/nonsimultaneity of brief stimuli in two parallel, spatially separate triplets. The dyslexic readers' average temporal performance was somewhat poorer in all six comparisons; in audiovisual comparisons the group differences were not statistically significant, however. A principal component analysis indicated that temporal acuity and phonological awareness are related in dyslexic readers. The impairment of temporal input processing seems to be a general correlative feature of dyslexia in children and adults, but the overlap in performance between dyslexic and normal readers suggests that it is not a sufficient reason for developmental reading difficulties.     

Temporal integration and contrast sensitivity in foveal and peripheral vision

Spatial contrast sensitivity functions and temporal integration functions for gratings with dark surrounds were measured at various eccentricities in photopic vision. Contrast sensitivity decreased with increasing eccentricity at all exposure durations and spatial frequencies tested. The decrease was faster at high than at low spatial frequencies, but similar at different exposure durations. When cortically similar stimulus conditions were produced at different eccentricities by M-scaling, contrast sensitivity became independent of visual field location at all exposure durations tested. The results support the view that in photopic vision spatiotemporal information processing is qualitatively similar across the visual field, and that quantitative differences result from retino-topical differences in ganglion cell sampling. For gratings of constant retinal area temporal integration (improvement of contrast sensitivity with increasing exposure duration) was more extensive at high than at low retinal spatial frequencies but independent of cortical spatial frequency and eccentricity. For M-scaled gratings temporal integration was more extensive at high than at low cortical spatial frequencies but independent of retinal spatial frequency and eccentricity. The results suggest that the primary determinant of temporal integration is not spatial frequency but grating value that is calculated as AF2 square cycles (cycle2), where A is grating area and F spatial frequency.     

The misperception of angles: Estimating the vertex of converging line segments

Estimates of the point of intersection of converging line segments depended upon the angle between lines and the orientation of the display. Main conclusion: The tilt of a line is perceptually altered to appear more nearly parallel to the more closely aligned axis, either horizontal or vertical, of an O’s visual field.     

Underestimation of curvature and task dependence in visual perception of form

Perceived curvatures of circular arcs were compared with those of circles. The length and orientation of the arcs varied. In all cases the curvatures of the arcs were underestimated, and the error was a decreasing exponential function of arc length. The results are consistent with the notion that tendency (efferent readiness) to perform rectilinear eye movements produces underestimation of curvature. The results discredited the explanation of the “Gibson normalization effect” as an instance of increase in perceptual accuracy. The overestimation of curvature found in earlier studies was interpreted as resulting from an inappropriate perceptual task and uncontrolled effects of illusions of extent.     

Tendencies to eye movement,and misperception of curvature,direction, and length

From previous studies of eye movements, three types of eye-movement tendency can be inferred: (1) tendency to rectilinear eye movements, (2) tendency to horizontal or vertical eye movements, and (3) tendency to center-of-gravity fixations. The possible influence of these eye-movement tendencies on perception was investigated in two experiments. In Experiment 1, errors in perceived location of intersection in arc figures were studied varying arc-point distance and are length. Tendencies 1 and 2 accounted very well for the resultant S-shaped functions. In Experiment 2, the Müller-Lyer illusion with three different oblique angles and a line-segment illusion were measured as a function of the distance between the vertex and the center of gravity of the arrowhead. Tendency 3 accounted well for the inverted-U forms of the obtained functions but not for the increase of error with increasing angle.     

Psycholphysical 'measurement' of cortical colour mechanisms: reply of Meyer.

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.     

Rate of information segregation in developmentally dyslexic children

Slowed processing of sequential perceptual information is related to developmental dyslexia. We investigated this unimodally and crossmodally in developmentally dyslexic children and controls ages 8-12 years. The participants judged whether two spatially separate trains of brief stimuli, presented at various stimulus onset asynchronies (SOA) in one or two senses, were synchronous or not. The stimulus trains consisted of light flashes in vision, clicks in audition, and indentations of the skin in the tactile sense. The dyslexic readers required longer SOAs than controls for successful performance in all six comparisons. The crossmodal spatiotemporal resolution of the groups differed more than unimodal performance. The dyslexic readers' segregation performance was also less differentiated than that of the controls. Our results show that not only sensory but also polysensory nonverbal information processing is temporally impaired in dyslexic children.