| The inability of the auditory system to process temporal cues can lead to deficits in speech recognition that can ultimately lead to learning and/or behavioral disorders. Therefore it is important to understand the mechanisms by which the auditory system processes time-varying signals. Echolocating bats provide a useful model to study the neural bases for the perception of time-varying signals because of their dependence on such sounds. Previous work has shown that the time-domain representation of sound undergoes a transformation between the inferior colliculus (IC) and the auditory cortex (AC) of the echolocating bat, Myotis lucifugus. Specifically, the tonic discharge patterns present in the IC are transformed into phasic patterns in the AC. There is also a degradation in the capacity for AC neurons to entrain their discharges to the temporal waveform of incoming signals when compared to IC neurons. This study examined the role of the intervening structure, the medial geniculate body (MGB), in producing these IC-to-AC transformations. Electrophysiological recordings from neurons in the MGB revealed that the unit selectivities to stimulus frequency and amplitude were not significantly different from those observed in the IC or AC. However, more than 90% of the neurons in the MGB displayed phasic discharge patterns. In addition, in response to trains of unmodulated tone pulses, the cutoff frequency for time-locked discharges (64.0 +/- 46.9 pulses per second or pps) and mean number of spikes per pulse (19.2 +/- 12.2 pps), were intermediate to those for the IC and AC. In response to amplitude-modulated pulse trains, MGB units displayed a degree of response facilitation that was intermediate to that of the IC and AC (IC: 1.32 +/- 0.33, MGB: 1.75 +/- 0.26, AC: 2.52 +/- 0.96, p < 0.01). Computational models incorporating the circuitry of the MGB suggest that co-activation of MGB intrinsic inhibitory interneurons and afferent inhibitory neurons within the IC are sufficient to reproduce the transformations observed in the experimental data. These results suggest that a portion of the IC-to-AC transformations in signal representation occurs in the MGB, and that the intrinsic circuitry of the MGB can support these transformations. |
