Poster
# 37
Poster |
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6th Internet World Congress for Biomedical Sciences |
Marcelle Bartolo Abela(1)
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[Neuroscience] |
[Physiology] |
The two main current theories of hypnosis are those of neo-dissociation and dissociated control, the former postulated from Janet´s theory (Hilgard 1976) and maintaining that responses are due to a division and co-existence of consciousness into two or more simultaneous streams which are separated by an amnestic barrier preventing access to suggestion-related executive functions, monitoring functions, or both (Kirsch and Lynn 1998, Woody and Bowers 1994), but which maintain realistic, logical relations among themselves. The dissociated part in Hilgard´s theory is the postulated "hidden observer," i.e., that part of a person´s mind knowing about the presence of pain but which that other conscious part of him knows nothing about (Woody and Bowers 1994). From this theory arose an advanced modified version known as the dissociated control theory of hypnosis (Bowers 1992), which maintains that hypnotic inductions weaken frontal control of behavioral schemas, thereby allowing direct activation of behavior by the hypnotist´s suggestions (Kirsch and Lynn 1998).
Support for this is found in two recent dissertations by Hughes (1988) and Miller (1986) respectively, which show that hypnotic behavior can be purposeful (i.e., the suggested state of affairs is [or can be] achieved) and nonvolitional (the suggested state of affairs is not achieved by high-level executive initiative and ongoing effort), thus making it easily integrated as a concept with current views of frontal executive function (Woody and Farvolden 1998) - i.e., the representative two-tier control model of volition (Norman and Shallice 1986). Briefly, this model maintains that multiple subsystems interact to coordinate goals and actions, which are controlled by two qualitatively different mechanisms, i.e, the decentralized lower-level contention-scheduling mechanism which handles relatively routine selections and behaviors not requiring conscious or attentional control, and the higher-level supervisory attentional system (SAS) which intervenes in novel or competitive situations to govern non-routine actions in a qualitatively different, centralized manner (Woody and Farvolden 1998, Zigmond et al 1999). So the SAS, posited to involve the frontal lobes and limbic system (Posner and Peterson 1990, Shallice and Burgess 1991), influences behavior indirectly by modulating the lower-level system and by contributing extra activation and inhibition to particular schemas, consequently biasing the schema selection process of the contention-scheduling system (Woody and Farvolden 1998).
According to this model, the experience of volition (i.e., will) is associated with SAS involvement in the initiation and control of behavior, and if the SAS is actively modulating the selection of schemas (what can be interpreted as conscious filtering), the individual has the phenomenal experience of will, or deliberate volitional control. Alternatively, if the SAS neither modulates nor monitors the contention-scheduling system, then the person experiences the action as automatic (ibid.), i.e., as immediately following the idea of it in the mind - a circumstance termed an ideo-motor act (Norman and Shallice 1986). However, such a model is only partially in keeping with findings from experimental intracranial electrophysiological studies, because these have shown that deliberate volitional control - the phenomenal experience of will - is not as free as traditionally defined (i.e., taken to include a conscious intention to act, and a conscious ability to control such an act), because results of investigations (Libet et al 1982, 1983a, b, Libet 1985) of cerebral "time-on" theory (this states that the transition from an unconscious mental event to one that reaches awareness and is consciously experienced, can be a function of a sufficient increase in the duration or "time-on" of appropriate neural activities [Libet 1989]) have demonstrated that the performance of even a freely voluntary act is initiated unconsciously, some 350 msec before the individual becomes consciously aware of wanting to move, and that it is the conscious control of whether to carry out the act, which will actually still be performed during the remaining 150 to 200 msec before activating the muscles (ibid.). This theory is also indirectly supported by the presence of an error detection system operating at an early, i.e., possibly pre-conscious stage during Stroop-like tasks with hypnosis, and which was not found to be compromised by the latter (Gruzelier 1998).
So since conscious (volitional) control appears only after awareness of the wish to move has developed - the control process depending upon prior awareness of the volitional direction, but not being an awareness in itself (ibid.) - in the case of the SAS its activation is also unconsciously-initiated, because volitional process starts with unconscious cerebral activity (Libet et al. 1991): the transition between psychological detection of a sensory signal without awareness, and detection with awareness, has been found to be controlled simply by differences in duration of repetitive ascending activations of the sensory cortex, with a minimum duration of up to approximately 500 msec being necessary to elicit a conscious experience of an event, while appropriate neural activity having a duration briefer than that required for awareness mediates unconscious mental functions, but without any subjective awareness of them (Libet 1993, Taylor and McCloskey 1990). Therefore, although the role of free will is not excluded, Freud´s deterministic stance is somewhat supported, as free will is changed from being an initiator of the voluntary act as commonly believed to one of only controlling the outcome of the volitional process, after the individual becomes aware of an intention or wish to act (Libet 1991). This also provides indirect support for the dissociated control theory of hypnosis, physiological support for which has been even more forthcoming from the rCBF increases in the caudal part of the right anterior cingulate gyrus (i.e., Brodmann´s area 32 - a powerful behavioral part of the limbic system), and the fact that hallucination of auditory stimuli also activates this area similarly to the actual hearing of such stimuli, but not similarly to what happens in imagined hearing (Szechtman et al 1998) - in fact, there is activation of the temporal areas, which is considered to reflect acoustical attention (Meyer et al 1989).
The anterior cingulate gyrus, found to be engaged in the processing of pain (Casey et al 1994, Davis et al 1995, Talbot et al 1991), is a portion of the limbic system that communicates between the prefrontal cerebral cortex and subcortical limbic structures - the limbic system being the entire neuronal circuitry controlling emotional behavior and motivational drives (Guyton 1992), and performs executive functions which are subdivided into affective and cognitive components - the former being involved in the regulation of autonomic and endocrine functions, assessment of motivational context, and the significance of sensory stimuli and emotional valence, while the latter are involved in response selection processing such as Stroop interference (Devinsky, Morrell, and Vogt 1995). Metabolic activity in this gyrus has been found to increase when people generate semantic associates to words, and when a situation requiring divided attention is present, as evidenced by divided-attention versus selective-attention tasks (Gazzaniga, Ivry, and Mangun 1998). Therefore, since divided-attention conditions require a higher-level attentional system which simultaneously monitors information across the specialized modules, this function conforms to the attributes of an SAS, causing the anterior cingulate to be implicated during planning or decision-making, error correction, novel and not-well-learned responses, situations regarded as difficult or dangerous, and the overcoming of habitual responses (Posner 1994) - during a PET study of language where activation in a repeat condition was subtracted from the generate condition, greater blood flow was found to consistently occur in the dorsolateral prefrontal cortex and anterior cingulate (Peterson et al 1988), a result similar to the rCBF increases found during hypnosis.
So activation of the anterior cingulate gyrus during hypnosis is related to the cingulate´s establishing a node in the working-memory system of the lateral prefrontal cortex to hold representations retrieved from longer-term semantic representations of word meanings in the posterior cortex (presumably Wernicke´s area), and as processing spreads among the latter´s semantic network, the working-memory system inhibits representations of irrelevant associates, allowing task-relevant associates to be sufficiently activated. This permits the SAS to allow the task´s goal to influence interactions between working and long-term memory (Gazzaniga, Ivry, and Mangun 1998) and is in keeping with the neuropsychological translation of hypnotic induction where hypnosis is initiated by engaging anterior executive control systems which orchestrate top down changes influencing thalamic and brainstem mechanisms (Gruzelier 1998). The anterior cingulate gyrus and the dorsolateral prefrontal cortex are also implicated in the volitional system - a totally different system from stimulus-driven action (Frith 1992, 1995), so under hypnosis it is considered that anterior, frontal lobe functions become engaged through instructions of focusing attention (left hemispheric frontotemporal processing [Gruzelier 1998]) and once engaged become inhibited, with such inhibition underpinning the suspension of reality testing, abdication of planning functions, and reduced attentional monitoring of external cues (Gruzelier and Warren 1993).
Even more support for the dissociated control theory is provided by the hypnotic responsiveness of high hypnotizables, who respond non-volitionally when compared to lows who seem to respond more intentionally (King and Council 1998), making the former group´s responsiveness more likely to result from dissociated control. In this case, hypnotic suggestions more often directly activate subsystems of cognitive control (ibid., Bowers 1992) in keeping with Norman and Shallice´s model, while compliance and social influence is more apt to account for the low hypnotizables´ responsiveness (Bowers 1992). Such agrees with evidence of dissociations between explicit and implicit memory and perception in hypnosis (Kilhstrom 1998) and is confirmed by the deterioration in performance by low hypnotizables when the structure of hypnotic suggestions precludes the use of absorption rather than dissociation (Bartis and Zamansky 1990). This implies that lows respond to suggestions only by assimilating them and not by dissociation, a finding supported by evidence showing that highs use imagery to dissociate effectively, while lows perceive and assimilate suggestions by means of mental math (Ray 1997), which can also be considered to account for the greater levels of emotional experiences of highs in comparison to lows, the former group´s increased ability to access affect (De Pascalis et al 1987, 1989, 1998, 1999), as well as their ability to reduce both the sensory and motivational components of pain (Price and Barber 1987) achieving better pain control (Crawford et al 1998), conversely to lows who are less able, or unable, to reduce the sensory-discriminative component of pain (Price and Barber 1987).
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[Neuroscience] |
[Physiology] |
