Response properties of neurons in the central nucleus and external send dorsal cortices of the inferior colliculus in guinea pig

The inferior colliculus (IC) represents a midbrain structure which integrat es information from many ascending auditory pathways, descending corticotec tal projections and intercollicular pathways. The processing of information is different in each of the three main subdivisions of the IC - the centra l nucleus (CNIC), the dorsal cortex (DCIC) and the external cortex (ECIC) w hich may be distinguished morphologically as well as by different inputs an d outputs. To assess the differences in information processing we compared the response properties of single neurons in individual subnuclei of the IC in anesthetized guinea pigs. In comparison with DCIC and ECIC neurons, the CNIC neurons as a group were characterized by a sharper frequency tuning l as expressed by Q(10) values), a lower average threshold, a shorter average first-spike latency of response to tones at the characteristic frequency ( CF). a higher occurrence of non-monotonic rate/level functions and a higher rate of spontaneous activity. CNIC neurons and DCIC neurons reacted to ton es at the CF more frequently by a sustained type of response than did ECIC neurons. The difference between the parameters of DCIC neuronal activity an d ECIC neuronal activity was found to be smaller. The frequency tuning (exp ressed in Q(10) values), spontaneous activity and dominance of monotonic ra te/level functions were very similar in both structures; ECIC neurons expre ssed a higher average threshold and a shorter average first-spike latency t han did DCIC neurons. Responsiveness expressed as the average maximal firin g rate to tones at the CF was significantly higher in the CNIC than in the ECIC. The results give additional support to the idea that the CNIC is a pa rt of a fast, frequency-tuned, low threshold and intensity-sensitive ascend ing pathway, whereas the other two subdivisions are involved in additional processing of information that involves feedback loops and polysensory path ways.