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.