How does subjective experience correlate with neural activity? Activity in early sensory cortex reflects accuracy in discriminating different stimuli, and is thought to be key in determining perceptual judgments. Now a study on page 1698 of this issue shows that in monkeys judging stimuli at the threshold of detection, activity in frontal rather than sensory cortex correlates with subjective experience.

Lafuente and Romo recorded single-neuron responses from primary somatosensory and medial premotor cortex in monkeys trained to report the presence or absence of a mechanical vibration applied to their fingertips. The strength of the vibration varied, and it was sometimes just at the threshold of detection, so that the monkeys made mistakes. Activity in the somatosensory cortex accurately reflected stimulus strength, but it did not correlate with the monkeys' perceptual reports. Medial premotor cortex activity was independent of stimulus strength, but covaried with the monkeys' subjective judgments, even when they gave an incorrect response. Electrically stimulating the medial premotor cortex caused the monkeys to respond almost exactly as they did to real mechanical stimulation. These results suggest that somatosensory cortex represents the physical properties of a stimulus, whereas frontal neurons are more involved in subjective perceptual judgments.

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