Functional organization of the avian auditory cortex analogue. I. Topographic representation of isointensity...

Bandwidth of auditory units in the chick forebrain (field L/Hv complex) was measured with isointensity tone stimuli. Isointensity bandwidth is topographically represented within the four-layered tonotopically organized structure. It declines continuously from rostrodorsal to caudoventral along the longitudinal axis of two-dimensional best frequency planes (frequency band laminae). Layer-specific differences along the radial axis are also obvious. In the input layer of field L and in Hv ON-response bandwidths are relatively broad. The narrower bandwidths of units in the two postsynaptic layers of field L are probably caused by lateral inhibition mechanisms, as derived from the different topographic representations of OFF-versus ON-response bandwidths. A quantitative comparison of the topographic representation of bandwidth is made with the geometry of the tonotopic organization of the chick auditory forebrain complex, as revealed by 2-deoxyglucose data in a former study. A number of possible input-output transformations are derived from this comparison.