Digital and Cephalic Vasomotor Orienting Responses to Indifferent, Signal, and Verbal Stimuli

Both digital and cephalic components of the vasomotor orienting response (VOR) were studied with the use of a habituation procedure. A habituation criterion of 8 nonresponses on 10 successive trials was used with respect to the digital vasomotor responses. All 12 subjects received the three stimulus conditions: a) a tone of moderate intensity (60 dB, 500 Hz, 10-sec duration) as an indifferent stimulus (IS), b) the same tone as a signal stimulus (SS) for clenching the hand, and c) a verbal stimulus (VS) for clenching the hand but no tone. The SS condition followed the IS condition, and the VS condition was administered either preceding the IS or following the SS condition. The tone produced a decrease in the digital blood volume and an increase in the cephalic blood volume which habituated more slowly under the SS condition. However, the VOR was weakened significantly faster in the digital than in the cephalic component. Different measures of habituation rate, including the max-rate score, were used and the outcomes were compared.     

Effect of stressor intensity on habituation and sensitization of glucocorticoid responses in rats

This experiment was designed to study the effect of stressor intensity on habituation/sensitization of the adrenocortical stress response in rats. Rats were given 18 shocks in 3-hr daily sessions for 8 days, and a single shock probe before the sessions was used to determine how adrenocortical responsiveness changed with repeated exposure to the stress sessions. When lower intensity shock was given, the changes in plasma corticosterone response to shock probes followed a U-shaped curve--with a response that first habituated to no-shock control levels but later returned to the same magnitude as seen on the 1st probe day. Plasma corticosterone responses in rats given higher intensity shock never habituated and instead demonstrated an increased response indicative of sensitization; a temporal delay of 1 week occurred before sensitization developed. Responsiveness to exogenous adrenocorticotropin 24 hr after the last stress session was monotonically related to the intensity of the stressor presented during the experimental sessions. These data are consistent with the rule from the habituation literature that stimulus intensity is inversely related to the magnitude of habituation. Thus the data extend the dual process theory of Groves and Thompson (1970) to an endocrine respondent. The data also suggest that an explanation as to discrepancies in the literature concerning adrenocortical response to repeated presentation of stressors may relate to differences in the stressor parameters used.     

Effects of a Change in Tone Frequency on the Habituated Orienting Response of the Sleeping Rat

EEG arousal and heart rate responses to change in tone frequency following habituation training were studied during slow wave sleep in the rat. Rats were exposed to sequences of 12 tone pulses. Habituation stimuli (trials 1-7) had a frequency of 16 kHz. The frequency of the following (test) trials was changed to either 5, 12, 14, 22, or 40 kHz. Hearing level was held constant for all frequencies. Test trials were repeated five times to study rehabituation. The results indicate that heart rate deceleration is an indicator of the orienting reflex and its magnitude is a positive function of the amount of frequency change of the stimulus. Large frequency changes in either direction relative to the habituation frequency lead to re-evocation of the habituated EEG arousal and heart rate deceleration, with subsequent rehabituation. Small tone-frequency changes do not result in EEG arousal or heart rate deceleration responses that differ from the responses to the habituated frequency. Heart rate acceleration shows neither short-term habituation nor significant reaction to any of the test frequencies.     

Motor imagery of gait: a quantitative approach

Motor imagery (MI) is widely used to study cognitive aspects of the neural control of action. Prior studies were mostly centred on hand and arm movements. Recently a few studies have used imagery tasks to explore the neurophysiology of human gait, but it remains unclear how to ascertain whether subjects actually perform imagery of gait as requested. Here we describe a new experimental protocol to quantify imagery of gait, by behaviourally distinguishing it from visual imagery (VI) processes and by showing its temporal correspondence with actual gait. Fourteen young healthy subjects performed two imagery tasks and an actual walking (AW) task. During both imagery tasks subjects were sitting on a chair and faced a computer screen that presented photographs of walking trajectories. During one task (MI), subjects had to imagine walking along the walking trajectory. During the other task (VI), subjects had to imagine seeing a disc moving along the walking trajectory. During the AW task, subjects had to physically walk along the same walking trajectory as presented on the photographs during the imagery tasks. We manipulated movement distance by changing the length of the walking trajectory, and movement difficulty by changing the width of the walking trajectory. Subjects reported onset and offset of both actual and imagined movements with a button press. The time between the two button presses was taken as the imagined or actual movement time (MT). MT increased with increasing path length and decreasing path width in all three tasks. Crucially, the effect of path width on MT was significantly stronger during MI and AW than during VI. The results demonstrate a high temporal correspondence between imagined and AW, suggesting that MI taps into similar cerebral resources as those used during actual gait. These results open the possibility of using this protocol for exploring neurophysiological correlates of gait control in humans.     

Prolonged reduction of motion sickness sensitivity by visual-vestibular interaction

The angular vestibulo-ocular reflex (aVOR) and optokinetic nystagmus (OKN) were elicited simultaneously at low frequencies to study effects of habituation of the velocity storage time constant in the vestibular system on motion sickness. Twenty-nine subjects, eleven of whom were susceptible to motion sickness from common transportation, were habituated by sinusoidal rotation at 0.017?Hz at peak velocities from 5 to 20°/s, while they watched a full-field OKN stimulus. The OKN stripes rotated in the same direction and at the same frequency as the subjects, but at a higher velocity. This produced an OKN opposite in direction to the aVOR response. Motion sickness sensitivity was evaluated with off-vertical axis rotation (OVAR) and by the response to transportation before and after 5?days of visual-vestibular habituation. Habituation did not induce motion sickness or change the aVOR gains, but it shortened the vestibular time constants in all subjects. This greatly reduced motion sickness produced by OVAR and sensitivity to common transport in the motion susceptible subjects, which persisted for up to 18?weeks. Two motion susceptible subjects who only had aVOR/OKN habituation without being tested with OVAR also became asymptomatic. Normal subjects who were not habituated had no reduction in either their aVOR time constants or?motion sickness sensitivity. The opposing aVOR/OKN stimulation, which has not been studied before, was well tolerated, and for the first time was an effective technique for rapid and prolonged habituation of motion sickness without exposure to drugs or other nauseating habituation stimuli.     

Anxiety and the Habituation of the Skin Conductance Response

The effect of threat of shock on the habituation of the skin conductance response (SCR) to a series of auditory stimuli was assessed. Skin resistance and heart rate were recorded during the presentation of 20, 1 sec, 1000 Hz tones in a control session and in a session at the end of which subjects were led to expect a painful electric shock. The effects of the threat of shock consisted of recovery of the SCR and an increase in skin conductance level and heart rate. They were restricted to the period during which subjects believed the shock to be imminent. Two possible reasons for the SCR recovery were offered, one in terms of Lader and Mathews' (1968) ‘maximal habituation’ hypothesis and a second in terms of a change in the stimulus complex.     

Prepulse inhibition decreases as startle reactivity habituates

In the present study, I investigated the effect of stimulus repetition on human startle eyeblink reflex inhibition (PPI) by a prepulse. Participants were assigned to one of three groups (n= 14 each). in which they received 18 trials of (a) noise startle stimuli presented alone or preceded by 1000-Hz tone prepulses, (b) startle stimuli presented alone, or (c) prepulses presented alone. All participants then received 18 more trials of startle stimuli presented alone or preceded by 1000-Hz tone prepulses, followed by 6 trials of startle stimuli presented alone or preceded by 2000-Hz tone prepulses. As trials progressed, startle eyeblink electromyogram magnitude habituated and PPI decreased. PPI was not affected by the repeated presentation of the prepulses alone but decreased with the repeated presentation of the startle stimulus alone. Changes in the frequency of the prepulse had no significant effect on PPI. These data suggest that the reduction of PPI that is seen across trials is not due to habituation of the prepulse but is related to startle reactivity in control trials, which is reduced by habituation.     

The influence of visual information on habituation of the electrodermal and the visual orienting reaction.

In this study the influence of the information value of visual stimuli on habituation of the visual orienting reaction (VOR) and the skin conductance reaction (SCR) was investigated. 28 subjects received two blocks of 14 trials. Half the subjects received the higher information condition first and then the lower information, the other half received the conditions in the reversed order. Subjects fixated the stimuli with the higher information value longer than the stimuli with the lower information value during the 14 trials and habituated slower. This effect of information was absent in the second block and VOR habituation was also faster in this block. There was no difference in SCR amplitudes nor in SCR habituation scores between the two information conditions, neither in block 1 nor in block 2. The results of this study are discussed in relation with a two-stage model of the OR.     

Relationship of the 'fear-inhibited light reflex' to the level of state/trait anxiety in healthy subjects.

RATIONALE: It has been shown that the amplitude of the pupillary light reflex response decreases when subjects anticipate an aversive stimulus (i.e. electric shock), compared to periods when subjects are resting ('fear-inhibited light reflex'). OBJECTIVE: To examine whether the sensitivity of the pupillary light reflex to the threat of an electric shock is related to the pre-existing levels of state and trait anxiety. METHODS: Thirty-two healthy volunteers participated in one experimental session. The possibility of an electric shock to the wrist was signalled by a tone. There were six blocks of three light stimuli: three SAFE blocks (no tone applied) and three THREAT blocks (tone applied). The State-Trait Anxiety Inventory was completed at the beginning and at the end of each session. RESULTS: There was a positive correlation between the state anxiety scores and the within-subject (SAFE-THREAT) difference in light reflex amplitude (P<0.05). There was no significant correlation between the trait anxiety scores and the within-subject differences in light reflex amplitude. CONCLUSIONS: Individual differences in state anxiety associated with the threat of an electric shock are reflected in the amplitude of the pupillary light reflex response. This observation strengthens the validity of the fear-inhibited light reflex as a model of human anxiety.     

Motor imagery beyond the joint limits: A transcranial magnetic stimulation study

The processes and neural bases used for motor imagery are also used for the actual execution of correspondent movements. Humans, however, can imagine movements they cannot perform. Here we explored whether plausibility of movements is mapped on the corticospinal motor system and whether the process is influenced by visuomotor vs. kinesthetic-motor first person imagery strategy. Healthy subjects imagined performing possible or biomechanically impossible right index finger movements during single pulse TMS of the left motor cortex. We found an increase of corticospinal excitability during motor imagery which was higher for impossible than possible movements and specific for the muscle involved in the actual execution of the imagined movement. We expand our previous action observation studies, suggesting that the plausibility of a movement is computed in regions upstream the primary motor cortex, and that motor imagery is a higher-order process not fully constrained by the rules that govern motor execution.     

Delayed Habituation of the Electrodermal Orienting Response as a Function of Increased Level of Arousal

Level of arousal was manipulated by having subjects perform arithmetic tasks during a habituation procedure, which consisted of 55 presentations or a 1000 Hz. 80 dB tone. For one group a threat of shock was added to the task performance in order to include aspects of stress in the arousal manipulation. These two groups were compared with a group who heard the same stimuli but were simply instructed to relax. The three groups differed in level of arousal according to KEG signs of drowsiness, skin conductance level, and frequency of spontaneous skin conductance responses, All three groups differed from each other in number of trials to habituation criterion for skin conductance responses to stimuli. Since the delay of habituation was seen tin- both the Task group and the Shock-threat group, it was concluded that the effect was not bound to aspects of stress but was a general effect of increased arousal. For vasomotor responses an analysis in terms of habituation was difficult to apply because the two high arousal groups were very unresponsive from the beginning.     

Effects of "Below-Zero" Habituation on the Electrodermal Orienting Response to a Test Stimulus

Two experiments were designed to investigate the effect of below-zero habituation training on skin conductance response (SCR) amplitude to a change in auditory stimulus frequency. In both experiments, subjects were trained with a 1000 Hz tone until zero responding and then received 5, 10, or IS further training trials. In Experiment 1 (N=45), subjects then received 1 presentation of a test stimulus of 1400 Hz, while in Experiment 2 (N=45), the test stimulus was a tone of 670 Hz. On the basis of dual-process theory, it was hypothesized that response amplitude to the test stimulus would be inversely related to amount of below-zero training. However, the results of both experiments indicate that SCR amplitude was positively related to amount of below-zero training. These results suggest that in situations of extended habituation training, an expectancy or subjective probability of stimulus occurrence gradient is important in determining response amplitude to a test stimulus.     

Habituation and Dishabituation of the Orienting Reaction to Between and Within Trial Changes in Pitch and Loudness

We investigated the effect of between and within trial changes in pitch and loudness on the electrodermal orienting reaction (OR), perceptions of movement, and their interaction. First, following 20 habituation trials (6-s, 80dB, 1000 Hz), four groups of 10 subjects experienced either an increase or a between-trial decrease in pitch (2000 or 500 Hz) or loudness (90 or 70 dB) on the 21st trial. Second, four additional groups of 20 subjects heard 20 tones that, within each 6-s trial, either increased or decreased in pitch (500 to 2000 or 2000 to 500 Hz) or loudness (70 to 90 or 90 to 70 dB). During a dishabituation phase each group was split into halves having a nonchanging tone either increasing or decreasing relative to mean pitch or loudness. After each session subjects rated whether they perceived tones as approaching, retreating, or motionless. Compared with groups experiencing constant stimuli, groups presented tones changing within trials had greater skin conductance responses that habituated slower irrespective of direction of change or its perception. During dishabituation a similar number of subjects dishabituated to a stimulus increase as compared to a decrease irrespective of whether the change was in pitch or loudness. Perceptions of movement treated as an independent variable did not predict habituation or dishabituation of the OR. The results generally support Sokolov, and are discussed in relation to hypotheses presented by Sokolov, O'Gorman, and Bernstein.     

The influence of task relevance and stimulus information on habituation of the visual and the skin conductance orienting reaction

This study investigated the hypothesis that task-relevant stimuli induce orienting reactions (ORs) that are stronger and more resistant to habituation when their information content is high than when it is low. Task-relevance was given to the stimuli by rewarding the subjects for correct recognition at the end of the experiment. The dependent variables in this study were the visual orienting reaction (VOR), the skin conductance reaction (SCR), their habituation scores and the number of spontaneous fluctuations in skin conductance (SFs). 28 subjects received two blocks of 14 trials. Half the subjects received the higher information condition first and then the lower information condition, while the other half received the reversed order. The VOR habituated quickly and was not significantly influenced by information value. SCR amplitudes were larger and SCR-habituation slower to stimuli containing more information. The results were interpreted as supporting the hypothesis that the SCR is associated with a secondary phase of the orienting process.     

Evaluation of Cortical Connectivity During Real and Imagined Rhythmic Finger Tapping

Accumulating evidence suggests the existence of a shared neural substrate between imagined and executed movements. However, a better understanding of the mechanisms involved in the motor execution and motor imagery requires knowledge of the way the co-activated brain regions interact to each other during the particular (real or imagined) motor task. Within this general framework, the aim of the present study is to investigate the cortical activation and connectivity sub-serving real and imaginary rhythmic finger tapping, from the analysis of multi-channel electroencephalogram (EEG) scalp recordings. A sequence of 250 auditory pacing stimuli has been used for both the real and imagined right finger tapping task, with a constant inter-stimulus interval of 1.5 s length. During the motor execution, healthy subjects were asked to tap in synchrony with the regular sequence of stimulus events, whereas in the imagery condition subjects imagined themselves tapping in time with the auditory cue. To improve the spatial resolution of the scalp fields and suppress unwanted interferences, the EEG data have been spatially filtered. Further, event related synchronization and desynchronization phenomena and phase synchronization analysis have been employed for the study of functionally active brain areas and their connectivity during real and imagery finger tapping. Our results show a fronto-parietal co-activation during both real and imagined movements and similar connectivity patterns among contralateral brain areas. The results support the hypothesis that functional connectivity over the contralateral hemisphere during finger tapping is preserved in imagery. The approach and results can be regarded as indicative evidences of a new strategy for recognizing imagined movements in EEG-based brain computer interface research.     

Dishabituating long-term memory for gustatory habituation in the cabbage looper, Trichoplusia ni

The gustatory rejection response of the cabbage looper, Trichoplusia ni (Lepidoptera: Noctuidae), habituates to antifeedant compounds, allowing for the consumption of deterrent yet nontoxic plant materials. In the present study, we demonstrate that habituation to an antifeedant compound (quinine) persists through the moult between larval instars. As an indirect test of whether the memory was protein synthesis-dependent, we tested whether disrupting protein synthesis would block memory reconsolidation after a reminder. The results indicated that disrupting protein synthesis in habituated larvae following a reminder treatment (reexposure to quinine) eliminated the memory for habituation and restored the antifeedant properties of the quinine. We then examined whether the learned memory could be dishabituated and whether this would disrupt or eliminate long-term memory. We show that 6 hr after exposure to a novel/noxious stimulus (a second antifeedant, xanthotoxin) habituated larvae showed a transient dishabituation-like effect in which the quinine deterred feeding again. However, this effect did not permanently eliminate the habituation produced by the extended exposure as larvae tested 72 hr after xanthotoxin exposure again showed a willingness to consume the quinine treated leaves, indicating that the earlier habituation was still present.     

Lack of contextual modulation of habituated neuroendocrine responses to repeated audiogenic stress

Exposure to stress reliably activates the hypothalamo-pituitary-adrenocortical (HPA) axis response in rodents, which is significantly reduced (habituated) following repeated exposures. In the current study, it was first established that HPA axis response habituation to repeated loud noise lasted for at least 4 weeks in rats. In the next experiment, a contextual extinction procedure following repeated loud noise exposures failed to restore the habituated HPA axis response. Although an additional study indicated some recovery of responses when the context was modified on a test day following habituation, this effect could be mostly attributed to the familiarity with the contextual cues. A final study confirmed that rats could distinguish between the contexts used and further indicated that context preexposures reduce acute HPA axis responses to loud noise. These studies therefore provide no support for the hypothesis that contextual cues regulate HPA axis response habituation.     

Mesencephalic reticular formation lesions made after habituation training abolish long-term habituation of the acoustic startle response in rats

Lesions to the mesencephalic reticular formation (MRF) in rats severely attenuate the acquisition of long-term habituation of the startle response when the lesions are made prior to habituation training. The present experiments extend the finding of habituation deficits to animals with MRF damage made after the animals have habituated to an auditory stimulus. Following habituation training, some animals received lesions to the MRF. The startle amplitudes of these animals immediately changed from control levels to levels indistinguishable from those of animals that never habituated across days--animals with MRF lesions made prior to habituation training. The mechanism responsible for long-term habituation appears to be a progressive increase in activity within a long-term habituation pathway extrinsic to the reflex circuit for the startle response, but the synaptic mechanisms responsible for this change are unknown.     

Habituation During Sleeping and Waking

EEG and autonomic habituation were studied during sleep stages 2 and REM. and during awake. Repeated presentations of the same stimulus to the same subjects within sleep stages and when awake permitted evaluation of habituation between tone sets within stag 2, REM, and awake, and from sleep to awake. Forty-six subjects were exposed to 800 Hz. 75 dB, 1- or 2-sec tones, presented in sets of 20 tones. During steep, there was no habituation of the EEC response. Habituation occurred for both FPR and HRR during stage 2, with no evidence of within-stage. between tone set habituation. No significant habituation occurred during REM for HRR or FPR. Evoked skin resistance responses were rare during both stage 2 and RKM. In the awake state, HRR was unresponsive but all other variables showed habituation. Tones presented during the night had no effect on the awake response.