Edward J. Schicatano

Dry Eye, Blinking, and Blepharospasm

this prediction by comparing the SO-evoked blinks of individuals with dry eye with those of age-matched controls. Given the frequent occurrence of dry eye at the onset of benign essential blepharospasm (BEB), 7 blink modifications associated with dry eye may play a role in the origin of BEB. We present a hypothesis that links the adaptive processes initiated by dry eye with the origin of BEB.

To blink or not to blink: inhibition and facilitation of reflex blinks

A reflex blink typically inhibits subsequent blinks. In this study, we investigated whether the nature and time course of this inhibition vary when different combinations of blink-evoking stimuli are used. We used the paired stimulus paradigm, in which two blink-evoking stimuli — a conditioning stimulus followed by a test stimulus — are presented with a variety of interstimulus intervals, to examine the interactions between blinks evoked by trigeminal and acoustic stimuli in rats and humans. In addition, we studied the effect of a blink-evoking trigeminal stimulus on subsequent gaze-evoked blinks in humans. The results revealed that long-lasting inhibition occurred when the conditioning and test stimuli were within the same modality. A shorter period of inhibition followed by facilitation occurred when the stimuli were in different modalities. The data demonstrate that a blink-evoking stimulus initiates a lengthy period of inhibition in its own sensory pathway and a shorter period of inhibition in the reticular formation and/or in blink motoneurons. In addition, the results show that the blink-evoking stimulus also initiates a facilitatory process. Thus, the magnitude of a blink reflects a balance between inhibitory and facilitatory processes.

PREPULSE EFFECTS ON MAGNITUDE ESTIMATION OF STARTLE-ELICITING STIMULI AND STARTLE RESPONSES

The present studies investigated the relationship between prepulse effects on the modification of the brainstem startle reflex and magnitude estimates of startle-eliciting stimuli. In Experiment 1, startle eyeblink responses were elicited in 24 students, half of whom were instructed to estimate the loudness of the startle stimulus (actual intensities of 80, 90, and 100 dB) and half of whom were instructed to estimate the magnitude of their eyeblink. When weak acoustic prepulses preceded the startle-eliciting stimulus, eyeblink amplitude was inhibited, and estimates of response magnitude decreased, but estimates of startle stimulus magnitude decreased only when 100-dB startle stimuli were presented. In Experiment 2, the same startle stimuli were preceded on some trials by a vibrotactile prepulse to the hand. In conditions in which startle amplitude was inhibited, startle stimulus magnitude estimates were not affected. This suggests that the effect of acoustic prepulses on 100-dB startle stimuli in Experiment 1 may have been due to loudness assimilation, an effect independent of the prepulse inhibition of startle responding.

The effects of caffeine and directed attention on acoustic startle habituation.

The present experiment tested the effects of caffeine on acoustic startle habituation during different attention tasks in which subjects either (a) attended to the acoustic startle stimulus (auditory attention; n = 9) (b) attended to a visual search task during presentation of acoustic startle stimuli (visual attention; n = 10), or (c) were given no specific instructions during acoustic startle testing (no attention; n = 9). Startle eyeblink responses were measured after subjects received either caffeine (1 mg/kg) or placebo. Caffeine significantly delayed response habituation in the no attention group and in the auditory attention group, but had no effect on habituation in the visual attention group. These data show that startle habituation can occur with minimal attention being directed to the acoustic startle stimulus, and that visual attention cancels the effects of caffeine on startle habituation.

The effects of different doses of caffeine on habituation of the human acoustic startle reflex

Research in this laboratory showed that caffeine (4 mg/kg) delays habituation of the acoustic startle reflex in humans. The present study examined the effects of 2- and 6-mg/kg doses of caffeine on acoustic startle habituation in moderate-high and low caffeine users. Eyeblink responses to 30 trials of 85-dB noise stimuli were measured beginning 30 min after oral ingestion of either placebo or 2 or 6 mg/kg of caffeine. The 2-mg/kg dose of caffeine delayed startle habituation in both moderate-high and low caffeine users. The 6-mg/kg dose produced no differential effects on startle responding from placebo. In moderate-high users, following habituation, startle responding was smaller in the placebo condition compared to both caffeine conditions. In low users there were no differences in posthabituation responding between doses, suggesting that this dose effect is dependent on a history of chronic caffeine usage.

Caffeine delays habituation of the human acoustic startle reflex

The acoustic startle reflex has proved to be an excellent system for studying habituation in mammals. In animal studies, startle habituation has been found to be sensitive to various pharmacological manipulations. The present experiments were designed to determine whether caffeine (4 mg/kg) modified startle habituation in low and high caffeine users. Human eyeblink responses were measured in a startle habituation paradigm in which 30 trials of 85-dB broadband noise stimuli with a duration of 50 msec and a rise time of .1 msec were presented. Caffeine delayed the habituation of startle amplitude in both low and high users and produced significant dishabituation in low users. These findings indicate that caffeine disrupts early sensory filtering.

Effects of caffeine on the trigeminal blink reflex.

The acoustic startle and trigeminal blink reflexes share the same motor output. Since caffeine has been shown to augment the startle reflex, it was proposed that caffeine would also increase the trigeminal blink reflex. In 6 humans, the effects of caffeine (100 mg) on the trigeminal blink reflex were investigated. Reflex blinks were elicited by stimulation of the supraorbital branch of the trigeminal nerve. Following ingestion of caffeinated coffee, reflex blinks increased in amplitude and duration and occurred at a shorter latency than reflex blinks following ingestion of decaffeinated coffee. Since the blink reflex is a brainstem reflex, these results suggest that the psychomotor effects of caffeine facilitate brainstem processing.

The Effects of Attention on the Trigeminal Blink Reflex

During top-down processing, higher cognitive processes modulate lower sensory processing. The present experiment tested the effects of directed attention on trigeminal reflex blinks in humans (n = 8). In separate sessions, participants either attended to blink-eliciting stimuli or were given no attentional instructions during stimulation of the supraorbital branch of the trigeminal nerve. Attention to blink-eliciting stimuli significantly increased reflex blink amplitude and duration and shortened blink latency compared with the no attention condition. These results suggested that higher processes such as attention can modify the trigeminal blink reflex circuit.