We found that excitatory response areas (ERAs) of sharply tuned neurons (below 1.0 in the square root transformation) with transient early on responses (latency 7-20 ms) were flanked by inhibitory regions at one or both frequency margins, which often partially overlapped the ERA. The extent and significance of these inhibitory regions depended on the sharpness of a neuron´s frequency tuning and could span from ¼ to several octaves. In most cases, a complete suppression of the neuronal response to tones from the ERA and an elevation of threshold of about 20-30 dB was observed with adequate intensities of the second tone. Additionally, the discharge pattern could change, in that late-, off-, or late-off-responses appeared when presenting second tones. These characteristics of two-tone inhibition were found regularly in sharply tuned neurons of the pars lateralis (LV) and the pars ovoidea (OV) of the MGB´s ventral division, whereas this was less often seen in sharply tuned neurons of the medial division (MGM). Interestingly, lateral inhibitory sidebands were generally not observed in broadly tuned neurons. Therefore, we conclude that the arrangement of response areas of narrowly tuned neurons in excitatory and inhibitory regions serves as a major mechanism for frequency sharpening in the MGB. This arrangement corresponds to the organization of neuronal receptive fields in other sensory systems, e.g. the visual and somatosensory systems. Our findings further support the view that lateral inhibitory processes constitute basic mechanisms for stimulus discrimination and contrast enhancement in various sensory systems.

Published: Eur J Neurosci Suppl. 5, p. 145