Hierarchical transformations in sound envelope encoding differ across cortical layers.

Amplitude-modulation (AM) is critical for the perception of complex sounds, and transformations in AM encoding may underlie primate-specific aspects of complex sound perception. The nonhuman primate (NHP) model provides an opportunity to understand what circuit mechanisms generate hierarchical and interhemispheric transformations of AM encoding in different circuits across the cortical hierarchy. To address this, here we report the encoding of AM signals as a function of cortical layer and hemisphere in primary area A1 and the tertiary parabelt (PB) area of five NHPs. We presented AM noise and click trains to awake NHPs while recording from linear array multielectrodes spanning cortical layers. A1 typically encoded all AM frequencies (1.6-200 Hz) with high (> 90%) classification accuracy, while the PB encoded lower (~1.6-25 Hz) frequencies. The laminar gradient observed in A1 (Granular > Infragranular > Supragranular) was inverted in PB (Supragranular > Infragranular > Granular), consistent with differences in thalamocortical input. Both areas displayed enhanced AM encoding in the left hemisphere, restricted to the supragranular layers. These results represent the first analysis of AM encoding in the PB, in which we identify circuits differing in their temporal sensitivity across the hierarchy, and suggest local supragranular neuronal populations contribute to hemispheric specialization.