Sumnall J H, Freeman T C A, Snowden R J, 2001, "Contrast and spatial frequency effects in the perception of head-centric speed" Perception 30 ECVP Abstract Supplement
Contrast and spatial frequency effects in the perception of head-centric speed
J H Sumnall, T C A Freeman, R J Snowden
Many stimulus properties modify the relationship between physical and perceived speeds of moving patterns. Here we investigate how contrast and spatial frequency affect perceived head-centric speed of pursued and non-pursued patterns. Two patches of Gaussian blobs were presented sequentially and their speed match determined. Observers tracked the first pattern with their eyes and viewed the second with eyes stationary. Eye movements were recorded and found to be independent of stimulus properties. For stimuli with identical contrast, the pursued pattern was perceived to move slower than the non-pursued pattern [the Aubert - Fleischl phenomenon (A - F)]. Varying the contrast of the non-pursued pattern resulted in a systematic change to the magnitude of the A - F. This change correlated with the size of the contrast effect obtained when the eyes were stationary in both intervals. Similar results were obtained for spatial-frequency manipulation. A parsimonious explanation is that stimulus properties modify perceived retinal speeds, but not perceived pursuit speeds (Freeman and Banks, 1998 Vision Research 38 941 - 945). Alternative models suggest that stimulus properties change the optokinetic potential of the pattern during pursuit. However, a further condition in which the contrast or spatial frequency of the pursued pattern was varied, resulted in no change to the A - F.
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