The
mathematical formulations used to study the neurophysiological signals
governing choice behavior fall under one of two major theoretical
frameworks: "choice probability" or "subjective value." These two
formulations represent behavioral quantities closely tied to the
decision process, but it is unknown whether one of these variables, or
both, dominates the neural mechanisms that mediate choice. Value and
choice probability are difficult to distinguish in practice, because
higher-valued options are chosen more frequently in free-choice tasks.
This distinction is particularly relevant for sensorimotor areas such
as parietal cortex, where both value information and motor signals
related to choice have been observed. We recorded the activity of
neurons in the lateral intraparietal area while monkeys performed an
intertemporal choice task for rewards differing in delay to
reinforcement. Here we show that the activity of parietal neurons is
precisely correlated with the individual-specific discounted value of
delayed rewards, with peak subjective value modulation occurring early
in task trials. In contrast, late in the decision process these same
neurons transition to encode the selected action. When directly
compared, the strong delay-related modulation early during decision
making is driven by subjective value rather than the monkey's
probability of choice. These findings show that in addition to
information about gains, parietal cortex also incorporates information
about delay into a precise physiological correlate of economic value
functions, independent of the probability of choice.
|