Response properties of single units in the dorsal nucleus of the lateral lemniscus and paralemniscal zone of an echolocating bat.

1. Connectional evidence suggests that the dorsal nucleus of the lateral lemniscus (DNLL) and the paralemniscal zone (PL) function as centers for binaural analysis interposed between the superior olivary complex and the midbrain. In addition, the DNLL is known to be a major source of inhibitory input to the midbrain. The aim of this study was to characterize the response properties of neurons in DNLL and PL of the echolocating bat Eptesicus fuscus, a species that utilizes high-frequency hearing and that might be expected to have a large proportion of neurons responsive to interaural differences in sound level. 2. Auditory stimuli were presented monaurally or binaurally to awake animals, and responses of single units were recorded extra-cellularly with the use of glass micropipettes. 3. Below the ventrolateral border of the inferior colliculus is a region that contains large gamma-aminobutyric acid-positive neurons. On the basis of its immunohistochemical reactivity, this entire region could be considered as DNLL. However, within the area, there was an uneven distribution of binaural responses. Caudally, binaural neurons made up 84% (41/49) of those tested, but rostrally only 29% (6/21). For this reason the rostral area is considered as a separate functional subdivision and referred to as the dorsal paralemniscal zone (DPL). PL is located ventral to DPL and medial to the intermediate and ventral nuclei of the lateral lemniscus; in PL 88% (14/16) of neurons were binaural. 4. Most neurons responded only to a contralateral stimulus when sounds were presented monaurally. Out of 49 neurons in DNLL, 42 responded only to a contralateral sound, 1 responded only to an ipsilateral sound, and 6 responded to sound at either ear. In the DPL, all of the 21 neurons tested responded to a contralateral sound and none to an ipsilateral sound. Out of 16 neurons in the PL, 11 responded only to a contralateral sound, 1 responded only to an ipsilateral sound, and 4 responded to sound at either ear. 5. When sounds were presented at both ears simultaneously, several different patterns of binaural interaction occurred. The most common pattern was suppression of the response to sound at one ear by sound at the other ear. In DNLL, 57% (28/49) of neurons showed this type of binaural interaction. Another 10% (5/49) showed facilitation at some interaural level differences and suppression at others, and another 10% (5/49) showed facilitation at some interaural level differences but no suppression.(ABSTRACT TRUNCATED AT 400 WORDS)