Connections of cat auditory cortex: III. Corticocortical system

Abstract

The mammalian auditory cortex (AC) is essential for computing the source and decoding the information contained in sound. Knowledge of AC corticocortical connections is modest other than in the primary auditory regions, nor is there an anatomical framework in the cat for understanding the patterns of connections among the many auditory areas. To address this issue, we investigated cat AC connectivity in thirteen auditory regions. Retrograde tracers were injected in the same area or in different areas to reveal the areal and laminar sources of convergent input to each region. Architectonic borders were established in Nissl and SMI-32 immunostained material. We assessed the topography, convergence, and divergence of the labeling. Intrinsic input constituted >50% of the projection cells in each area, and extrinsic inputs were strongest from functionally related areas. Each area received significant convergent ipsilateral input from several fields (5 to 8; mean 6). These varied in their laminar origin and projection density. Major extrinsic projections were preferentially from areas of the same functional type (tonotopic to tonotopic, non-tonotopic to non-tonotopic, limbic-related to limbic-related, multisensory-to-multisensory), while smaller projections link areas belonging to different groups. Branched projections between areas were <2% with deposits of two tracers in an area or in different areas. All extrinsic projections to each area were highly and equally topographic and clustered. Intrinsic input arose from all layers except layer I, and extrinsic input had unique, area-specific infragranular and supragranular origins. The many areal and laminar sources of input may contribute to the complexity of physiological responses in AC and suggest that many projections of modest size converge within each area rather than a simpler area-to-area serial or hierarchical pattern of corticocortical connectivity.

Connections of cat auditory cortex: II. Commissural system

“…11C). Comparably sparse branching is also found in the auditory thalamic (Lee and Winer, 2008a) and corticocortical (Lee and Winer, 2008b) pathways. Similar numerical values in physiologically-mapped cases intended to maximize double-labeling (Lee et al, 2004a) pertain for the sparse intra-and interareal divergence, and argue against false negatives as an explanation for the low values in mismatched injections.…”

“…This is unexpected because of the apparent commissural specificity in other studies (Rouiller et al, 1991;Imig and Reale, 1980;reviewed in Winer, 1992). Heterotopic projections preferentially link areas with similar thalamic (Lee and Winer, 2008a) and corticocortical (Lee and Winer, 2008b) affiliations, e.g., in non-primary perivisual areas such as ED and in limbicrelated regions such as area In (Figs. 4, 9).…”

“…Common sources of commissural input would not be predicted since In has predominantly limbic thalamic and amygdaloid relations (Clascá et al, 1997), whereas ED has mixed auditory and perivisual connections and no known limbic thalamic or cortical relation (Bowman and Olson, 1988a). Heterotopic convergence from these sources could explain the apparent physiological activation of an area by a broad array of commissural input , which can enable commissural integration of information across many cortical areas (Lee and Winer, 2008b).…”

Cortico‐cortical connectivity within ferret auditory cortex

“…Projections from the PAF and VPAF are largely similar, connecting strongly with other tonotopic fields. One distinguishing feature is that the VPAF has stronger connections with nontonotopic multisensory areas on the posterior ectosylvian gyrus (Lee and Winer, 2008b). …”

Reciprocal Modulatory Influences between Tonotopic and Nontonotopic Cortical Fields in the Cat

“…Based on neuroanatomical studies, we hypothesized that the weak projections from A1 to A2 (Lee and Winer, 2008) would result in small changes in A2 neuronal response properties during A1 deactivation. Conversely, the larger number of projections from A2 to A1 would substantially influence the response properties of A1 neurons during A2 deactivation (Lee and Winer, 2008). However, the results did not support our hypothesis and demonstrated a comparable reduction in response strength in both fields during epochs of deactivation.…”