Temporal acuity in auditory function in the rat: Reflex inhibition by brief gaps in noise.

Previous results show that the acoustic startle reflex in the rat is inhibited if a relatively weak stimulus precedes the startle-eliciting tone burst. The present 5 experiments explored the effect of brief silent periods (gaps) in white noise on the startle reflex in order to describe the limits of temporal resolution in the auditory system of 12 Long-Evans hooded rats. Brief silent periods did depress reflex behavior, and 2 responsible processes were identified. One was most evident at a 190-msec lead time between gap and startle tone. It yielded a linear decrement in reflex expression over a dynamic range of 0–7 msec and an estimate for the threshold of temporal acuity of 3.5 msec. The 2nd was evident primarily at a 40-msec interstimulus interval and had a linear effect over a dynamic range of at least 40 msec. Brief gaps had a greater inhibitory effect at the 190-msec interval between gap and startle stimulus; prolonged gaps had their greater effect at the 40-msec interval. The 1st process was identified as reflex inhibition, which is sensitive to the sensory properties of the lead stimulus. The 2nd process was identified as sensory adaptation, produced by noise exposure but unmasked by silence. (47 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Modification of the pigeon's visual startle reaction by the sensory environment.

Seven experiments related amplitude and latency of 20 racing pigeons' startle response, elicited by an intense visual stimulus, to antecedent auditory and visual events in the sensory environment. Data indicate that (a) within broad limits the amplitude of the reflex was a positive function of the intensity of the sensory background prevailing at the time of startle elicitation; (b) a change in the sensory environment occurring 15-2,000 msec prior to the startle-eliciting stimulus inhibited the amplitude of the response; and (c) a change in the sensory environment less than 10 msec prior to the startle-eliciting stimulus reduced the latency of the response. Findings are consistent with previous research on acoustic elicited startle in the rat. The overall configuration of the results suggests that a pathway including the reticulospinal tract and the bulbopontine reticular nuclei could be the major mediator of startle. In these terms, latency-reduction effects would occur because of partial activation of this pathway, amplitude inhibition would occur because of cerebellar influence, and amplitude facilitation would reflect cerebral or striatal influences. (49 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temporal integration of acoustic stimulation obtained in reflex inhibition in rats and humans.

Investigated whether inhibition provided by initial stimuli of various durations conforms to established temporal integration functions. Initial stimuli (S1) were noise bursts varying in duration (2, 20, or 200 msec) and intensity (55 or 85 db). Eliciting stimuli (S2) for 6 Holtzman rats were intense tone bursts, which elicited the acoustic startle reflex, and for 9 19–24 yr old humans were electrotactile stimuli to the forehead, which elicited the eye blink. Findings reveal that inhibition was greater with the 85-db S1 stimulus and increased linearly with log increases in duration. Data suggest that the acoustic substrate for reflex inhibition has a long-time constant. There was one exception to this general finding. For 7 human Ss, inhibition declined when the duration of the 85-db S1 was increased from 20 to 200 msec. Postexperimental questioning and video monitoring suggest that this anomaly resulted from a reflex-enhancing arousal process. (38 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Modulation of prepulse inhibition by an augmented acoustic environment in DBA/2J mice.

DBA/2J (DBA) mice exhibit progressive hearing loss, evident for high frequencies (>20 kHz) at age 3–4 weeks and severe by 12–16 weeks. From age 25 days to 12 weeks, DBA mice were exposed for 12 hr nightly to an augmented acoustic environment (AAE): moderately intense broadband noise bursts. After AAE treatment, prepulse inhibition (PPI) to tone prepulses (4–24 kHz, 70 dB SPL) was stronger, and baseline acoustic startle responses were larger, compared with results for age-matched DBA mice (testing performed with AAE off). Nightly AAE treatment was then terminated, and both AAE effects were largely gone 1 week later. Reinstatement of AAE treatment after the 4-week period had no significant effect on startle magnitude, but PPI improved significantly, with the AAE effect reacquired after 3 weeks. It is proposed that AAE modulates neural plasticity induced by high-frequency hearing loss in auditory system components of the PPI pathway. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Feedback control of the auditory periphery: anti-masking effects of middle ear muscles vs. olivocochlear efferents

Both MEM and MOC systems are sound-evoked reflexes to the auditory periphery which can be elicited by sound in either ear. Both MEM and MOC systems can increase thresholds in the auditory periphery: the MEM system acts by stiffening the ossicular chain, the MOC system by decreasing outer hair cell amplification of sound-induced motion in the inner ear. MEM-induced attenuations are largest for low frequency stimuli, MOC-induced attenuations are largest for mid- to high-frequency sounds. Both MEM and MOC systems can have anti-masking effects. The MEM reflex can decrease the masking of high-frequency signals by low-frequency noise (i.e., the upward spread of masking). The MOC reflex is complementary in that it minimizes masking of high-frequency transient signals by high-frequency continuous noise. MEM anti-masking arises by reducing suppressive masking and can improve masked thresholds at high frequencies. MOC anti-masking arises by counteracting excitatory masking. It does not improve masked thresholds, but can improve the detectability of small suprathreshold intensity increments. Anti-masking effects of both MEM and MOC systems should be reduced in cases of sensorineural hearing loss.     

Acoustic startle threshold of the albino rat (Rattus norvegicus).

The startle threshold of the albino Sprague-Dawley rat runs parallel to the curve of the hearing threshold. The difference between the startle and hearing threshold is 87 dB (SPL) at a background noise level of 75 dB (SPL). At 110 dB (SPL), the threshold has a range from 2 kHz to 50 kHz with a minimum at 10 kHz and a second minimum at 40 kHz. Amplitude and latency of the startle response are not only dependent on the sensation level of the acoustic stimulus but also on the frequency. At threshold, only the head movement component of the startle response is elicited. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Activation of a distinct arousal state immediately after spontaneous awakening from sleep

In contrast to the many neural studies into the mechanisms of sleep onset and maintenance, few studies have focused specifically on awakening from sleep. However, the abrupt electrographic changes and large brief cardio-respiratory activation at awakening suggest that a distinct, transiently aroused, awake state may exist compared to later wakefulness. To test this hypothesis we utilized the acoustic startle reflex, a standard un-conditioned reflex elicited by a sudden loud noise. This reflex is modulated under specific conditions, one being a diminution of startle when a quieter pre-stimulus is presented immediately before the loud stimulus. This pre-pulse inhibition (PPI) is used as a measure of sensorimotor gating, with smaller PPI indicating less filtering of sensory inputs and increased responsiveness to external stimuli. Eight rats with electrodes for recording sleep-wake state were studied. An accelerometer measured startle responses. The startle reflex was elicited by 115 dB, 40 ms tones. PPI was produced by 74 dB, 20 ms tones preceding the 115 dB tone by 100 ms. Responses within 100 ms were measured. Stimuli were applied either 3-10 s after spontaneous awakenings, or in established wakefulness (> 30 s). Responses to the startle stimuli alone were similar in the different awake states (P = 0.821). However, PPI was smaller at awakening from non-REM sleep compared to established wakefulness (45.4 +/- 7.5% vs. 74.3 +/- 6.1%, P = 0.0002). PPI after awakening from REM sleep (52.8 +/- 17.9%) was not significantly different than established wakefulness (P = 0.297). Reduced PPI of the startle reflex at awakening from non-REM sleep supports the hypothesis that wakefulness immediately after spontaneous sleep episodes is neurophysiologically distinct from later wakefulness and associated with reduced gating of motor responses to sensory inputs. Spontaneous activation of this distinct, transiently aroused, state upon awakening may serve a protective function, preparing an animal to respond immediately to potentially threatening stimuli.     

Remote masking in normal-hearing and noise-exposed chinchillas

Remote masking (RM), the phenomenon whereby an intense high-frequency masking noise elevates thresholds for low-frequency signals, has been shown to be sensitive to various types of hearing loss in humans. We performed two experiments to evaluate the chinchilla as a model of RM and to examine changes in RM associated with temporary threshold shifts (TTSs) induced by low-frequency noise exposure. Thresholds for 0.5-, 1- and 2-kHz tones were measured in quiet, then in the presence of a narrow-band (300-Hz-wide) masking noise centered at 3 kHz. In Experiment I, effective masking was measured as a function of masker level, from 48 to 98 dB sound pressure level (SPL; referenced to 20 microPa), to determine threshold and rate of growth of RM in the chinchilla. In Experiment II, RM was measured before, during and after exposure to a low-frequency noise known to produce TTSs in chinchillas (i.e., a 0.5-kHz octave band noise at 90 dB SPL for 6 h/day for 10 days). The results show that normal-hearing chinchillas have the same pattern of RM as humans, and that a noise exposure that produces TTSs also produces rapid and significant changes in RM.     

Modification of the rat's startle reaction by an antecedent change in the acoustic environment.

Conducted 3 experiments with 6 male albino rats each in which inhibition and facilitation of the startle response, elicited by an intense auditory signal, was related to a change of the frequency characteristic of a 70-db continuous acoustic signal. Data indicated that if a frequency change occurred in the acoustic environment 64 msec before the startle-eliciting stimulus, the amplitude of the startle response was reduced; if frequency change occurred 4 msec prior to the startle-eliciting stimulus, the response latency was reduced. Results extend the generality of previous research employing weak antecedent acoustic signal onset and offset. Results indicate that neural mechanisms mediating the startle reflex may be activated by any change in the acoustic environment and that these mechanisms may be a component of the orienting reflex arc. (21 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Reflex modification in the rat: The inhibitory effects of intensity and frequency changes in steady tones.

Four experiments with 50 male albino Charles River rats investigated the relation between the inhibitory effects of tonal frequency change and the length of the silent period (gap) preceding it. It is noted that, in both laboratory rats and in humans, a low-intensity tone that precedes a high-intensity burst of noise by approximately 100 msec can reduce the amplitude of the startle reaction elicited by the burst of noise. Overall results show that a gap in an otherwise continuous pure tone inhibited startle when the gap occurred approximately 100 msec prior to the noise burst. Although an increase in gap duration increased the inhibition afforded by the gap, the maximum inhibition was yielded by gaps of 100 msec and greater; this maximum was equivalent to the inhibition yielded by the presentation of a postgap tone alone. A shift in tonal frequency across a 10-msec gap yielded more inhibition than did the same gap with no frequency shift; again, the shift yielded equivalent inhibition to the presentation of the postgap tone alone. An increase in the frequency shift increased inhibition when the shift occurred across a 10-msec gap but not across a 100-msec gap. It is concluded that the amount of inhibition afforded by a complex acoustic event is more than a mere additive function of the inhibitory effects of the different elements that make up the event. For example, a tonal onset is just as inhibitory as a frequency shift across a gap, which includes a tonal onset, tonal offset, and frequency shift. (15 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Stimulus quality affects expression of the acoustic startle response and prepulse inhibition in mice.

The relationship between stimulus intensity and startle response magnitude (SIRM) can assess the startle reflex and prepulse inhibition (PPI) with advantages over more commonly used methods. The current study used the SIRM relationships in mice to determine differences between white noise and pure tone (5 kHz) stimuli. Similarly to rats, the SIRM relationship showed a sigmoid pattern. The SIRM-derived reflex capacity (RMAX) and response efficacy (slope) of the white noise and pure tone stimuli in the absence of prepulses were equivalent. However, the pure tone startle response threshold (DMIN) was increased whereas the stimulus potency (1/ES??) was decreased when compared to white noise. Prepulses of both stimulus types inhibited RMAX and increased DMIN, but the white noise prepulses were more effective. Both stimulus intensity gating and motor capacity gating processes are shown to occur, dependent on prepulse intensity and stimulus onset asynchrony. Prepulse intensities greater than 10 dB below the startle threshold appear to produce PPI via stimulus intensity gating, whereas a motor capacity gating component appears at prepulse intensities near to the startle threshold. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of stapedius-muscle contractions on the masking of auditory-nerve responses

There has been little exploration of the mechanisms by which stapedius muscle contractions reduce the masking of responses to high-frequency sounds by low-frequency sounds. To fill this gap in knowledge, controlled stapedius contractions were elicited with direct shocks in anesthetized cats, and measurements were made of the effects of these contractions on the masking of single auditory-nerve fibers and on the attenuation of middle-ear transmission. The results show that the stapedius-induced reductions of masking can be much larger than the attenuations of low-frequency sound. With a 300-Hz band of masking noise centered at 500 Hz, and signal tones at 6 or 8 kHz, unmasking effects over 40 dB were observed for sounds 100 dB SPL or less. The data suggest that much larger unmasking might occur. The observed unmasking can be explained completely by a linear stapedius-induced attenuation of sound transmission through the middle ear and a nonlinear growth rate of masking for auditory-nerve fibers. No central effects are required. It is argued that the reduction of the upward spread of masking is probably one of the most important functions of the stapedius muscle.     

Habituation of the Acoustic and the Tactile Startle Responses in Mice: Two Independent Sensory Processes.

To test whether habituation is specific to the stimulus modality, the authors analyzed cross-habituation between the tactile startle response' (TSR) and the acoustic startle response (ASR). The acoustic artifacts of airpuffs used to elicit the TSR were reduced by using a silencer and were effectively masked by background noise of 90-100 dB sound-pressure level. ASR was elicited by 14-kHz tones. TSR and ASR habituated in DBA and BALB mice: both the TSR and ASR habituated to a greater extent in DBA mice than in BALB mice. In both strains, habituation of the TSR did not generalize to the ASR, and vice versa. From this, the authors concluded that habituation of startle is located in the sensory afferent branches of the pathway. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Modification of the rat's startle reaction by termination of antecedent acoustic signals.

Conducted 4 experiments with 24 male albino rats relating inhibition and facilitation of the startle response, elicited by an intense auditory stimulus, to the offset of a 70-db continuous acoustic signal. Data indicate that if the antecedent signal terminated 10-1000 msec before the startle-eliciting stimulus, the amplitude of the startle response was reduced. If offset occurred less than 10 msec prior to the startle-eliciting stimulus, the response latency was reduced. Results are consistent with previous research employing weak antecedent acoustic signal onset. The overall configuration of the results suggests that activity in brain centers mediating the startle reflex may be an early component of the orienting reflex arc. (19 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Prepulse inhibition of acoustic or trigeminal startle of rats by unilateral electrical stimulation of the inferior colliculus.

Transient electrical stimulation of the inferior colliculus (IC) of adult male rats had a strong and long-lasting inhibitory effect on startle responses elicited by either intense noise bursts or unilateral electrical stimulation of the principal trigeminal nucleus. Startle elicited by noise bursts was inhibited over a wide range of interstimulus intervals (ISIs) with the maximum inhibition at ISIs between 15 and 30 msec. Startle elicited by trigeminal stimulation was inhibited more sharply than acoustic startle, with the maximum inhibition at ISIs between 20 and 35 msec. These data support the view that the IC is a critical part of the pathway mediating prepulse inhibition (D. S. Leitner & M. E. Cohen, 1985). More important, the data reveal the time course of the inhibitory influence of the IC on startle and indicate that the inhibitory effects of IC stimulation have higher temporal resolution on trigeminal startle than on acoustic startle. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Sensitivity of the mouse to changes in azimuthal sound location: Angular separation, spectral composition, and sound level.

Auditory spatial acuity was measured in mice using prepulse inhibition (PPI) of the acoustic startle reflex as the indicator response for stimulus detection. The prepulse was a “speaker swap” (SSwap), shifting a noise between two speakers located along the azimuth. Their angular separation, and the spectral composition and sound level of the noise were varied, as was the interstimulus interval (ISI) between SSwap and acoustic startle reflex elicitation. In Experiment 1 a 180° SSwap of wide band noise (WBN) was compared with WBN Onset and Offset. SSwap and WBN Onset had near equal effects, but less than Offset. In Experiment 2 WBN SSwap was measured with speaker separations of 15, 22.5, 45, and 90°. Asymptotic level and the growth rate of PPI increased with increased separation from 15 to 90°, but even the 15° SSwap provided significant PPI for the mean performance of the group. SSwap in Experiment 3 used octave band noise (2–4, 4–8, 8–16, or 16–32 kHz) and separations of 7.5 to 180°. SSwap was most effective for the highest frequencies, with no significant PPI for SSwap below 8–16 kHz, or for separations of 7.5°. In Experiment 4 SSwap had WBN sound levels from 40 to 78 dB SPL, and separations of 22.5, 45, 90, and 180°: PPI increased with level, this effect varying with ISI and angular separation. These experiments extend the prior findings on sound localization in mice, and the dependence of PPI on ISI adds a reaction time-like dimension to this behavioral analysis. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Auditory sensitivity in the great tit: perception of signals in the presence and absence of noise

Absolute and masked auditory thresholds (critical masking ratios) were determined behaviourally in the great tit, Parus major, using a GO/NOGO-procedure. Absolute sensitivity was measured between 0.25 and 10 kHz. In the absence of noise, great tits were most sensitive to frequencies between 2 and 4 kHz. In background noise, however, the sensitivity was only a function of the noise level and was independent of frequency. Critical masking ratios determined for signals between 0.25 and 8 kHz were almost constant (median values varied between 23.8 and 25.9 dB) irrespective of signal frequency. Therefore, in contrast to the majority of bird species, great tits have unusually low critical masking ratios at high frequencies. This means that great tits can use high-frequency vocalizations to communicate efficiently in noisy (i.e. natural) environments. Copyright 1998 The Association for the Study of Animal Behaviour.     

Facilitation and inhibition of the acoustic startle reflex in the rat after a momentary increase in background noise level

Small increments in background noise were shown to increase the amplitude of a subsequently elicited acoustic startle reflex (ASR) in rats by as much as 100% under optimal conditions. Increment lead time (5-160 ms) and level (1.5-15 dB), initial noise level (30-70 dB), startle level (95-125 dB), number of test days (1-5), and drug condition (diazepam or saline ip) were varied in 6 experiments. Prepulse facilitation (PPF), measured by difference scores, was greatest for intermediate increments (3 dB) and lead times (20-40 ms) and was replaced by prepulse inhibition (PPI) for higher values, especially in the later test days. Diazepam reduced baseline ASR and diminished PPI, but it did not affect PPF. These data argue against hypotheses that attribute PPF of this sort to either temporal integration within the ASR pathways or to the elicitation of a nonspecific arousal reaction by the prepulse.     

Effect of vestibular nerve section on cytochrome oxidase activity in the vestibular ganglion cells of the squirrel monkey

Estimates of behavioral thresholds of infants are elevated relative to those of adults. Explanations for the differences include auditory sensory factors and non-sensory factors, but no direct estimates of the relative contributions of these two factors have been made. In this investigation, thresholds in quiet and in increasing levels of a masking noise for a 1 kHz tone, in infants 8 to 11 months old and in adults, were determined. The infant-adult differences in unmasked threshold was compared to the infant-adult difference in an estimate of the minimum masking level (MML) that was derived from the masking data. The intensity level of a masking noise at which masking begins is assumed to be independent of the non-sensory factors that impact on the threshold value itself. Therefore, it is reasoned that the infant-adult difference in MML reflects more closely differences in auditory sensory factors than does the infant-adult difference in unmasked threshold. In the region of 1 kHz, the infant-adult difference in behavioral threshold was 12 dB and the infant-adult difference in MML was 8 dB. Therefore, according to our assumptions, 8 dB of the infant-adult difference in unmasked threshold is accounted for by sensory factors and the remaining 4 dB must be attributable to non-sensory factors.     

Startle reflex inhibition in the rat: Its persistence after extended repetition of the inhibitory stimulus.

Startle reflexes to intense sound bursts are inhibited by weak stimuli that briefly precede their elicitation. In 3 experiments, with 36 Long-Evans hooded rats, the startle stimulus (a 110-db tone burst) was presented 100 msec after the final link in a train of stimuli, the length of the train varying from 1 to 1,000, its repetition rate varying from 1 to 10 per sec, and its constituents being 40 db or 50 db white noise bursts of 25 msec duration. Inhibition was invariant across train length and repetition rate. In Exp IV, the startle stimulus was presented a variable interval after the final link, from 40 to 1,280 msec with 1 or 100 noise bursts (50 db) in the train. Inhibition developed more rapidly following the last member of the 100-stimulus train, suggesting a "priming" or sensitization effect of stimulus repetition, but its overall strength and subsequent rate of decay were not different in the 2 conditions. The general persistence of inhibition following these extended series of stimuli reveals that reflex inhibition must be the outcome of a fixed and obligatory process associated with sensory input. (29 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)