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R E Passingham

Publications and source records attributed to R E Passingham.

132 records · Page 8Linked to original sources
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Premotor cortex: sensory cues and movement.

There are suggestions that premotor cortex might play a role either in the execution of axial and proximal movements or in the sensory guidance of movement. Monkeys with bilateral premotor lesions were therefore studied on a series of behavioural tests; the tasks differed either in the movements to be performed or in the nature of the sensory cues that guided the animals. It was found that the success or failure of the monkeys depended on the way in which they received instructions as to what to do. These results are consistent with the view that premotor cortex is involved in the direction of movement but not in its execution.

Animals↗
PubMedDate not supplied

Entropy and cortical activity: information theory and PET findings.

Functional segregation requires convergence and divergence of neuroanatomical connections. Furthermore, the nature of functional segregation suggests that (1) signals in convergent afferents are correlated and (2) signals in divergent efferents are uncorrelated. The aim of this article is to show that this arrangement can be predicted mathematically, using information theory and an idealized model of cortical processing. In theory, the existence of bifurcating axons limits the number of independent output channels from any small cortical region, relative to the number of inputs. An information theoretic analysis of this special (high input:output ratio) constraint indicates that the maximal transfer of information between inputs, to a cortical region, and its outputs will occur when (1) extrinsic connectivity to the area is organized such that the entropy of neural activity in afferents is optimally low and (2) connectivity intrinsic to the region is arranged to maximize the entropy measured at the initial segments of projection neurons. Under the constraints of the model, a low entropy is synonymous with high correlations between axonal firing rates (and vice versa). Consequently this antisymmetric arrangement of functional activity in convergent and divergent connections underlying functional segregation is exactly that predicted by the principle of maximum preservation of information, considered in the context of axonal bifurcation. The hypothesis that firing in convergent afferents is correlated (has low entropy) and spatially coherent was tested using positron emission tomographic measurements of cortical synaptic function in man. This hypothesis was confirmed.

Afferent Pathways↗
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Signal-, set- and movement-related activity in the human brain: an event-related fMRI study.

Electrophysiological studies on monkeys have been able to distinguish sensory and motor signals close in time by pseudorandomly delaying the cue that instructs the movement from the stimulus that triggers the movement. We have used a similar experimental design in functional magnetic resonance imaging (fMRI), scanning subjects while they performed a visuomotor conditional task with instructed delays. One of four shapes was presented briefly. Two shapes instructed the subjects to flex the index finger; the other two shapes coded the flexion of the middle finger. The subjects were told to perform the movement after a tone. We have exploited a novel use of event-related fMRI. By systematically varying the interval between the visual and acoustic stimuli, it has been possible to estimate the significance of the evoked haemodynamic response (EHR) to each of the stimuli, despite their temporal proximity in relation to the time constant of the EHR. Furthermore, by varying the phase between events and image acquisition, we have been able to achieve high temporal resolution while scanning the whole brain. We dissociated sensory and motor components of the sensorimotor transformations elicited by the task, and assessed sustained activity during the instructed delays. In calcarine and occipitotemporal cortex, the responses were exclusively associated with the visual instruction cues. In temporal auditory cortex and in primary motor cortex, they were exclusively associated with the auditory trigger stimulus. In ventral prefrontal cortex there were movement-related responses preceded by preparatory activity and by signal-related activity. Finally, responses associated with the instruction cue and with sustained activity during the delay period were observed in the dorsal premotor cortex and in the dorsal posterior parietal cortex. Where the association between a visual cue and the appropriate movement is arbitrary, the underlying visuomotor transformations are not achieved exclusively through frontoparietal interactions. Rather, these processes seem to rely on the ventral visual stream, the ventral prefrontal cortex and the anterior part of the dorsal premotor cortex.

Adult↗