Hierarchical effects of attention on amplitude modulation encoding in auditory cortex

Abstract

How attention influences single neuron responses in the auditory system remains unresolved. We found that when monkeys actively discriminated temporally amplitude modulated (AM) from unmodulated sounds, primary auditory (A1) and middle lateral belt (ML) cortical neurons better discriminated those sounds than when the monkeys were passively listening. This was true for both rate and temporal codes. Differences in AM responses and effects of attentional modulation on those responses suggest: (1) attention improves neurons’ ability to temporally follow modulation (2) non-synchronized responses play an important role in AM discrimination (3) ML attention-related increases in activity are stronger and longer-lasting for more difficult stimuli consistent with stimulus specific attention, whereas the results in A1 are more consistent with multiplicative nonlinearity, and (4) A1 and ML code AM differently; ML uses both increases and decreases in firing rate to encode modulation, while A1 primarily uses activity increases. These findings provide a crucial step to understanding both how the auditory system encodes temporal modulation and how attention impacts this code. Further, our findings support a model where rate and temporal coding work in parallel, permitting a multiplexed code for temporal modulation. [Work supported by NIDCD RO1 DC-02514.]