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Richard E Passingham

Publications and source records attributed to Richard E Passingham.

24 records · Page 2Linked to original sources
PubMed2002

Multiple movement representations in the human brain: an event-related fMRI study.

Neurovascular correlates of response preparation have been investigated in human neuroimaging studies. However, conventional neuroimaging cannot distinguish, within the same trial, between areas involved in response selection and/or response execution and areas specifically involved in response preparation. The specific contribution of parietal and frontal areas to motor preparation has been explored in electrophysiological studies in monkey. However, the associative nature of sensorimotor tasks calls for the additional contributions of other cortical regions. In this article, we have investigated the functional anatomy of movement representations in the context of an associative visuomotor task with instructed delays. Neural correlates of movement representations have been assessed by isolating preparatory activity that is independent from the performance of an actual motor act, or from the presence of a response's target. Movement instruction (specified by visual cues) and motor performance (specified by an auditory cue) were separated by a variable delay period. We have used whole-brain event-related fMRI to measure human brain activity during the performance of such a task. We have focused our analysis on specific preparatory activity, defined as a sustained response over variable delay periods between a transient visual instruction cue and a brief motor response, temporally independent from the transient events. Behavioral and electrophysiological controls ensured that preparatory activity was not contaminated by overt motor responses or working memory processes. We report suggestive evidence for multiple movement representations in the human brain. Specific sustained activity in preparation for an action was found not only in parieto-frontal regions but also in extrastriate areas and in the posterior portion of the superior temporal sulcus. We suggest that goal-directed preparatory activity relies on both visuomotor and visuoperceptual areas. These findings point to a functional-anatomical basis for the integration of perceptual and executive processes.

Adult↗
PubMed2002

Initial demonstration of in vivo tracing of axonal projections in the macaque brain and comparison with the human brain using diffusion tensor imaging and fast marching tractography.

Diffusion tensor imaging (DTI), a magnetic resonance imaging technique, is used to infer major axonal projections in the macaque and human brain. This study investigates the feasibility of using known macaque anatomical connectivity as a "gold-standard" for the evaluation of DTI tractography methods. Connectivity information is determined from the DTI data using fast marching tractography (FMT), a novel tract-tracing (tractography) method. We show for the first time that it is possible to determine, in an entirely noninvasive manner, anatomical connection pathways and maps of an anatomical connectivity metric in the macaque brain using a standard clinical scanner and that these pathways are consistent with known anatomy. Analogous human anatomical connectivity is also presented for the first time using the FMT method, and the results are compared. The current limitations of the methodology and possibilities available for further studies are discussed.

Animals↗
PubMed2002

Changes of cortico-striatal effective connectivity during visuomotor learning.

It has been suggested that the cortico-striatal system might play a crucial role in learning behavioural plans of action. We have tested this hypothesis by studying the dynamics of functional coupling among the neural elements of cortico-striatal circuitry. Human cerebral activity was measured with functional magnetic resonance imaging (fMRI) during the learning of an associative visuomotor task. Structural equation modelling of regional fMRI time-series was used to characterize learning-related changes in effective connectivity. We report that learning to associate visual instructions with motor responses significantly altered cortico-striatal functional couplings. Specific learning-related increases of effective connectivity were found in temporo-striatal and fronto-striatal circuits. Connectivity among portions of the frontal cortex decreased as a function of learning. Temporo-frontal and parieto-frontal couplings were not altered during learning. We infer that novel visuomotor associations are established through the enhancement of specific cortico-striatal circuits, rather than through the alteration of direct temporo-frontal or parieto-frontal connectivity.

Cerebral Cortex↗
PubMed2002

Learning of sequences of finger movements and timing: frontal lobe and action-oriented representation.

Motor sequence learning involves learning of a sequence of effectors with which to execute a series of movements and learning of a sequence of timings at which to execute the movements. In this study, we have segregated the neural correlates of the two learning mechanisms. Moreover, we have found an interaction between the two learning mechanisms in the frontal areas, which we claim as suggesting action-oriented coding in the frontal lobe. We used positron emission tomography and compared three learning conditions with a visuo-motor control condition. In two learning conditions, the subjects learned either a sequence of finger movements with random timing or a sequence of timing with random use of fingers. In the third condition the subjects learned to execute a sequence of specific finger movements at specific timing; we argue that it was only in this condition that the motor sequence was coded as an action-oriented representation. By looking for condition by session interactions (learning vs. control conditions over sessions), we have removed nonspecific time effects and identified areas that showed a learning-related increment of activation during learning. Learning of a finger sequence was associated with an increment of activation in the right intraparietal sulcus region and medial parietal cortex, whereas learning of a timing sequence was associated with an increment of activation in the lateral cerebellum, suggesting separate mechanisms for learning effector and temporal sequences. The left intraparietal sulcus region showed an increment of activation in learning of both finger and timing sequences, suggesting an overlap between the two learning mechanisms. We also found that the mid-dorsolateral prefrontal cortex, together with the medial and lateral premotor areas, became increasingly active when subjects learned a sequence that specified both fingers and timing, that is, when subjects were able to prepare specific motor action. These areas were not active when subjects learned a sequence that specified fingers or timing alone, that is, when subjects were still dependent on external stimuli as to the timing or fingers with which to execute the movements. Frontal areas may integrate the effector and temporal information of a motor sequence and implement an action-oriented representation so as to perform a motor sequence accurately and quickly. We also found that the mid-dorsolateral prefrontal cortex was distinguished from the ventrolateral prefrontal cortex and anterior fronto-polar cortex, which showed sustained activity throughout learning sessions and did not show either an increment or decrement of activation.

Adult↗