The human visual system is able to extract an object from its surrounding using a number of cues. These include foreground/background gradients in disparity, motion, texture, colour, and luminance. We have investigated normal subjects' ability to detect objects defined by either motion, texture, or luminance gradients. The effects of manipulating cue density and cue foreground/background gradient on both detection and recognition accuracy were also investigated. The results demonstrate a simple additive relationship between cue density and cue gradient across forms defined by motion, luminance, and texture. The results are interpreted as evidence for the notion that form parsing is achieved via a similar algorithm across anatomically distinct processing streams.

This paper provides data on a family in which three members, all female, have been diagnosed as having agenesis of the corpus callosum. That all three acallosal individuals came from the same family and showed relatively uniform neuropsychological impairment and could be compared in similar terms with their “callosal” siblings, also female, provides a unique sample. Inter-hemispheric transfer, psychometric measures, and motor and cognitive function were examined in the acallosal individuals, all of whom had borderline to low-average intelligence, with results compared to their non-acallosal siblings. The data indicated that all acallosal individuals exhibited deficits with the cognitive tests indicating difficulties of inter-hemispheric transfer of tactile information, difficulties in some areas of memory and, at least as far as the children are concerned, a marked difference in Verbal IQ and Performance IQ.

Event-related potentials were measured in response to an interference task in which unattended stimulus items were compatible, incompatible or neutral with regard to the attended stimulus items. Two stimulus items were presented simultaneously and bilaterally—one in each visual field. This allowed examination of the event-related potential waveform according to whether recording sites were contralateral to the attended or unattended location. The first experiment used sustained cueing with 3.5° separation between attended and unattended locations. Attentional modulation of the N1 was observed but not for the P1. In the second experiment, separation between attended and unattended locations was increased to 11.5°. In both experiments, the hemisphere contralateral to the unattended material (unattended hemisphere) showed a greater negativity in the N2 latency range in the temporal regions to compatible and incompatible conditions compared to a neutral condition. These data are inconsistent with findings suggesting the filtering of material early in visual processing.

A series of psychophysical and electrophysiological experiments is reported using the apparent motion (AM) breakdown effect. Breakdown describes an effect in AM in which, during continuous viewing, the percept of smooth motion of a single stimulus alternates with the percept of two discrete alternating stimuli. Visual evoked potentials (VEPs) were recorded during periods of motion or breakdown (“nonmotion”) in horizontal and vertical displays. VEPs were compared with synthetic VEPs (“composite-flash”) produced by adding VEPs to each element of the display recorded in isolation. Subtraction of VEPs was used in an attempt to compare the electrical responses with the processing of information relating to the form of the stimulus, subthreshold motion processing, and suprathreshold motion processing. The results, presented as scalp electrical potential distribution maps, were interpreted as consistent with a central adaptation process underlying the breakdown effect. The results also indicated that the hemispheric asymmetries in AM VEPs described by Manning, Finlay, and Fenelon (1988) were most likely due to the position of the stimuli in the visual field, rather than as a lateralization of motion processes per se. The results also provided evidence that the subthreshold and suprathreshold motion responses to the display were the product of different populations of motion units.

The initial experiment in this research used psychophysical responses to dynamic random-dot stereograms (DRDS) to examine disparity detection and disparity or depth discrimination in stereopsis. Using identical crossed and uncrossed disparity stimuli across conditions at 150 ms exposure, we found that while disparity could be detected and form discriminated, observers were unable to discriminate depth direction. These results suggest that disparity detection is a faster process than disparity discrimination and that form can be discriminated prior to the assignment of depth direction. In a second experiment we examined disparity (correlation) and noncorrelation in stereopsis and used visual evoked potentials (VEPs) in response to three DRDS displays: (a) a target containing disparity on a correlated background, (b) an uncorrelated target on a correlated background, and (c) a target containing disparity on an uncorrelated background. At 200 ms stimulus exposures, VEPs to regions of disparity and regions of noncorrelation on correlated backgrounds were largely undifferentiated. There was not, however, any discernible VEP to stimuli incorporating disparity on an uncorrelated background. These findings imply that while form may be discriminated on the basis of a region of disparity (correlation) in stereopsis, the uncorrelated background condition suggests this is a slow process, since there was no apparent response to form or depth in the recording epoch. Early processes may regard the entire DRDS as a region of noncorrelation with both target and background being mismatched, and hence initially undifferentiated. A faster process may discriminate form on the basis of a region of noncorrelation, as suggested by the correlated background conditions.

The experiment reported here examined an interference paradigm using a bilateral stimulus presentation in which stimuli were presented simultaneously in the left and right visual fields. The lateralization of the early visual components allowed an ERP examination of material presented in each field. Attention was directed to one field or the other on each trial by a 100% valid cue. Two letters were nominated as targets and the simultaneous presentation allowed presentation of material compatible, incompatible or neutral with reference to the target. A negative peak was observed at 230 ms post stimulus at occipital and temporal sites. There was a variation in this peak for unattended stimuli, with compatible and incompatible target letters being significantly different to non-target letters. Contrary to previous research, this finding suggests that material is not filtered out at an early stage as proposed by early selection. It was found that the response to unattended target material also varied according to the type of item presented at the attended location. These findings were discussed in relation to previous studies which found no difference in the processing of unattended target and non-target material, and also in relation to suggestions that automatic processing of unattended material occurs only when there is controlled processing occurring simultaneously.

The paper describes aspects of individual variability in Visual Evoked Potentials (VEPs) in terms of variability in cortical anatomy. VEPs were obtained from 6 subjects using small circular stimuli, adjusted in size for cortical magnification factor, and presented at various eccentricities along a vertical meridian. The eccentricities were 0°, ±3°, ±5°, ±10°, and −15°. The scalp topography of the first major component of the VEP, at 118 ms poststimulus onset, differed between upper and lower visual field stimulation with a polarity reversal at midline occipital/parietal sites. However, the degree of individual variability made the interpretation of the group averaged response difficult, especially for the 0° and −3° conditions. Using a three concentric sphere model of the head, equivalent dipole sources of the VEPs were estimated. These were interpreted in conjunction with magnetic resonance images of the brain. Results for individual subjects were consistent with the VEP generator lying within primary visual cortex. However, the cruciform model of primary visual cortex anatomy was inadequate as a predictor of these results. The advantages of interpreting individual rather than group averaged data are emphasised.

Differential motion thresholds were measured at eccentricities of 9° and 16.6° using computer-generated sinusoidal gratings. Three spatial frequencies (0.51, 0.25, and 0.13 cycles/deg) were examined at reference velocities of 2, 4, 8, 16, 52, and 48 deg/sec. Minimum differential velocity thresholds were between 20 and 30% of the reference velocities for the three spatial frequencies at both eccentricities Increasing eccentricity produced an increase in the velocity at which minimum velocity discrimination occurred. Temporal frequency tuning was between 4 and 8 Hz, regardless of eccentricity.

Varying short-range apparent motion (AM) stimulus displacements from 2.7 to 21.6 min it was found that VEP amplitudes varied as a function of the limits for short-range AM described using time-till-breakdown as a behavioural measure of AM strength. This VEP amplitude difference was, however, in the reverse direction to that predicted as the "motion" condition elicited lower VEP amplitude responses than the "non-motion" conditions (which did not significantly differ from each other). This direction of VEP amplitude difference was supported by an intensive study of a single subject. The "breakdown effect" enabled VEPs to be gathered during periods in which the subjective experience was of either coherent lateral motion, or breakdown (incoherent motion) without changing any stimulus parameter. The VEP component identified in expt 2, as predicted, was of lower amplitude during motion with respect to periods of breakdown. The results of these experiments are discussed in terms of describing motion and breakdown in short-range AM displays as "coherent" and "incoherent" motion, rather than as "motion" and "non-motion".

Motion thresholds were determined at 9 degrees eccentricity in infants (mean = 14 weeks old). The stimuli used were computer-generated sinusoidal gratings presented through a 7.45 degrees aperture at a contrast ratio of .83. The range of velocities (.5, 1, 2, 4, and 6 degrees per s) was examined at only one spatial frequency (1 cycle per degree). At low velocities (less than 2 degrees per s), the infants showed no clear preference for the moving stimulus over the stationary stimulus. At faster velocities (2-6 degrees per s), the infants exhibited a clear preference for the moving stimulus. The results were interpreted as indicating that infants at 3 months of age are relatively insensitive to slow motions for low spatial frequency stimuli.

Temporal detection thresholds are reported from two subjects to dynamic random dot stereograms (DRDSs) of both crossed and uncrossed disparity. The stimuli were 1 degree square, of 0.25 degrees disparity and were presented at 81 positions in a 9 degree square central region. In both subjects crossed disparity stimuli were detected at shorter durations. One subject displayed particularly acute stereoscopic vision and presented a pattern of temporal thresholds increasing gradually with eccentricity. The other subject displayed evidence, particularly in response to uncrossed stimuli, of a 'stereo-scotoma' which was not evident in monocular testing or ophthalmologic examination. Criteria used to define 'stereo-scotoma' are discussed. It was concluded that, although left-right field differences may be found with a large subject sample, individual factors may be more important in the distribution of stereo sensitivity throughout the portion of visual field tested.