Death feigning by grass snakes (Natrix natrix) in response to handling by human "predators."

Death feigning, a variant of tonic immobility, is usually interpreted as a last-resort antipredator measure. The authors describe death feigning in grass snakes (Natrix natrix) and test some of its potential correlates. Death feigning was seen in 66% of wild-caught snakes but was not seen in hatchlings from laboratory-incubated eggs. Minimal indication of death feigning was mouth gaping, often with the tongue hanging free, but more dramatic cases involved voluntary supination and/or lack of muscle tone. Aside from hatchlings, which did not feign death, there was little variation in frequency or intensity of death feigning with body size. There was no effect of body temperature on death feigning nor were snakes that were moving when caught less likely to feign death than those that were not moving. Interpretation of the adaptive value of death feigning in grass snakes or in other animals is hampered by lack of evidence of this behavior in the field in response to natural predators.     

Cholinergic modulation of tonic immobility in the rabbit (Oryctolagus cuniculus).

The present study was performed to determine the effects of the anticholinergic agent scopolamine and the cholinergic agent physostigmine on tonic immobility in rabbits. Recordings of the electroencephalographic (EEG) activity from cortex and hippocampus were also made before, during, and after each test session. Scopolamine significantly prolonged the response and produced large amplitude slow wave activity in the EEG of both cortex and hippocampus. Physostigmine significantly shortened the duration of immobility and increased rhythmic slow activity in the frequency range of 5.5-9.1 Hz in the hippocampus while producing a desynchronized cortical rhythm. It is suggested that the cortex and hippocampus play a role in modulating tonic immobility duration by inhibiting the brain-stem structures thought to control this response.     

Tonic immobility in the blue crab (Callinectes sapidus, Rathbun) its relation to threat of predation.

The tonic immobility. response in the blue crab (Callinectes sapidus, Rathbun) was investigated in a series of seven experiments. Although reported to be a powerful variable in other species, preinduction electric shock produced inconsistent increases in the duration of tonic immobility with the blue crab; Manipulations that were more directly relevant to the fear of predation had considerably greater effects than shock. Physical damage to the chelipeds, mirror image stimulation, and immobilization beneath artificial glass eyes all produced significant prolongation of the immobility episode; Crabs immobilized on a bed of sand rather than on a hard surface showed shorter immobility durations, suggesting that opportunity for escape is an important variable affecting the immobility reactionmthe present results support the contention that threat of predation is the organizing principle behind tonic immobility     

The tonic immobility reaction in chickens: Response characteristics and methodology

A total of 379 chickens were run in five studies to assess the effect of different procedures on tonic immobility and to measure some of the response components of the reaction. The optimal period of manual restraint, by way of producing the most durable immobility response, was found to be 15 sec, and pretest holding conditions were shown to exert a significant influence on the reaction in naive birds. Birds who defecated during immobility or shortly after termination, remained immobile longer than comparable nondefecators. Duration of eye closure and latency of vocalization were also found to be good predictors of the duration of an immobility episode.     

Changes in Immobility Responses in Rat Pups with Maternal Stimuli

Presentation of maternal stimuli to an isolated rat pup results in what has been called “comfort” responses, as indicated by both behavioral and physiological quieting. This experiment investigated the role of passive maternal stimuli on tonic immobility and dorsal immobility in 9- and 15-day-old rat pups. Although these stimuli appeared ineffective in inducing tonic immobility, presentation of the mother did produce an increase in duration of dorsal immobility in 15-day-old pups. This increase in the dorsal immobility response may reduce struggling in the presence of the mother and thereby aid the mother in transporting the infant back to the nest.     

"Scared stiff": catatonia as an evolutionary-based fear response

Catatonia, long viewed as a motor disorder, may be better understood as a fear response, akin to the animal defense strategy tonic immobility (after G. G. Gallup & J. D. Maser, 1977). This proposal, consistent with K. L. Kahlbaum's (1874/1973) original conception, is based on similarities between catatonia and tonic immobility ("death feint") as well as evidence that catatonia is associated with anxiety and agitated depression and responds dramatically to benzodiazepines. It is argued that catatonia originally derived from ancestral encounters with carnivores whose predatory instincts were triggered by movement but is now inappropriately expressed in very different modern threat situations. Found in a wide range of psychiatric and serious medical conditions, catatonia may represent a common "end state" response to feelings of imminent doom and can serve as a template to understand other psychiatric disorders.     

Effect of different parameters of shock on tonic immobility

Four separate experiments were conducted with chickens to explore the effects of various parameters of electric shock on tonic immobility. Immobilizing subjects at varying times after the receipt of shock affected the duration of immobility only after delays of over 15 min. Duration of shock affected immobility differently, depending upon whether alternating current or continuous direct current was used. The effects of continuous dc, pulsating dc. and ac shock were also examined. The results were discussed in terms of shock onset-offset characteristics being an important consideration.     

Traumatic events and tonic immobility

Tonic immobility is a basic defense strategy which has not been studied in depth in humans. Data suggest that it may be a relatively frequent phenomenon in victims of rape and sexual abuse, but its occurrence has not been systematically explored in other types of trauma. We carried out a retrospective study in a sample of 100 university students to establish whether tonic immobility varies depending on the nature of the worst trauma experienced, defined subjectively by each participant. Immobility was assessed using the Tonic Immobility Scale and traumas were assessed using the modified Traumatic Events Questionnaire. Seventy percent of the sample had experienced trauma of some kind. There were no significant differences in tonic immobility between different types of trauma (e.g., physical abuse, assault or aggression, serious accident), except that the mean tonic immobility score was significantly higher in the group with trauma due to physical/psychological or sexual abuse than in the group with trauma due to receiving news of the mutilation, serious injury, or violent or sudden death of a loved one. We conclude tentatively that tonic immobility may be typical not only of sexual traumas, but of other kinds of directly experienced traumas as well.     

Tonic immobility in chickens: possible involvement of monoamines.

Tonic immobility in chickens was affected by a variety of drugs that act on monoaminergic systems. Compounds that enhanced the duration of tonic immobility were d-LSD, BOL-148, pargyline, and iproniazid; 5-hydroxytryptophan, p-CPA, and atropine had no effect. Injections of serotonin depressed response duration. A suggestive parallel was noted between the results of the present study and those of previous work reporting drug-induced suppression of raphe electrical activity. The data appear to implicate monoamines, especially serotonin, in the mediation of behavioral activation and suppression.     

Immobility reactions: A modified classification

This theoretical paper sets the stage for subsequent experimental reports on mobility and immobility in the Arkansas Line of Nervous Pointer dogs as contrasted with kennel mates of the normal line. Exactly opposite to the normal animals, the nervous dogs show marked inhibition of heart rate and musculoskeletal responses to man, including reduced ambulation and durable immobility following inversion and brief manual restraint in an open sling. The sling immobility of the nervous dogs (which may not differ basically from their freezing in upright position) might be designated as “tonic immobility” (TI). We hypothesize that such immobility, common in laboratory and natural conditions in many species, should be divided into two classes: hypotonic (cataleptic) and hypertonic (catatonic). We provide examples of such behaviors, particularly in man and dog, and cursorily review TI studies of other species. Neurophysiologic and neurochemical studies which bear on possible immobility mechanisms are briefly noted. We suggest that inconsistencies in reported concommitants of TI might result from failure to discriminate between types of behavioral responses.     

Tonic immobility and childhood sexual abuse: a preliminary report evaluating the sequela of rape-induced paralysis

The present study examined tonic immobility (TI) in victims of childhood sexual abuse (CSA). Female undergraduates (n=39) and female psychiatric inpatients (n=41) who experienced CSA completed a series of questionnaires assessing aspects of their victimization history, psychological functioning, and TI symptoms. Over fifty-two percent of all participants reported TI in response to CSA. Episodes of CSA involving attempted or completed penile/vaginal penetration were more likely to be associated with an increased likelihood of experiencing TI, and report of TI was associated with greater current psychological impairment. The implications of these findings are discussed, and suggestions for future research are offered.     

Fear of snakes and facial reactions: A case of rapid emotional responding

The aim of the present study was to explore how rapid emotional responses are manifested as facial electromyographic (EMG) reactions when people with explicit fear of snakes are exposed to their fear relevant stimuli. Fifty-six subjects, high or low in fear of snakes, were exposed to pictures of snakes and flowers while facial EMG activity from the corrugator supercilii and the zygomatic major muscle regions was recorded. Measures of autonomic activity and ratings of the stimuli were also collected. Pictures of snakes evoked a rapid corrugator supercilii muscle reaction which was larger in the High fear group as early as 500ms after stimulus onset. The High fear group also rated snakes as more unpleasant and displayed larger skin conductance responses (SCRs) and increased heart rate (HR) when exposed to snakes. Pictures of flowers tended to evoke increased zygomatic major muscle activity which did not differ among the groups. The present results demonstrate that the facial EMG technique is sensitive enough to detect rapidly evoked negative emotional reactions. The results support the hypothesis that people high in fear of snakes are disposed to react very rapidly with a negative emotional response to their fear relevant stimuli.     

The importance of the peritraumatic experience in defining traumatic stress

In the Diagnostic and Statistical Manual of Mental Disorders (4th ed., text rev., DSM-IV-TR; American Psychiatric Association, 2000), posttraumatic stress disorder (PTSD) Criterion A2 stipulates that an individual must experience intense fear, helplessness, or horror during an event that threatened the life or physical integrity of oneself or others to be eligible for the PTSD diagnosis. In considering this criterion, we describe its origins, review studies that have examined its predictive validity, and reflect on the intended purpose of the criterion and how it complements the mission of the DSM. We then assert that the predictive validity of Criterion A2 may not be an appropriate metric for evaluating its worth. We also note that the current Criterion A2 may not fully capture all the salient aspects of the traumatic stress response. To support this claim, we review empirical research showing that individuals adapt to extreme environmental events by responding in a complex and coordinated manner. This complex response set involves an individual's appraisal regarding the degree to which the event taxes his or her resources, as well as a range of other cognitions (e.g., dissociation), felt emotions (e.g., fear), physiological reactions (e.g., heart rate increase), and behaviors (e.g., tonic immobility). We provide evidence that these response components may be associated with the subsequent development of PTSD. We then describe the challenges associated with accurately assessing an individual's traumatic stress response. We conclude with a discussion of the need to consider the individual's immediate response when defining a traumatic stressor.     

Functional referents and acoustic similarity revisited: the case of Barbary macaque alarm calls

Barbary macaques (Macaca sylvanus) utter "shrill barks" in response to disturbances in their surroundings. In some cases, the majority of group members react by running away or climbing up a tree. In many other instances, however, group members show no overt reaction to these calls. We conducted a series of playback experiments to identify the factors underlying subjects' responses. We presented calls given in response to dogs that had elicited escape responses and calls that had failed to do so. We also presented calls given in response to snakes and to the observer approaching the sleeping-trees at night. An acoustic analysis of the calls presented in the playback experiments (electronic supplementary material, audioclip S1) revealed significant differences among calls given in response to dogs, the observer approaching at night, and snakes. However, the analysis did not detect any differences between calls given in response to dogs that were related to whether or not they had elicited escape responses in the first place. Correspondingly, after playback of calls given in response to dogs, we observed no difference in subjects' responses in relation to whether or not the calls had initially elicited escape responses. Subjects showed startle or escape responses significantly more often after playbacks of calls given in response to dogs than after calls given in response to observers. Playbacks of calls given in response to snakes failed to elicit specific responses such as standing bipedally or scanning the grass. Although these findings may imply that responses depend on the external referent, they also indicate that there is no clear-cut relationship between the information available to the listeners and their subsequent responses. This insight forces us to extend current approaches to identifying the meaning of animal signals. Accepted after revision: 15 May 2001 Electronic Publication     

What's so special about slithering serpents? Children and adults rapidly detect snakes based on their simple features

Snakes are among the most common targets of fears and phobias around the world. In visual search tasks, both adults and young children have repeatedly been found to visually detect snakes more rapidly than other kinds of stimuli. An important question that remains unstudied is what accounts for humans’ rapid response to snakes? Here we suggest that specific features of snakes themselves lead to their rapid detection. The results of five experiments suggest that a snake's shape is the crucial factor in their rapid detection.     

Adaptation to optically-increased interocular separation under naturalistic viewing conditions

Mirror spectacles which enhance binocular disparity by optically doubling the normal separation between the eyes were used to create conditions of combined perceptual and oculomotor conflict. Apparent depth and distance, as well as tonic accommodation, tonic vergence, and accommodative-vergence gain (response AC/A ratio), were assessed immediately before and after a 30 min exposure period of naturalistic viewing with the spectacles. Wearing the spectacles produced an increase in tonic vergence, and perceptual aftereffects consisting of increased apparent distance and depth. The results indicate that oculomotor conflict associated with enhanced interocular separation may be resolved through adaptation of tonic vergence, rather than through alteration of accommodative-vergence gain. The results also demonstrate that perceptual conflict between disparity and multiple veridical depth cues does not necessarily produce adaptive modification of the relationship between binocular disparity and apparent depth.     

Facial reactions to fear-relevant stimuli for subjects high and low in specific fear

This study was performed to investigate whether subjects high and low in fear of snakes react with different facial electromyographic (EMG) responses when exposed to snakes. Two groups of subjects were exposed to slides of snakes or flowers. After the experiment they rated their fear of snakes on a questionnaire and based on these ratings they were divided into high and low fear groups. Facial EMG was measured from the corrugator and zygomatic muscle regions. As predicted the high fear group reacted with a facial response interpretable as a negative emotional reaction whereas the low fear group did not. This difference was specific to snakes because high and low groups did not differ in responses to slides of flowers. The results are consistent with the proposition that facial EMG technique is a sensitive tool to distinguish reactions between subjects high and low in specific fears.     

Assessment of retention capacities in old rats

Young (3-6 month) and old (24-27 month) barrier reared Wistar rats were tested for their ability to retain an inhibitory (passive) avoidance, acquired immobility, and a conditioned taste aversion response as a function of time. Old rats exhibited accelerated forgetting of both the inhibitory avoidance and acquired immobility response in comparison to young rats. In contrast, old rats displayed good retention of the conditioned taste aversion response at all time intervals tested. It appears that the dynamic aspects of retention are altered in aged rats depending on the task, and in some instances may be expressed as accelerated forgetting.     

The detection of fear-relevant stimuli: are guns noticed as quickly as snakes?

Potentially dangerous stimuli are important contenders for the capture of visual-spatial attention, and it has been suggested that an evolved fear module is preferentially activated by stimuli that are fear relevant in a phylogenetic sense (e.g., snakes, spiders, angry faces). In this study, a visual search task was used to test this hypothesis by directly contrasting phylogenetically (snakes) and ontogenetically (guns) fear-relevant stimuli. Results showed that the modern threat was detected as efficiently as the more ancient threat. Thus, both guns and snakes attracted attention more effectively than neutral stimuli (flowers, mushrooms, and toasters). These results support a threat superiority effect but not one that is preferentially accessed by threat-related stimuli of phylogenetic origin. The results are consistent with the view that faster detection of threat in visual search tasks may be more accurately characterized as relevance superiority effects rather than as threat superiority effects.     

Divergent selection for duration of tonic immobility and social reinstatement behavior in Japanese quail (Coturnix coturnix japonica) chicks

Japanese quail (Coturnix coturnix japonica) were selected over 8 generations for long or short duration of tonic immobility (TI) weighted for independence from social reinstatement (SR) behavior (as measured in a treadmill) and for high or low levels of SR behavior weighted for independence from duration of TI. Divergent selection for long or short duration of TI and for high or low levels of SR behavior resulted in immediate and significant responses that continued throughout the 8 generations. There was no evidence of a plateau in the response to selection in any of the lines. SR responses did not vary in lines selected for duration of TI and vice versa. Selection for short duration of TI led to in an increase in the number of inductions required to induce TI. Selection for low levels of SR behavior led to a decrease in the tendency to move toward conspecifics but not to an increase in the tendency to move away from conspecifics.