Set-related activity in the premotor cortex of rhesus monkeys: effect of triggering cues and relatively long delay intervals

"Set-related activity" has been defined as a significant alteration in neuronal discharge rate during an "instructed delay period," a period when a previously instructed movement is being withheld. It has been argued that set-related activity in the primate premotor cortex, or at least a significant proportion of it, reflects motor preparation. In most previous investigations, however, in which visual stimuli have triggered the movement and simultaneously indicated its target, set-related activity might reflect either the anticipation of or attention to the trigger stimulus. The present report shows that set-related activity is robust and can be directionally selective when trigger stimuli do not indicate the target and when a trigger stimulus is absent. Another feature of previous studies has been the relatively brief intervals between the instruction and trigger stimuli (typically 3 sec or less). In the present study, we were able to observe the activity of a small number of cells during longer delay periods. Set-related activity persists, although it becomes less consistent, for as much as 7.5 sec after an instruction stimulus. These results support the hypothesis that set-related activity reflects the preparation for specific limb movements.     

Posterior parietal cortex encodes autonomously selected motor plans

The posterior parietal cortex (PPC) of rhesus monkeys has been found to encode the behavioral meaning of categories of sensory stimuli. When animals are instructed with sensory cues to make either eye or hand movements to a target, PPC cells also show specificity depending on which effector (eye or hand) is instructed for the movement. To determine whether this selectivity retrospectively reflects the behavioral meaning of the cue or prospectively encodes the movement plan, we trained monkeys to autonomously choose to acquire a target in the absence of direct instructions specifying which effector to use. Activity in PPC showed strong specificity for effector choice, with cells in the lateral intraparietal area selective for saccades and cells in the parietal reach region selective for reaches. Such differential activity associated with effector choice under identical stimulus conditions provides definitive evidence that the PPC is prospectively involved in action selection and movement preparation.     

Neuronal Activity Preceding Directional and Nondirectional Cues in the Premotor Cortex of Rhesus Monkeys

Pre-cue activity, the neuronal modulation that precedes a predictable stimulus, was studied in the premotor cortex of three rhesus monkeys. In one condition, a directional cue dictated the timing and target of a forelimb movement. In another condition, a non-directional cue provided identical timing information but did not indicate the target. Of 501 task-related neurons recorded in premotor cortex, 168 showed pre-cue activity. The onset time of pre-cue activity varied markedly from trial to trial and cell to cell, ranging from trial initiation to 4.8 sec later. No pre-cue activity reflected the direction of limb movement; thus, the data argue against the hypothesis that pre-cue activity reflects preparation for specific limb movements. A small number of cells showed greater pre-cue activity before directional than before nondirectional cues, and this difference may reflect anticipation of the cue's directional information. However, the vast majority (84%) of neurons lacked such differences. We therefore hypothesize that most pre-cue activity reflects or contributes to a facet of behavior common to the two conditions: anticipation of the time and/or nature of events.     

Auditory event-related potentials dissociate early and late memory processes

Event-related potentials (ERPs) to environmental sounds were recorded from 15 young control subjects in an auditory recognition memory task. Subjects listened to a continuous string of binaurally presented sounds, 20% of which were presented once and 80% were repeated. Of the repeated sounds, some repeated immediately after the initial presentation (2 sec; short delay repetition) while others repeated after 2–6 intervening sounds (4–12 sec; long delay repetition). Subjects were instructed to indicate whether they had heard the sounds before by pressing a “yes” or “no” button.The initial stimulus presentation and long delay repetition stimuli generated both an N4 component and a prolonged latency P3 component while the short delay repetition stimuli elicited no N4 component and an earlier latency P3 component. Subjects' responses were faster and more accurate for short delay repetition. All stimuli generated a sustained frontal negative component (SFN). These data indicate that auditory recognition memory for environmental sounds may involve two processes. The P3 generated by both short and long delay repetition stimuli may index activation of a neocortical template matching system. The N4 generated by initial stimulus presentations and long delay repetition is proposed to measure additional activation of limbic memory systems at long retention intervals.     

Set-related neuronal activity in the premotor cortex of rhesus monkeys: effects of changes in motor set

Previous studies have suggested that the premotor cortex plays a role in motor preparation. We have tested this hypothesis in macaque monkeys by examining neuronal activity during an enforced, 1.5-3.0 s delay period between the presentation of an instruction for movement and the onset of that movement. Two targets for movement were available to the monkey, one on the left and one on the right. Illumination of one of the targets served as the instruction for a forelimb movement. It is known that there are cells in the premotor cortex that have directionally specific, sustained activity increases or decreases following such instructions. If the premotor cortex is involved in the preparation for movement in a particular direction, then changing the target from one to the opposite side during the delay period should lead to a pronounced change in sustained neuronal activity. Further, removing the instruction, while still requiring movement to the target, should have little or no sustained effect. Seventy cells showed the predicted activity patterns, thus supporting the view that the premotor cortex plays a role in motor preparation.     

Unit activity of the prefrontal cortex during delayed alternation performance in monkey.

Unit activity in the dorsolateral regions of the prefrontal cortex (s. principalis) of the monkey was analysed by multineuronal recording technique. The sequence during delayed-response alternation included anticipatory stimulus, non-specific expectancy, conditioned cue, delay, trigger stimulus and alternation performance. Food reward completed the sequence. Unit activity in s. principalis was found to be involved in such learning processes as integration of behavioural acts into an accomplished programme and storage of traces in short-term memory. The latter can be observed in successive involvement of neuronal populations during a 10 sec delay as well as in rearrangements of unit activity, the maximum of which shifts to the latest part of the delay period. Unit activity in s. principalis reflects the level of correct response, corresponding correlates of which depend on the level of attention. The dorsolateral prefrontal cortex has structural and functional characteristics for short-term storage of external signals in its neuronal nets according to the level of attention.     

Studies on the excitability of the central program generator in the spinal cord of the terrapin Pseudemys scripta elegans

We present observations on the multicyclic scratch reflex in spinal terrapins as produced by electrical stimuli applied to the shell at the specific regions at which a mechanical stimulus produces the reflex. EMGs and hip and knee movements are recorded. The responses to the electrical stimuli are similar to the responses to mechanical stimuli. There is a three phase EMG pattern (Stein and Grossman, 1980), to which the movement pattern is related (Bakker and Crowe, 1982). A response may consist of a series of up to 25 movement cycles with a total time course of up to about 30 sec. The initial cycles of a response are relatively fast (less than 1 sec), but the cycles at the expiration of the response may have a duration of 2-3 sec. A single electrical stimulus pulse is often insufficient to trigger a series response. Instead, a weak EMG burst of a few tenths of a second duration, together with a slight movement, is often seen. However, a second pulse can set the cycle series in motion even after an interval of 40 sec between the pulses. A further booster stimulus pulse given while a reflex response is taking place can increase the speed of the movement. If the booster pulse is given just after cessation of reflex activity it can restart the activity, but this second cycle series is often shorter than the first one. The results indicate that the excitability of the central program generator is not constant. Long duration changes in the excitability are produced within the spinal cord.     

Neuronal activity preceding directional and nondirectional cues in the premotor cortex of rhesus monkeys

Pre-cue activity, the neuronal modulation that precedes a predictable stimulus, was studied in the premotor cortex of three rhesus monkeys. In one condition, a directional cue dictated the timing and target of a forelimb movement. In another condition, a nondirectional cue provided identical timing information but did not indicate the target. Of 501 task-related neurons recorded in premotor cortex, 168 showed pre-cue activity. The onset time of pre-cue activity varied markedly from trial to trial and cell to cell, ranging from trial initiation to 4.8 sec later. No pre-cue activity reflected the direction of limb movement; thus, the data argue against the hypothesis that pre-cue activity reflects preparation for specific limb movements. A small number of cells showed greater pre-cue activity before directional than before nondirectional cues, and this difference may reflect anticipation of the cue's directional information. However, the vast majority (84%) of neurons lacked such differences. We therefore hypothesize that most pre-cue activity reflects or contributes to a facet of behavior common to the two conditions: anticipation of the time and/or nature of events.     

Effect of motor stimulation and stretching on afferent activity of the neuromuscular spindle isolated from the frog]

The arrangement of muscle spindles in m. ext. long. dig. IV has been examined by microdissection. It is confirmed that spindle systems generally appear to consist of individual receptors. Stimulation effects of fast motor fibres (conduction velocities greater than 12 m/sec) on the spindles of the same muscle were studied. Receptors were isolated with their nerves and the appropriate spinal roots, the latter ones were used for stimulating efferent fibres and recording sensory discharges. Single shocks to the ventral root filaments caused afferent responses ranging from a single action potential to a train of impulses. During repetitive stimulation (train of stimuli at frequency of 10 to 150/sec) a marked increase in afferent activity was found. Afferent activity could be driven by the frequency of stimuli ("driving") and the stimulus/action potentials ratio varied from 1:1 to 1:3 or more. The rate of sensory discharge depended on the frequency of stimuli: the maximum effect, was attained at 30 to 50 stimuli/sec and, in the most responsive receptors, up to 80 stimuli/sec. Slight increases of the initial lengths of the receptors caused facilitation of sensory responses to motor stimulation. Moreover, impairing effects, which appear during sustained or high-frequency stimulation, possibly related to fatigue in intrafusal neuromuscular transmission, could be relieved by increasing the initial length. The repetitive stimulation of fast fusimotor fibres increased both dynamic and static responses and also raised the afferent activity after a period of stretching, when usually a depression occurs; these effects varied according to the preparation, its initial tension and the frequency of stimulation. The main feature of the examined motor fibres, when stimulated, is the constant excitatory action on muscle spindle static response. Results are discussed. It is suggested that the different characteristics of intrafusal muscle fibres, the receptor initial tension and the frequency of motor units discharges, may together affect muscle spindles static or dynamic performance.     

Continuity of Visual and Auditory Rhythms Influences Sensorimotor Coordination

People often coordinate their movement with visual and auditory environmental rhythms. Previous research showed better performances when coordinating with auditory compared to visual stimuli, and with bimodal compared to unimodal stimuli. However, these results have been demonstrated with discrete rhythms and it is possible that such effects depend on the continuity of the stimulus rhythms (i.e., whether they are discrete or continuous). The aim of the current study was to investigate the influence of the continuity of visual and auditory rhythms on sensorimotor coordination. We examined the dynamics of synchronized oscillations of a wrist pendulum with auditory and visual rhythms at different frequencies, which were either unimodal or bimodal and discrete or continuous. Specifically, the stimuli used were a light flash, a fading light, a short tone and a frequency-modulated tone. The results demonstrate that the continuity of the stimulus rhythms strongly influences visual and auditory motor coordination. Participants'' movement led continuous stimuli and followed discrete stimuli. Asymmetries between the half-cycles of the movement in term of duration and nonlinearity of the trajectory occurred with slower discrete rhythms. Furthermore, the results show that the differences of performance between visual and auditory modalities depend on the continuity of the stimulus rhythms as indicated by movements closer to the instructed coordination for the auditory modality when coordinating with discrete stimuli. The results also indicate that visual and auditory rhythms are integrated together in order to better coordinate irrespective of their continuity, as indicated by less variable coordination closer to the instructed pattern. Generally, the findings have important implications for understanding how we coordinate our movements with visual and auditory environmental rhythms in everyday life.     

Behavioral,Cognitive, and Motor Preparation Deficits in a Visual Cued Spatial Attention Task in Autism Spectrum Disorder

Abnormalities in motor skills have been regarded as part of the symptomatology characterizing autism spectrum disorder (ASD). It has been estimated that 80 % of subjects with autism display “motor dyspraxia” or clumsiness that are not readily identified in a routine neurological examination. In this study we used behavioral measures, event-related potentials (ERP), and lateralized readiness potential (LRP) to study cognitive and motor preparation deficits contributing to the dyspraxia of autism. A modified Posner cueing task was used to analyze motor preparation abnormalities in children with autism and in typically developing children (N = 30/per group). In this task, subjects engage in preparing motor response based on a visual cue, and then execute a motor movement based on the subsequent imperative stimulus. The experimental conditions, such as the validity of the cue and the spatial location of the target stimuli were manipulated to influence motor response selection, preparation, and execution. Reaction time and accuracy benefited from validly cued targets in both groups, while main effects of target spatial position were more obvious in the autism group. The main ERP findings were prolonged and more negative early frontal potentials in the ASD in incongruent trials in both types of spatial location. The LRP amplitude was larger in incongruent trials and had stronger effect in the children with ASD. These effects were better expressed at the earlier stages of LRP, specifically those related to response selection, and showed difficulties at the cognitive phase of stimulus processing rather that at the motor execution stage. The LRP measures at different stages reflect the chronology of cognitive aspects of movement preparation and are sensitive to manipulations of cue correctness, thus representing very useful biomarker in autism dyspraxia research. Future studies may use more advance and diverse manipulations of movement preparation demands in testing more refined specifics of dyspraxia symptoms to investigate functional connectivity abnormalities underlying motor skills deficits in autism.     

Representation of moving stimuli by somatosensory neurons.

The findings obtained in neurophysiological and psychophysical investigations using tactile stimuli that move at constant velocity across the skin are reviewed. For certain neurons in the postcentral gyrus of the cerebral cortex (S-I) of macaque monkeys, direction of stimulus motion is a "trigger feature" i.e., moving tactile stimuli evoke vigorous discharge activity in these neurons only if the stimuli are moved in a particular direction across the receptive field. This directional selectivity is maximal when stimulus velocity is between 5 and 50 cm/sec, and falls off rapidly at lower or higher velocities. The capacity for human subjects to correctly identify the direction of stimulus motion on the skin exhibits a similar dependence on stimulus velocity. The similar effects of velocity on neural and psychophysical measures of directional sensitivity support the idea that direction of stimulus motion on the skin can only be recognized if the moving stimulus optimally activates the group of S-I neurons for which that directions of simulus motion is the trigger feature.     

Relation between warning stimuli and contingent negative variation in man

Seven right handed male students were submitted to standard and target paired auditory stimuli to determine the relationship between contingent negative variation (CNV) and the slow negative waves following non paired stimuli. The warning stimuli informed subjects whether or not the impending unconditional stimulus implied a motor response. Subjects were instructed to execute the motor task at less than one second after unconditional stimulus. They were informed that the standard warning stimulus would be followed by an additional stimulus which confirmed its detection. Recordings of standard warning stimulus condition showed a subsequent increase in CNV amplitude. This significant increase appeared to be related to an enhancement in the subject's expectation towards the additional stimulus. The lowered CNV voltage in target condition seemed to be linked to the instructions, which involved a motor preparation after the imperative stimuli occurrence. It is suggested that the CNV may represent not only a general anticipatory but also an orienting process related to the signal value of the warning stimulus.     

Automatic versus voluntary motor imitation: effect of visual context and stimulus velocity

Automatic imitation is the tendency to reproduce observed actions involuntarily. Though this topic has been widely treated, at present little is known about the automatic imitation of the kinematic features of an observed movement. The present study was designed to understand if the kinematics of a previously seen stimulus primes the executed action, and if this effect is sensitive to the kinds of stimuli presented. We proposed a simple imitation paradigm in which a dot or a human demonstrator moved in front of the participant who was instructed either to reach the final position of the stimulus or to imitate its motion with his or her right arm. Participants' movements were automatically contaminated by stimulus velocity when it moved according to biological laws, suggesting that automatic imitation was kinematic dependent. Despite that the performance, in term of reproduced velocity, improved in a context of voluntary imitation, subjects did not replicate the observed motions exactly. These effects were not affected by the kind of stimuli used, i.e., motor responses were influenced in the same manner after dot or human observation. These findings support the existence of low-level sensory-motor matching mechanisms that work on movement planning and represent the basis for higher levels of social interaction.     

Five classes of visual interneurons in the optic nerve of the hermit crab

Response characteristics and trigger features are described for five operational classes of visual interneurons in the optic nerve of the hermit crab. The units were initially segregated on the basis of response sign (“on”, “off”, “on-off”) and duration (phasic, tonic). Tonic “on” and tonic “off” units were observed to be indistinguishable from the previously described sustaining and dimming fibers of other decapod crustacea. Phasic “off” units closely resemble either jittery motion or novelty detectors also previously documented. Phasic “on” units are characterized by an excitable antagonistic surround, a slow rate of habituation, and a sustained low level discharge during periods of increasing luminous flux at any point within their receptive fields. Highly sensitive to moving contours, they occasionally revealed a directional preference. The phasic “on” units are also capable of following brief stimuli at rates of up to 20/sec. Phasic “on-off” units were characterized by coextensive receptive fields for “on” and “off” responses and movement sensitivity independent of contrast.     

Mechanisms of interference in vibrotactile working memory

In previous studies of interference in vibrotactile working memory, subjects were presented with an interfering distractor stimulus during the delay period between the target and probe stimuli in a delayed match-to-sample task. The accuracy of same/different decisions indicated feature overwriting was the mechanism of interference. However, the distractor was presented late in the delay period, and the distractor may have interfered with the decision-making process, rather than the maintenance of stored information. The present study varies the timing of distractor onset, (either early, in the middle, or late in the delay period), and demonstrates both overwriting and non-overwriting forms of interference.     

Amplitude-temporal parameters of the evoked potentials of the visual and motor areas in the visual perception and mental representation of an image

Amplitude-temporal analysis was carried out of the EP components of the visual and motor areas elicited by neutral (diffuse light) and structural (checker board pattern) stimuli in different situations, defined by instruction. Interserial comparisons showed that at any instruction, the latency of the first EP component of the motor areas is reduced; as a result it can appear here simultaneously with the EP of the visual areas. At the instruction involving the subject in the process of active change of perception, activation of the right hemisphere, including the motor area, is manifest by EP parameters, while the right occipital area is activated in response to the structural stimulus, and the left one--in response to the neutral stimulus. At complication of the stimulus or instruction, the period is prolonged when the latency of EP components of the motor area is shorter than the latency of the isopolar components of the visual area--from 120 to 150 ms in response to the neutral stimuli and the neutral with their counting; from 90 to 150 ms in response to the structural stimuli; from 80 to 210 ms in response to the neutral stimuli with mental representation of the structural one.     

The “Plantar Test” Apparatus (Ugo Basile Biological Apparatus), a Controlled Infrared Noxious Radiant Heat Stimulus for Precise Withdrawal Latency Measurement in the Rat,as a Tool for Humans?

In the present study, we precisely and automatically measured the withdrawal latency to noxious radiant heat application in unrestrained male rats and in human subjects of both sexes, by means of the “plantar test” apparatus (Ugo Basile Biological Apparatus). The infrared light stimulus of this tool was applied underneath the hindpaws of rats and the middle fingers of human subjects. With one right and one left stimulation every 10 min, we observed a decrease in latency over a 40-min testing period in rats; the latency reached a mean value of 5.08 ± 0.25 sec after 40 min with a 36-W stimulus, which corresponded to 46.5d`C. In pilot experiments, also performed on rats, we showed that the opiate morphine (10 mg/kg, i.p.) produced remarkable increases of the withdrawal latency only in “naive” animals (i.e., ones that had never experienced the plantar test stimulus) and not in animals “habituated” to it. Among humans, we noted gender differences, such as less sensitivity to the infrared noxious radiant heat for women, particularly during the menstrual period. A difference from rats was that there was no significant latency modification along the 40-min testing period for either women or men, with a mean latency of 5.61 ± 0.18 sec (47.5d`C) for the women and 4.39 ± 0.10 sec (45.5d`C) for the men. These data confirm the reliability of the plantar test in rats, and demonstrate the possible use of an infrared source in human subjects as a noxious heat stimulus; the withdrawal reaction to this stimulus is emphasized as a good index of nociception in humans     

The preparation and storage of cytochrome oxidase

The preparation of cytochrome oxidase by the method of Keilin and Hartree has been modified to give an increased yield and activity of the enzyme. The enzyme can be dried from the frozen state and stored for more than a year provided it is reconstituted in pH 9.5 buffer and treated with sound waves at 9,000 cycles/sec. This material is “soluble” in that it resists centrifugation at 11,000 × g for 2.5 hr. Its “solubility” depends in no way on autolysis of the heart muscle prior to extraction of the enzyme.     

Cardiac response of domestic chickens to hawk and goose models

The response of precocial birds to configurational stimuli has been a source of controversy for decades. In this experiment we measured cardiac response of domestic chicks to “hawk” and “goose” silhouettes. The chicks' heart rates varied more in response to the hawk model than to the goose model, suggesting that the hawk silhouette is a more fearful stimulus than that of the goose. Our data document the recognition of a configurational stimulus without prior, pertinent experience.