Summation between acoustic and trigeminal stimuli evoking startle.

Electrical stimulation of the spinal trigeminal pathway evokes a short-latency startle-like response in rats. To explore the relationship between acoustic and tactile systems mediating startle, we studied temporal summation between pairs of startle-evoking stimuli in awake rats by varying the interstimulus interval. The stimuli were: (i) two noise bursts; (ii) two unilateral electrical stimuli near the principal nucleus of the trigeminal nerve; (iii) electrical stimulation of the left and right trigeminal nucleus; or (iv) a noise burst and unilateral stimulation of the trigeminal nucleus. Following two noise bursts, the amplitude of startle increased as the interval increased from 0 to 4 ms, then declined smoothly as the interval increased to 15 ms. Unilateral stimulation of the trigeminal nucleus resulted in a sharper summation effect, with maximal summation at 2 ms, and refractory periods estimated at 0.4-0.8 ms. Bilateral stimulation of the trigeminal nucleus resulted in broader summation without a refractory period, and maximal summation when the stimuli on both sides of the trigeminal nucleus were presented simultaneously. The combination of acoustic and trigeminal stimulation was most effective in enhancing startle amplitudes, and summation peaked when the noise burst preceded the trigeminal stimulation by 5 ms. Similarly, electromyogram latencies measured in the hindlimb were 3-4 ms shorter for trigeminal stimulation than for the noise burst. Startle appears to be optimally activated by simultaneous acoustic and tactile stimuli, as occurs during head blows.