Motion adaptation effects suggest an explicit representation of velocityP Schrater and E P SimoncelliPublished in Investigative Opthalmology and Visual Science Supplement (ARVO), vol.35 pp. 1268, May 1994.© Elsevier Science Ltd. This paper has been superseded by:
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| Purpose: The perceived direction of motion (DOM) of a drifting sinusoidal grating can be altered by first adapting to a grating or plaid drifting in a different direction (Lew, et. al., ARVO, 1991). We have designed experiments using similar motion-adaptation effects to demonstrate that the visual system represents motion information with a population of mechanisms that are tuned for velocity (i.e., both speed and direction). Methods: Subjects viewed an adapting stimulus followed by a drifting sinusoidal test grating, and were asked to report (by positioning a directional arrow with a mouse) the perceived DOM of the test. Test gratings were of variable spatial and temporal frequency, and the adapting stimulus was either a drifting grating, plaid, or correlated random dots. Results: We found that the effect on the perceived DOM of the test was always of the same form, regardless of the type of adapting stimulus. In particular, for test gratings with DOMs less than 90° from that of the adapting stimulus1, the DOM was strongly biased away from the adapting DOM. For gratings with orientations more than 90° from the adapting DOM, the DOM was biased slightly toward the adapting DOM. For grating adaptation, shifts of the test spatial frequency of up to an octave from that of the adapting grating modestly reduce the magnitude of the effect. Conclusions: Since these effects are not strongly dependent on the type of adapting stimulus (i.e., grating, plaid or correlated dot pattern), or the spatial frequency (in the case of grating adaptation), we argue that they cannot be simply explained by adaptation of spatio-temporal energy mechanisms. They are, however, consistent with a mechanism that explicitly represents velocities. |
| 1Adapting DOM is defined as the normal direction for gratings, and the pattern (or intersection-of-constraints) direction for plaids. |