Etienne Olivier

Visual Responses on Neck Muscles Reveal Selective Gating that Prevents Express Saccades

Express saccades promote the acquisition of visual targets at extremely short reaction times. Because of the heads considerable inertia, it is unknown whether express saccades are accompanied by a parallel command to the head. Here, by recording electromyographic (EMG) activity from monkey neck muscles, we demonstrate that visual target presentation elicits time-locked, lateralized recruitment of neck muscles at extremely short latencies (55-95 ms). Remarkably, such recruitment not only accompanies express saccades, but also precedes nonexpress saccades, occasionally by up to 150 ms. These results demonstrate selective gating of components of descending commands from the superior colliculus to prevent express saccades yet permit recruitment of a head orienting synergy. We conclude that such selective gating aids eye-head coordination by permitting force development at neck muscles while a decision to commit to a gaze shift is being made, optimizing the contribution of the more inertial head to the ensuing gaze shift.

Actions, Words, and Numbers A Motor Contribution to Semantic Processing?

Recent findings in neuroscience challenge the view that the motor system is exclusively dedicated to the control of actions, and it has been suggested that it may contribute critically to conceptual processes such as those involved in language and number representation. The aim of this review is to address this issue by illustrating some interactions between the motor system and the processing of words and numbers. First, we detail functional brain imaging studies suggesting that motor circuits may be recruited to represent the meaning of action-related words. Second, we summarize a series of experiments demonstrating some interference between the size of grip used to grasp objects and the magnitude processing of words or numbers. Third, we report data suggestive of a common representation of numbers and finger movements in the adult brain, a possible trace of the finger-counting strategies used in childhood. Altogether, these studies indicate that the motor system interacts with several aspects of word and number representations. Future research should determine whether these findings reflect a causal role of the motor system in the organization of semantic knowledge.

Lateral interactions in the superior colliculus, not an extended fixation zone, can account for the remote distractor effect

Recordings of neuronal activity in the monkey superior colliculus (SC) suggest that the two apparently independent effects of a visual distracter on both temporal (latency) and spatial (metrics) saccade parameters may be the result of lateral interactions between subpopulations of saccade-related neurons located at different sites on the motor map of the superior colliculus. One subpopulation is activated during the planing and initiation of a saccade; the other is activated by the appearance of a distractor. The inhibitory or facilitative nature of this interaction depends on the distance between the distracter and the target and is consistent with the complex pattern of intrinsic and commissural collicular connections.

Recording an identified pyramidal volley evoked by transcranial magnetic stimulation in a conscious macaque monkey

A descending volley in response to non-invasive transcranial magnetic stimulation has been recorded from the pyramidal tract in a conscious monkey and identified by means of a collision test. The short latency of the earliest wave was inconsistent with a trans-synaptically mediated activation of pyramidal tract neurones. Considerable variability in the size of this wave was seen in response to a constant stimulus, and isoflurane anaesthetic was shown to depress it markedly. These results are consistent with direct activation of pyramidal tract neurones at a site close to the cell body.

Emergent oscillations in a realistic network: the role of inhibition and the effect of the spatiotemporal distribution of the input.

We have simulated a network of 10,000 two-compartment cells, spatially distributed on a two-dimensional sheet; 15% of the cells were inhibitory. The input to the network was spatially delimited. Global oscillations frequently were achieved with a simple set of connectivity rules. The inhibitory neurons paced the network, whereas the excitatory neurons amplified the input, permitting oscillations at low-input intensities. Inhibitory neurons were active over a greater area than excitatory ones, forming a ring of inhibition. The oscillation frequency was modulated to some extent by the input intensity, as has been shown experimentally in the striate cortex, but predominantly by the properties of the inhibitory neurons and their connections: the membrane and synaptic time constants and the distribution of delays.In networks that showed oscillations and in those that did not, widely distributed inputs could lead to the specific recruitment of the inhibitory neurons and to near zero activity of the excitatory cells. Hence the spatial distribution of excitatory inputs could provide a means of selectively exciting or inhibiting a target network. Finally, neither the presence of oscillations nor the global spike activity provided any reliable indication of the level of excitatory output from the network.

Comparison of the distribution and somatodendritic morphology of tectotectal neurons in the cat and monkey.

The presence of a commissure connecting the two superior colliculi suggests they do not act independently, but the function of the tectotectal connection has never been firmly identified. To develop a better understanding of this commissural system, the present study determined the distribution and morphology of tectotectal neurons in the cat and macaque monkey, two animals with well-studied, but different orienting strategies. First, we compared the distribution of tectotectal cells retrogradely labeled following WGA-HRP injections into the contralateral superior colliculus. In monkeys, labeled tectotectal cells were found in all layers, but were concentrated in the intermediate gray layer (75%), particularly dorsally, and the adjacent optic layer (12%). Tectotectal cells were distributed throughout nearly the entire rostrocaudal extent of the colliculus. In cats, tectotectal cells were found in all the layers beneath the superficial gray, but the intermediate gray layer contained the greatest concentration (56%). Labeled cells were almost exclusively located in the rostral half of the cat superior colliculus, in contrast to the monkey distribution. In the context of the representation of visuomotor space in the colliculus, the distribution of monkey and cat tectotectal cells suggests a correspondence with oculomotor range. So these neurons may be involved in directing orienting movements performed within the oculomotor range. The somatodendritic morphology of tectotectal cells in these two species was revealed by homogeneous retrograde labeling from injections of biocytin or biotinylated dextran amine into the contralateral colliculus. The cell classes contributing to this pathway are fairly consistent across the two species. A variety of neuronal morphologies were observed, so there is no single tectotectal cell type. Instead, cell types similar to those found in each layer, excepting the largest neurons, were present among tectotectal cells. This suggests that a sample of each layers output is sent to the contralateral colliculus.

Evidence for glutamatergic tectotectal neurons in the cat superior colliculus: a comparison with GABAergic tectotectal neurons

The tectotectal commissural pathway is commonly regarded as responsible for the reciprocal inhibition that takes place between the two superior colliculi (SC). Although this hypothesis has received strong support from electrophysiological studies, more recent investigations have suggested that some collicular cells, e.g. fixation neurons, may establish excitatory connections with cells in the contralateral SC through the collicular commissure. The goal of the present study was to seek immunohistochemical evidence for glutamatergic tectotectal cells in the cat SC by using a double‐labelling technique. Tectotectal cells were retrogradely labelled with wheat germ agglutinin (WGA) –horseradish peroxidase (HRP) coupled to colloidal gold injected in the contralateral SC, and neurons containing glutamate or γ‐aminobutyric acid (GABA) were then identified with immunohistochemical techniques. The present study provides evidence that, in the cat SC, equal numbers of tectotectal cells are immunopositive to glutamate and GABA, suggesting that the tectotectal pathway may consist of two distinct functional components. The finding that an equal number of tectotectal cells are GABAergic and glutamatergic is somewhat surprising as electrophysiological studies have invariantly indicated that the inhibitory component of the tectotectal projection predominates. Another striking feature of the GABAergic and glutamatergic tectotectal cell populations is their identical topographic distribution in the SC. These results suggest that not only cells in the rostral fixation zone establish excitatory connections with the contralateral SC. Tectotectal projections could be potentially important to shape the spatial pattern of saccade‐related activity that may occur simultaneously in the two SC during vertical and oblique orienting movements.

Excitability of human upper limb motoneurones during rhythmic discharge tested with transcranial magnetic stimulation.

1. The activity of thirty‐one single motor units (SMUs) was recorded from forearm and hand muscles of three volunteers. The excitability of the rhythmically firing motoneurones supplying these SMUs was examined after voluntary discharge using transcranial magnetic stimulation (TMS). 2. The magnetic stimulus was delivered either at a fixed delay (range: 1‐60 ms) after SMU discharge (triggered mode) or at random with respect to voluntary SMU discharge (random mode). Post‐stimulus time histograms (PSTHs) of responses to 50‐100 stimuli were constructed for each experimental condition. 3. In the triggered mode, the probability of response to TMS progressively decreased as the spike‐to‐stimulus interval was shortened. Shortening of the interval also resulted in redistribution of responses within the different subpeaks characterizing the short‐latency response of motor units to TMS: the relative response probability of the first subpeak decreased with the shorter spike‐to‐stimulus intervals. 4. In the triggered mode, the probability of SMU responding to TMS was significantly higher when the firing rate of the motor unit was increased from a slow rate (< 10 impulses s‐1) to a faster rate (> 12 impulses s‐1), irrespective of the spike‐to‐stimulus interval. In contrast, in the random mode, the response probability was greater at the slower discharge rate. 5. The higher excitability of motoneurones at the fast rate in the triggered mode is consistent with findings in cat motoneurones suggesting a shallower after‐hyperpolarization, but other mechanisms could contribute. Furthermore, our results suggest that there is an asymptotic recovery in the excitability of slow firing motoneurones towards the end of the interspike interval.

Disrupting the Supplementary Motor Area Makes Physical Effort Appear Less Effortful

The perception of physical effort is relatively unaffected by the suppression of sensory afferences, indicating that this function relies mostly on the processing of the central motor command. Neural signals in the supplementary motor area (SMA) correlate with the intensity of effort, suggesting that the motor signal involved in effort perception could originate from this area, but experimental evidence supporting this view is still lacking. Here, we tested this hypothesis by disrupting neural activity in SMA, in primary motor cortex (M1), or in a control site by means of continuous theta-burst transcranial magnetic stimulation, while measuring effort perception during grip forces of different intensities. After each grip force exertion, participants had the opportunity to either accept or refuse to replicate the same effort for varying amounts of reward. In addition to the subjective rating of perceived exertion, effort perception was estimated on the basis of the acceptance rate, the effort replication accuracy, the influence of the effort exerted in trial t on trial t+1, and pupil dilation. We found that disruption of SMA activity, but not of M1, led to a consistent decrease in effort perception, whatever the measure used to assess it. Accordingly, we modeled effort perception in a structural equation model and found that only SMA disruption led to a significant alteration of effort perception. These findings indicate that effort perception relies on the processing of a signal originating from motor-related neural circuits upstream of M1 and that SMA is a key node of this network.

Tracing premotor brain stem networks of orienting movements

Methods allowing a direct matching of movement-related firing patterns and connectivity of individual neurons have been used in the analysis of premotor networks controlling orienting movements. Advances have been made in the description of coding properties of orienting-related tectal output neurons, as well as in specifying their distributed connections in the brain stem and possible modes of coupling to saccadic pattern generators in the reticular formation. New data on the properties of signals and connectivity patterns have also been obtained for the tecto-recipient reticulo-spinal neurons. At least a small portion of the network performing the spatio-temporal transformations of orienting-related tectal efferent signals can now be described both in functional and in morphological terms.