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.     

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.     

A microphotometer for measuring luminance distributions on a CRT

A relatively inexpensive microphotometer is described. The output voltage of the microphotometer is a linear function of luminance, and the sensitivity of the photometer is sufficient for the measurement spot size of .2 mm without special averaging devices. The rise and decay time of the photometer response is less than 100 microsec, and the spectral sensitivity of the photometer can be corrected to approximate that of the human photopic eye.     

Perceived curvature of arcs and dot patterns as a function of curvature, arc length, and instructions

Arcs of circles, with six arc lengths and four radii of curvature, and an equivalent set of figures composed of three dots were used as stimuli. Subjects in Group I imagined the circle from which an arc or dot triplet was taken and indicated the centre of the circle. Group II subjects estimated the location of the point that was equidistant from the middle and ends of an arc, or equidistant from the three dots of a triplet. The results from arcs showed, in Group I, an underestimation of curvature that decreased as a function on the length of the arc. In Group II, however, overestimation of the curvature of most arcs occurred, indicating a strong influence of the difference in the perceptual task on the results. The effect of instructions was similar with the dot figures but, in general, more errors resembling overestimation of curvature occurred with these figures.     

On mathematical illusions

A criticism of Walker’s (1973) paper on the grounds that its physiological premises lack support, that it misrepresents the theoretical position of Blakemore, Carpenter, and Georgeson (1970), and that its theory is contradicted by existing evidence.     

GEOMETRIC ILLUSIONS

The effects of instruction and training on the magnitude of illusion were investigated using as stimuli Müller-Lyer, Müller-Lyer-Ebbinghaus, and Oppel's figure, and a perspective illusion. The subjects estimated the difference between the standard and the variable. The results suggest that changes in judging attitudes, and perceptual learning, contribute to the decrement of illusion during repeated presentation; adaptation to a two-dimensional stimulus world seems to affect the size of illusions which can be explained on the basis of constancy theory. The interactions between the treatments were fairly high.     

NOTE ON THE SYSTEMATIC ERROR OF ESTIMATION AS A FUNCTION OF STIMULUS MAGNITUDE

On the basis of a discussion of Weber's law the prediction was made that the systematic errors of estimation are linear functions of stimulus magnitude. As a special case it is suggested that the size of a geometrical illusion generally is a linear function of the size of the illusion figure. The result of an experiment with Oppel's illusion is in agreement with this prediction.—During successive estimations practice did not decrease the amount of illusion.