CONNECTIONS AMONG FUNCTIONAL AREAS IN THE MOUSTACHED BAT AUDITORY-CORTEX

Connections among functional areas in the mustached bat's auditory cor tex were examined by placing anatomical tracers in physiologically def ined locations. We identified at least two and probably three channels connecting the various areas. One channel is formed by interconnectio ns among areas containing neurons sensitive to frequency-modulated com ponents (FMs) of the pulse and echo. These neurons are tuned to echo d elay, a cue for target range, and thus define a ranging channel. An ad ditional one or two channels are formed by interconnections among area s that contain neurons sensitive to the constant frequency components (CFs) of echoes. These neurons are of two main types: either sensitive to CFs of both pulse and echo (CF/CF neurons) or sensitive to a pulse FM and echo CF (FM-CF neurons). There was only a weak. connection bet ween the largest area of each type, suggesting they lie in different c hannels. Connections among areas in the ranging channel and echo CF-se nsitive channel(s) were weak. Thus, the interconnections among functio nal areas in the mustached bat's auditory cortex define parallel chann els for processing different types of biosonar information. Most corti cocortical connections were patchy in a manner suggestive of a columna r organization. The average width of the patches was approximately 360 mu m. Based on the sizes of the functional areas, we estimate the aud itory cortex contains a total of approximately 150 columns. Individual areas contain from as many as approximately 20 to as few as 13 column s. Each area had abundant projections outside of the auditory cortex. Connections within the cortex included the frontal, anterior cingulate , retrosplenial and perirhinal cortices, and the claustrum. Subcortica l targets included the amygdyla, auditory thalamus, pens, pretectum, s uperior and inferior colliculi, and central gray. Projections within t he cortex were of modest strength compared with several of the subcort ical projections. Thus, the auditory areas themselves are the primary source of cortically processed biosonar information to the rest of the brain. (C) 1998 Wiley-Liss, Inc.