Devin Blair Terhune

Enhanced cortical excitability in grapheme-color synesthesia and its modulation.

Summary Synesthesia is an unusual condition characterized by the over-binding of two or more features and the concomitant automatic and conscious experience of atypical, ancillary images or perceptions [1–3]. Previous research suggests that synesthetes display enhanced modality-specific perceptual processing [4–7], but it remains unclear whether enhanced processing contributes to conscious awareness of color photisms. In three experiments, we investigated whether grapheme-color synesthesia is characterized by enhanced cortical excitability in primary visual cortex and the role played by this hyperexcitability in the expression of synesthesia. Using transcranial magnetic stimulation, we show that synesthetes display 3-fold lower phosphene thresholds than controls during stimulation of the primary visual cortex. We next used transcranial direct current stimulation to discriminate between two competing hypotheses of the role of hyperexcitability in the expression of synesthesia. We demonstrate that synesthesia can be selectively augmented with cathodal stimulation and attenuated with anodal stimulation of primary visual cortex. A control task revealed that the effect of the brain stimulation was specific to the experience of synesthesia. These results indicate that hyperexcitability acts as a source of noise in visual cortex that influences the availability of the neuronal signals underlying conscious awareness of synesthetic photisms.

Dissociative tendencies and individual differences in high hypnotic suggestibility

Introduction. Inconsistencies in the relationship between dissociation and hypnosis may result from heterogeneity among highly suggestible individuals, in particular the existence of distinct highly suggestible subtypes that are of relevance to models of psychopathology and the consequences of trauma. This study contrasted highly suggestible subtypes high or low in dissociation on measures of hypnotic responding, cognitive functioning, and psychopathology. Methods. Twenty-one low suggestible (LS), 19 low dissociative highly suggestible (LDHS), and 11 high dissociative highly suggestible (HDHS) participants were administered hypnotic suggestibility scales and completed measures of free recall, working memory capacity, imagery, fantasy-proneness, psychopathology, and exposure to stressful life events. Results. HDHS participants were more responsive to positive and negative hallucination suggestions and experienced greater involuntariness during hypnotic responding. They also exhibited impaired working memory capacity, elevated pathological fantasy and dissociative symptomatology, and a greater incidence of exposure to stressful life events. In contrast, LDHS participants displayed superior object visual imagery. Conclusions. These results provide further evidence for two highly suggestible subtypes: a dissociative subtype characterised by deficits in executive functioning and a predisposition to psychopathology, and a subtype that exhibits superior imagery and no observable deficits in functioning.

The incidence and determinants of visual phenomenology during out-of-body experiences

The visual content of out-of-body experiences (OBEs) has received little attention but a number of theories of OBEs include implicit predictions regarding the determinants of this phenomenological feature. Hypnagogic imagery and unusual sleep experiences, weak synaesthesia and preference for employing object and spatial visual imagic cognitive styles were psychometrically measured along with the incidence of self-reported OBEs and the absence or presence of visual content therein, in a sample of individuals drawn from the general population. Seventy percent of individuals who had experienced an OBE reported that the experience included some form of visual content. These individuals exhibited greater scores on the measures of preference for object visual imagic cognition and weak synaesthesia than those who reported an absence of visual content during their OBE. Subsequent analysis revealed that the measure of weak synaesthesia was the stronger discriminator of the two cohorts. The results are discussed within the context of the synaesthetic model of visual phenomenology during OBEs (Brugger, 2000; Irwin, 2000). This account proposes that visual content appears during these experiences through a process of cognitive dedifferentiation in which visual hallucinations are derived from available non-visual sensory cues and that such dedifferentiation is made possible through an underlying characteristic hyperconnectivity of cortical structures regulating vestibular and visual representations of the body and those responsible for the rotation of environmental objects. Predictions derived from this account and suggestions for future research are proffered.

The neurophenomenology of neutral hypnosis

INTRODUCTION After a hypnotic induction, medium and highly hypnotizable individuals often report spontaneous alterations in various dimensions of consciousness. Few studies investigating these experiences have controlled for the inherent demands of specific hypnotic suggestions and fewer still have considered their dynamic properties and neural correlates. METHODS We adopted a neurophenomenological approach to investigate neutral hypnosis, which involves no specific suggestion other than to go into hypnosis, with 37 individuals of high, medium, and low hypnotizability (Highs, Mediums, and Lows). Their reports of depth and spontaneous experience at baseline, following a hypnotic induction, and then after multiple rest periods were analyzed and related to EEG frequency band power and global functional connectivity. RESULTS Hypnotizability was marginally associated with lower global functional connectivity during hypnosis. Perceived hypnotic depth increased substantially after the induction especially among Highs and then Mediums, but remained almost unchanged among Lows. In the sample as a whole, depth correlated moderately to strongly with power and/or power heterogeneity for the fast EEG frequencies of beta2, beta3, and gamma, but independently only among Highs. The spontaneous phenomenology of Lows referred primarily to the ongoing experiment and everyday concerns, those of Mediums to vestibular and other bodily experiences, and those of Highs to imagery and positive affect/exceptional experiences. The latter two phenomena were associated with lower global functional connectivity during hypnosis. Imagery correlated positively with gamma power heterogeneity and negatively with alpha1 power heterogeneity. Generally, the pattern of correlations for the Highs was the opposite of that for the Lows. CONCLUSIONS Experienced hypnotic depth and spontaneous phenomena following a neutral hypnotic induction vary as a function of hypnotizability and are related to global functional connectivity and EEG band wave activity.

Differential patterns of spontaneous experiential response to a hypnotic induction: A latent profile analysis

A hypnotic induction produces different patterns of spontaneous experiences across individuals. The magnitude and characteristics of these responses covary moderately with hypnotic suggestibility, but also differ within levels of hypnotic suggestibility. This study sought to identify discrete phenomenological profiles in response to a hypnotic induction and assess whether experiential variability among highly suggestible individuals matches the phenomenological profiles predicted by dissociative typological models of high hypnotic suggestibility. Phenomenological state scores indexed in reference to a resting epoch during hypnosis were submitted to a latent profile analysis. The profiles in the derived four-class solution differed in multiple experiential dimensions and hypnotic suggestibility. Highly suggestible individuals were distributed across two classes that exhibited response patterns suggesting an inward attention subtype and a dissociative subtype. These results provide support for dissociative typological models of high hypnotic suggestibility and indicate that highly suggestible individuals do not display a uniform response to a hypnotic induction.

Differential frontal-parietal phase synchrony during hypnosis as a function of hypnotic suggestibility.

Spontaneous dissociative alterations in awareness and perception among highly suggestible individuals following a hypnotic induction may result from disruptions in the functional coordination of the frontal-parietal network. We recorded EEG and self-reported state dissociation in control and hypnosis conditions in two sessions with low and highly suggestible participants. Highly suggestible participants reliably experienced greater state dissociation and exhibited lower frontal-parietal phase synchrony in the alpha2 frequency band during hypnosis than low suggestible participants. These findings suggest that highly suggestible individuals exhibit a disruption of the frontal-parietal network that is only observable following a hypnotic induction.

The induction of synaesthesia with chemical agents: a systematic review

Despite the general consensus that synaesthesia emerges at an early developmental stage and is only rarely acquired during adulthood, the transient induction of synaesthesia with chemical agents has been frequently reported in research on different psychoactive substances. Nevertheless, these effects remain poorly understood and have not been systematically incorporated. Here we review the known published studies in which chemical agents were observed to elicit synaesthesia. Across studies there is consistent evidence that serotonin agonists elicit transient experiences of synaesthesia. Despite convergent results across studies, studies investigating the induction of synaesthesia with chemical agents have numerous methodological limitations and little experimental research has been conducted. Cumulatively, these studies implicate the serotonergic system in synaesthesia and have implications for the neurochemical mechanisms underlying this phenomenon but methodological limitations in this research area preclude making firm conclusions regarding whether chemical agents can induce genuine synaesthesia.

Subjective duration as a signature of coding efficiency: Emerging links among stimulus repetition, predictive coding, and cortical GABA levels

Immediate repetition of a stimulus reduces its apparent duration relative to a novel item. Recent work indicates that this may reflect suppressed cortical responses to repeated stimuli, arising from neural adaptation and/or the predictive coding of expected stimuli. This article summarizes recent behavioral and neurobiological studies linking perceived time to the magnitude of cortical responses, including work suggesting that variations in GABA-mediated cortical inhibition may underlie some of the individual differences in time perception. We suggest that the firing of cortical neurons can be modified using simple recurrent networks with time-dependent processes that are modulated by GABA levels. These local networks feed into a core-timing network used to integrate across stimulus inputs/modalities, thereby allowing for the coordination of multiple duration ranges and effector systems.

Disruption of synaesthesia by posthypnotic suggestion: An ERP study

This study examined whether the behavioral and electrophysiological correlates of synaesthetic response conflict could be disrupted by posthypnotic suggestion. We recorded event-related brain potentials while a highly suggestible face-color synaesthete and matched controls viewed congruently and incongruently colored faces in a color-naming task. The synaesthete, but not the controls, displayed slower response times, and greater P1 and sustained N400 ERP components over frontal-midline electrodes for incongruent than congruent faces. The behavioral and N400 markers of response conflict, but not the P1, were abolished following a posthypnotic suggestion for the termination of the participants synaesthesia and reinstated following the cancellation of the suggestion. These findings demonstrate that the conscious experience of synaesthesia can be temporarily abolished by cognitive control.

Hypnotic Experience is Related to Emotional Contagion.

Abstract The authors conducted 2 studies to evaluate whether emotional contagion, the propensity to automatically imitate the emotional expressions of others and experience the corresponding emotions, is related to behavioral and experiential indices of hypnotizability and whether such a relationship is influenced by administration context. In Study 1, behavioral and subjective measures of hypnotizability were measured alongside emotional contagion in the same context. In Study 2, different measures of hypnotizability and hypnotic depth were administered, whereas emotional contagion was independently measured in a different (nonhypnotic) context. Emotional contagion correlated with behavioral and experiential indices of hypnotizability in Study 1 but only with the latter in Study 2. The authors interpret the results as reflecting a positive relationship between emotional contagion and, at least, experiential features of hypnotizability and strengthening the case for the importance of affectivity in hypnotic responsiveness.