Abstract
Spatial hearing aids a listener in disentangling multiple competing sound sequences. We find that separation of around 10° between target and masker sound sources permits humans and cats to hear interleaved sound sequences as segregated streams, thus enabling a “rhythmic masking release” task requiring recognition of target rhythms. In cats and rats, neurons in primary auditory cortex (A1) exhibit spatial stream segregation in that they synchronize selectively to one of two interleaved sequences of noise burst originating from spatially separated sources. Cortical spatial selectivity is markedly sharper under competing-sound conditions compared to that observed with single sound sources. Cortical responses are predicted well by a model that incorporates moderate spatial selectivity inherited from the brainstem sharpened by forward suppression at the level of thalamocortical synapses. Consistent with that model, spatial stream segregation in rats is stronger in cortical area A1 than in the ventral division of the medial geniculate body, its principal source of thalamic input. In cats, psychophysical performance was better for high-frequency sounds, and cortical stream segregation was stronger for neurons having high characteristic frequencies. In contrast, human psychophysics was better for low-frequency sounds, suggesting that the larger heads of humans provide them with greater interaural time differences.
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