we look with our next fixation or our selection of which movie to see or
book to read is decidedly non random.
What controls this selection when an individual is not hungry,
avoiding harm, engaged in deliberate search, etc.?
And how can the selection be manifested in real time, at the rate
of three visual fixations per second?
The surprising discovery of a gradient of mu-opioid
receptors (possible ligand termed "endomorphin") in the
macaque ventral cortical visual pathway, a system presumed to subserve
visual recognition, may
provide the key for understanding the spontaneous selectivity of
perception and thought. These
receptors are sparse in the earliest stages, e.g., V1 and V2, but dense
in the later stages (parahippocampus), where perceptual information
activates the products of past experience. A simple mechanism may
account for a vast range of spontaneous perceptual selectivity:
Experiences are preferred that maximize the rate of endomorphin release.
Such inputs will tend to be those that are richly interpretable (not
just complex) insofar as they would produce high activation of
associative connections in areas that have the greatest density of
mu-opiate receptors. Once an input is experienced, however, competitive
learning would serve to reduce activity, resulting in less endomorphin
release, leading to habituation and boredom. Ratings
of scene preference and their decline with repetition reliably correlate
with fMRI activity in the parahippocampal gyrus during passive 1 s
viewing of scenes. These are
not feedforward effects in that nonpreferred and repeated scenes produce
equivalent activity in more posterior areas associated with early visual
processing and attention. Because
this system is never satiated and it maximizes the rate at which we
acquire new but richly interpretable information, it renders us