PubMed · 13436656
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B BAGOLINI. [After-image transfer test].. https://pubmed.ncbi.nlm.nih.gov/13436656/
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When only a featureless straight contour of a moving object is visible, one cannot tell its true velocity and the object seems to be moving perpendicularly to its orientation. Using psychophysics and brain imaging, Goltz et al. have now demonstrated that this aperture problem also occurs in visual representations in egocentric coordinates. An afterimage of inclined lines that is perceived to move with smooth-pursuit eye movements appears to move perpendicularly to the lines rather than in the tracking direction.
The study is of brain activity in a blindsight subject (D.B.), who reports conscious visual afterimages of stimuli of which he is unaware when they are presented. This contrast offered a unique opportunity to study event-related potential recordings of conscious versus unconscious visual phenomena generated by the very same stimulus in the identical locus of the visual field. The behavioral results confirmed the reliability of the difference in the subject's report for inducing stimuli versus their aftereffects. The rationale of the event-related potential analysis was to subtract "on" signals from "off" signals, the latter associated with the onset of conscious events and the former for events that remained unconscious. Because there are inherent differences in on and off potentials, the subtractive resultants for the blind hemifield were compared with the same subtractions for the good hemifield when the subject was aware both of the stimuli and their afterimages. A differential pattern in subtractive resultants emerged with a strong anterior left frontal focus for the blind field and a posterior focus for the intact field. The results are compared with other studies suggesting an anterior focus for conscious visual events.
The context of a target can modulate behavioral as well as neural responses to that target. For example, target processing can be suppressed by iso-oriented surrounds whereas it can be facilitated by collinear contextual elements. Here, we present experiments in which collinear elements exert strong suppression whereas iso-oriented contextual surrounds yield no contextual modulation--contrary to most studies in this field. We suggest that contextual suppression depends strongly on the spatial arrangement of the context pointing to the influence of Gestalt factors in contextual modulation.