Frequency and space representation in the inferior colliculus of the FM bat, Eptesicus fuscus

Abstract

The tonotopic organization and spatial sensitivity of 217 inferior collicular (IC) neurons of Eptesicus fuscus were studied under free field stimulation conditions. Acoustic stimuli were delivered from a loudspeaker placed 21 cm ahead of the bat to determine the best frequency (BF) and minimum threshold (MT) of isolated IC neurons. A BF stimulus was then delivered as the loudspeaker was moved horizontally across the frontal auditory space of the bat to locate the best azimuthal angle (BAZ) at which the neuron had its lowest MT. The stimulus was then raised 3 dB above the lowest MT to determine the horizontal extent of the auditory space within which a sound could elicit responses from the neurons. This was done by moving the loudspeaker laterally at every 5 degrees or 10 degrees until the neuron failed to respond. These measurements also allowed us to redetermine the BAZ at which the neuron fired maximal number of impulses. Electrodes were placed evenly across the whole IC surface and IC neurons were sampled as many as possible within each electrode penetration. Tonotopic organization and spatial sensitivity were examined among all 217 IC neurons as a whole as well as among IC neurons sequentially sampled within individual electrode penetrations. The whole population of 217 IC neurons is organized tonotopically along the dorsoventral axis of the IC. Thus, low frequency neurons are mostly located dorsally and high frequency neurons ventrally with median frequency neurons intervening in between. The BAZ of these 217 IC neurons tend to shift from lateral to medial portions of the contralateral frontal auditory space with increasing BF.(ABSTRACT TRUNCATED AT 250 WORDS)

GABAergic disinhibition changes the recovery cycle of bat inferior collicular neurons

“…High BF neurons tended to have high MTs and short response latencies. These findings are similar to those obtained from previous studies on the same species of bat (Casseday and Covey 1992;Jen and Schlegel 1982;Jen et al 1987;Pinheiro et al 1991;Poon et al 1990;Wu and Jen 1991).…”

Encoding repetition rate and duration in the inferior colliculus of the big brown bat, Eptesicus fuscus

“…The BFs of these neurons ranged from 12.1 to 87.6 kHz; 202 neurons (47.5%) had BFs between 20 and 30 kHz which corresponds to the range of the 'pseudo-constant' frequency component in Eptesicus fuscus (Obrist 1989). As in previous studies (Jen and Schlegel 1982;Jen et al 1987;Poon et al 1990;Wu and Jen 1991), these neurons were tonotopically organized along the dorsoventral axis of the IC according to their BFs and recording depths (between 40 and 2370 ~tm). Thus, BFs of sequentially isolated neurons changed from low to high with increasing recording depths (Fig.…”

Involvement of Monkey Inferior Colliculus in Spatial Hearing

“…5, 6a), but, unlike in bats (Poon et al, 1990), we obtained no evidence for a spatial topography (Fig. 7).…”

“…2). This tonotopic order, together with the short response latencies, indicates that most recordings were taken from the central nucleus (Ryan and Miller, 1978;Poon et al 1990). The number of sensitive neurons (n ϭ 8) does not exceed chance.…”