Camille-Aimé Possamaï

The time-course of preparatory spinal and cortico-spinal inhibition: an H-reflex and transcranial magnetic stimulation study in man

Abstract In a previous study where reaction-time methods were combined with transcranial magnetic stimulation (TMS) of the motor cortex, cortico-spinal excitability was shown to reflect time preparation. Provided that subjects can accurately estimate time, the amplitude of motor-evoked potentials (MEPs) diminish progressively during the interval separating the warning signal from the response signal (i.e., the foreperiod). On the other hand, several experiments have demonstrated that the amplitude of the Hoffman (H) reflex elicited in prime movers diminishes during the foreperiod of reaction-time tasks. The aim of the present study was to compare the time course of the respective decrements of H-reflex and MEP amplitude during a constant 500-ms foreperiod. The subjects (n=8) participated in two experimental sessions. In one session, H-reflexes were induced in a tonically activated, responding hand muscle, the flexor pollicis brevis, at different times during the foreperiod of a visual-choice reaction-time task. In the other session, motor potentials were evoked in the same muscle by TMS of the motor cortex delivered in the same behavioral conditions and at the same times as in the first session. The results show that both H-reflexes and MEPs diminish in amplitude during the foreperiod, which replicates and extends previous findings. Interestingly, the time constants of the two decrements differed. There was a facilitatory effect of both electrical and magnetic stimulations on the subject’s performance: reaction time was shorter for the trials during which a stimulation was delivered than for the no-stimulation trials. This facilitation was maximal when the stimulations were delivered simultaneously with the warning signal and vanished progressively with stimulation time.

Early modulation of visual input: A study of attentional strategies

Despite agreement among many attentional theories that processing resources are limited and allocated according to task demands, controversy continues about the locus of selectivity. Studies of spatial orientation of attention suggest an early effect. These results, however, can be explained instead by effects of decision processes. The present study avoids this difficulty by directly manipulating attention in a dual-task paradigm and by using SDT to dissociate sensory tuning from criterion shifts. Ten subjects judged whether two lines to the left of fixation were the same or different in length; they also judged two lines presented simultaneously to the right. In a given block of 64 trials, the subject was to allocate 80%, 50%, or 20% of attention to one pair of lines and the rest to the other. On every trial, the subject judged both pairs. Results showed that d′ increased from 0.77 with 20% allocation to 1.69 with 80%, indicating that sensitivity is modulated by attentional instructions. These results are predicted quantitatively by Luces sample-size model.

Effect of the irrelevant location of the response signal on choice reaction time : An electromyographic study in humans

Choice reaction time (RT) is shorter when the stimulus corresponds spatially to the response than when the stimulus does not, even when the stimulus location is irrelevant to the task. We used electromyographic measures to document that this effect is the result of a response conflict. The activity of the prime movers of two alternative responses was recorded during the performance of a visual RT task in which the irrelevant spatial correspondence between the stimuli and the responses was varied. Only the premotor component of RT was affected by the stimulus-response correspondence. Correct trials were distinguished according to whether or not the activation of the prime mover involved in the required response was preceded by an activation of the prime mover involved in the alternative response. Double muscular activation trials were more numerous for noncorresponding than for corresponding stimulus-response associations. Furthermore, these trials yielded longer RTs than the single muscular activation trials.

Relationship between inhibition and facilitation following a visual cue

Abstract Several recent studies have shown that a brief event in the visual field (cue) can facilitate the detection of a target presented at that same place within about 100 msec. This initial facilitation was followed by an inhibition - i.e., targets presented at that same place were processed less efficiently than the ones presented elsewhere. As it happens, this crossover between facilitation and inhibition may well have been artefactual since it occurred precisely when the cue was turned off. Moreover, the relationship between late inhibition and prior facilitation was not clear. In the present experiment, a given location was cued by a luminance increment, for only 35 msec, of one out of 3 diodes located at fixation, 7° to the left and 7° to the right of fixation ( p = 1 3 for each diode). The subsequent target, a small cross on a video screen, occurred in one of the 3 locations ( p = 1 3 ). There was no correlation between the location of the cue and that of the target. Subjects had to press a single key, as quickly as possible, whenever a target was turned on. For peripheral targets, the same pattern of results as in the previous studies was found. This showed that this pattern was not artefactual. Cued central targets were always responded to slower than uncued ones. The implications of the latter finding for the relationship between facilitation and inhibition are discussed.

Changes in spinal excitability during choice reaction time: The H reflex as a probe of information transmission

The aim of the present study was to investigate the modulations in amplitude of H reflexes elicited in a hand muscle, the flexor pollicis brevis, during the performance of a choice reaction time (RT) task in which this muscle was directly involved. Ten subjects were to choose between a left- or a right-thumb key-press according to the lateral location of a flash of light. The stimulus-response mapping was either compatible or incompatible. Hoffman reflexes were elicited at different times during the RT by stimulation of the median nerve. Twenty-five milliseconds before the voluntary response, the amplitude of the H reflex suddenly increased when the muscle was involved in the response and decreased symmetrically when the muscle was not involved in the response. Mapping compatibility exerted no detectable influence on the changes in spinal excitability. The latter result supports the assumptions that are at the core of Sternbergs additive factor method.

Stimulus preprocessing and response selection in depression: A reaction time study ☆

Depressed subjects are slower than normal controls in reaction time (RT) tasks. However, it is not clear whether depression affects all stages of information-processing or only some of them. In the present study, this question was addressed by using the additive factor method. Ten inpatients and ten control subjects performed a two-choice visual RT task. Stimulus intensity and stimulus-response compatibility were manipulated. The effect of intensity was similar in both groups whereas the effect of compatibility was larger for the patients than for the controls. This suggests that stimulus preprocessing is unaffected by depression whilst response selection is impaired.

Intensity to force translation: a new effect of stimulus-response compatibility revealed by analysis of response time and electromyographic activity of a prime mover

In reaction time studies of stimulus-response compatibility, emphasis has been placed on the influence of spatial stimulus-response relationships, but what seems to be essential for the emergence of an effect of stimulus-response compatibility is the existence of a conceptual match between stimulus and response variables. This notion was at the origin of the present study to assess the compatibility relationship between the intensity of a visual stimulus and the force of a voluntary muscle contraction. A stimulus-response compatibility effect was demonstrated. This effect was entirely due to premotoric processes.

A responding hand effect in a simple-RT precueing experiment: evidence for a late locus of facilitation

Reaction time (RT) to a peripheral flash target is often shortened by a shortly leading (about 100 msec) non-informative cue presented at the same location; later on (500 msec), to the contrary, RT is lengthened. In this paper, an interaction between target position and responding hand is reported. Subjects made speeded key-press responses to targets presented either 7 degrees to the left or 7 degrees to the right of fixation. A valid (same location) or invalid (mirror-symmetric location) cue preceded the target by either 80 or 500 msec. Depending on the trial, subjects used either the left or the right hand. The crossover in RT, from facilitation to inhibition, is observed only when the target is on the same side as the responding hand. An analysis of the RT distributions suggests that (a) facilitation is due to decision rather than perceptual processes, (b) inhibition is due to motoric processes.

N-methyl-d-aspartate receptors and information processing: human choice reaction time under a subanaesthetic dose of ketamine

Ketamine is an N-methyl-D-aspartate antagonist that induces cognitive dysfunctions. The purpose of the present study was to investigate the effects of a subanesthetic dose of ketamine on human information processing, using the additive factor method. During perfusion of a subanesthetic dose of ketamine (0.5 mg/kg over 60 min) or a placebo (randomized double-blind, cross-over design), eight adults (aged 22-33, mean=27) performed a two-choice visual reaction time (RT) task. Signal intensity, stimulus-response mapping, and foreperiod duration were manipulated. The effects of these three variables were found to be additive on RT, indicating that three independent stages - namely, stimulus preprocessing, response selection and motor selection- were manipulated. Ketamine altered RT performance in a specific way: it interacted with foreperiod duration but its effect was additive with those of signal intensity and stimulus-response mapping. These results show that ketamine specifically affects the stage of motor adjustment, which suggests that the glutamatergic system plays an important role in motor processes.

An electromyographic investigation of the effect of stimulus-response mapping on choice reaction time.

The activity of the agonist muscles was recorded during the performance of a two-choice visual reaction time (RT) task in which the compatibility of the stimulus-response mapping was manipulated. Correct trials were distinguished according to whether or not the activation of the agonist of the required response was preceded by an activation of the agonist of the nonrequired response. Double activation trials were more numerous for the incompatible than for the compatible mapping. Furthermore, these trials yielded longer RTs than the single muscular activation trials. These results suggest that initial activations of nonrequired responses are more frequently aborted and corrected when the mapping is incompatible than when it is compatible. This finding supports the dimensional overlap model of stimulus-response compatibility (S. Kornblum, T. Hasbroucq, & A. Osman, 1990).