Ipsilateral sensorimotor regions and motor sequence learning.

Recently, Boyd and Winstein tested three groups of individuals with damage to unilateral sensorimotor areas on a version of the serial reaction time task performed with the ipsilesional hand. Only when the individuals were provided in advance with explicit knowledge of the motor sequence were they able to benefit behaviorally from the sequence. Despite aspects of the experimental procedure and the subject selection that make it difficult to draw strong conclusions, these results add to growing evidence that sensorimotor structures contribute to the formation of abstract representations that affect more than ipsilateral effectors.

Neural fate of seen and unseen faces in visuospatial neglect: a combined event-related functional MRI and event-related potential study.

To compare neural activity produced by visual events that escape or reach conscious awareness, we used event-related MRI and evoked potentials in a patient who had neglect and extinction after focal right parietal damage, but intact visual fields. This neurological disorder entails a loss of awareness for stimuli in the field contralateral to a brain lesion when stimuli are simultaneously presented on the ipsilateral side, even though early visual areas may be intact, and single contralateral stimuli may still be perceived. Functional MRI and event-related potential study were performed during a task where faces or shapes appeared in the right, left, or both fields. Unilateral stimuli produced normal responses in V1 and extrastriate areas. In bilateral events, left faces that were not perceived still activated right V1 and inferior temporal cortex and evoked nonsignificantly reduced N1 potentials, with preserved face-specific negative potentials at 170 ms. When left faces were perceived, the same stimuli produced greater activity in a distributed network of areas including right V1 and cuneus, bilateral fusiform gyri, and left parietal cortex. Also, effective connectivity between visual, parietal, and frontal areas increased during perception of faces. These results suggest that activity can occur in V1 and ventral temporal cortex without awareness, whereas coupling with dorsal parietal and frontal areas may be critical for such activity to afford conscious perception.

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.

Neural activation during response competition.

The flanker task, introduced by Eriksen and Eriksen [Eriksen, B. A., & Eriksen, C. W. (1974). Effects of noise letters upon the identification of a target letter in a nonsearch task. Perception & Psychophysics, 16, 143--149], provides a means to selectively manipulate the presence or absence of response competition while keeping other task demands constant. We measured brain activity using functional magnetic resonance imaging (fMRI) during performance of the flanker task. In accordance with previous behavioral studies, trials in which the flanking stimuli indicated a different response than the central stimulus were performed significantly more slowly than trials in which all the stimuli indicated the same response. This reaction time effect was accompanied by increases in activity in four regions: the right ventrolateral prefrontal cortex, the supplementary motor area, the left superior parietal lobe, and the left anterior parietal cortex. The increases were not due to changes in stimulus complexity or the need to overcome previously learned associations between stimuli and responses. Correspondences between this study and other experiments manipulating response interference suggest that the frontal foci may be related to response inhibition processes whereas the posterior foci may be related to the activation of representations of the inappropriate responses.

Abstract and effector-specific representations of motor sequences identified with PET.

Positron emission tomography was used to identify neural systems involved in the acquisition and expression of sequential movements produced by different effectors. Subjects were tested on the serial reaction time task under implicit learning conditions. In the initial acquisition phase, subjects responded to the stimuli with keypresses using the four fingers of the right hand. During this phase, the stimuli followed a fixed sequence for one group of subjects (group A) and were randomly selected for another group (group B). In the transfer phase, arm movements were used to press keys on a substantially larger keyboard, and for both groups, the stimuli followed the sequence. Behavioral indices provided clear evidence of learning during the acquisition phase for group A and transfer when switched to the large keyboard. Sequence acquisition was associated with learning-related increases in regional cerebral blood flow (rCBF) in a network of areas in the contralateral left hemisphere, including sensorimotor cortex, supplementary motor area, and rostral inferior parietal cortex. After transfer, activity in inferior parietal cortex remained high, suggesting that this area had encoded the sequence at an abstract level independent of the particular effectors used to perform the task. In contrast, activity in sensorimotor cortex shifted to a more dorsal locus, consistent with motor cortex somatotopy. Thus, activity here was effector-specific. An increase in rCBF was also observed in the cingulate motor area at transfer, suggesting a role linking the abstract sequential representations with the task-relevant effector system. These results highlight a network of areas involved in sequence encoding and retrieval.

Attention and stimulus characteristics determine the locus of motor-sequence encoding. A PET study.

PET revealed the effects of stimulus characteristics on the neural substrate of motor learning. Right-handed subjects performed a serial reaction time task with colour-coded stimuli to eliminate the potential for learned eye-movements. The task was performed with the right hand under two different conditions. In one condition, subjects simultaneously performed a distractor task. Although they did show behavioural evidence of learning, they were not explicitly aware of the stimulus-response sequence. In the second condition, there was no distractor task, and seven out of the 11 subjects then became explicitly aware of the stimulus sequence. Metabolic correlates of learning were distinct in the two conditions. When learning was implicit under dual-task conditions, learning-related changes were observed in left motor and supplementary motor cortex as well as in the putamen. These regions are similar to those observed in a previous study in which the stimuli were cued by spatial position. Under single-task conditions, metabolic changes were found in the right prefrontal cortex and premotor cortex, as well as in the temporal lobe. A similar shift to the right hemisphere was observed in the spatial study during single-task learning. However, explicit learning of the task with colour stimuli activated more ventral regions. The areas supporting motor-sequence learning are contingent on both stimulus properties and attentional constraints.

Neural mechanisms of timing.

A crucial step in timing research is to isolate clock components from other sources of temporal variability. Significant progress has been made both behaviorally and neurologically, using elaborate experimental designs that separate timing mechanisms from motoric sensory and mnemonic processes. Marked similarities between the temporal characteristics of the clock in perception and production tasks implicate a common timing system. Similar conclusions can be reached from clinical studies, indeed individuals with neocerebellar damage are impaired at discriminating and reproducing short intervals. However, other patient populations, especially those with disorders affecting the basal ganglia, also exhibit deficits in timing tasks. It therefore appears that temporal computation may be distributed throughout the brain with specific roles for different neural structures.

Functional mapping of sequence learning in normal humans.

The brain localization of motor sequence learning was studied in normal subjects with positron emission tomography. Subjects performed a serial reaction time (SRT) task by responding to a series of stimuli that occurred at four different spatial positions. The stimulus locations were either determined randomly or according to a 6-element sequence that cycled continuously. The SRT task was performed under two conditions. With attentional interference from a secondary counting task there was no development of awareness of the sequence. Learning-related increases of cerebral blood flow were located in contralateral motor effector areas including motor cortex, supplementary motor area, and putamen, consistent with the hypothesis that nondeclarative motor learning occurs in cerebral areas that control limb movements. Additional cortical sites included the rostral prefrontal cortex and parietal cortex. The SRT learning task was then repeated with a new sequence and no attentional interference. In this condition, 7 of 12 subjects developed awareness of the sequence. Learning-related blood flow increases were present in right dorsolateral prefrontal cortex, right premotor cortex, right ventral putamen, and biparieto-occipital cortex. The right dorsolateral prefrontal and parietal areas have been previously implicated in spatial working memory and right prefrontal cortex is also implicated in retrieval tasks of verbal episodic memory. Awareness of the sequence at the end of learning was associated with greater activity in bilateral parietal, superior temporal, and right premotor cortex. Motor learning can take place in different cerebral areas, contingent on the attentional demands of the task.