Sound frequency representation in primary auditory cortex 6 is level - tolerant for moderately loud , complex sounds 7 8 9 10

40 The distribution of neuronal characteristic frequencies over the area of primary 41 auditory cortex (AI) roughly reflects the tonotopic organization of the cochlea. 42 However, because the area of AI activated by any given sound frequency in43 creases erratically with sound level, it has generally been proposed that fre44 quency is represented in AI not with a rate-place code, but with some more com45 plex, distributed code. Here, on the basis of both spike and local field potential 46 (LFP) recordings in the anesthetized cat, we show that the tonotopic representa47 tion in AI is much more level-tolerant when mapped with spectrotemporally dense 48 tone pip ensembles rather than with individually presented tone pips. That is, we 49 show that the tuning properties of individual unit and LFP responses are less 50 variable with sound level under dense compared to sparse stimulation, and that 51 the spatial frequency resolution achieved by the AI neural population at moderate 52 stimulus levels (65 dB SPL) is better with denselythan sparsely-presented 53 sounds. This implies that nonlinear processing in the central auditory system can 54 compensate (in part) for the level-dependent coding of sound frequency in the 55 cochlea, and suggests that there may be a functional role for the cortical 56 tonotopic map in the representation of complex sounds. 57