Processing of acoustic motion in the auditory cortex of the rufous horseshoe bat, Rhinolophus rouxi
b'This study investigated the representation of acoustic motion in different fields of auditory\\ncortex of the rufous horseshoe bat, Rhinolophus rouxi. Motion in horizontal direction\\n(azimuth) was simulated using successive stimuli with dynamically changing interaural\\nintensity differences presented via earphones. The mechanisms underlying a specific\\nsensitivity of neurons to the direction of motion were investigated using microiontophoretic\\napplication of \\u03b3-aminobutyric acid (GABA) and the GABAA receptor antagonist bicuculline\\nmethiodide (BMI).\\nIn the first part of the study, responses of a total of 152 neurons were recorded. Seventy-one\\npercent of sampled neurons were motion-direction sensitive. Two types of responses could be\\ndistinguished. Thirty-four percent of neurons showed a directional preference exhibiting\\nstronger responses to one direction of motion. Fifty-seven percent of neurons responded with\\na shift of spatial receptive field position depending on the direction of motion. Both effects\\ncould occur in the same neuron depending on the parameters of apparent motion. Most\\nneurons with contralateral receptive fields exhibited directional preference only with motion\\nentering the receptive field from the opposite direction (i.e. the ipsilateral part of the azimuth).\\nReceptive field shifts were opposite to the direction of motion. Specific combinations of\\nspatio-temporal parameters determined the motion-direction-sensitive responses. Velocity\\nwas not encoded as a specific parameter.\\nTemporal parameters of motion and azimuthal position of the moving sound source were\\ndifferentially encoded by neurons in different fields of auditory cortex. Neurons with a\\ndirectional preference in the dorsal fields can encode motion with short interpulse intervals,\\nwhereas direction preferring neurons in the primary field can best encode motion with\\nmedium interpulse intervals. Furthermore, neurons with a directional preference in the dorsal\\nfields are specialized for encoding motion in the midfield of azimuth, whereas direction\\npreferring neurons in the primary field can encode motion in lateral positions.\\nIn the second part of the study, responses were recorded from additional 69 neurons.\\nMicroiontophoretic application of BMI influenced the motion-direction sensitivity of 53 % of\\nneurons. In 21 % of neurons the motion-direction sensitivity was decreased by BMI by\\ndecreasing either directional preference or receptive field shift. In neurons with a directional\\npreference, BMI increased the spike number for the preferred direction in about the same\\namount as for the non-preferred direction. Thus, inhibition was not direction specific. In contrast, BMI increased motion-direction sensitivity by either increasing directional\\npreference or magnitude of receptive field shifts in 22 % of neurons. An additional 10 % of\\nneurons changed their response from a receptive field shift to a directional preference under\\nBMI. In these 32 % of neurons, the observed effects could often be better explained by\\nadaptation of excitation than by inhibition.\\nThe results suggest, that motion information is differentially processed in different fields of\\nthe auditory cortex of the rufous horseshoe bat. Thus, functionally organized pathways for the\\nprocessing of different parameters of auditory motion seem to exist. The fact that cortex\\nspecific GABAergic inhibition contributes to motion-direction sensitivity in at least a part of\\ncortical neurons is supportive for the notion that the auditory cortex plays an important role in\\nfurther processing the neural responses to apparent motion brought up from lower levels of\\nthe auditory pathway.'