The proposition
that reentrant interactions into the early visual system are necessary
for visual awareness has lately been under close scrutiny. We examine
this proposition in the context of a neuronal model which explains the
phenomenology of feature inheritance.
Feature inheritance (Herzog and Koch) is a class of visual illusions
in which a mask stimulus acquires a feature from a preceding brief target
stimulus. We focus on the case where the target stimulus is a single
tilted bar (on for 30 ms), and the mask stimulus is a grating of straight
bars (on for 300 ms). In this case, only a tilted grating is perceived.
If the mask is on for a short time (60 ms), target and mask are perceived
superimposed.
Feature inheritance thus exhibits aspects of both backward masking and
of temporal integration. Correspondingly, our model consists of two
pathways: a multiplicative hypothesis-testing one and an activity-accumulating
one. The multiplication pathway multiplies a "template" (a
hypothesis about the world, based on early stimuli) with the current
bottom-up input, in order to test the hypothesis. The template can be
influenced by high-level expectations. If the match is good, the result
is a bound object which is subsequently sent to perception. If it is
bad, such as when the input is very brief or rapidly changing, the brain
uses the output of the second pathway, which accumulates activity during
the time the hypothesis-testing has not yet been completed. This yields
temporal integration.
Although it is possible that the multiplicative pathway involves descending
connections, no feedback in a graph-theoretical sense is necessary.
Our model can explain much of the psychophysics of brief visual stimuli.
In particular, we claim that the reconstruction of the contents of the
perceptual gap due to the hypothesis-testing is exactly Efron's ``minimal
perceptual moment''. Furthermore, our model is consistent with the computational
model of object substitution masking as presented by DiLollo et al.
References
M.H. Herzog and C. Koch, Seeing properties of an invisible object: Feature
inheritance and shine-through, Proc. Natl. Acad. Sci. USA 98 (2001),
4271
Di Lollo, J.T. Enns, and R.A. Rensink, Competition for Consciousness
Among Visual Events: The Psychophysics of Reentrant Visual Processes,
J. of Experimental Psychology 129 (2000), 481-507
