Processing of acoustic motion in the auditory cortex of the rufous horseshoe bat, Rhinolophus rouxi

This study investigated the representation of acoustic motion in different fields of auditory cortex of the rufous horseshoe bat, Rhinolophus rouxi. Motion in horizontal direction (azimuth) was simulated using successive stimuli with dynamically changing interaural intensity differences presented via earphones. The mechanisms underlying a specific sensitivity of neurons to the direction of motion were investigated using microiontophoretic application of γ-aminobutyric acid (GABA) and the GABAA receptor antagonist bicuculline methiodide (BMI). In the first part of the study, responses of a total of 152 neurons were recorded. Seventy-one percent of sampled neurons were motion-direction sensitive. Two types of responses could be distinguished. Thirty-four percent of neurons showed a directional preference exhibiting stronger responses to one direction of motion. Fifty-seven percent of neurons responded with a shift of spatial receptive field position depending on the direction of motion. Both effects could occur in the same neuron depending on the parameters of apparent motion. Most neurons with contralateral receptive fields exhibited directional preference only with motion entering the receptive field from the opposite direction (i.e. the ipsilateral part of the azimuth). Receptive field shifts were opposite to the direction of motion. Specific combinations of spatio-temporal parameters determined the motion-direction-sensitive responses. Velocity was not encoded as a specific parameter. Temporal parameters of motion and azimuthal position of the moving sound source were differentially encoded by neurons in different fields of auditory cortex. Neurons with a directional preference in the dorsal fields can encode motion with short interpulse intervals, whereas direction preferring neurons in the primary field can best encode motion with medium interpulse intervals. Furthermore, neurons with a directional preference in the dorsal fields are specialized for encoding motion in the midfield of azimuth, whereas direction preferring neurons in the primary field can encode motion in lateral positions. In the second part of the study, responses were recorded from additional 69 neurons. Microiontophoretic application of BMI influenced the motion-direction sensitivity of 53 % of neurons. In 21 % of neurons the motion-direction sensitivity was decreased by BMI by decreasing either directional preference or receptive field shift. In neurons with a directional preference, BMI increased the spike number for the preferred direction in about the same amount as for the non-preferred direction. Thus, inhibition was not direction specific. In contrast, BMI increased motion-direction sensitivity by either increasing directional preference or magnitude of receptive field shifts in 22 % of neurons. An additional 10 % of neurons changed their response from a receptive field shift to a directional preference under BMI. In these 32 % of neurons, the observed effects could often be better explained by adaptation of excitation than by inhibition. The results suggest, that motion information is differentially processed in different fields of the auditory cortex of the rufous horseshoe bat. Thus, functionally organized pathways for the processing of different parameters of auditory motion seem to exist. The fact that cortex specific GABAergic inhibition contributes to motion-direction sensitivity in at least a part of cortical neurons is supportive for the notion that the auditory cortex plays an important role in further processing the neural responses to apparent motion brought up from lower levels of the auditory pathway.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Ludwig-Maximilians-Universität

Year

2001

Author dc:creator

• Firzlaff, Uwe

Contributors dc:contributor

• Schuller, Gerd

Repository record source_url

https://edoc.ub.uni-muenchen.de/35/

OAI identifier oai:identifier

oai:edoc.ub.uni-muenchen.de:35