Jean Requin

The predictive value for performance speed of preparatory changes in neuronal activity of the monkey motor and premotor cortex

Three monkeys were trained in a precued reaction time (RT) paradigm. An initial preparatory signal (PS) provided complete, partial or no information about direction and extent of a wrist flexion/extension movement which was executed after the second, response signal (RS). A PS providing information about direction shortened the RT much more than a PS indicating movement extent. The activity of 464 task-related neurons was recorded in the primary motor (MI) and premotor (PM) cortex. Not only the timing and amplitude of mean activity changes were analyzed, but also trial-by-trial correlation analyses between RT and discharge frequency during the PS-RS interval were conducted. Correlations were stronger in the condition of information about direction than in conditions of information about extent or no information. Considering directionally selective neurons, correlations were stronger when the neurons preferred direction than the opposite direction was precued. Correlation distributions were similar for MI and PM. Correlations were negative when preparatory activity increased during the PS-RS interval, and positive when activity decreased. Correlation analyses between behavioral performance and neuronal activity can thus be considered as a powerful tool to obtain a deeper insight into the functional mechanism of motor preparation.

Effects of preliminary perceptual output on neuronal activity of the primary motor cortex.

Observations of single neurons in the primary motor cortex of 1 monkey provided evidence that preliminary perceptual information reaches the motor system before perceptual analysis is complete. Neurons were recorded during a task in which 1 stimulus was assigned to a wrist flexion response and another was assigned to wrist extension. Two stimuli were assigned to a no-go response; each was visually similar to either the flexion or the extension stimulus. When a no-go stimulus was presented, neurons responded with weaker versions of the discharge patterns exhibited to the visually similar stimulus requiring a movement, suggesting that neurons receive partial perceptual information favoring that movement. Functionally separable neuronal populations were identified, and differences in the activations of these provide evidence about the functional effects of preliminary perceptual output on movement control processes.

Are extent and force independent movement parameters ? Preparation- and movement-related neuronal activity in the monkey cortex

Movement extent and movement force can be independently controlled in motor performance. Therefore, independent representations of extent and force should exist in the central nervous system (CNS). To test this hypothesis, microelectrode recordings were made in sensorimotor cortex of monkeys trained to perform visually cued wrist flexion movements of two extents, against two levels of frictional resistance. An initial preparatory signal (PS) provided complete, partial or no information about extent and/or force of the movement, which had to be performed in response to a second, response signal (RS). The activity of 511 neurons of the primary motor cortex (MI), the premotor cortex (PM), the postcentral cortex (PC), and the posterior parietal cortex (PA) was recorded in two monkeys. Both reaction time (RT) and neuronal data suggest that there exist independent, neuronal mechanisms responsible for the programming of either parameter. On the one hand, partial information about either movement parameter shortened RT when compared with the condition of no prior information. On the other hand, there were, among others, two discrete populations of neurons, one related only to extent, the other only to force. Preparatory changes in activity related to either movement parameter were mainly located in the frontal cortex, especially in the PM. After occurrence of the RS, the percentage of selective changes in activity increased and tended to extend to the parietal cortex. In particular during the movement, force-related changes in activity have been encountered in PA. Furthermore, we conducted trial-by-trial correlation analyses between RT and preparatory neuronal activity for all conditions of prior information. The mean correlation coefficient was significantly higher in the condition of information about movement extent than of information about movement force and it was significantly higher in MI/ PM than in PC/PA.

Neuronal correlates of sensorimotor association in stimulus-response compatibility.

Neuronal mechanisms underlying stimulus-response (S-R) associations in S-R compatibility tasks were identified in 2 experiments with monkeys. Visual stimuli were presented on the left and right calling for left-right movements under congruent and incongruent S-R mapping instructions. High- and low-pitched tones calling for left-right movements were presented to the left and right ear, and the stimulus side was irrelevant. Single neurons sensitive to the S-R mapping rule were found in the primary motor cortex. The large overlap between the neuronal populations sensitive to the stimulus side, the S-R mapping rule, and the response side, respectively, is consistent with the idea that sensory-to-motor transformation is a continuous rather than a discrete process. Results partly support the hypothesis that the increase in reaction time with incongruent mapping is caused by the automatic activation of the congruent, but erroneous, response.

A critical re-examination of the concept of function within the neocortex of the monkey

Abstract Combined neurophysiological/psychophysiological experiments have been applied to the study of how motor acts are planned. The justification for these experiments was the structural isomorphism between theoretical stage models of information processing and the supposed functional specialization of cortical areas. However, recent developments in both fields now suggest that the assumptions on which this isomorphism is based may be erroneous. Describing the results of our own combined experiments, we attempt to show how the concept of function within the cerebral cortex needs to be revised. We present new interpretations of data from single neuron recording experiments in three cortical areas which challenge the traditional idea of structure/function. In its place, we propose the hypothesis of a continuum of function going from sensory input to motor output.

Signatures of Dynamic Cell Assemblies in Monkey Motor Cortex

It has been suggested that neuronal representations are embedded in dynamically coupled ensembles of simultaneously active neurons, commonly called cell assemblies. Multiple single-neuron recordings in the primary motor cortex of the monkey support the hypothesis that processing of information is reflected in the activity of cell assemblies. Individual neurons may belong to multiple assemblies. Furthermore, synchronized neuronal activity as an expression of the activation of a cell assembly depends on the behavioral context but not always on changes in the firing rate of neurons.