Temporal integration of acoustic and cutaneous stimuli shown in the blink reflex

Temporal integration of pairs of brief blink-eliciting acoustic and cutaneous stimuli was investigated to determine if there was integration of stimuli from different modalities. Reflexes elicited by a tone burst or by a brief electrical shock to the supraorbital nerve followed by a second tone burst or shock at short stimulus onset asynchronies (SOAs) were larger and faster than control reflexes elicited by a single stimulus identical to the lead stimulus of the stimulus pairs. Reflex amplitude was augmented at longer SOAs where there was no effect on latency. Temporal integration was evident for all stimulus pairs, showing that it is due, at least in part, to processes that occur outside specific sensory pathways. Heterogeneous stimulus pairs produced greater reflex enhancement than did homogeneous stimulus pairs. This finding was examined further in Experiment 2, which showed that reflex enhancement with pairs of acoustic pulses was unaffected by the frequency of the second stimulus, suggesting that sensory masking was not acting to suppress reflex expression with acoustic pulse pairs. Integration of reflexogenic acoustic stimuli shown in the blink reflex is restricted to shorter intervals than is integration of acoustic stimuli shown by psychophysical procedures, suggesting that the two methods reflect different aspects of stimulus processing. Integration of reflexogenic stimuli may result from summation of activity associated more directly with reflex expression than with perceptual awareness.     

Modulation of the acoustic startle reflex in humans in the absence of anticipatory changes in the middle ear reflex

If a weak tone precedes an intense tone, then the acoustic startle eyeblink reflex elicited by the stronger stimulus is inhibited. It has been suggested that the leading stimulus gives rise to a protective middle ear reflex that attenuates the effective intensity of the second. This hypothesis was tested and disproved. In seven subjects intense tone bursts sufficient to elicit both intratympanic and eyeblink responses were presented sometimes alone and sometimes preceded at various lead times (25 to 400 msec) by a weak tone. The weak tone inhibited the amplitude of the eye blink to the strong tone, maximally at intervals of 100 to 200 msec, but was never seen to produce any of the anticipatory impedance changes that would be characteristic of middle ear reflex activity during the interval between the two stimuli.     

Spectral frequency and the modulation of the acoustic startle reflex by background noise

The rat's (Long-Evans) acoustic startle reflex to a high-frequency tone burst (10.5 kHz) was depressed by intense high-frequency band-pass noise (8-16 kHz) but enhanced by low frequency noise (1-2 kHz). However, contrary to the hypothesis that the depression of startle in intense background noise is produced by sensory masking, the reflex to a low-frequency tone burst (at 1 kHz) was depressed by both high- and low-frequency band-pass noise. Two additional hypotheses are offered to supplement sensory masking in order to explain the asymmetry in these data. The first is that the intratympanic reflex, which acts as a high pass filter on acoustic input, is elicited in intense backgrounds. The second is that acoustic startle reflexes elicited by intense low-frequency tones are in part elicited by their high-frequency distortion products and that these distortion products are then masked by high-frequency background noise.     

The role of small changes in the acoustic environment in modifying the startle reflex.

When either the intensity or frequency spectrum of an approximately 70-db. SPL narrow-band noise was abruptly changed by a small amount, the rat's response to a startle stimulus presented 64 msec later was inhibited. When similar small frequency changes preceded the startle stimulus by ony 5 msec, the latency of the startle response was reduced, but even relatively large changes in intensity of the antecedent stimulus had no effect on response latency. These findings provide added support for the generalization that the neural processes associated with startle are engaged by small changes in the auditory environment. They also point to a measure of separation between the processes responsible for inhibition and those responsible for latency shift.     

Potentiation of the acoustic startle response by a conditioned stimulus paired with acoustic startle stimulus in rats

The hypothesis that the standard acoustic startle habituation paradigm contains the elements of Pavlovian fear conditioning was tested. In a potentiated startle response paradigm, a startle stimulus and a light conditioned stimulus (CS) were paired. A startle stimulus then was tested alone or following the CS. Freezing behavior was measured to index conditioned fear. The startle response was potentiated on CS trials, and rats froze more in CS than in non-CS periods. In Experiment 1, response to a previously habituated, weak startle stimulus was potentiated. In Experiment 2, response to the same stimulus used as the unconditioned stimulus (US) in training was potentiated. This CS-potentiated response retarded the course of response decrements over training sessions as compared with an explictly unpaired control group. Conditioned fear is a standard feature of this habituation paradigm, serves to potentiate the startle response, and provides an associative dimension lacking in the habituation process per se.     

Emotion, attention, and the startle reflex

This theoretical model of emotion is based on research using the startle-probe methodology. It explains inconsistencies in probe studies of attention and fear conditioning and provides a new approach to emotional perception, imagery, and memory. Emotions are organized biphasically, as appetitive or aversive (defensive). Reflexes with the same valence as an ongoing emotional state are augmented; mismatched reflexes are inhibited. Thus, the startle response (an aversive reflex) is enhanced during a fear state and is diminished in a pleasant emotional context. This affect-startle effect is not determined by general arousal, simple attention, or probe modality. The effect is found when affects are prompted by pictures or memory images, changes appropriately with aversive conditioning, and may be dependent on right-hemisphere processing. Implications for clinical, neurophysiological, and basic research in emotion are outlined.     

Measurement of acoustic startle response in mice

Apparatus for measurement of acoustic startle response in mice is described. The technique utilizes a spring suspended lever, commercially available strain-gauge transducer, and polygraph. Individual startle responses are recorded as upward (flexion) and downward (extension) pen deflections, the force of either component being linearly related to the amplitude of the vertical pen deflections. The apparatus, with minor modifications, may be used to measure startle responses in larger rodents.     

Potentiation of acoustic startle behavior in the rat (Rattus norvegicus) at the onset of darkness

In Experiment 1 (N = 8), a 20-ms light pulse, given at various times before a noise burst, inhibited reflex expression with a single trough at a lead time of 70 ms, whereas a dark pulse facilitated the reflex with two peaks at 40 and 160 ms. In Experiment 2 (N = 18) facilitation by dark onset had a single peak, and inhibition by light onset a single trough; thus, the double peak of the dark pulse may result because inhibition from light onset at the end of the dark pulse was briefly impressed on the facilitatory effect of dark onset. In Experiment 3 (N = 12), diazepam (2.5 mg/kg, but not 1 mg/kg) eliminated dark facilitation but not light inhibition. These diazepam data reveal a basic similarity, perhaps identity, of the mechanisms responsible for the effect of dark onset and those producing reflex facilitation by Pavlovian fear conditioning and prolonged background noise, because all are moderated by a GABAergic system.     

Directed attention influences the modification of startle reflex probability

The present study was designed to investigate the effect of directed attention on elicitation and modification of the startle reflex. 30 adult human subjects received 90 dB(A) broadband noise startle stimuli either alone or preceded by a 60 dB(A) prepulse (either 2000-Hz tone, 1000-Hz tone, or broadband noise). Subjects were instructed to attend to one of the three prepulses during half the trials and to ignore all stimuli during the rest of the trials. The probability of responding while attending to a prepulse was significantly lower than the probability of responding while ignoring the prepulses. Responding to the prepulses was also more probable while subjects were attending to the prepulses, and these effects were more pronounced for tone than for noise prepulses. These results suggest that directed attention can influence the probability of the startle reflex without influencing startle amplitude or latency.     

Affective individual differences and startle reflex modulation

Potentiation of startle has been demonstrated in experimentally produced aversive emotional states, and clinical reports suggest that potentiated startle may be associated with fear or anxiety. To test the generalizability of startle potentiation across a variety of emotional states as well as its sensitivity to individual differences in fearfulness, the acoustic startle response of 17 high- and 15 low-fear adult subjects was assessed during fear, anger, joy, sadness, pleasant relaxation, and neutral imagery. Startle responses were larger in all aversive affective states than during pleasant imagery. This effect was enhanced among high fear subjects, although followup testing indicated that other affective individual differences (depression and anger) may also be related to increased potentiation of startle in negative affect. Startle latency was reduced during high- rather than low-arousal imagery but was unaffected by emotional valence.     

Personality and modulation of the startle reflex by emotionally-toned filmclips

Several researchers have found that pleasant foreground stimuli attenuate the eyeblink component of the startle reflex while unpleasant foreground stimuli potentiate it. The effects of personality factors on such modulation of the eyeblink response, as measured by electromyographic (EMG) activity in reaction to loud acoustic startle probes, were examined in subjects viewing emotionally-toned (pleasant, unpleasant and neutral) filmclips. During the main part of the experiment, introverts had higher baseline EMG activity and lower response probability than extraverts; no differences were observed at the beginning of the experiment, during an acclimatization session. Reflex modulation, as measured by response latency, was influenced by the Psychoticism (P) factor of the Eysenck Personality Questionnaire: subjects high on P showed longer latencies to eyeblink onset when probed during viewing of pleasant filmclips than subjects low on this dimension; no significant differences were observed between subjects low and high on P for neutral and unpleasant filmclips. No influence of personality factors was found on affective modulation as measured by response amplitude/magnitude. The results are discussed in relation to Gray's and Eysenck's theories of personality.     

Inhibition of acoustic startle using different mechanoreceptive channels

In this experiment, we investigated the impact of vibrotactile prepulse frequency and intensity on the acoustically elicited startle response in humans. Mechanoreceptive channels differing in their sensitivity to transient stimulation have been identified in the skin. The Pacinian channel is optimally sensitive to vibrations at approximately 300 Hz and is specialized for the detection of stimulus transients, whereas the non-Pacinian I and III channels are optimally sensitive to vibrations at approximately 30 Hz. Vibrotactile prepulses with frequencies of 30 and 300 Hz and intensities of 95 and 130 mV were presented for 50 msec to the dominant hand of college students (N = 31), followed on some trials by a 95-dB broadband acoustic startle stimulus. The 300-Hz prepulses resulted in significantly more pronounced inhibition of startle magnitude, amplitude, and probability, whereas only the 30-Hz prepulses significantly facilitated blink latency. These results support the idea that the inhibition of acoustic startle is determined more by transient than by sustained aspects of vibrotactile prepulse stimuli. This study also demonstrates that different aspects of the startle response differentially reflect stimulus characteristics of the prepulse.