Electrophysiological recordings show that any visual stimulus produces a response in a large population of differently tuned sensory neurons. However, it remains unclear how this population response is used to make perceptual decisions. Major theories, such as probabilistic population codes and neural sampling, assume that the population response can be flexibly used for decision making. However, another possibility is that decision-making circuits do not have access to the sensory population

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... de but only to a summary of this code. These possibilities can be disentangled for choices among n>2 discrete stimulus categories. In two experiments, we asked subjects to choose between n=4 colors or n=6 symbols. We then used the n-alternative condition to predict the performance on a different, 2-alternative condition where only two colors or symbols were given as available alternatives on each trial. A population model assuming that decision-making circuits have access to the whole distribution of the sensory activation levels across all colors or symbols significantly overestimated the performance in the 2-alternative condition. Instead, a summary model assuming that decision- making circuits only have access to the sensory activity associated with the dominant color predicted the 2-alternative condition very well and was preferred in Bayesian model comparison. Finally, a third experiment asked subjects to make two decisions in a row and also found the summary model to provide a better description of the data. These results show that the full population code in sensory cortex may not be available for deliberate decision making, at least within the context of decisions between discrete alternatives.