Effects of movement in the background field on long-range apparent motion

Time-till-breakdown and scalp electrical potential maps of long-range apparent motion

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 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.

Time-till-breakdown and VEP measures of short-range apparent motion

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".

Visual evoked potentials to stimuli in apparent motion

Following an extensive single subject pilot study 12 Ss viewed a continuously cycling (3 Hz), two stimulus, apparent motion (AM) display. The "AM breakdown effect" was utilised to gather visual evoked potentials (VEPs) from sites O1, O2, T5, T6, P3, and P4 during periods in which the display elicited either: the percept of motion; or the percept of two discrete alternating stimuli. VEPs displayed components of larger amplitude during motion periods in all right (but no left) hemisphere sites during periods 60-82 msec post left field stimulus onset (positive component) and 100-126 msec post right field stimulus onset (negative component). Results were interpreted as indicating initial extraction of motion information within the occipital lobe, with further motion processing taking place in temporal and parietal lobes.

Individual differences in responses of untrained observers to stroboscopic apparent motion

Two large groups of inexperienced subjects (n = 208 and n = 50) and a small group of experienced subjects (n = 5) were tested using time-till-breakdown as a measure of long-range apparent motion across a range of temporal frequencies. One group of inexperienced subjects was retested after one week and demonstrated quite stable patterns of response. Large intersubject variability was observed in terms of the amount of motion seen, with most inexperienced subjects reporting very little apparent motion. A raster display produced a peak frequency 1 Hz higher than a standard tachistoscope display. The role of experience was also examined with a small group of inexperienced subjects (n = 8) tested once daily over five consecutive days. There was high intersubject variability and intrasubject consistency, demonstrating little influence of learning and experience. The results are discussed in terms of current ideas on the breakdown effect.