Neural encoding of amplitude modulation within the auditory midbrain of squirrel monkeys

The neuronal responses to amplitude modulated (AM) sounds were investigated in the auditory midbrain of the squirrel monkey. Sinusoidally modulated tones and noise served as acoustic stimuli. In order to describe the response properties of collicular neurons, Fast-Fourier-Transformation (FFT), a cross-correlation algorithm and spike-rate counts were applied to translate the neuronal reactions into modulation transfer functions. FFT and cross-correlation defined a measure for synchronicity of the neuronal discharges with the modulation cycles. All neurons (542) responded selectively to AM-sounds insofar as all displayed a best modulation frequency (BMF). Most of them furthermore had a band-pass-like modulation transfer function, whose center frequencies were mainly between 8 and 128 Hz. Transfer functions obtained by spike-rate showed less selectivity: a relatively great number of neurons did not change their spike rate as a function of modulation frequency. The results show that encoding of amplitude-modulated sounds occurs to a greater extent via phase locking of discharges than via changes in spike number. In the same way, changing modulation depth is processed: whereas spike rate on average remains constant between 100% and 0% modulation, there is a drastic reduction in synchronicity. No clear relationship was found between a unit's characteristic frequency and BMF; the same applied to BMF and recording place. The results furthermore show that amplitude modulations are encoded selectively in a band pass function in a non-human primate. The midbrain thereby occupies an intermediate position within the pathway from the periphery to the cortex. This form of temporal resolution probably underlies mechanisms caused by the increasing synaptic activity in the course of the pathway. This may indicate adaptation since those modulation frequencies embedded in this species' vocal repertoire fit quite well with the system's tuning properties for amplitude modulation.