Whole-Body Prepulse Inhibition Protocol to Test Sensorymotor Gating Mechanisms in Monkeys

<div><p>Prepulse inhibition (PPI) is the decrease of startle reflex amplitude when a slight stimulus is previously generated. This paradigm may provide valuable information about sensorimotor gating functionality. Here we aimed at determining the inhibited and uninhibited startle response of capuchin monkeys (<i>Sapajus spp</i>.), and to evaluate the role of the superior colliculus in PPI. Capuchin monkeys were tested in a whole-body protocol, to determine the best startle amplitude and interstimuli interval. Additionally we tested two subjects with bilateral superior colliculus damage in this protocol. Results show that 115 dB auditory pulse has induced the best startle response. In contrast to reports in other species, no habituation to the auditory stimuli was observed here in capuchins. Also, startle reflex inhibition was optimal after 120 msec interstimuli interval. Finally, there was a downward tendency of percentage inhibition in superior colliculus-lesioned monkeys. Our data provides the possibility of further studies with whole-body protocol in capuchin monkeys and reinforces the importance of the superior colliculus in PPI.</p></div

Monkeys startle response with and without prepulse stimuli.

<p>A - Mean relative startle responses, collapsed across interstimulus intervals, across 7 blocks of test trials. B - Mean relative startle responses in each test situation. * difference of pulse-alone response (p<0.05). C – Mean relative percent of startle inhibition provoked by each interval between prepulse and pulse stimuli. (n = 8).</p

Monkeys startle response amplitude.

<p>A – Mean relative startle responses, collapsed across stimulus intensities, across repeated blocks of test trials. B - Mean relative startle responses across repeated blocks. * basal activity vs. all acoustic intensities (90–120 dB); # no statistical difference. (n = 6).</p

Primate test chamber.

<p>Monkeys were positioned with the neck at the neck-hole in a standing position on the accelerometer platform.</p

Distributions of the 6 visual stimuli in a two-dimensional space resulting from multidimensional scaling using responses of the 78 neurons to these stimuli in epoch 1 (A), epoch 2 (B).

<p>In epochs 1 and 2 (A, B), the snakes in threat display were separated from the non-threat-displaying snakes.</p

Visual stimuli (A) and delayed nonmatching-to-sample (DMNS) task (B) used in the present study.

<p>(A) Six photos of two categories of the stimuli including snakes in threat display and non-threat-displaying postures. (B) Stimulus sequence in the DMNS task in which stimuli were sequentially presented with a delay.</p