Fig. 4.

MUA recordings of VPM thalamo-cortical excitatory neurons reveal sustained thalamic burst activity at low frequency (1 Hz) and a transition to tonic activity at high frequencies (20 Hz and 40 Hz). (A) MUA measured during optogenetic stimulation at five frequencies. Twenty-one-second trace of VPM MUA from a representative animal during VPM optogenetic stimulation (1 Hz, 10% duty cycle; 5, 10, 20, and 40 Hz, 30% duty cycle; Top Left to Bottom Left). Corresponding 2-s VPM MUA trace magnified at the beginning (Bottom Left) of stimulation and illustration of evoked thalamic burst (multiple spikes) and tonic activity (single spike) (Bottom Right) for each stimulation frequency, respectively. Similar levels of MUA responses were evoked at all frequencies by the first stimulation pulse. Analysis of optogenetically evoked spikes revealed an onset latency of 3.3 ± 1.4 ms (mean ± SD), consistent with VPM thalamo-cortical neuron characteristics. Burst activity was sustained at 1 Hz, decreased at 5 Hz and 10 Hz, and transitioned to tonic activity at 20 Hz and 40 Hz. VPM spiking summary for two animals (Top Right to Bottom Right). Spike-rate analysis showed that spikes can be successfully generated at high frequencies (20 Hz and 40 Hz). (B) MUA measured using short-pulse stimulation (10 ms). Corresponding 2-s VPM MUA trace magnified at the beginning of stimulation for 1 Hz (Left), 5 Hz (Middle), and 10 Hz (Right). These findings demonstrate that similar spiking characteristics and occurrence of evoked burst activity in VPM are determined by interstimulus intervals, not by stimulus duration.