A Rate Code for Sound Azimuth in Monkey Auditory Cortex: Implications for Human Neuroimaging Studies

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Abstract
Is sound location represented in the auditory cortex of humans and monkeys? Human neuroimaging experiments have had only mixed success at demonstrating sound location sensitivity in primary auditory cortex. This is in apparent conflict with studies in monkeys and other animals, in which single-unit recording studies have found stronger evidence for spatial sensitivity. Does this apparent discrepancy reflect a difference between humans and animals, or does it reflect differences in the sensitivity of the methods used for assessing the representation of sound location? The sensitivity of imaging methods such as functional magnetic resonance imaging depends on the following two key aspects of the underlying neuronal population: (1) what kind of spatial sensitivity individual neurons exhibit and (2) whether neurons with similar response preferences are clustered within the brain. To address this question, we conducted a single-unit recording study in monkeys. We investigated the nature of spatial sensitivity in individual auditory cortical neurons to determine whether they have receptive fields (place code) or monotonic (rate code) sensitivity to sound azimuth. Second, we tested how strongly the population of neurons favors contralateral locations. We report here that the majority of neurons show predominantly monotonic azimuthal sensitivity, forming a rate code for sound azimuth, but that at the population level the degree of contralaterality is modest. This suggests that the weakness of the evidence for spatial sensitivity in human neuroimaging studies of auditory cortex may be attributable to limited lateralization at the population level, despite what may be considerable spatial sensitivity in individual neurons.