Anatomical and physiological evidence for a relationship between the 'cingular' vocalization area and the auditory cortex in the squirrel monkey

With the aid of the autoradiographic tracing technique the projections from cortical limbic vocalization areas to the auditory cortex in the superior temporal gyrus were studied in the squirrel monkey. The vocalization areas were identified by exploring the anterior limbic cortex with moving electrodes until a site was found where electrical stimulation yielded vocalization. Projections from the region around the cingulate sulcus and supracallosal anterior cingulate gyrus have their terminal fields in the lower part of the superior temporal gyrus (STG) and upper bank of the superior temporal sulcus. Injections just in front of the genu of the corpus callosum and in the subcallosal gyrus and gyrus rectus lead to terminal fields in the middle part of STG. No projections were found in the upper part of STG, i.e. the primary auditory cortex. To test the functional properties of this pathway, action potentials of single neurons in the auditory cortex were recorded during electrical stimulation of the cingular vocalization area. From a total of 135 STG neurons, an effect on spontaneous activity was seen in 27 cells. All except one of these neurons also reacted to acoustic stimuli. In most cases, stimulation of the cingular area caused a decrease in the discharge rate of the STG neurons. In 4 neurons, stimulation of the vocalization area had an influence on the acoustic reactivity of the STG neurons. The results provide evidence that during phonation the 'cingular' vocalization area exerts a predominantly inhibitory influence on auditory cortex neurons. This effect probably is mediated via the extreme capsule. Its possible function is discussed.