Aspects of sensory guidance in sequence learning

We explored the effects of sensory information upon procedural learning using three versions of the serial reaction-time task (SRTT): a standard task where the position of the stimulus cued the response; a non-standard task where the color of the stimulus was related to the correct response; and a combined task where both the color and position simultaneously cued the response. Despite these differences, each task had the same temporal pattern of a repeating ten-item sequence. We refer to each of these tasks based upon the cues available for guiding learning: position, color, and combined tasks. Procedural sequence learning was greater for the combined than for the other two tasks, suggesting that learning is enhanced when multiple sources of sensory information cue consistently and simultaneously for the same response. Transfer of skill occurred across all the tasks, except from the position to the color task. These results suggest that a fundamental neural algorithm is responsible for acquiring knowledge about a temporal sequence of responses rather than forming an associative relationship amongst stimuli.

Prefrontal repetitive transcranial magnetic stimulation as add on treatment in depression

A growing number of studies report antidepressant effects of repetitive transcranial magnetic stimulation (rTMS) in patients with major depression. The hypothesis that high frequency (20 Hz) rTMS (HF-rTMS) may speed up and strengthen the therapeutic response to sertraline in MD was tested. Twenty eight patients who had not yet received medication for the present depressive episode (n=12) or had failed a single trial of an antidepressant medication (n=16) were started on sertraline and randomised to receive either real of sham HF-rTMS. HF-rTMS was applied to the left dorsolateral prefrontal area in daily sessions (30 trains of 2 s, 20-40 s intertrain interval, at 90% motor threshold) on 10 consecutive working days. The results suggest that in this patient population, HF-rTMS does not add efficacy over the use of standard antidepressant medication.

Transcranial magnetic stimulation coregistered with MRI: a comparison of a guided versus blind stimulation technique and its effect on evoked compound muscle action potentials

INTRODUCTION AND METHODS

Compound muscle action potentials (CMAPs) elicited by transcranial magnetic stimulation (TMS) are characterized by enormous variability, even when attempts are made to stimulate the same scalp location. This report describes the results of a comparison of the spatial errors in coil placement and resulting CMAP characteristics using a guided and blind TMS stimulation technique. The former uses a coregistration system, which displays the intersection of the peak TMS induced electric field with the cortical surface. The latter consists of the conventional placement of the TMS coil on the optimal scalp position for activation of the first dorsal interossei (FDI) muscle.

RESULTS

Guided stimulation resulted in significantly improved spatial precision for exciting the corticospinal projection to the FDI compared to blind stimulation. This improved precision of coil placement was associated with a significantly increased probability of eliciting FDI responses. Although these responses tended to have larger amplitudes and areas, the coefficient of variation between guided and blind stimulation induced CMAPs did not significantly differ.

CONCLUSION

The results of this study demonstrate that guided stimulation improves the ability to precisely revisit previously stimulated cortical loci as well as increasing the probability of eliciting TMS induced CMAPs. Response variability, however, is due to factors other than coil placement.

Intracortical inhibition and facilitation in human facial motor area: difference between upper and lower facial area

OBJECTIVE

To investigate the intracortical inhibitory and excitatory systems in the motor cortical representation of upper and lower facial muscles.

METHODS

Paired-pulse transcranial magnetic stimulation (TMS) was applied to 7 healthy volunteers, with the interstimulus interval (ISI) between the conditioning stimulus (CS) and test stimulus, varied from 1 to 20 ms. CS was set at 90% of motor threshold. Muscle evoked potentials (MEPs) were recorded from first dorsal interosseus (FDI), orbicularis oculi (o. oculi) and mentalis muscles.

RESULT

TMS evoked MEPs in o. oculi on both ipsi- and contralateral sides in all subjects. In the paired-pulse study, MEP amplitude in the mentalis decreased at short ISIs of 1-3 ms, followed by increases at 12-20 ms. These effects were similar to those in the FDI. O. oculi did not show a distinct inhibitory period at short ISIs and facilitation at long ISIs was detected but was significantly less than in FDI and mentalis. In o. oculi, there was no significant difference between the effects of ipsilateral and contralateral CS on the MEPs.

CONCLUSION

The bi-hemispheric control of volitional movement and the modulation from brainstem projections appear to markedly influence intracortical inhibitory and excitatory systems in the motor cortical representation of o. oculi.

Enhanced visual spatial attention ipsilateral to rTMS-induced 'virtual lesions' of human parietal cortex

The breakdown of attentional mechanisms after brain damage can have drastic behavioral consequences, as in patients suffering from spatial neglect. While much research has concentrated on impaired attention to targets contralateral to sites of brain damage, here we report the ipsilateral enhancement of visual attention after repetitive transcranial magnetic stimulation (rTMS) of parietal cortex at parameters known to reduce cortical excitability. Normal healthy subjects received rTMS (1 Hz, 10 mins) over right or left parietal cortex. Subsequently, detection of visual stimuli contralateral to the stimulated hemisphere was consistently impaired when stimuli were also present in the opposite hemifield, mirroring the extinction phenomenon commonly observed in neglect patients. Additionally, subjects' attention to ipsilateral targets improved significantly over normal levels. These results underline the potential of focal brain dysfunction to produce behavioral improvement and give experimental support to models of interhemispheric competition in the distributed brain network for spatial attention.

Transcranial magnetic stimulation in young persons: a review of known cases

There are no published data about transcranial magnetic stimulation (TMS) as a treatment for psychiatric disorders in young persons. The aim of this article is to collate available information about TMS in this population. Information was sought, by placing a message on the TMS Listserver, from investigators who had used TMS in patients 18 years of age or younger. Only one group reported experience in this area; it had treated seven patients, ages 16-18 years. Three patients had unipolar depression, three had schizophrenia, and one had bipolar disorder. Five of the seven patients had improved by the conclusion of the TMS course. Adverse events were reported in one patient. Further studies are needed first to investigate systematically the safety of TMS in children and adolescents and second to examine its potential therapeutic effects in this population.

Grammatical distinctions in the left frontal cortex

Selective deficits in producing verbs relative to nouns in speech are well documented in neuropsychology and have been associated with left hemisphere frontal cortical lesions resulting from stroke and other neurological disorders. The basis for these impairments is unresolved: Do they arise because of differences in the way grammatical categories of words are organized in the brain, or because of differences in the neural representation of actions and objects? We used repetitive transcranial magnetic stimulation (rTMS) to suppress the excitability of a portion of left prefrontal cortex and to assess its role in producing nouns and verbs. In one experiment subjects generated real words; in a second, they produced pseudowords as nouns or verbs. In both experiments, response latencies increased for verbs but were unaffected for nouns following rTMS. These results demonstrate that grammatical categories have a neuroanatomical basis and that the left prefrontal cortex is selectively engaged in processing verbs as grammatical objects.

Transcranial magnetic stimulation evidence of a potential role for progesterone in the modulation of premenstrual corticocortical inhibition in a woman with catamenial seizure exacerbation

A transcranial magnetic stimulation paired-pulse paradigm was used to determine that cortical excitability was less during the late luteal phase than in the early follicular phase in a woman with epilepsy who had premenstrual seizure exacerbation. The data are consistent with the possibility that a reduction in GABA-mediated cortical inhibitory activity may be responsible. The administration of progesterone, a reproductive steroid with potent GABAergic metabolites, during the luteal phase restored cortical excitability to normal range.

The role of the dorsolateral prefrontal cortex during sequence learning is specific for spatial information

Many studies have implicated the dorsolateral prefrontal cortex in the acquisition of skill, including procedural sequence learning. However, the specific role it performs in sequence learning has remained uncertain. This type of skill has been intensively studied using the serial reaction time task. We used three versions of this task: a standard task where the position of the stimulus cued the response; a non-standard task where the color of the stimulus was related to the correct response; and a combined task where both the color and position simultaneously cued the response. We refer to each of these tasks based upon the cues available for guiding learning as position, color and combined tasks. The combined task usually shows an enhancement of skill acquisition, a result of being driven by two simultaneous and congruent cues. Prior to the performance of each of these tasks the function of the dorsolateral prefrontal cortex was disrupted using repetitive transcranial magnetic stimulation. This completely prevented learning within the position task, while sequence learning occurred to a similar extent in both the color and combined tasks. So, following prefrontal stimulation the expected learning enhancement in the combined task was lost, consistent with only a color cue being available to guide sequence learning in the combined task. Neither of these effects was observed following stimulation at the parietal cortex. Hence the critical role played by the dorsolateral prefrontal cortex in sequence learning is related exclusively to spatial cues. We suggest that the dorsolateral prefrontal cortex operates over the short term to retain and manipulate spatial information to allow cortical and subcortical structures to learn a predictable sequence of actions. Such functions may emerge from the broader role the dorsolateral prefrontal cortex has in spatial working memory. These results argue against the dorsolateral prefrontal cortex constituting part of the neuronal substrate responsible for general aspects of implicit or explicit sequence learning.

Increased variability of paced finger tapping accuracy following repetitive magnetic stimulation of the cerebellum in humans

Imaging and lesion studies suggest that the cerebellum is involved in the self-generation of timed motor responses. Using repetitive transcranial magnetic stimulation (rTMS), we studied the effects of transient disruption of the lateral or medial cerebellum on a paced-finger-tapping task (PFT). Results show greater variability on the PFT task following a 5 min train of 1 Hz rTMS to the medial cerebellum. Magnetic stimulation of the lateral cerebellum or motor cortex, and sham stimulation, had no effect on performance. Expanding the results of neuroimaging studies, these data show the causal link between activity in the medial cerebellum and the production of timed movements. This is the first demonstration of the feasibility of transiently disrupting the cerebellum by rTMS and inducing behavioral effects. This method of 'virtual lesions' can expand the study of the role of the cerebellum in motor control and cognition.

The brain that plays music and is changed by it

Playing a musical instrument demands extensive procedural and motor learning that results in plastic reorganization of the human brain. These plastic changes seem to include the rapid unmasking of existing connections and the establishment of new ones. Therefore, both functional and structural changes take place in the brain of instrumentalists as they learn to cope with the demands of their activity. Neuroimaging techniques allow documentation of these plastic changes in the human brain. These plastic changes are fundamental to the accomplishment of skillful playing, but they pose a risk for the development of motor control dysfunctions that may give rise to overuse syndromes and focal, task-specific dystonia.

Phase-specific modulation of cortical motor output during movement observation

The effects of different phases of an observed movement on the modulation of cortical motor output were studied by means of transcranial magnetic stimulation (TMS). A video-clip of a reaching-grasping action was shown and single TMS pulses were delivered during its passive observation. Times of cortical stimulation were related to the phases of the shown movement, locking them to the appearance of specific kinematic landmarks. The amplitude of the motor evoked potentials (MEPs) induced by TMS in the first dorsal interosseus (FDI) muscle was modulated by the amount of the observed finger aperture. The presence of such an effect is consistent with the notion of a mirror neuron system in premotor areas that couples action execution and action observation also in terms of temporal coding.

Fast backprojections from the motion to the primary visual area necessary for visual awareness

Much is known about the pathways from photoreceptors to higher visual areas in the brain. However, how we become aware of what we see or of having seen at all is a problem that has eluded neuroscience. Recordings from macaque V1 during deactivation of MT+/V5 and psychophysical studies of perceptual integration suggest that feedback from secondary visual areas to V1 is necessary for visual awareness. We used transcranial magnetic stimulation to probe the timing and function of feedback from human area MT+/V5 to V1 and found its action to be early and critical for awareness of visual motion.

Abnormalities of spatial and temporal sensory discrimination in writer's cramp

Clinical observations of patients with writer's cramp suggest that abnormalities of the sensory system may be a frequent finding in this disorder. Neurophysiological data from an animal model of focal dystonia have revealed cells in somatosensory cortex with enlarged and overlapping tactile receptive fields. However, psychophysical studies so far have been unable to document a clinical correlate supporting a similar enlargement of receptive fields in humans. We compared the fingertip discrimination of the orientation of fine spatial gratings between writer's cramp and control subjects and found a significant decrease in grating sensitivity in the patients, consistent with the possibility of enlarged tactile receptive fields. In addition, we duplicated previous experiments showing an abnormality of tactile temporal discrimination. The results provide psychophysical measures which may relate to the development of sensory cortical reorganization in patients with writer's cramp.

Brain cortical activation during guitar-induced hand dystonia studied by functional MRI

Focal hand dystonia in musicians is a strongly task-related movement disorder. Typically, symptoms become apparent only when players execute specific overpracticed skilled exercises on their instrument. We therefore examined five guitarists with functional MRI during dystonic symptom provocation by means of an adapted guitar inside the magnet. The activation patterns obtained in comparable nondystonic guitarists and in the study patients when performing normal-hand exercise served as references. A 1.5-T system equipped with echo-speed gradients and single-shot echoplanar imaging software was used. Data acquisition was centered on the cortical motor system encompassed in eight contiguous slices. Dystonic musicians compared with both control situations showed a significantly larger activation of the contralateral primary sensorimotor cortex that contrasted with a conspicuous bilateral underactivation of premotor areas. Our results coincide with studies of other dystonia types in that they show an abnormal recruitment of cortical areas involved in the control of voluntary movement. However, they do suggest that the primary sensorimotor cortex, rather than being underactive in idiopathic dystonic patients, may be overactive when tested during full expression of the task-induced movement disorder.

Self-recognition and the right prefrontal cortex

Although the anatomical and functional substrates subserving face recognition have been subject to extensive investigation, the underpinnings of self-face recognition are not well understood. Given the evidence that own-face recognition has been demonstrated by a select number of species, it is intriguing to speculate whether self-face recognition is accomplished via a 'self-network' or simply a 'face-network' within the brain. Furthermore, the relationship of self-recognition to other self-processes, such as self-evaluation and autobiographical retrieval, are not clearly defined. However, data from fMRI, ERPs and repetitive transcranial magnetic stimulation as well as from split-brain studies and patients with focal lesions, indicate that the prefrontal cortex, with possible right hemisphere lateralization, may be a preferential component in self-recognition. Studies using these methods, as well as PET, have indicated that the self-processes of self-evaluation and autobiographical memory preferentially engage networks within the right fronto-temporal region. Although it is highly improbable that there is a 'self-recognition' or 'self' center, it appears that there may be a bias for the processing of 'self' within the right prefrontal cortex.

Electrical inhibition of basal ganglia nuclei in Parkinson's disease: long-term results

Since 1995 a group of 57 patients with familial tremor and Parkinson's disease have been operated in two main sets: the first, with 11 cases, with predominant tremor, where the ventro-oralis posterior (Vop) thalamic nucleus was stimulated, and the second, with 44 cases, with mainly rigidity and akinesia, where the globus pallidus internus (GPi) was unilaterally (10 cases) or bilaterally (34 cases) stimulated. In 2 cases, a combined thalamic-pallidal stimulation was achieved. In the last cases, an image fusion software with 3D correlation of MR and CT images and volumetric image matching has been used to correct MRI distortion. In all cases, an Itrell II stimulation system (Medtronic) was implanted. Result analysis shows that Vop stimulation is mostly effective for tremor, and GPi stimulation is better for rigidity and akinesia. Transcortical magnetic stimulation tests suggest that GPi pallidal electrical inhibition increases cortical excitability, as opposed to Vop thalamic stimulation, which implies a different mechanism of action.

Interindividual variability of the modulatory effects of repetitive transcranial magnetic stimulation on cortical excitability

Repetitive transcranial magnetic stimulation (rTMS) appears to have effects on cortical excitability that extend beyond the train of rTMS itself. These effects may be inhibitory or facilitatory and appear to depend on the frequency, intensity, duration and intertrain interval of the rTMS. Many studies assume facilitatory effects of high-frequency rTMS and inhibitory effects of low-frequency rTMS. Nevertheless, the interindividual variability of this modulation of cortical excitability by rTMS has not been systematically investigated. In this study, we applied 240 pulses of rTMS at 90% of the subjects' motor threshold to their motor cortex at different frequencies (1, 10, 15 and 20 Hz) and examined the effects on motor evoked potentials (frequency tuning curve). Although the averaged group data showed a frequency-dependent increase in cortical excitability, each subject had a different pattern of frequency tuning curve, i.e. a different modulatory effect on cortical excitability at different rTMS frequencies. The interindividual variability of these modulatory effects was still high, though less so, when the number of rTMS pulses was increased to 1,600. These findings illustrate the degree of variability of the rTMS effects in the human brain.

Interhemispheric asymmetry of motor cortical excitability in major depression as measured by transcranial magnetic stimulation

BACKGROUND

Neuroimaging studies of major depressive disorder (MDD) indicate interhemispheric differences in prefrontal cortical activity (right greater than left).

AIMS

To investigate whether there are any interhemispheric differences of motor cortical excitability in MDD.

METHOD

Eight patients with treatment-refractory MDD off medication were assessed for the severity of their depression, and transcranial magnetic stimulation studies (bilateral motor threshold and paired-pulse studies) were conducted. Eight normal controls were also studied.

RESULTS

MDD patients showed significant interhemispheric differences in motor threshold and paired-pulse curves, both of which showed lower excitability on the left hemisphere. Such differences were absent in controls.

CONCLUSIONS

Our findings may aid the further understanding of the neurophysiology underlying MDD.

Hand response differences in a self-face identification task

Evidence has indicated that the right frontal cortex is preferentially involved in self-face recognition. To test this further, we employed a face identification task and examined hand response differences (N=10). Pictures of famous faces were combined with pictures of the participants' faces (self) and their co-workers' faces (familiar). These images were presented as a 'movie' in which one face transformed into another. Under the first instruction set, the movies began with either the participant's face or a co-worker's face, and the sequences gradually morphed into a famous face. When told to stop the movie when the face in the sequence became famous, a significantly later 'frame' was identified when the movies were composed of self-faces and the participants responded with their left hand. When the movies started with the famous faces and participants had to stop the movie when it became their own or their familiar co-worker's image (Instruction set 2), a significantly earlier frame was identified in the 'Self: Left hand' condition. The data suggest that participants are inclined to identify images as their own when the right hemisphere is preferentially accessed.

Self-face identification is increased with left hand responses

Evidence suggests that autobiographical memory, self-related semantic category judgements, and self-identification tasks may be lateralised, with preferential activity in the right anterior temporal and prefrontal cortex. To test this hypothesis, participants (N=10) were presented with morphed images of themselves (self) combined with a famous face. A further set of images was generated in which the face of one of the participant's co-workers (familiar) was combined with a famous face. When compared to morphed images composed of a familiar face, the participants identified images less often as being famous if the images were composed of self, but only when responding with their left hands. This greater "self-effect" found in left-hand responses may imply that when the right hemisphere is preferentially active, participants have a tendency to refer images to self. These data provide further support for a preferential role of the right hemisphere in processing self-related material.

Tactile spatial resolution in blind braille readers

OBJECTIVE

To determine if blind people have heightened tactile spatial acuity.

BACKGROUND

Recently, studies using magnetic source imaging and somatosensory evoked potentials have shown that the cortical representation of the reading fingers of blind Braille readers is expanded compared to that of fingers of sighted subjects. Furthermore, the visual cortex is activated during certain tactile tasks in blind subjects but not sighted subjects. The authors hypothesized that the expanded cortical representation of fingers used in Braille reading may reflect an enhanced fidelity in the neural transmission of spatial details of a stimulus. If so, the quantitative limit of spatial acuity would be superior in blind people.

METHODS

The authors employed a grating orientation discrimination task in which threshold performance is accounted for by the spatial resolution limits of the neural image evoked by a stimulus. The authors quantified the psychophysical limits of spatial acuity at the middle and index fingers of 15 blind Braille readers and 15 sighted control subjects.

RESULTS

The mean grating orientation threshold was significantly (p = 0.03) lower in the blind group (1.04 mm) compared to the sighted group (1.46 mm). The self-reported dominant reading finger in blind subjects had a mean grating orientation threshold of 0.80 mm, which was significantly better than other fingers tested. Thresholds at non-Braille reading fingers in blind subjects averaged 1.12 mm, which were also superior to sighted subjects' performances.

CONCLUSION

Superior tactile spatial acuity in blind Braille readers may represent an adaptive, behavioral correlate of cortical plasticity.

[The application of transcranial magnetic stimulation in neuropsychological investigation]

OBJECTIVE

In this review we describe the main studies in which transcranial magnetic stimulation (TMS) has been used in the study of superior cognitive function.

DEVELOPMENT

The various studies published in the literature show that TMS can modulate neuropsychological processes such as attention, different types of memory such as working memory, declarative memory, memory of procedures and language. In most cases TMS acts on the different cognitive abilities blocking or making them difficult. Thus TMS may be used as a method of causing transient lesions bringing the relationship brain-conduct to a dimension of cause and avoiding certain limitations of the classical method for creating lesions. The positive effects of TMS in certain tasks involving language and memory has also been shown. The latter offer new possibilities of future application in cognitive rehabilitation.

CONCLUSIONS

TMS has an obvious effect on neuropsychological functions. Over the past ten years studies in this field have increased progressively. At the present time the results obtained by using TMS in cognitive neuroscience are of a basic type, limited to experimental laboratory work. It has mainly been used on normal persons. However, it cannot be long before it is used clinically in neuropsychological patients.