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

The Effects of Repetitive Transcranial Magnetic Stimulation (rTMS) on Procedural Memory and Dysphoric Mood in Patients With Major Depressive Disorder

OBJECTIVE

To study the effects of depression and treatment with repetitive transcranial magnetic stimulation (rTMS) on sequence learning.

BACKGROUND

Prefrontal dysfunction in depression may affect sequence learning and be amenable to normalization by rTMS.

METHOD

The serial reaction time test (SRTT) was administered to 19 patients with major depressive disorder (MDD) and 20 nondepressed control participants. MDD patients were examined before and following treatment with rTMS to the left dorsolateral prefrontal cortex in daily sessions of 1600 stimuli at 10 Hz and at an intensity of 110% of the motor threshold. Treatment occurred over a 2-week interval of time.

RESULTS

MDD and nondepressed groups differed significantly with respect to baseline response speed. Following treatment with rTMS, MDD participants demonstrated significantly improved mood, improved response speed, and improved procedural learning.

CONCLUSIONS

Findings suggest that rTMS over a 2-week period improves performance on tasks of response speed and procedural memory in patients with MDD. These cognitive effects are greater in those patients who showed a significant antidepressant effect to rTMS intervention.

Dyslexics are impaired on implicit higher-order sequence learning, but not on implicit spatial context learning

Developmental dyslexia is characterized by poor reading ability and impairments on a range of tasks including phonological processing and processing of sensory information. Some recent studies have found deficits in implicit sequence learning using the serial reaction time task, but others have not. Other skills, such as global visuo-spatial processing may even be enhanced in dyslexics, although deficits have also been noted. The present study compared dyslexic and non-dyslexic college students on two implicit learning tasks, an alternating serial response time task in which sequential dependencies exist across non-adjacent elements and a spatial context learning task in which the global configuration of a display cues the location of a search target. Previous evidence indicates that these implicit learning tasks are based on different underlying brain systems, fronto-striatal-cerebellar circuits for sequence learning and medial temporal lobe for spatial context learning. Results revealed a double dissociation: dyslexics showed impaired sequence learning, but superior spatial context learning. Consistent with this group difference, there was a significant positive correlation between reading ability (single real and non-word reading) and sequence learning, but a significant negative correlation between these measures and spatial context learning. Tests of explicit knowledge confirmed that learning was implicit for both groups on both tasks. These findings indicate that dyslexic college students are impaired on some kinds of implicit learning, but not on others. The specific nature of their learning deficit is consistent with reports of physiological and anatomical differences for individuals with dyslexia in frontal and cerebellar structures.

Stimulus-specific sequence representation in serial reaction time tasks

Some recent evidence has favoured purely response-based implicit representation of sequences in serial reaction time tasks. Three experiments were conducted using serial reaction time tasks featuring four spatial stimuli mapped in categories to two responses. Deviant items from the expected sequence that required the expected response resulted in increased response latencies. The findings demonstrated a stimulus-specific form of representation that operates in the serial reaction time task. No evidence was found to suggest that the stimulus-specific learning was contingent on explicit knowledge of the sequence. Such stimulus-based learning would be congruent with a shortcut within an information-processing framework and, combined with other research findings, suggests that there are multiple loci for learning effects.

Neural correlates of encoding and expression in implicit sequence learning

In the domain of motor learning it has been difficult to separate the neural substrate of encoding from that of change in performance. Consequently, it has not been clear whether motor effector areas participate in learning or merely modulate changes in performance. Here, using a variant of the serial reaction time task that dissociated these two factors, we report that encoding during procedural motor learning does engage cortical motor areas and can be characterized by distinct early and late encoding phases. The highest correlation between activation and subsequent changes in motor performance was seen in the motor cortex during early encoding, and in the basal ganglia during the late encoding phase. Our results show that rapid encoding during procedural motor learning involves several distinct processes, and is represented primarily within motor system structures.

The implicit sequence learning deficit in patients with Parkinson's disease: a matter of impaired sequence integration?

Despite the wealth of research investigating the serial reaction time (SRT) learning abilities of people with Parkinson's disease (PD), the role of the basal ganglia in implicit sequence learning remains largely unclear. The present research sought to examine the ability of people with PD to implicitly learn simultaneously operating sequences and integrate patterned information from each sequence dimension. Using a version of the SRT which reduced motor demands, the present experiment investigated the implicit learning of a spatial sequence, a stimulus-response sequence, and an integrated spatial/stimulus-response sequence, all of which are usually confounded in the standard SRT task. Whereas both PD and control groups demonstrated robust learning for the individual spatial and response sequences, only control participants evidenced learning for the integrated sequence. Further, unlike implicit learning for the spatial and object sequences, impaired integrated sequence acquisition was specifically related to the severity of patients' PD symptomatology. The implicit learning deficits of PD patients are discussed with regard to the role played by the basal ganglia in integrative sequence learning in the SRT.

Dorsal posterior parietal rTMS affects voluntary orienting of visuospatial attention

Patients with lesions in posterior parietal cortex (PPC) are relatively unimpaired in voluntarily directing visual attention to different spatial locations, while many neuroimaging studies in healthy subjects suggest dorsal PPC involvement in this function. We used an offline repetitive transcranial magnetic stimulation (rTMS) protocol to study this issue further. Ten healthy participants performed a cue-target paradigm. Cues prompted covert orienting of spatial attention under voluntary control to either a left or right visual field position. Targets were flashed subsequently at the cued or uncued location, or bilaterally. Following rTMS over right dorsal PPC, (i) the benefit for target detection at cued versus uncued positions was preserved irrespective of cueing direction (left- or rightward), but (ii) leftward cueing was associated with a global impairment in target detection, at all target locations. This reveals that leftward orienting was still possible after right dorsal PPC stimulation, albeit at an increased overall cost for target detection. In addition, rTMS (iii) impaired left, but (iv) enhanced right target detection after rightward cueing. The finding of a global drop in target detection during leftward orienting with a spared, relative detection benefit at the cued (left) location (i-ii) suggests that right dorsal PPC plays a subsidiary rather than pivotal role in voluntary spatial orienting. This finding reconciles seemingly conflicting results from patients and neuroimaging studies. The finding of attentional inhibition and enhancement occurring contra- and ipsilaterally to the stimulation site (iii-iv) supports the view that spatial attention bias can be selectively modulated through rTMS, which has proven useful to transiently reduce visual hemispatial neglect.

Cerebral activation related to skills practice in a double serial reaction time task: striatal involvement in random-order sequence learning

We used functional Magnetic Resonance Imaging (fMRI) to examine the distribution of cerebral activation related to prolonged skill practice. In a bimanual variant of the Serial Reaction Time Task (SRT), simultaneous finger movements of the two hands were made in response to randomly ordered pairs of visual stimuli (Double SRT, DoSRT). Extended practice by a week of daily performance resulted in gradual decrease of reaction times, associated with an increased involvement of the ventral putamen and globus pallidus, reaching statistical significance only on the left side (Statistical Parametric Mapping, SPM99). This increase was complementary to a decrease of cortical activations. The striatal activation after training on random order stimuli indicates that the striatum is not exclusively involved in sequence learning. This extended function implies a role in the acquisition of basic visuomotor skills that includes the specific selection of the appropriate muscles in response to independent stimuli.

Overt propositional speech in chronic nonfluent aphasia studied with the dynamic susceptibility contrast fMRI method

This study examined activation levels in the left (L) supplementary motor area (SMA) and the right (R) SMA (separately), and activation in nine R perisylvian language homologues during overt, propositional speech in chronic nonfluent aphasia patients. Previous functional imaging studies with a variety of chronic aphasia patients have reported activation in these regions during different language tasks, however, overt propositional speech has not been examined. In the present research, four nonfluent aphasia patients were studied during overt elicited propositional speech at 4-9 years post-single L hemisphere stroke, which spared the SMA. The dynamic susceptibility contrast (DSC) method of functional MRI was used to calculate relative cerebral blood volume (relCBV) for cortical regions of interest (ROIs) during the first-pass bolus of gadolinium during two conditions: (1) pattern (silent viewing of checkerboard patterns) and (2) story (overt, elicited propositional speech describing sequential pictures, which formed a story). During the story condition, controls had significantly higher relCBV in L SMA than in R SMA; aphasics, however, had significantly higher relCBV in R SMA than in L SMA. During the pattern condition, no significant differences were observed between the L SMA and the R SMA for either controls or aphasics. In addition, aphasics had significantly higher relCBV in the R sensorimotor mouth during story than pattern. This R sensorimotor mouth relCBV was also significantly higher in aphasics than controls during story, and the two groups did not differ during pattern. The overall mean relCBV for the nine R perisylvian ROIs was significantly higher for aphasics than controls during both story and pattern. These results suggest that poor modulation, including possible over-activation of R sensorimotor mouth and other R perisylvian language homologues may underlie in part, the hesitant, poorly articulated, agrammatic speech associated with nonfluent aphasia.

Implicit spatial contextual learning in healthy aging

Three experiments investigated the aging of implicit spatial and spatiotemporal context learning in 2 tasks. In contextual cuing, people learn to use repeated spatial configurations to facilitate search for a target, whereas in higher order serial learning, they learn to use subtle sequence regularities to respond more quickly and accurately to a series of events. Results reveal a dissociation; overall contextual cuing is spared in healthy aging, whereas higher order sequence learning is impaired in the same individuals. This finding suggests that these 2 forms of implicit learning rely on different neural substrates that age differently; the results are also consistent with recent evidence that fronto-striatal circuits are particularly susceptible to decline in health aging.

Studies in cognition: the problems solved and created by transcranial magnetic stimulation

The application of transcranial magnetic stimulation (TMS) to investigate important questions in cognitive neuroscience has increased considerably in the last few years. TMS can provide substantial insights into the nature and the chronometry of the computations performed by specific cortical areas during various aspects of cognition. However, the use of TMS in cognitive studies has many potential perils and pitfalls. Although TMS can help bridge the gap between psychological models and brain-based arguments of cognitive functions, hypothesis-driven carefully designed experiments that acknowledge the current limitations of TMS are critical.

Facilitation of probabilistic classification learning by transcranial direct current stimulation of the prefrontal cortex in the human

The aim of our study was to test if the electrical stimulation of the prefrontal cortex (PFC) could modify probabilistic classification learning (PCL). Transcranial direct current stimulation (tDCS) was administered to the left prefrontal and to the primary visual cortex of 22 healthy subjects while they performed a PCL task. In this task subjects learned which of two outcomes would occur on each trial after presentation of a particular combination of cues. Ten minutes of anodal, but not cathodal, stimulation improved implicit learning only when the left PFC was stimulated. Our results show that implicit PLC can be modified by weak anodal tDCS, which probably increases neural excitability, as has been shown in the motor and visual cortices previously. Our results suggest that further studies on the facilitation of learning and memory processes by tDCS are warranted.

Correlation between evening and morning waking EEG and spatial orientation

The prefrontal, frontal, and parietal EEG of 16 healthy young adults (seven men, nine women; age=22.57+/-4.2) was recorded during the waking state (eyes closed) in the evening before and the morning following a second consecutive night spent in a sleep laboratory. Following the morning EEG recording session, participants were tested in a human-size maze upon five learning trials of a four-intersection route. Results on the fifth trial served as the learning index. We found a significant positive correlation between time taken to carry out the route and prefrontal, frontal EEG alpha-2 (10.0-12.75 Hz), and sigma (11.5-14.5 Hz) frequency bands. We also found that prefrontal and frontal theta activity correlated negatively with number of errors. No correlation was found between performance and neither alpha-1 (8.0-9.75 Hz) nor parietal EEG activity. These results confirm the involvement of the prefrontal and frontal cortices in the mechanisms responsible for modulating spatial orientation.

Organization of action sequences and the role of the pre-SMA

To understand the contribution of the human presupplementary motor area (pre-SMA) in sequential motor behavior, we performed a series of finger key-press experiments. Experiment 1 revealed that each subject had a spontaneous tendency to organize or "chunk" a long sequence into shorter components. We hypothesized that the pre-SMA might have a special role in initiating each chunk but not at other points during the sequence. Experiment 2 therefore examined the effect of 0.5-s, 10-Hz repetitive transcranial magnetic stimulation (rTMS) directed over the pre-SMA. As hypothesized, performance was disrupted when rTMS was delivered over the pre-SMA at the beginning of the second chunk but not when it was delivered in the middle of a chunk. Contrary to the hypothesis, TMS did not disrupt sequence initiation. Experiments 3 and 4 examined whether the very first movement of a sequence could be disrupted under any circumstances. Pre-SMA TMS did disrupt the initiation of sequences but only when subjects had to switch between sequences and when the first movement of each sequence was not covertly instructed by a learned visuomotor association. In conjunction, the results suggest that for overlearned sequences the pre-SMA is primarily concerned with the initiation of a sequence or sequence chunk and the role of the pre-SMA in sequence initiation is only discerned when subjects must retrieve the sequence from memory as a superordinate set of movements without the aid of a visuomotor association. Control experiments revealed such effects were not present when rTMS was applied over the left dorsal premotor cortex.

Age deficits in learning sequences of spoken words

Previous research has demonstrated age-related deficits in implicit learning of visual sequences characterized by subtle predictive relationships among the sequence elements. This study investigates whether this reflects modality-specific, or more general, sequence learning deficits by using an auditory sequence-learning task. Young and old adults responded with a key press to each of a series of unrelated spoken words. Unknown to the participants, every other word was presented in a fixed, repeating order with alternate words chosen at random. Both groups responded more quickly and accurately to the predictable than to unpredictable words, revealing sequence learning. However, elderly participants showed less learning than young participants on several measures. This suggests that age-related deficits in implicit sequence learning reflect a general impairment in learning subtle environmental covariations rather than a modality-specific visual impairment.

Neocortical mechanisms in motor learning

The ability to learn novel motor skills has fundamental importance for adaptive behavior. Neocortical mechanisms support human motor skill learning, from simple practice to adaptation and arbitrary sensory-motor associations. Behavioral and neural manifestations of motor learning evolve in time and involve multiple structures across the neocortex. Modifications of neural properties, synchrony and synaptic efficacy are all related to the development and maintenance of motor skill.

Implicit learning deficit in children with developmental dyslexia

Several neuropsychological deficits have been reported as characteristic of the cognitive profile of dyslexic children. Phonological and visual processing are often impaired as well as auditory processing, attention and information processing speed. We investigated whether implicit learning, is impaired in dyslexic children and adolescents. Tests of implicit and declarative learning were administered to 18 clinically defined dyslexics and 18 similar age controls. Dyslexics showed a reduced learning rate in the implicit but not in the declarative task, suggesting a specific deficit of implicit learning. Although alternative hypothesis cannot be ruled out, considering that implicit learning is a cognitive function primarily processed by the cerebellum and that recent neurological and physiological data suggest a cerebellar dysfunction in dyslexia, the present results suggest an impairment of cerebellar system in reading disabilities.

Prefrontal cortex: procedural sequence learning and awareness

Activation of the prefrontal cortex has been linked to awareness during sequence-learning tasks. A recent study, however, finds activation of the prefrontal cortex during such tasks regardless of awareness. So what is the neurophysiological basis of awareness, and what is the role of the prefrontal cortex in sequence learning?

Transcranial magnetic stimulation: neurophysiological applications and safety

TMS is a non-invasive tool for measuring neural conduction and processing time, activation thresholds, facilitation and inhibition in brain cortex, and neural connections in humans. It is used to study motor, visual, somatosensory, and cognitive functions. TMS does not appear to cause long-term adverse neurological, cardiovascular, hormonal, motor, sensory, or cognitive effects in healthy subjects. Single-pulse (<1Hz) TMS is safe in normal subjects. High frequency, high-intensity repetitive TMS (rTMS) can elicit seizures even in normal subjects. Safety guidelines for using rTMS have been published.

rTMS to the supplementary motor area disrupts bimanual coordination

Bimanual coordination tasks form an essential part of our behaviour. One brain region thought to be involved in bimanual coordination is the supplementary motor area (SMA). We used repetitive transcranial magnetic stimulation (rTMS) at 1 Hz for 5 min to create a temporary virtual lesion of the rostral portion of the human SMA immediately prior to performance of a goal-directed bimanual coordination task. In two control conditions, participants underwent sham stimulation or stimulation over the primary motor cortex (MI). The experimental task was to open a drawer with the left hand, catch a ball with the right hand, and reinsert the ball into the drawer through an aperture just big enough for the ball to pass through, again with the right hand. Hence, the actions of one hand depend upon the actions of the other. We calculated time intervals between the successive component actions of one hand (unimanual intervals) and actions of both hands (bimanual intervals) and analyzed these intervals separately. Interestingly, none of the unimanual intervals were affected by the rTMS, but the variability of a critical bimanual interval--the time between the left hand opening the drawer and the right hand starting to move to catch the ball--was increased by rTMS over the rostral parts of the SMA. No such effect was seen following rTMS over MI or after sham rTMS. Our results suggest that the rostral parts of the SMA play an important role in aspects of functional bimanual tasks, which involve tight temporal coordination between different motor actions of the two hands.

Direct comparison of neural systems mediating conscious and unconscious skill learning

Procedural learning, such as perceptual-motor sequence learning, has been suggested to be an obligatory consequence of practiced performance and to reflect adaptive plasticity in the neural systems mediating performance. Prior neuroimaging studies, however, have found that sequence learning accompanied with awareness (declarative learning) of the sequence activates entirely different brain regions than learning without awareness of the sequence (procedural learning). Functional neuroimaging was used to assess whether declarative sequence learning prevents procedural learning in the brain. Awareness of the sequence was controlled by changing the color of the stimuli to match or differ from the color used for random sequences. This allowed direct comparison of brain activation associated with procedural and declarative memory for an identical sequence. Activation occurred in a common neural network whether initial learning had occurred with or without awareness of the sequence, and whether subjects were aware or not aware of the sequence during performance. There was widespread additional activation associated with awareness of the sequence. This supports the view that some types of unconscious procedural learning occurs in the brain whether or not it is accompanied by conscious declarative knowledge.

Basic mechanisms of TMS

Transcranial magnetic stimulation (TMS) is now established as an important noninvasive measure for neurophysiologic investigation of the central and peripheral nervous systems in humans. Magnetic stimulation can be used for stimulating peripheral nerves with a similar mechanism of activation as for electrical stimulation. When TMS is applied to the cerebral cortex, however, some features emerge that distinguish it from transcranial electrical stimulation. One of the most important features is designated the D and I wave hypothesis, which is now widely accepted as a mechanism of TMS of the motor cortex. Transcranial electrical stimulation excites the pyramidal tract axons directly, either at the initial segment of the neuron or at proximal internodes in the subcortical white matter, giving rise to D (direct) waves, whereas TMS excites the pyramidal neurons transsynaptically, giving rise to I (indirect) waves. There are still other phenomena with mechanisms that remain to be elucidated. First, not only excitatory effects but also inhibitory effects can be elicited by TMS of the cerebral cortex (e.g., the silent period and intracortical inhibition). The inhibitory effect may also be used to investigate cerebral functions other than the motor cortex, such as the visual, sensory cortices, and the frontal eye field, from which no overt response like the motor evoked potential can be elicited. Second, there is an abundance of intraregional functional connectivities among different cortical areas that can also be revealed by TMS, or TMS in combination with neuroimaging techniques. Last, repetitive transcranial stimulation exerts a lasting effect on brain function even after the stimulation has ceased. With further investigation of the neural mechanisms of TMS, these techniques will open up new possibilities for investigating the physiologic function of the brain as well as opportunities for clinical application.

Central mechanisms of motor skill learning

Recent studies have shown that frontoparietal cortices and interconnecting regions in the basal ganglia and the cerebellum are related to motor skill learning. We propose that motor skill learning occurs independently and in different coordinates in two sets of loop circuits: cortex-basal ganglia and cortex-cerebellum. This architecture accounts for the seemingly diverse features of motor learning.