Phase-locked responses to low frequency tones in the medial geniculate body

Over 2500 extracellular single unit recordings were obtained from the medial geniculate body (MGB) of cats anaesthetized with nitrous oxide. Three out of four units were activated by tone bursts, most of them presenting a transient 'on' response. Only about 10% of the units showed a sustained excitatory 'through' response. Some of these neurons, when activated by low-frequency tone bursts, had discharge synchronized with the phase of the tonal stimulus. Such phase-locked units were principally found in the pars lateralls, but also in the pars magnocellularis and pars ovoidea of the MGB. Significant phase-locking (vector strength R ⩾ 0.5) was observed up to 1000 Hz. About 20% of all units, responding in a sustained fashion to tone bursts below that frequency, were phase-locked. Monaural stimulation led to a shift of the mean phase angle compared to that measured when the tone bursts were delivered simultaneously to both ears. Implications of these experimental results on the synaptic jitter and the information processing through the auditory pathways are discussed.