Personalized transcranial alternating current stimulation improves sleep quality: Initial findings.

Insufficient sleep is a major health issue. Inadequate sleep is associated with an array of poor health outcomes, including cardiovascular disease, diabetes, obesity, certain forms of cancer, Alzheimer's disease, depression, anxiety, and suicidality. Given concerns with typical sedative hypnotic drugs for treating sleep difficulties, there is a compelling need for alternative interventions. Here, we report results of a non-invasive electrical brain stimulation approach to optimizing sleep involving transcranial alternating current stimulation (tACS). A total of 25 participants (mean age: 46.3, S.D. ± 12.4, 15 females) were recruited for a null-stimulation controlled (Control condition), within subjects, randomized crossed design, that included two variants of an active condition involving 15 min pre-sleep tACS stimulation. To evaluate the impact on sleep quality, the two active tACS stimulation conditions were designed to modulate sleep-dependent neural activity in the theta/alpha frequency bands, with both stimulation types applied to all subjects in separate sessions. The first tACS condition used a fixed stimulation pattern across all participants, a pattern composed of stimulation at 5 and 10 Hz. The second tACS condition used a personalized stimulation approach with the stimulation frequencies determined by each individual's peak EEG frequencies in the 4-6 Hz and 9-11 Hz bands. Personalized tACS stimulation increased sleep quantity (duration) by 22 min compared to a Control condition (p = 0.04), and 19 min compared to Fixed tACS stimulation (p = 0.03). Fixed stimulation did not significantly increase sleep duration compared to Control (mean: 3 min; p = 0.75). For sleep onset, the Personalized tACS stimulation resulted in reducing the onset by 28% compared to the Fixed tACS stimulation (6 min faster, p = 0.02). For a Poor Sleep sub-group (n = 13) categorized with Clinical Insomnia and a high insomnia severity, Personalized tACS stimulation improved sleep duration by 33 min compared to Fixed stimulation (p = 0.02), and 30 min compared to Control condition (p < 0.1). Together, these results suggest that Personalized stimulation improves sleep quantity and time taken to fall asleep relative to Control and Fixed stimulation providing motivation for larger-scale trials for Personalized tACS as a sleep therapeutic, including for those with insomnia.

Comparing the electric fields of transcranial electric and magnetic perturbation.

Significance.Noninvasive brain stimulation (NIBS) by quasistatic electromagnetic means is presently comprised of two methods: magnetic induction methods (transcranial magnetic perturbation or TMP) and electrical contact methods (transcranial electric perturbation or TEP). Both methods couple to neuronal systems by means of the electric fields they produce. Both methods are necessarily accompanied by a scalp electric field which is of greater magnitude than anywhere within the brain. A scalp electric field of sufficient magnitude may produce deleterious effects including peripheral nerve stimulation and heating which consequently limit the spatial and temporal characteristics of the brain electric field. Presently the electromagnetic NIBS literature has produced an accurate but non-generalized understanding of the differences between the TEP and TMP methods.Objective.The aim of this work is to contribute a generalized understanding of the differences between the two methods which may open doors to novel TEP or TMP methods and translating advances, when possible, between the two methods.Approach.This article employs a three shell spherical conductor head model to calculate general analytical results showing the relationship between the spatial scale of the brain electric fields and: (1) the scalp-to-brain mean-squared electric field ratio for the two methods and (2) TEP-to-TMP scalp mean-squared electric field ratio for similar electric fields at depth.Main results.The most general result given is an asymptotic limit to the TEP-to-TMP ratio of scalp mean-squared electric fields for similar electric fields at depth. Specific example calculations for these ratios are also given for typical TEP electrode and TMP coil configurations. While TMP has favorable mean-squared electric field ratios compared to TEP this advantage comes at an energetic cost which is briefly elucidated in this work.

Individual differences in TMS sensitivity influence the efficacy of tDCS in facilitating sensorimotor adaptation.

BACKGROUND: Transcranial direct current stimulation (tDCS) can enhance cognitive function in healthy individuals, with promising applications as a therapeutic intervention. Despite this potential, variability in the efficacy of tDCS has been a considerable concern. OBJECTIVE: /Hypothesis: Given that tDCS is always applied at a set intensity, we examined whether individual differences in sensitivity to brain stimulation might be one variable that modulates the efficacy of tDCS in a motor learning task. METHODS: In the first part of the experiment, single-pulse transcranial magnetic stimulation (TMS) over primary motor cortex (M1) was used to determine each participant's resting motor threshold (rMT). This measure was used as a proxy of individual sensitivity to brain stimulation. In an experimental group of 28 participants, 2 mA tDCS was then applied during a motor learning task with the anodal electrode positioned over left M1. Another 14 participants received sham stimulation. RESULTS: M1-Anodal tDCS facilitated learning relative to participants who received sham stimulation. Of primary interest was a within-group analysis of the experimental group, showing that the rate of learning was positively correlated with rMT: Participants who were more sensitive to brain stimulation as operationalized by our TMS proxy (low rMT), showed faster adaptation. CONCLUSIONS: Methodologically, the results indicate that TMS sensitivity can predict tDCS efficacy in a behavioral task, providing insight into one source of variability that may contribute to replication problems with tDCS. Theoretically, the results provide further evidence of a role of sensorimotor cortex in adaptation, with the boost from tDCS observed during acquisition.

Evidence of a novel somatopic map in the human neocerebellum during complex actions.

The human neocerebellum has been hypothesized to contribute to many high-level cognitive processes including attention, language, and working memory. Support for these nonmotor hypotheses comes from evidence demonstrating structural and functional connectivity between the lateral cerebellum and cortical association areas as well as a lack of somatotopy in lobules VI and VII, a hallmark of motor representations in other areas of the cerebellum and cerebral cortex. We set out to test whether somatotopy exists in these lobules by using functional magnetic resonance imaging to measure cerebellar activity while participants produced simple or complex movements, using either fingers or toes. We observed a previously undiscovered somatotopic organization in neocerebellar lobules VI and VIIA that was most prominent when participants executed complex movements. In contrast, activation in the anterior lobe showed a similar somatotopic organization for both simple and complex movements. While the anterior somatotopic representation responded selectively during ipsilateral movements, the new cerebellar map responded during both ipsi- and contralateral movements. The presence of a bilateral, task-dependent somatotopic map in the neocerebellum emphasizes an important role for this region in the control of skilled actions.

Timing of rhythmic movements in patients with cerebellar degeneration.

A distinction in temporal performance has been identified between two classes of rhythmic movements: those requiring explicit timing of salient events marking successive cycles, i.e., event timing, and continuous movements in which timing is hypothesized to be emergent. Converging evidence in support of this distinction is reviewed, including neuropsychological studies showing that individuals with cerebellar damage are selectively impaired on tasks requiring event timing (e.g., tapping). Recent behavioral evidence in neurologically healthy individuals suggests that for continuous movements (e.g., circle drawing), the initial cycle is marked by a transformation from event to emergent timing, allowing the participant to match their movement rate to an externally defined cycle duration. We report a new experiment in which individuals with cerebellar ataxia produced rhythmic tapping or circle drawing movements. Participants were either paced by a metronome or unpaced. Ataxics showed a disproportionate increase in temporal variability during tapping compared to circle drawing, although they were more variable than controls on both tasks. However, two predictions of the transformation hypothesis were not confirmed. First, the ataxics did not show a selective impairment on circle drawing during the initial cycles, a phase when we hypothesized event timing would be required to establish the movement rate. Second, the metronome did not increase variability of the performance of the ataxics. Taken together, these results provide further evidence that the integrity of the cerebellum is especially important for event timing, although our attempt to specify the relationship between event and emergent timing was not successful.

Further evidence that Whorfian effects are stronger in the right visual field than the left.

The Whorf hypothesis holds that differences between languages induce differences in perception and/or cognition in their speakers. Much of the experimental work pursuing this idea has focused on the domain of color and has centered on the issue of whether linguistically coded color categories influence color discrimination. A new perspective has been cast on the debate by recent results that suggest that language influences color discrimination strongly in the right visual field but not in the left visual field (LVF). This asymmetry is likely related to the contralateral projection of visual fields to cerebral hemispheres and the specialization of the left hemisphere for language. The current study presents three independent experiments that replicate and extend these earlier results by using different tasks and testing across different color category boundaries. Our results differ in one respect: although we find that Whorfian effects on color are stronger for stimuli in the right visual field than in the LVF, we find that there are significant category effects in the LVF as well. The origin of the significant category effect in the LVF is considered, and two factors that might account for the pattern of results are proposed.

Ipsilateral corticospinal projections do not predict congenital mirror movements: a case report.

Congenital mirror movements (CMMs) are involuntary, symmetric movements of one hand during the production of voluntary movements with the other. CMMs have been attributed to a range of physiological mechanisms, including excessive ipsilateral projections from each motor cortex to distal extremities. We examined this hypothesis with an individual showing pronounced CMMs. Mirror movements were characterized for a set of hand muscles during a simple contraction task. Transcranial magnetic stimulation (TMS) was then used to map the relative input to each muscle from both motor cortices. Contrary to our expectations, CMMs were most prominent for muscles with the strongest contralateral representation rather than in muscles that were activated by stimulation of either hemisphere. These findings support a bilateral control hypothesis whereby CMMs result from the recruitment of both motor cortices during intended unimanual movements. Consistent with this hypothesis, bilateral motor cortex activity was evident during intended unimanual movements in an fMRI study. To assess the level at which bilateral recruitment occurs, motor cortex excitability during imagined unimanual movements was assessed with TMS. Facilitory excitation was only observed in the contralateral motor cortex. Thus, the bilateral recruitment of the hemispheres for unilateral actions in individuals with CMMs appears to occur during movement execution rather than motor planning.

The human striatum is necessary for responding to changes in stimulus relevance.

Various lines of evidence suggest that the striatum is implicated in cognitive flexibility. The neuropsychological evidence has, for the most part, been based on research with patients with Parkinson's disease, which is accompanied by chemical disruption of both the striatum and the prefrontal cortex. The present study examined this issue by testing patients with focal lesions of the striatum on a task measuring two forms of cognitive switching. Patients with striatal, but not frontal lobe lesions, were impaired in switching between concrete sensory stimuli. By contrast, both patient groups were unimpaired when switching between abstract task rules relative to baseline nonswitch trials. These results reveal a dissociation between two distinct forms of cognitive flexibility, providing converging evidence for a role of the striatum in flexible control functions associated with the selection of behaviorally relevant stimuli.

Illusory conjunctions are alive and well: a reply to Donk (1999).

When presented with a red T and a green O, observers occasionally make conjunction responses and indicate that they saw a green T. These errors have been interpreted as reflecting separable processing stages of feature detection and integration with the illusory conjunctions arising from a failure at the integration stage. Recently, M. Donk (1999) asserted that the phenomenon of illusory conjunctions is an artifact. Conjunction reports are actually the result of confusing a nontarget item (O in the example above) for a target item (the letter T) and (correctly) reporting the color associated with the (incorrectly) selected target. The authors demonstrate that although target-nontarget confusion errors are a potential source of conjunction reports, there is a plethora of findings that cannot be accounted for by this confusion model. A review of the literature indicates that in many studies, illusory conjunctions do result from a failure to properly integrate features.

Comparison of the basal ganglia and cerebellum in shifting attention.

The basal ganglia and cerebellum have traditionally been associated with motor performance. Recently, there has been considerable interest regarding the contributions of these subcortical structures to aspects of cognition. In particular, both the basal ganglia and cerebellum have been hypothesized to be involved in the control of attentional set. To date, no neuropsychological studies have directly compared the effects of basal ganglia and cerebellar dysfunction on the same attention shifting tasks. To this end, we employed an alternating attention task that has been used to demonstrate putative attentional control deficits in children with cerebellar pathology, either related to autism or neurological insult. When adult patients with either Parkinson's disease or cerebellar lesions were tested on this task, a similar pattern of deficits was observed for both groups. However, when the motor demands were reduced, cerebellar patients showed a significant improvement on the alternating attention task, whereas the Parkinson patients continued to exhibit an impairment. This dissociation suggests that attentional deficits reported previously as being due to cerebellar dysfunction may be, at least in part, secondary to problems related to coordinating successive responses. In contrast, attention-shifting deficits associated with basal ganglia impairment cannot be explained by recourse to the motor demands of the task.

The coupled oscillator model of between-hand coordination in alternate-hand tapping: a reappraisal.

Single and alternating hand tapping were compared to test the hypothesis that coordination during rhythmic movements is mediated by the control of specific time intervals. In Experiment 1, an auditory metronome was used to indicate a set of timing patterns in which a 1-s interval was divided into 2 subintervals. Performance, measured in terms of the deviation from the target patterns and variability, was similar under conditions in which the finger taps were made with 1 hand or alternated between the 2 hands. In Experiment 2, the modality of the metronome (auditory or visual) was found to influence the manner in which the produced intervals deviated from the target patterns. These results challenge the notion that bimanual coordination emerges from coupling constraints intrinsic to the 2-hand system. They are in accord with a framework that emphasizes the control of specific time intervals to form a series of well-defined motor events.

Moving to directly cued locations abolishes spatial interference during bimanual actions.

Interference is frequently observed during bimanual movements if the two hands perform nonsymmetric actions. We examined the source of bimanual interference in two experiments in which we compared conditions involving symmetric movements with conditions in which the movements were of different amplitudes or different directions. The target movements were cued either symbolically by letters or directly by the onset of the target locations. With symbolic cues, reaction times were longer when the movements of the two hands were not symmetric. With direct cues, reaction times were the same for symmetric and nonsymmetric movements. These results indicate that directly cued actions can be programmed in parallel for the two hands. Our results challenge the hypothesis that the cost to initiate nonsymmetric movements is due to spatial intetference in a motor-programming stage. Rather the cost appears to be caused by stimulus identification, response-selection processes connected to the processing of symbolic cues, or both.

Both sides of human cerebellum involved in preparation and execution of sequential movements.

Using an event-related fMRI procedure, we investigated the role of the human cerebellum in sequential finger movements. Subjects performed a delayed sequential finger movement task in which an instructive cue preceded the imperative signal by 16.5 s. Bilateral activation was observed in the cerebellum following both the cue and imperative signals. The activated regions overlapped within the cerebellum, extending across intermediate and lateral regions corresponding to lobules HV-HVII. In contrast, activation in primary motor cortex was primarily restricted to the execution phase and most prominent in the contralateral hemisphere. These results indicate that the cerebellum is bilaterally recruited for the preparation and execution of sequential movements.

Exploring the role of the cerebellum in sensory anticipation and timing: commentary on Tesche and Karhu.

The past decade has witnessed a paradigm shift concerning the study of the cerebellum. Results from various studies employing a variety of methodologies suggest that the functional role of this structure is not limited to motor control. The article by Tesche and Karhu appearing in this issue, provides strong evidence that the cerebellum in humans is activated in anticipation of somatosensory events, even when these events do not require overt responses. In their study, the sensory response is observed when the stimuli fail to occur at expected points in time, consistent with the hypothesis that the cerebellum is specialized for representing the temporal relationships between events, motoric or otherwise. Timing and sensory expectancy likely reflect nested hypotheses, and it remains to be seen if one provides a more encompassing yet specific view of cerebellar function.

Asymmetries in a unilateral flanker task depend on the direction of the response: the role of attentional shift and perceptual grouping.

Four experiments were conducted using a flanker task with 1 distractor appearing either on the left or right side of a central target. Responses were made on a keyboard aligned parallel to the displays. A larger flanker effect was obtained when the distractor was on the same side as the response. Two factors account for this asymmetry. First, when the flanker and target are identical, the 2 form a group that is assigned a spatial tag, creating a form of the Simon effect on the basis of the compatibility between the response keys and the group. Second, preparation of a lateralized response appears to entail a shift of visual attention in the corresponding direction, thus enhancing processing of the flanker on the response side. Consistent with the 2nd hypothesis, participants were more likely to correctly recognize letters that were briefly presented at the distractor position on the same side as the response.

Sequential priming in hierarchically organized figures: effects of target level and target resolution.

Three experiments are reported in which participants identified target letters that appeared at either the global or local level of hierarchically organized stimuli. It has been previously reported that response time is facilitated when targets on successive trials appear at the same level (L. M. Ward, 1982; L. C. Robertson, 1996). Experiments 1 and 2 showed that this sequential priming effect can be mediated by target-level information alone, independent of the resolution, or actual physical size, of targets. Target level and resolution were unconfounded by manipulating total stimulus size, such that global elements of the smaller stimuli subtended the same amount of visual angle as local elements of the larger stimuli. Experiment 3, however, showed that when level information is less useful than resolution in parsing targets from distractors, resolution does become critical in intertrial priming. These data are discussed as they relate to the role of attention in local vs. global (part vs. whole) processing.

Manual laterality and hitting performance in major league baseball.

Asymmetrical hand function was examined in the context of expert sports performance: hitting in professional baseball. An archival study was conducted to examine the batting performance of all Major League Baseball players from 1871 to 1992, focusing on those who batted left (n = 1,059) to neutralize the game asymmetry. Among them, left-handers (n = 421) were more likely to hit with power and to strike out than right-handers (n = 638). One possible account, based on the idea of hand dominance and an analogy to tennis, is that batting left involves a double-handed forehand for left-handers and a weaker and more reliable double-handed backhand for right-handers. The results are also interpretable in the light of Y. Guiard's (1987) kinematic chain model of a between-hands asymmetrical division of labor, which provides a detailed account of why left batting is optimal for left-handers.

Effects of divided attention on temporal processing in patients with lesions of the cerebellum or frontal lobe.

Prefrontal cortex and cerebellum have both been implicated in temporal processing tasks although the exact contribution of each system remains unclear. To investigate this issue, control participants and patients with either prefrontal or cerebellar lesions were tested on temporal and nontemporal perceptual tasks under 2 levels of attentional load. Each trial involved a comparison between a standard tone and a subsequent comparison tone that varied in frequency, duration, or both. When participants had to make concurrent judgments on both dimensions, patients with frontal lobe lesions were significantly impaired on both tasks whereas the variability of cerebellar patients increased in the duration task only. This dissociation suggests that deficits on temporal processing tasks observed in frontal patients can be related to the attention demands of such tasks; cerebellar patients have a more specific problem related to timing.