1 Department of Cell Biology and Anatomy, University of
Arizona College of Medicine, Tucson, Arizona 85724, and
2 Department of Speech and Hearing Science, Arizona State
University, Tempe, Arizona 85287
Several functional maps have been described in primary auditory
cortex, including those related to frequency, tuning, latency, binaurality, and intensity. Many of these maps are arranged in a
discontinuous or patchy manner. Similarly, thalamocortical projections arising from the ventral division of the medial geniculate body to the
primary auditory cortex are also patchy. We used anterograde labeling
and electrophysiological methods to examine the relationship between
thalamocortical patches and auditory cortical maps. Biotinylated dextran-amine was deposited into physiologically characterized sites
in the ventral division of the medial geniculate body of New Zealand
white rabbits. Approximately 7 d later, the animal was
again anesthetized and the ipsilateral auditory cortex was mapped with
tungsten microelectrodes. Multi-unit physiological data were obtained
for the following characteristics: best frequency (BF), binaurality,
response type, latency, sharpness of tuning, and threshold.
Immunocytochemical methods were used to reveal the injection site in
the ventral division of the medial geniculate body as well as the
anterogradely labeled thalamocortical afferents in the auditory cortex.
In 86% of the cases (12 of 14), entry into a thalamocortical patch was
associated with a marked change in physiological responses. A
consistent BF and binaural class were usually observed within a patch.
The patches appear to innervate distinct functional regions coding
frequency and binaurality. A model is presented showing how patchy
thalamocortical projections participate in the formation of tonotopic
and binaural maps in primary auditory cortex.
Key words: medial geniculate body; audition; neocortex; frequency map; thalamus; patches