Biomedical subjects

M F Rushworth

Publications and source records attributed to M F Rushworth.

10 recordsLinked to original sources

PubMed1999

Signal-, set-, and movement-related activity in the human premotor cortex.

Single unit recording studies in non-human premotor cortex have revealed neurons with motor-related activity. Other neurons, however, seem to be involved in prior movement selection and preparation processes, and have activity related to visual instruction signals or movement preparation ('set'). We have used single pulse transcranial magnetic stimulation (TMS) to identify similar processes in human subjects. In Experiment 1 subjects performed a cued movement task while being stimulated with TMS over three sites: sensorimotor cortex, posterior premotor cortex and anterior premotor cortex. TMS slowed movements when applied at 140 ms after the visual cue over the anterior premotor site, at 180 ms after the visual cue over the posterior premotor site, and at 220 ms and later after the visual cue over the sensorimotor cortex. The results are consistent with a change from signal to movement-related processing when moving from premotor to motor cortex. In Experiment 2 there was a preparatory set period between the instruction signal that informed subjects which movement to make and the 'go' signal that informed them when to actually make the movement. TMS was applied over the anterior premotor site and the sensorimotor site during the set period. At both sites TMS had similar effects on slowing subsequent movements. The results suggest set activity in both premotor and motor cortices in human subjects.

Brain↗

PubMed1998

How do visual instructions influence the motor system?

The paper distinguishes the use of visual cues to guide reaching and grasping, and the ability to learn to associate arbitrary sensory cues with movements. Using positron emission tomography (PET), we have shown that the arbitrary association of visual cues and movements involves the ventral visual system (prestriate, inferotemporal and ventral prefrontal cortex), the basal ganglia and the dorsal premotor cortex. Using functional magnetic resonance imaging (fMRI), we have shown that the evoked haemodynamic responses in the ventral visual system are time-locked to the presentation of the visual cues, that the response in the motor cortex is locked to the time of response, and that the response in the dorsal premotor cortex shows cuerelated, movement-related and set-related components. Using PET we have shown that there are learning-related changes in activation in both the ventral prestriate cortex and the basal ganglia (globus pallidus) when subjects learn a visuomotor associative task. We argue that the basal ganglia may act as a flexible system for learning the association of sensory cues and movements.

Humans↗

PubMed1998

The preparation, execution and suppression of copied movements in the human brain.

We used positron emission tomography (PET) to measure movement set-related changes in regional cerebral blood flow (rCBF) when human subjects were asked to copy hand movements. Movement set-related activity in the brain is thought to reflect the processes of movement selection, preparation and inhibition. Four conditions were used. In the first condition, prepare and execute (PE), the hand stimulus to be copied was shown to subjects 3 s before an auditory "go"-cue instructed subjects to execute the movement; a large part of the scanning time was therefore spent in preparing to move. In the immediate execution condition (E), the hand stimulus and the go cue were presented simultaneously. The prepare-only condition (P) was similar to PE, except subjects only prepared to make the movement and did not actually execute any movement when they heard the auditory go-cue. The same stimuli were presented in a baseline condition (B), but the subjects were instructed to neither prepare nor execute movements. There were 5 principle findings: (1) In contrast to a previous study of human set-related activity in which movements were instructed by an arbitrary pattern of LEDs, preparing to make a copied movement causes rCBF changes in area 44 in posterior Broca's area; (2) set-related activity can be recorded in the cerebellar hemispheres and midline; (3) we confirmed that the supramarginal gyrus has a general role in preparing movements - there was more rCBF in the P than the E condition; (4) the cerebellar nuclei and the basal ganglia may be particularly involved in the initiation and execution of a planned movement; these regions were more active in the PE condition than the P condition; (5) the ventrolateral prefrontal cortex and a left anterior cingulate area are part of a distributed system involved in the suppression of a motor response; these areas were significantly more active in the P than the PE condition.

Adult↗

PubMed1998

The left hemisphere and the selection of learned actions.

The left hemisphere's dominance for movement is well known. The basis of its dominance is less clear. We have tested 16 left hemisphere (LH) patients, 17 right hemisphere (RH) patients and 12 neurologically normal controls on a battery of five tasks. The tasks were based on animal lesion and recording studies, and human imaging and magnetic stimulation studies that identified two distributed systems that are important for the selection of motor responses and object-oriented responses. The LH patients were impaired on three response selection tasks: learning to select between joystick movement responses instructed by visual cues; learning to select between analogous object-oriented responses instructed by visual cues; learning to select movements in a sequence. Although we replicated the finding that LH damage impairs sequencing, some of the impaired tasks had no sequencing element. We therefore argue that the LH deficits are best explained as an impairment of response selection. This was confirmed by showing that LH subjects were unimpaired on a more demanding task-object discrimination learning-which imposed a greater memory load but had no response selection element. Moreover, the LH deficits could not be attributed to disorganization of movement kinematics. The lesions of the impaired LH group were widespread but always included the distributed systems known to be important for response selection-the dorsolateral frontal and parietal cortices, striatum, thalamus and white matter fascicles.

Adult↗

PubMed1998

Temporary interference in human lateral premotor cortex suggests dominance for the selection of movements. A study using transcranial magnetic stimulation.

It is known that damage to the left hemisphere can lead to movement deficits, and that patients with apraxia have difficulty in selecting movements. Neurophysiological recording studies and lesion studies have shown that the premotor cortex is important for the selection of movements in monkeys. In this study we used transcranial magnetic stimulation (TMS) to disrupt the processing in human premotor cortex. We applied TMS to normal healthy volunteers over the premotor and primary motor areas while they carried out choice reaction time and simple reaction-time tasks. We measured response times of either hand as subjects were stimulated over the left and right hemisphere separately. We found that we were able to delay responses by stimulating at short cue-stimulus intervals (100-140 ms) over premotor cortex and at longer cue-stimulus intervals (300-340 ms) over primary motor cortex while subjects performed the choice reaction-time task with the contralateral hand. We were also able to delay responses with the ipsilateral hand while stimulating over the left premotor cortex, but not while stimulating over the right premotor cortex or either sensorimotor cortex. Premotor cortex stimulation alone disrupts an early stage of movement selection; motor cortex stimulation disrupts the movements at a later stage of execution. There was no distinguishing short cue-stimulus interval effect when premotor cortex was stimulated in the simple reaction time paradigm, where the movement selection demands of the task are kept to a minimum. We conclude that the premotor cortex is important for selecting movements after a visual cue and that the left hemisphere is dominant for the rapid selection of action.

Analysis of Variance↗

PubMed1998

Parietal cortex and spatial-postural transformation during arm movements.

Cells in the parietal motor areas 5, MIP, and 7b have spatially tuned activity during movements. Lesions, however, do not disrupt visual reaching or learned nonspatial movement selection. The role of such parietal cells in sensorimotor coordinate transformations is unclear. The present experiment investigates whether the parietal motor areas are concerned with the following: 1) the transformation between the desired position in space of the hand and the limb's postural configuration during movement and 2) interjoint coordination. Six macaque monkeys were trained to reach in the dark. Spatial-postural transformations assume a simple form in the absence of vision and so may be most easily studied when animals reach in the dark. A lesion was placed in the parietal cortex that included areas 5, MIP, and 7b of three macaques. The simple relation between hand position and limb postural configuration seen in controls was disrupted after the lesion. The intercoordination of movements of the hand with those of the rest of the arm was also affected. The lesion did not affect the range or velocity of joint movements or the curvature of the hand's trajectory. The cell activity in parietal areas 5, MIP, and 7b may not be essential for the transformation between retinocentric representation of the target and shoulder centered representations of the desired position of the hand, but it is essential for both the subsequent transformation between desired hand position and the postural configuration of the arm and for interjoint integration.

Animals↗

PubMed1997

Ventral prefrontal cortex is not essential for working memory.

It is widely held that the prefrontal cortex is important for working memory. It has been suggested that the inferior convexity (IC) may play a special role in working memory for form and color (). We have therefore assessed the ability of monkeys with IC lesions to perform visual pattern association tasks and color-matching tasks, both with and without delay. In experiment 1, six monkeys were trained on a visual association task with delays of up to 2 sec. Conservative IC lesions that removed lateral area 47/12 in three animals had no effect on the task. Further experiments showed that these lesions had no effect on the postoperative new learning of a color-matching task with delays of up to 2 sec or versions of the visual association task involving delays of up to 8 sec. In experiment 2, larger lesions of both areas 47/12 and 45A were made in the three control animals. This lesion caused a profound deficit in the ability to relearn simultaneous color matching, but subsequent matching with delays of up to 8 sec was clearly unimpaired. We suggest that the IC may be more important for stimulus selection and attention as opposed to working memory.

Animals↗

PubMed1997

Parietal cortex and movement. I. Movement selection and reaching.

Recording studies in the parietal cortex have demonstrated single-unit activity in relation to sensory stimulation and during movement. We have performed three experiments to assess the effect of selective parietal lesions on sensory motor transformations. Animals were trained on two reaching tasks: reaching in the light to visual targets and reaching in the dark to targets defined by arm position. The third task assessed non-standard, non-spatial stimulus response mapping; in the conditional motor task animals were trained to either pull or turn a joystick on presentation of either a red or a blue square. We made two different lesions in the parietal cortex in two groups of monkeys. Three animals received bilateral lesions of areas 5, 7b and MIP, which have direct connections with the premotor and motor cortices. The three other animals subsequently received bilateral lesions in areas 7a, 7ab and LIP. Both groups were still able to select between movements arbitrarily associated with non-spatial cues in the conditional motor task. Removal of areas 7a, 7ab and LIP caused marked inaccuracy in reaching in the light to visual targets but had no effect on reaching in the dark. Removal of areas 5, 7b and MIP caused misreaching in the dark but had little effect on reaching in the light. The results suggest that the two divisions of the parietal cortex organize limb movements in distinct spatial coordinate systems. Area 7a/7ab/LIP is essential for spatial coordination of visual motor transformations. Area 5/7b/MIP is essential for the spatial coordination of arm movements in relation to proprioceptive and efference copy information. Neither part of the parietal lobe appears to be important for the non-standard, non-spatial transformations of response selection.

Animals↗

PubMed1997

Parietal cortex and movement. II. Spatial representation.

Lesions in the two divisions of parietal cortex, 5/7b/MIP and 7a/LIP, produce dissociable reaching deficits. Monkeys with 5/7b/MIP removals were tested on reaching in the dark under two different conditions. All the reaches made on any day were from the same starting position to the same target position in the control condition. In the "transfer" condition, all the reaches were made to the same target position but consecutive reaches were made from different starting positions. The target could be represented as a constant pattern of joint and muscle positions in the control condition. The transfer condition required a representation of the starting position of the hand and/or a representation of the target in terms of its position in space. Removal of areas 5, 7b and MIP produced only a very mild impairment in the control condition and a severe impairment in the transfer condition. This suggests that 5/7b/MIP does not represent the limb in simple sensory or motor coordinates but in terms of its spatial position.

Animals↗

PubMed1997

The left parietal cortex and motor attention.

The posterior parietal cortex, particularly in the right hemisphere, is crucially important for covert orienting; lesions impair the ability to disengage the focus of covert orienting attention from one potential saccade target to another (Posner, M. I. et al., Journal of Neuroscience, 1984, 4, 1863-1874). We have developed a task where precues allow subjects to covertly prepare subsequent cued hand movements, as opposed to an orienting or eye movement. We refer to this process as motor attention to distinguish it from orienting attention. Nine subjects with lesions that included the left parietal cortex and nine subjects with lesions including the right parietal cortex were compared with control subjects on the task. The left hemisphere subjects showed the same ability as controls to engage attention to a movement when they were forewarned by a valid precue. The left hemisphere subjects, however, were impaired in their ability to disengage the focus of motor attention from one movement to another when the precue was incorrect. The results support the existence of two distinct attentional systems allied to the orienting and limb motor systems. Damage to either system causes analogous problems in disengaging from one orienting/movement target to another. The left parietal cortex, particularly the supramarginal gyrus, is associated with motor attention. All the left hemisphere subjects had ideomotor apraxia and had particular problems performing sequences of movements. We suggest that the well documented left hemisphere and apraxic impairment in movement sequencing is the consequence of a difficulty in shifting the focus of motor attention from one movement in a sequence to the next.

Adult↗