Background
Working memory (WM) is regarded as the “sketchpad of conscious thought” [
1], a cognitive capacity that holds and manipulates a limited amount of information for a short time in order to produce a response [
2]. Multiple brain regions are involved in WM, and WM impairment is one of the most prevalent symptoms after acquired brain injury (ABI) [
3], regardless of lesion etiology, localization, and severity [
4]. WM is a central cognitive ability upon which rehabilitation for other functions depends and is a predictor for rehabilitation outcome, activity of daily living (ADL), physical rehabilitation, need for community services after discharge from hospital, community participation, and occupational status [
5‐
10]. Rehabilitation efforts to improve high-level cognitive functioning following ABI have yielded limited clinically relevant effects so far [
11‐
13]. Despite repeated administrations, reviews of biological interventions such as pharmaceuticals [
14], noninvasive brain stimulation [
15], physical exercise [
16], and nutrition [
17] show effect sizes in the zero to moderate range. Mindfulness can potentially reduce ABI-related fatigue. However, no significant effect on WM is found [
18]. Treatment of WM through computerized training programs is widespread, regardless of systematic reviews reporting small to no clinically WM-relevant changes [
19]. A comprehensive review of the brain-training literature reported no evidence for improvements on “far-transfer” cognitive abilities [
19]. In other words, the improvements failed to generalize to other capacity-dependent activities [
20]. Previous WM rehabilitation efforts have typically focused on “bottom-up” training programs that aim to restore function through repeated drills and graded exercise [
12], where transfer of effects to untrained functional domains has not been documented.
In contrast to the train and drill bottom-up approach, the use of clinical hypnoses represents a “top-down” approach and has been suggested by Lindeløv and colleagues [
4], to improve WM performance and to potentially enable generalization to other contexts.
The hypnotic state is a condition of openness to suggestions so that they may be used to elicit changes in a diverse array of psychological and bodily functions [
21]. Reviews of hypnotic treatment in psychology [
22] and medicine [
23] are impressive, with demonstrated efficacy for anxiety [
24], depression [
25,
26], chronic pain [
27], and headache [
28] as an adjunct or alternative to anesthesia during surgery [
29], in neurorehabilitation for motor disorders [
30,
31], pain and vertigo [
32], and aphasia [
33]. Recent publications have made advancements in identifying the neural correlates underlying the hypnotic state [
34]. Recently, hypnotic suggestions were found to enhance updating in WM in healthy adults, which was associated with changes in event-related potentials (ERP) in WM-related regions of the brain [
35], indicating that hypnotic suggestions can be potent in altering cognitive functions
. A few hypnosis studies have been conducted aiming at enhancing cognitive and psychological functions after ABI [
36‐
38]. However, previous studies suffer from one or more major methodological weaknesses, such as a lack of control groups, lack of detailed descriptions of participants, randomization, blinding of testers, statistical procedures, and suggestions used during hypnosis.
The only randomized controlled trial (RCT) to date to include hypnosis in WM rehabilitation was performed by Lindeløv and colleagues [
4]. This RCT included 68 participants with ABI across 3 treatment arms: an intervention group, an active control group, and a passive control group. The intervention included techniques such as age regression and visualizations of brain plasticity with suggestions about enhancing WM functions through the instantiation of preinjury WM ability in the present. The non-targeted suggestions contained no explicit mention of ABI or WM-related abilities but was otherwise matched in length and procedure, thus serving as an active control. Contrary to previous research, this study reported large effect sizes after only four 1-h sessions in favor of the intervention group compared to both active controls (Bayes factors of 342 and 37.5 on the two aggregate outcome measures) and the passive control group (Bayes factor = 1.7 × 10
13). The long-term effect remained at approximately 6-week follow-up (Bayes factors = 7.1 and 1.3 in favor of no change). The outcome measures were neuropsychological tests, i.e., the WM index from WAIS-IV and Trail Making Test A and B. The authors suggest that hypnosis can improve WM following ABI, and that the speed and magnitude of the improvements indicate that there may be a potential for releasing residual cognitive capacity after ABI rather than “building” it anew. However, this study lacked medical data such as brain injury characteristics; it included only a few outcome measures (i.e., neuropsychological tests) and, most importantly, no measures of everyday WM capacity. Furthermore, Lindeløv and colleagues did not explore the potential underlying mechanisms associated with improved WM among the participants.
Self-efficacy theory asserts that an individual’s beliefs in personal competency predict actual performance [
24], and ABI survivors often display negative self-expectancies and show lower WM self-efficacy than healthy controls [
25]. Self-efficacy influences coping style and quality of life, satisfaction with functioning [
26], social participation [
27], functional independence [
28], and community integration [
29] after ABI. Thus, changed expectations about the possibility of dealing with the consequences of brain injury, i.e., improved self-efficacy, are a potential candidate in mediating generalization effects in cognitive rehabilitation [
26]. Hypnosis seems to be particularly well positioned to enhance self-efficacy [
30] and change expectations of psychological and behavioral outcomes in order to build confidence in one’s ability to cope with or solve problems [
31,
32,
39,
40]. A person’s brain injury expectations may influence their cognitive performances through mechanisms similar to the nocebo effect [
41]. Nocebo is a self-fulfilling prophecy where adverse effects are produced by expectations [
42,
43]. The Cognitive Activation Theory of Stress (CATS) [
44] suggests that expectancies regulate bodily stress responses [
45]. This is relevant to the current study given the established negative influence of stress-related cortisol on WM capacity [
46]. Specifically, expectations about coping (i.e., self-efficacy) may reduce the stress response, while expectations of helplessness or hopelessness may sustain it [
44,
47]. This study will test the hypothesis that hypnosis in WM rehabilitation exerts its effect partly through changes in WM self-efficacy.
Objectives and hypothesis
The main objective of this study is to explore the effect of hypnotic suggestion on WM capacity in adults with ABI. The following three research questions will be explored:
1.
Are the strong effect of hypnotic suggestion on WM performance in ABI patients found by Lindeløv et al. (2017) replicable?
Our hypothesis is that the direction and the magnitude of the effects on WM found by Lindeløv and colleagues after four treatment sessions will be replicated. Given the large changes seen in Lindeløv et al.’s study, even weaker results are of clinical interest.
2.
Does hypnotic suggestions improve everyday WM, everyday functioning and participation, emotional status, and quality of life in ABI patients?
The intervention is hypothesized to improve WM performance through “top-down” processes, which will allow improved WM capacity to be applied across diverse tasks and contexts. Thus, we expect to see treatment-related change in the domains of daily life, emotional status, and quality of life.
3.
Does increased self-efficacy predict the effect of hypnosis on WM?
We hypothesize that changes in self-efficacy predict WM improvements following hypnosis.