Spatial interaction between spectral integration and frequency gradient in primary auditory cortex.

Primary sensory cortical areas are characterized by orderly and largely independent representations of several receptive field properties. This is expressed in multiple, spatially overlaying parameter distributions, such as orientation preference, spatial frequency, and ocular dominance maps in the primary visual cortex. In the auditory cortex, two main and presumably independent representational parameters are the center frequency and the frequency extent of spectral tuning curves. Here we demonstrate interactions between cortical tonotopic gradient and spectral bandwidth modules in cat primary auditory cortex (AI). First, the spatial representation of spectral integration is not equally expressed across the whole frequency range in AI. Narrow-bandwidth modules are found only in the mid-frequency region (5-20 kHz). Thus spectral integration properties delineate three frequency regions (<5, 5-20, and >20 kHz) in cat AI. Second, the extent of spectral integration covaries with the local tonotopic gradient in the low- and mid-frequency ranges. Regions with a shallow frequency gradient tend to have narrower spectral integration than those with a steep gradient. These relationships between spectral selectivity and frequency gradient constrain forebrain models of thalamo- and corticocortical convergence and connectivity and may reflect the processing of behaviorally relevant stimulus constellations.