Monday, November 3,  12:00 – 1:30 pm, in 489 Minor Hall

Postdiction and Perceptual Awareness

presented by

Shinsuke (Shin) Shimojo, PhD

 Gertrude Baltimore Professor of Experimental Psychology
Division of Biology and Biological Engineering
California Institute of Technology


There are a few postdictive perceptual phenomena known, in which a stimulus presented later seems causally to affect the percept of another stimulus presented earlier (=the operational definition of “postdictive phenomenon”). While backward masking and apparent motion provide classical examples, the flash lag effect and its variations have stimulated theorists. The TMS-triggered scotoma together with “backward filling-in” of it offer a unique neurophysiological case. Findings suggest that various visual attributes – not just spatial locations, but shape, temporal sequence, motion, and even color – are vigorously reorganized in a postdictive fashion to be consistent with each other, or to be consistent in a causality framework. It is highly related to the ideas of “object updating” (by J. Enns & his colleagues) and “backward referral (B. Libet), but with different emphases and implications.

In terms of the underlying neural mechanisms, four prototypical models have been considered: the “catch up,” the “reentry,” the “different pathway” and the “memory revision” models. It may also be argued that “perceptual awareness” can be understood as a postdictive construct. If so(, together with the operational definition of “postdictive phenomenon” above), one may expect structurally similar phenomena of backward reconstrunction across a wide variety of time scale, not just limited to less than few hundred milliseconds for perception. By extending the list of postdictive phenomena to memory, sensory-motor and higher-level cognition, indeed one may note that such a postdictive reconstruction may be a general principle of neural computation, ranging from milliseconds to months in a time scale, from local neuronal interactions to long-range connectivity, in the complex brain.

The mechanisms and functions of such postdictive processes may be an intriguing “unsolved” target of research for the next several decades of perceptual/neural sciences.

Host: Stanley Klein

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