[Study of the organization of sensorimotor stages in man using delayed evoked potentials]

Event-related potentials elicited by passive movements in humans: characterization, source analysis, and comparison to fMRI

Cortical areas responsive to proprioceptive stimulation were assessed by ERP technique in normals and in selected patients with stroke and were compared to fMRI data. Repetitive extension of right and left forefinger elicited a P1/N1/P2 complex wave pattern. This pattern was absent in patient with complete sensory loss and present but spatially modified in patient with recovered sensory deficit. Source localization with a simple model showed three sources starting in the contralateral rolandic area (SI), then involving the inferior parietal lobe unilaterally and the supplementary motor area (10 to 134 ms). It was followed by a bilaterally distributed pattern of two sources located in the ipsilateral parietal region and in the contralateral insula. Right and left stimulation led to very symmetrical patterns. Comparison to fMRI obtained from passive extension of the wrist in normals showed very compatible data. We described in this paper, a sequential processing of proprioceptive inputs after passive movements involving primary and secondary sensory motor areas.

Implicit short-lived motor representations of space in brain damaged and healthy subjects

This article reviews experimental evidence for a specific sensorimotor function which can be dissociated from higher level representations of space. It attempts to delineate this function on the basis of results obtained by psychophysical experiments performed with brain damaged and healthy subjects. Eye and hand movement control exhibit automatic features, such that they are incompatible with conscious control. In addition, they rely on a reference frame different from the one used by conscious perception. Neuropsychological cases provide a strong support for this specific motor representation of space, which can be spared in patients with lesions of primary sensory systems who have lost conscious perception of visual, tactile or proprioceptive stimuli. Observation of these patients also showed that their motor behavior can be "attracted" by a goal only under specific conditions, that is, when the response is immediate and when no cognitive representation of this goal is elaborated at the same time. Beyond the issue of the dissociation between an implicit motor representation and more cognitive processing of spatial information, the issue of the interaction between these two systems is thus a matter of interest. It is suggested that the conscious, cognitive representation of a stimulus can contaminate or override the short-lived motor representation, but no reciprocal influence seem to occur. The interaction observed in patients can also be investigated in normals. The literature provides examples of interaction between sensorimotor and cognitive framing of space, which confirm that immediate action is not mediated by the same system as delayed action, and that elaborating a categorial representation of the action goal prevents the expression of the short-lived sensorimotor representation. It is concluded that action can be controlled by a sensory system which is specialized for on-line processing of relevant goal characteristics. The temporal constraints of this system are such that it can affect the action before a full sensory analysis of this goal has been completed. The performance obtained on the basis of this spatial sensory processing suggests that short-lived motor representations may rather be considered as real "presentation" of the action world, which share its metric properties.

P3, positive slow wave and working memory load: a study on the functional correlates of slow wave activity

Parietal positivities of the 'slow wave' type are known to emerge after the P300 whenever target detection leads to a complex subsidiary task. Although the functional correlates of these 'positive slow waves' (PSW) are not known, it has been suggested that they may index (a) the selection or decision processes, (b) the preparation of the response or (c) the evaluation of its correctness. We investigated whether PSW could be dissociated from each of these putative steps of information processing by means of a paradigm devoid of motor components and needing very long reaction times. In our protocol, target stimuli acted as the triggering signal to perform silently one of 4 different tasks, namely (a) simple updating of a target count; (b) counting backward in threes; (c) simultaneous updating of two items (day of the week and ordinal of the month) and (d) updating of 3 items (the two above plus the month of the year). Reaction times to the same stimuli were obtained in 5 subjects during separate sessions. The different tasks did not modify the latencies of N2 or P3b components, but attenuated the amplitude of P3 as a mirror image of the subjective difficulty scores. A conspicuous parietal PSW appeared in conditions where two or 3 items had to be updated. This PSW developed 1-2 s earlier than the reaction times to the same experiments and could be therefore dissociated from the selection and decision processes. PSW latency was correlated with the number of items to be updated, but not with subjective difficulty. In the present paradigm PSW appeared to index the retrieval of information from working memory; however, in more general terms our results suggest that PSW is a non-specific activity that signals the completion of any synchronized operation immediately following target detection. Our data suggest a functional link between P3 and PSW, also supported by the similarity of their respective scalp topographies. The present paradigm proved to be easy to implement and suitable to study the 'executive' functions governing attentional and working-memory control during the performance of multiple tasks.