Optical and FDG mapping of frequency-specific activity in auditory cortex

WITH optical recording of intrinsic signals (ORIS) and tone stimulation, we analysed frequency- and intensity-dependent spatio-temporal activity in primary auditory cortex Al of awake Mongolian gerbils. We found a two-dimensional and assymetric propagation of ORIS activity (1) from ventral to dorsal along isofrequency contours at all frequencies and (2) from the low-frequency end across the complete tonotopic gradient at low frequencies. High-frequency representations remained tonotopic. Tonotopic asymmetries were independent of stimulus intensity. By contrast, in fluoro-2-deoxyglucose images from the same animals metabolic activity was tonotopically confined at high and low frequencies. The assymetric tonotopic propagation of ORIS activity suggests subthreshold directional interaction mechanisms in the cortical network which may serve to relate low- to high-frequency components of complex sounds