Transformation of Temporal Processing Across Auditory Cortex of Awake Macaques

The anatomy and connectivity of the primate auditory cortex has been modeled as a core region receiving direct thalamic input surrounded by a belt of secondary fields. The core contains multiple tonotopic fields (including the primary auditory cortex, AI, and the rostral field, R), but available data only partially address the degree to which those fields are functionally distinct. This report, based on single-unit recordings across four hemispheres in awake macaques, argues that the functional organization of auditory cortex is best understood in terms of temporal processing. Frequency tuning, response threshold, and strength of activation are similar between AI and R, validating their inclusion as a unified core, but the temporal properties of the fields clearly differ. Onset latencies to pure tones are longer in R (median, 33 ms) than in AI (20 ms); moreover, synchronization of spike discharges to dynamic modulations of stimulus amplitude and frequency, similar to those present in macaque and human vocalizations, suggest distinctly different windows of temporal integration in AI (20-30 ms) and R (100 ms). Incorporating data from the adjacent auditory belt reveals that the divergence of temporal properties within the core is in some cases greater than the temporal differences between core and belt.