Strychnine increases acoustic startle amplitude but does not alter short-term or long-term habituation.

Used the glycine antagonist strychnine (1.0 mg/kg, ip, 10 min before treatment) to investigate the involvement of glycinergic neurons in the development and/or expression of short-term (within-session) habituation (Exp I) and long-term (between-sessions) habituation (Exps II and III) of the acoustic startle response in 120 male albino Sprague-Dawley rats. Over a range of eliciting-stimulus intensities (95, 105, and 115 db) and interstimulus intervals (3, 7, 13, and 27 sec), strychnine markedly increased startle amplitude, relative to water injection, whereas it failed to attenuate the rate of within-session habituation (Exp I). In Exp II, Ss that were exposed to daily sessions of startle-eliciting stimuli for 4 days and then tested on Day 5 showed lower levels of startle amplitude than Ss with no prior habituation training. Strychnine injected prior to the test session again increased startle amplitude but did not block the expression of between-sessions habituation. In Exp III, Ss injected with either strychnine or water prior to each of 3 daily habituation training sessions and subsequently tested on Day 4 showed similar between-sessions habituation. In general, results show that strychnine increased startle amplitude without affecting either within-session or between-sessions habituation of acoustic startle. (12 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Habituation of the acoustic startle response in rats after lesions in the mesencephalic reticular formation or in the inferior colliculus.

Extensive damage to the mesencephalic reticular formation (MRF) in 24 male albino rats altered long-term habituation of the acoustic startle response without disrupting short-term habituation. The MRF lesions did not alter initial startle amplitudes, but the Ss with lesions were unable to attain as low a long-term asymptote of habituation as could 21 control Ss. Subsequent manipulation of stimulus intensity and interstimulus interval revealed no differences in short-term habituation between the 2 groups. Large lesions to the inferior colliculus (IC) of 22 Ss did not disrupt long-term habituation of the acoustic startle response, but these Ss were unable to suppress responding as much as controls to intense stimuli presented rapidly. The deficits in long-term habituation following MRF lesions suggest a disruption of an extrinsic, inhibitory mechanism of habituation. The deficits following IC lesions could have been due either to a disruption of a short-term habituation mechanism or to an increase in response sensitization produced by the lesions. (30 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Signal-to-noise ratio as a predictor of startle amplitude and habituation in the rat.

Measured the startle response in 63 male Sprague-Dawley albino rats using tones of various intensities at various levels of background noise. Tone intensity and background noise interacted in their effects on startle amplitude, but overall startle bore a nonmonotonic relationship to the signal-to-noise (S/N) ratio (tone intensity minus background noise). The inflection point of this function changed with habituation; and under some conditions habituation curves of separate groups, when presented with tones at different background-noise levels, actually crossed. Habituation was directly related to the S/N ratio in training, and S/N ratio was a better predictor of habituation than tone intensity alone or background noise alone. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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)     

Effects of mesencephalic reticular formation lesions on habituation of startle and lick suppression responses in the rat.

In 2 experiments with 43 male albino Holtzman rats, extensive damage to the mesencephalic reticular formation (MRF) altered long-term habituation of the acoustic startle response without disrupting short-term habituation. Ss with MRF lesions, although initially neither more nor less responsive than controls, could not attain as low a long-term asymptote of habituation as could control Ss with repeated presentations of an auditory stimulus. Changing the quality of the auditory stimulus abolished the asymptotic difference in responsiveness. With repeated presentations of the 2nd auditory stimulus, control Ss again reached a significantly lower long-term habituation asymptote than did Ss with MRF lesions. The course of long-term habituation for the 2 groups suggested a disruption of an extrinsic, inhibitory habituation process by the lesions. The effects of MRF lesions were specific to the acoustic startle response. Control Ss and those with lesions showed comparable response levels, short-term habituation, and long-term habituation of the lick suppression response. Responsiveness and habituation to tactile stimuli were comparable for the 2 groups on both lick suppression and startle response measures. (32 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effect of lesions in the medial forebrain bundle on three measures of pain sensitivity and noise-elicited startle.

Subjected 137 male Holtzman albino rats to bilateral lesions of the medial forebrain bundle (MFB) or to surgical control procedures. Lesions produced equivalent decreases in paw-lick latency to heat and in jump and high-magnitude-response thresholds to footshock. Flinch and detection thresholds to footshock were also significantly decreased, though the lesion did not alter the magnitude or habituation of noise-elicited startle. Injection of 75 mg/kg serotonin into lesioned Ss returned the high-magnitude-response threshold to normal values but did not affect detection threshold. It is concluded that the increased sensitivity to painful stimuli resulted from the decrease in telencephalic content of serotonin following MFB lesions. Previous exposure of lesioned Ss to footshock resulted in a potentiation of the startle response to noise, further suggesting that the lesion had increased the aversiveness of footshock. (27 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Sensitization of the rat startle response by noise.

In a series of 6 experiments with a total of 160 male Sprague-Dawley albino rats, it was found that the startle response showed a progressive increase in amplitude when tones were presented against a high level of background noise. This sensitization effect was not a result of repetitive exposure to tones but rather a result of continuous exposure to noise. The size of the effect was directly related to noise intensity and required about 30-45 min to reach a maximum. The effect did not dissipate when the noise was maintained but did dissipate once the noise was turned down. Results are discussed in terms of the experimental conditions under which repetitive stimulus exposure produces either sensitization or habituation of the startle reflex. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Emotion, novelty, and the startle reflex: Habituation in humans.

Previous research with both animal and human Ss has shown that startle reflex magnitude is potentiated in an aversive stimulus context, relative to responses elicited in a neutral or appetitive context. In the present experiment, the same pleasant, unpleasant, and neutral picture stimuli were repeatedly presented to human Ss. Startle reflex habituation was assessed in each stimulus context and was compared with the habituation patterns of heart rate, electrodermal, and facial corrugator muscle responses. All systems showed initial differentiation among affective picture contents and general habituation over trials. The startle reflex alone, however, continued to differentiate among pleasant, neutral, and unpleasant pictures throughout the presentation series. These results suggest that (1) the startle probe reflex is relatively uninfluenced by stimulus novelty, (2) the startle modulatory circuit (identified with amygdala-reticular connections in animals) varies systematically with affective valence, and (3) the modulatory influence is less subject to habituation than is the obligatory startle pathway or responses in other somatic and autonomic systems. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Lesions of the amygdala, but not of the cerebellum or red nucleus, block conditioned fear as measured with the potentiated startle paradigm.

In Exp I, 97 male Sprague-Dawley albino rats were given 10 light–shock pairings on 2 successive days. At 24–48 hrs following training, groups of Ss received bilateral transection of the cerebellar peduncles, bilateral lesions of the red nucleus (which receives most of the cerebellar efferents), or bilateral lesions of the central nucleus of the amygdala. Controls were sham operated. At 3–4 days after surgery, Ss were tested for potentiated startle (PS [increased acoustic startle in the presence of the light previously paired with shock]). PS was blocked by lesions of the central nucleus of the amygdala but not by transection of the cerebellar peduncles or lesions of the red nucleus. Exp II, in which a visual prepulse test was used with 14 Ss, indicated that the blockade of PS observed in Ss with amygdala lesions could not be attributed to optic tract damage. Exp III, with 20 Ss, demonstrated that the absence of potentiation in Ss with amygdala lesions was not simply due to a lowered startle level ceiling, because these Ss could show increased startle with increased stimulus intensity and with administration of intraperitoneal strychnine, (0.75 mg/kg), a drug that increases startle. Results are consistent with the hypothesis that the amygdala is involved in fear conditioning. (64 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of single oral administrations of haloperidol and d-amphetamine on prepulse inhibition of the acoustic startle reflex in healthy male volunteers

The effects of acute administration of an indirect dopamine-agonist, d-amphetamine, and a non-selective dopamine receptor antagonist, haloperidol, were investigated in normal male volunteers on habituation and prepulse inhibition (PPI) of the acoustic startle reflex in two experiments. In Experiment 1, 40 male non-smoker volunteers were tested for habituation and PPI (defined as percentage reduction of the pulse-alone amplitude; prepulses 9 dB above background) before and after double-blind administration of either 2 mg haloperidol or placebo. No influence of haloperidol was observed on either habituation or PPI of the startle reflex in this experiment. In Experiment 2, 60 male volunteers underwent startle testing before and after double-blind administration of a single oral dose of 5 mg haloperidol, 5 mg d-amphetamine or placebo. Habituation and PPI (prepulses 15 dB above background) for the placebo group did not differ significantly from that observed for the d-amphetamine or for the haloperidol group. However, in a subgroup of smoking subjects, both d-amphetamine and haloperidol reduced PPI as compared to that observed prior to drug administration. The implications of these findings in relation to animal pharmacological studies and observed sensorimotor gating deficits in schizophrenia are discussed.     

Refractory period and habituation of acoustic startle response in rats.

Conducted 4 experiments, each with 10 male albino rats, to determine the time course of the refractory period of the acoustic startle response. The refractory effect was shown to be highly dependent upon the interval between 2 startle stimuli but independent of the intensity of the stimuli at a given interval, provided that the intensity of both stimuli was the same. A model based on summation of refractory effects to repetitive stimulation did not predict habituation to repetitive stimulation. The interaction between refractory effect and habituation was discussed, and it was suggested that short interstimulus intervals reduce the effective intensity of the habituation stimulus so that the effects of frequency and intensity become confounded in habituation studies where the interval between habituation stimuli invades the refractory period of the response being studied. (20 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Ontogenetic dissociation of two classes of habituation.

Determined the rates of decrement of 2 classes of response (an elicited startle reflex and emitted exploratory behavior) in rats of 2 different ages (15 and 36 days). Ss were 31 Sprague-Dawley albino rats. The rate of decrement in the startle reflex was not clearly differentiated as a function of age. In contrast, there was no evidence of habituation of exploration in the younger Ss, whereas older Ss uniformly showed profound response decrements. This ontogenetic dissociation of the 2 instances of response decrement indicates that accounts of both instances in terms of a common process called habituation may be unwarranted. In addition, these data, in conjunction with earlier findings, indirectly support the possibility that reflex decrements may be relatively more dependent on brain serotonin, whereas decrements in exploration may be more dependent on normal cholinergic activity in brain. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Habituation of prepulse inhibition of the startle reflex using an auditory prepulse close to background noise.

This study examined whether prepulse inhibition habituates with repeated presentation of the prepulse alone. Prepulse inhibition was determined by measuring the decrement in the startle response when the acoustic startle-eliciting stimulus was preceded by an auditory prepulse. Rats received repetitive exposures of the same auditory prepulse alone (experimental condition) and of a visual prepulse alone (control condition). To reduce habituation of startle itself and the possible dishabituating influence the startle stimulus might have on habituation of prepulse inhibition, startle stimulus presentations were infrequently interspersed among a much larger number of prepulse-alone presentations. Stimulus-specific habituation of prepulse inhibition occurred using an auditory prepulse 2.5 dB, but not 13 dB, above background noise. Implications are discussed for the role of prepulse inhibition in sensory gating. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Habituation of the EEG arousal response in rats: Short- and long-term effects, frequency specificity, and wake–sleep transfer.

In 3 experiments with 78 male Holtzman albino rats, presentation of auditory stimuli over interstimulus intervals (ISIs) of several minutes to sleeping Ss produced significant habituation of the EEG arousal response. The arousal response habituated to an asymptote after only 2 or 3 stimulus presentations. Little or no spontaneous recovery occurred between sessions separated by 24 or 72 hrs. The habituation produced by a single stimulus presentation was retained for at least 24 hrs, and orderly habituation was shown with a 24-hr ISI. Approximately 90% retention of habituation was shown 32 days following 10 stimulus presentations, and some degree of retention was shown for as long as 50 days. After habituation had reached a long-term asymptote, 600 stimulus presentations over 2-sec ISIs produced further response decrements, but these decrements recovered completely within a matter of minutes and responsiveness returned to the previously established long-term asymptote. Habituation was shown to be frequency-specific over both 24-hr and 32-day intervals. Habituation produced by stimulus presentations to awake Ss transferred to the condition in which stimuli were presented to sleeping Ss. (18 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Prepulse inhibition of acoustic startle in rats after lesions of the pedunculopontine tegmental nucleus.

Prepulse inhibition (PPI) of startle is impaired in schizophrenics, which suggests they have disturbances in circuitry that controls PPI. How activity in forebrain circuitry is communicated to the primary startle circuit to modulate PPI was explored. Subpallidal cells innervate the pedunculopontine tegmental nucleus (PPTg). Infusion of the γ-aminobutyric acid antagonist picrotoxin into the subpallidum impaired PPI. In other rats, electrolytic PPTg lesions decreased or eliminated PPI, potentiated startle amplitude, and did not alter habituation. The disruption of PPI correlated significantly with the extent of PPTg damage. PPTg lesions reduced PPI when startle stimuli were weak or intense (104 or 140 db) and when prepulse stimuli ranged from 2 to 17 db above background but were most profound with prepulses 5–8 db above background. The PPTg modulates sensorimotor gating and may process and transmit information from forebrain structures to the primary startle curcuit. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Neuroethological studies of reflex plasticity in intact Aplysia..

Multi-unit recording of siphon nerve activity in intact Aplysia californica (a marine gastropod) with chronically implanted cuff electrodes provided a monitor of activity in a central pattern generator, the Interneuron II (Int II) network, which produces large siphon and gill contractions both spontaneously and after tactile stimulation of the siphon. The phase–response curve of the Int II oscillator for single stimuli at different phases of the cycle showed a "refractory" period early in the cycle, after which most stimuli phase advanced the oscillator and caused a short-latency Int II burst and a large contraction. The amplitude of gill withdrawal and the duration of siphon withdrawal in response to different stimulus intensities depended on whether an Int II burst was triggered. Activation of the Int II oscillator transformed the reflex from one that was graded smoothly with stimulus intensity to one in which nearly maximal responses were elicited even by weak stimuli. Entrainment and habituation training both involved monotonous repetition of a stimulus at specific intervals. With repeated siphon stimuli, nearly maximal reflex responses were maintained in intact Ss as long as the Int II oscillator was entrained, whereas habituation was associated primarily with failure to entrain the oscillator. Sensitized Ss showed significantly more triggered Int II burst than did controls. Digital spike-train analysis indicated that individual siphon motoneurons showed significantly increased background activity that often persisted for several minutes. (41 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Habituation of startle response, lick suppression, and exploratory behavior in rats with hippocampal lesions.

34 male albino Holtzman rats with bilateral hippocampal lesions (HLs) and 22 controls with neocortical lesions (NLs) were compared on the habituation of lick suppression and startle response. Ss with HLs showed no consistent differences from controls on any measure within these 2 response systems. Ss with HLs showed significant retention of habituation over periods of 24 hrs and 21 days. Experimental and control differences were not revealed when stimulation was presented on a 1-sec interstimulus interval. None of these results varied with the extent of the HLs, which ranged from relatively small lesions restricted to the dorsal hippocampus to large lesions that damaged the hippocampus in its dorsal, posterior, and ventral aspects. In contrast to the startle response and lick suppression results, HLs significantly disrupted Y-maze exploratory behavior, and the disruption was directly related to the extent of hippocampal damage. Data suggest that the hippocampus is not involved in any important way in the control of either short- or long-term habituation of elicited, reflex-like behaviors but is importantly involved in the control of emitted, exploratory behaviors. (39 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Localization of telencephalic regions concerned with habituation of cardiac and ventilatory responses associated with arousal in the goldfish (Carassius auratus).

54 goldfish were presented with a light-on stimulus at 2-min intervals, and their cardiac and ventilatory responses were monitored. Normal Ss, Ss with sham operations for telencephalic lesions, and Ss with bilateral electrolytic lesions of the telencephalon (anterior commissural and posterior telencephalic groups) were compared for responsiveness and habituation to regularly presented stimuli. There was no evident difference in responsiveness between the 4 groups, although 3 Ss with lesions in the regions ventralis pars dorsalis and ventralis pars ventralis gave fright responses to novel stimuli. Lesions in posterior telencephalic regions dorsalis pars centralis and ventralis pars intermedialis and at the telencephalic/diencephalic border close to the habenula caused significant impairment of habituation. (26 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Topographic changes in responding during habituation to water-stream simulation in sea anemones (Anthopleura elegantissima).

Investigated habituation of contraction responses in 2 experiments with a total of 7 sea anemones using response-independent and response-dependent intertrial intervals (ITIs). In Exp I, repeated presentation of water-stream stimulation at 4 stimulus durations produced replicable response decrements in 5 out of 6 Ss in the response-independent ITI conditions. 3 Ss habituated repeatedly in the response-dependent ITI condition. Instances of both dishabituation and retention of habituation were observed, and habituation was often accompanied by changes in the topography of responding. In Exp II, habituation retention intervals were assigned values of 30, 60, or 90 min. 2 out of 3 Ss exhibited less responding at the shorter intervals. The data constitute nonassociative learning in individuals of a species lacking a central nervous system. (30 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Habituation of the orienting response as a function of individual differences in anxiety and autonomic lability.

Evaluated the role of individual differences in anxiety and autonomic lability on the habituation of GSR responses to auditory stimuli of moderate and very low intensity. 60 male undergraduate Ss were used. They were divided into high- and low-anxious groups on the basis of Taylor MA scale scores and high and low autonomic lability groups on the basis of spontaneous GSR activity during a rest period. No relationship between Taylor score and GSR habituation was obtained for either stimulus intensity. Autonomically labile Ss showed no habituation to the moderate tone, but did habituate to the low intensity tone. Autonomically stabile Ss habituated to both tones. It was concluded that individual differences in autonomic lability may predict whether an S will respond to moderate intensities with orienting or defensive reactions. (33 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)