Limitations of short range Mexican hat connection for driving target selection in a 2D neural field: activity suppression and deviation from input stimuli.

Dynamic Neural Field models (DNF) often use a kernel of connection with short range excitation and long range inhibition. This organization has been suggested as a model for brain structures or for artificial systems involved in winner-take-all processes such as saliency localization, perceptual decision or target/action selection. A good example of such a DNF is the superior colliculus (SC), a key structure for eye movements. Recent results suggest that the superficial layers of the SC (SCs) exhibit relatively short range inhibition with a longer time constant than excitation. The aim of the present study was to further examine the properties of a DNF with such an inhibition pattern in the context of target selection. First we tested the effects of stimulus size and shape on when and where self-maintained clusters of firing neurons appeared, using three variants of the model. In each model variant, small stimuli led to rapid formation of a spiking cluster, a range of medium sizes led to the suppression of any activity on the network and hence to no target selection, while larger sizes led to delayed selection of multiple loci. Second, we tested the model with two stimuli separated by a varying distance. Again single, none, or multiple spiking clusters could occur, depending on distance and relative stimulus strength. For short distances, activity attracted toward the strongest stimulus, reminiscent of well-known behavioral data for saccadic eye movements, while for larger distances repulsion away from the second stimulus occurred. All these properties predicted by the model suggest that the SCs, or any other neural structure thought to implement a short range MH, is an imperfect winner-take-all system. Although, those properties call for systematic testing, the discussion gathers neurophysiological and behavioral data suggesting that such properties are indeed present in target selection for saccadic eye movements.

Facilitation of cutaneous inputs during the planning phase of gait initiation.

It has been shown that during the planning of a voluntary movement the transmission of cutaneous afferent inputs to the somatosensory cortex is attenuated shortly before the motor output as well as during movement execution. However, it is not known whether the sensory suppression observed during the planning phase (i.e., before any movement execution) is a systemic phenomenon or whether it is dependent on movement context. For example, movements such as step initiation are controlled based on information received from cutaneous receptors in the feet. Because afferent information emerging from these receptors is critical for movement initiation, we hypothesized that suppression of these inputs may not occur during the planning phase prior to gait initiation. To examine this hypothesis we measured the cortical response to somatosensory stimulation during the planning phase of step initiation and during movement execution. Sensitivity to cutaneous stimulation was assessed by measuring the amplitude of the cortical somatosensory-evoked potential (SEP, over the Cz electrode) following electrical stimulations of the plantar sole of one foot. Two stimulations were provided during the planning phase of a step movement and two stimulations during movement execution. It was found that the P50-N80 SEP was facilitated in the early planning phase (-700 ms before motor execution) compared with when participants remained still (control standing task). This mechanism might contribute to an enhanced perception of cutaneous input leading to a more accurate setting of the forces to be exerted onto the ground to shift the body's weight toward the supporting side prior to foot-off.

Tactile stimulations and wheel rotation responses: toward augmented lane departure warning systems.

When an on-board system detects a drift of a vehicle to the left or to the right, in what way should the information be delivered to the driver? Car manufacturers have so far neglected relevant results from Experimental Psychology and Cognitive Neuroscience. Here we show that this situation possibly led to the sub-optimal design of a lane departure warning system (AFIL, PSA Peugeot Citroën) implemented in commercially available automobile vehicles. Twenty participants performed a two-choice reaction time task in which they were to respond by clockwise or counter-clockwise wheel-rotations to tactile stimulations of their left or right wrist. They performed poorer when responding counter-clockwise to the right vibration and clockwise to the left vibration (incompatible mapping) than when responding according to the reverse (compatible) mapping. This suggests that AFIL implements the worse (incompatible) mapping for the operators. This effect depended on initial practice with the interface. The present research illustrates how basic approaches in Cognitive Science may benefit to Human Factors Engineering and ultimately improve man-machine interfaces and show how initial learning can affect interference effects.

On the reduced influence of contour on saccade metrics and its competition with stimulus size.

It is well known that the metrical properties of saccadic eye movements are strongly influenced by the extraction of low-level visual features (e.g., luminance). Higher-level visual features (e.g., contour) also play a role, but their relative contribution and time course remain undetermined. Here, we investigated this issue, by testing the influence of contour on saccade metrics. We used a saccade-targeting task in which a peripheral target was, on some trials, simultaneously displayed with a less eccentric distractor. This paradigm is known to yield a global effect, that is a deviation of the eyes towards an intermediate location between the stimuli. The novelty was to test whether this effect would vary with the alignment of the distractor's elementary features. Distractors were of high vs. low luminance, and composed of 16 pixels that were either aligned or misaligned by 0.23° or 0.43°. Our prediction, under the hypothesis that contour intervenes, was that aligned distractors, which formed a definite contour, would deviate the eyes more strongly than misaligned distractors. On the contrary, we found that distractors of high luminance produced greater eye deviations when they were misaligned, and hence more largely spread, than when they were aligned. Furthermore, low-luminance distractors deviated the eyes to the same extent irrespective of their alignment, though showing a reversed, contour-like, effect of alignment for early-triggered saccades. We proposed that contour has only limited influence on saccade metrics, when other, lower-level and more salient visual features, such as the extent of the stimulus pattern, are available.

On the limited effect of stimulus boundaries on saccade metrics.

How the endpoint of saccadic eye movements is determined out of many potential peripheral locations is a crucial issue in the field of vision. Models of saccade generation account for this seemingly selective process in terms of competitive interactions between populations of neurons that encode respectively for different saccade amplitudes and directions. However, these models do not specify which visual stimulus properties other than the relative location of the stimuli are involved and how these properties contribute to ultimately determine a single saccade endpoint. We addressed this issue by contrasting the respective contributions of the 2-D spatial extent of the stimuli and the location of their boundaries in a global-effect paradigm. Participants were presented a to-be-looked-at peripheral target stimulus with or without a less eccentric visually invariant distractor. The extent of the target stimulus was manipulated in either one or two dimensions, such that targets differed either by their 2-D spatial extent (small, medium, or large circle) or the location of their boundaries (circle vs. horizontal or vertical ellipse of medium size). Results showed that the distractor deviated the eyes away from the target with the deviation varying with the 2-D spatial extent of the target but not the location of its boundaries. This finding suggests that the spatial distribution of luminance contrast and/or the number of elementary features that compose the stimuli prevails over visual boundaries in specifying the saccade endpoint. Implications for models of saccade generation are discussed.

Decision making and action implementation: evidence for an early visually triggered motor activation specific to potential actions.

To make a decision may rely on accumulating evidence in favor of one alternative until a threshold is reached. Sequential-sampling models differ by the way of accumulating evidence and the link with action implementation. Here, we tested a model's prediction of an early action implementation specific to potential actions. We assessed the dynamics of action implementation in go/no-go and between-hand choice tasks by transcranial magnetic stimulation of the motor cortex (single- or paired-pulse TMS; 3-ms interstimulus interval). Prior to implementation of the selected action, the amplitude of the motor evoked potential first increased whatever the visual stimulus but only for the hand potentially involved in the to-be-produced action. These findings suggest that visual stimuli can trigger an early motor activation specific to potential actions, consistent with race-like models with continuous transmission between decision making and action implementation.

Stimulus properties and saccade metrics: when local features are more critical than global features.

One major issue in the field of vision is how stimulus properties contribute to determine where the eyes move. Here, we examined the role of local versus global visual features in the computation of saccade metrics in the light of the well-known tendency for saccade to vary with the size of the stimuli. We used a saccade-target task in which we varied the properties of a visual distractor simultaneously displayed with the target stimulus. Both the size and the luminance contrast of the distractor were varied but the number of elementary features that composed the distractor was held constant. Our results showed that under such controls mean saccades' landing position remained unaffected by stimulus size irrespective of the level of luminance contrast. These findings suggest that the local visual features of a stimulus may be more critical than global features to specify a particular location to look at. These results are consistent with the notion that local features contribute to determine the amplitude and the width of the neuronal activity patterns associated with the visual stimulation and hence the computation of saccade metrics.

When larger visual distractors become less disruptive: behavioral evidence for lateral inhibition in saccade generation.

How neuronal activity is integrated over time may largely rely on excitatory and inhibitory mechanisms. Dynamic neural field models assume that local excitation and lateral inhibition (i.e., the "Mexican hat") shape the output of neural networks. Most models of saccade generation assume that such interactions in the superior colliculus play a key role in determining both the metrics and the latency of saccades. Here, we investigated the role of lateral inhibition in saccade metrics in humans. We used a saccade target task in which a visual distractor line was presented close to a peripheral visual target (i.e., a small circle). Models assuming lateral inhibition predict that beyond a critical size larger distractors induce less perturbation than smaller ones. To assess this prediction, we varied the length of the distractor. Results confirmed that a distractor presented along with the target deviated the saccade's landing position away from the target. This perturbation increased with distractor length but only up to a critical size as the effect reversed for larger distractors, leading to a reduced perturbation on saccade metrics. These results suggest that larger distractors induce a neuronal activity pattern wide enough to involve lateral inhibition, thereby decreasing the distractor's weight in the spatial integration of distractor and target locations. They are consistent with a critical role of lateral inhibition in the computation of saccade metrics.

Age-related differences in corticomotor excitability and inhibitory processes during a visuomotor RT task.

This study tested the postulation that change in the ability to modulate corticospinal excitability and inhibitory processes underlie age-related differences in response preparation and generation during tasks requiring either rapid execution of a motor action or actively withholding that same action. Younger (n = 13, mean age = 26.0 years) and older adults (n = 13, mean age = 65.5 years) performed an RT task in which a warning signal (WS) was followed by an imperative signal (IS) to which participants were required to respond with a rapid flexion of the right thumb (go condition) or withhold their response (no-go condition). We explored the neural correlates of response preparation, generation, and inhibition using single- and paired-pulse TMS, which was administered at various times between WS and IS (response preparation phase) and between IS and onset of response-related muscle activity in the right thumb (response generation phase). Both groups exhibited increases in motor-evoked potential amplitudes (relative to WS onset) during response generation; however, this increase began earlier and was more pronounced for the younger adults in the go condition. Moreover, younger adults showed a general decrease in short-interval intracortical inhibition during response preparation in both the go and no-go conditions, which was not observed in older adults. Importantly, correlation analysis suggested that for older adults the task-related increases of corticospinal excitability and intracortical inhibition were associated with faster RT. We propose that the declined ability to functionally modulate corticospinal activity with advancing age may underlie response slowing in older adults.

How does temporal preparation speed up response implementation in choice tasks? Evidence for an early cortical activation.

We investigated the influence of temporal preparation on information processing. Single-pulse transcranial magnetic stimulation (TMS) of the primary motor cortex was delivered during a between-hand choice task. The time interval between the warning and the imperative stimulus varied across blocks of trials was either optimal (500 ms) or nonoptimal (2500 ms) for participants' performance. Silent period duration was shorter prior to the first evidence of response selection for the optimal condition. Amplitude of the motor evoked potential specific to the responding hand increased earlier for the optimal condition. These results revealed an early release of cortical inhibition and a faster integration of the response selection-related inputs to the corticospinal pathway when temporal preparation is better. Temporal preparation may induce cortical activation prior to response selection that speeds up the implementation of the selected response.

Age-related differences in corticospinal excitability during a Go/NoGo task.

Age-related slowing of reaction times (RTs) is well documented but whether the phenomenon reflects deficits in movement preparation and/or response generation processes is unclear. To gain further insight into this issue, transcranial magnetic stimulation (TMS) was used to investigate motor cortex (M1) excitability and short-interval intracortical inhibitory (SICI) processes during a Go/NoGo RT task in younger and older adults. Single- and paired-pulse TMS was delivered over the left M1 during preparation and response generation periods in a right-hand muscle. Younger adults had shorter RTs and a larger increase in corticospinal excitability at response generation period than older adults. SICI modulation for both groups showed a large reduction in inhibition immediately prior to EMG onset. These findings indicate age-related differences in corticospinal excitability during the response generation stage of sensorimotor information processing.

Selective suppression of the incorrect response implementation in choice behavior assessed by transcranial magnetic stimulation.

Selecting the adequate alternative in choice situations may involve an inhibition process. Here we assessed response implementation during the reaction time of a between-hand choice task with single- or paired-pulse (3 or 15 ms interstimulus intervals [ISIs]) transcranial magnetic stimulation of the motor cortex. The amplitude of the single-pulse motor evoked potential (MEP) initially increased for both hands. At around 130 ms, the single-pulse MEP kept increasing for the responding hand and decreased for the nonresponding hand. The paired-pulse MEP revealed a similar pattern for both ISIs with no effect on short intracortical inhibition and intracortical facilitation measures. The results suggest that the incorrect response implementation was selectively suppressed before execution of the correct response, preventing errors in choice context. The results favor models assuming that decision making involves an inhibition process.

Emotion and motor preparation: A transcranial magnetic stimulation study of corticospinal motor tract excitability.

In the present study, we examined whether preparing motor responses under different emotional conditions alters motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation delivered to the motor cortex. Analyses revealed three findings: (1) Reaction times were expedited during exposure to unpleasant images, as compared with pleasant and neutral images; (2) force amplitude was greater during exposure to unpleasant images, as compared with pleasant and neutral images; and (3) MEPs were larger while participants viewed unpleasant images, as compared with neutral images. Hence, coupling the preparation of motor responses with the viewing of emotional images led to arousal-driven changes in corticospinal motor tract excitability, whereas movement speed and force production varied as a function of emotional valence. These findings demonstrate that the effects of emotion on the motor system manifest at varying sensitivity levels across behavioral and neurophysiological measures. Moreover, they validate the action readiness component of emotional experience by demonstrating that emotional states influence the execution of future movements but, alone, do not lead to overt movement.

The dual nature of time preparation: neural activation and suppression revealed by transcranial magnetic stimulation of the motor cortex.

Single-pulse transcranial magnetic stimulations (TMSs) of the motor cortex (M1) were performed in order to decipher the neural mechanisms of time preparation. We varied the degree to which it was possible to prepare for the response signal in a choice reaction time (RT) task by employing either a short (500 ms) or a long (2500 ms) foreperiod in separate blocks of trials. Transcranial magnetic stimulations were delivered during these foreperiods in order to study modulations in both the size of the motor evoked potential (MEP) and the duration of the silent period (SP) in tonically activated response agonists. Motor evoked potential area and silent period duration were assumed to reflect, respectively, the excitability of the cortico-spinal pathway and the recruitment of inhibitory cortical interneurons. Shorter reaction times were observed with the shorter foreperiod, indicating that a better level of preparation was attained for the short foreperiod. Silent period duration decreased as time elapsed during the foreperiod and this decrement was more pronounced for the short foreperiod. This result suggests that time preparation is accompanied by a removal of intracortical inhibition, resulting in an activation. Motor evoked potential area decreased over the course of the short foreperiod, but not over the long foreperiod, revealing that time preparation involves the inhibition of the cortico-spinal pathway. We propose that cortico-spinal inhibition secures the development of cortical activation, preventing erroneous premature responding.

Knowing when to respond and the efficiency of the cortical motor command: a Laplacian ERP study.

The objective was to test whether motor preparation can modulate the efficiency of the cortical motor command. The electroencephalogram (EEG) was recorded from electrodes located over the primary sensorimotor cortices during the performance of a between-hand choice reaction time task in which foreperiod duration (the interval between the warning and the imperative signals, 800 vs. 2800 ms) was varied across blocks of trials. In order to increase the spatial resolution of the EEG traces, surface Laplacian was estimated. The amplitude of the negative wave developing over the hemisphere contralateral to the response was smaller for the short foreperiod associated with the best performance level. These results indicate that the activation of the primary sensorimotor cortex involved in the response is less pronounced for the short foreperiod, suggesting that temporal advance information increases the efficiency of the cortical motor command.

Physiological evidence for response inhibition in choice reaction time tasks.

Inhibition is a widely used notion proposed to account for data obtained in choice reaction time (RT) tasks. However, this concept is weakly supported by empirical facts. In this paper, we review a series of experiments using Hoffman reflex, transcranial magnetic stimulation and electroencephalography to study inhibition in choice RT tasks. We provide empirical support for the idea that inhibition does occur during choice RT, and the implications of those findings for various classes of choice RT models are discussed.

An electromyographic analysis of the effect of levodopa on the response time of healthy subjects.

RATIONALE: Recently, we have shown that oral absorption of levodopa shortens reaction time (RT), measured as the interval between the response signal and the onset of voluntary electromyographic (EMG) activity. The motor time (MT) interval that elapses from the EMG activity to the mechanical response was not analysed. OBJECTIVE: The purpose of the present study was to analyse the effect of the dose of levodopa administrated in our previous study on the MT. Eight healthy adults (aged 21-28, mean=25), performed a two-choice visual RT task after oral absorption of a single dose of levodopa (200 mg) or a placebo (randomized, double-blind, cross-over design). RESULTS: Like RT, MT was shorter under levodopa than under placebo. Statistical analyses demonstrated that this effect was present for all deciles of the RT and MT distributions. CONCLUSION: Levodopa shortens not only RT but also MT, which points at the implication of the dopaminergic system in both premotor and motor processes.