The representation of value is a
critical component of decision making. Rational choice theory assumes
that options are assigned absolute values, independent of the value or
existence of other alternatives. However, context-dependent choice
behavior in both animals and humans violates this assumption,
suggesting that biological decision processes rely on comparative
evaluation. Here we show that neurons in the monkey lateral
intraparietal cortex encode a relative form of saccadic value,
explicitly dependent on the values of the other available alternatives.
Analogous to extra-classical receptive field effects in visual cortex,
this relative representation incorporates target values outside the
response field and is observed in both stimulus-driven activity and
baseline firing rates. This context-dependent modulation is precisely
described by divisive normalization, indicating that this standard form
of sensory gain control may be a general mechanism of cortical
computation. Such normalization in decision circuits effectively
implements an adaptive gain control for value coding and provides a
possible mechanistic basis for behavioral context-dependent violations
of rationality.
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