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S N Baker

Publications and source records attributed to S N Baker.

At least 19 recordsLinked to original sources
PubMed2001

Precise burst synchrony in the superior colliculus of the awake cat during moving stimulus presentation.

This study aimed to characterize the synchrony that occurs between cell discharges in the superior colliculus of the awake cat. We trained cats to perform a visual fixation in the presence of a visual moving stimulus and then recorded 686 pairs of neighboring cells in the superior colliculus during task performance. A new method to assess the significance of precise discharge synchronization is described, which permits analysis of nonstationary data. Of 181 pairs with sufficient data for quantitative analysis, 125 showed a cross-correlation histogram (CCH) with features assessed as significant using this approach. CCHs frequently showed an isolated central peak (41 of 125) or a peak flanked by one or two troughs (68 of 125), and in a few cases an oscillatory pattern of approximately 65 Hz (16 of 125). This is in contrast to the oscillation frequency reported for the visual cortex and shows that oscillations in the superior colliculus probably arise from a cortex-independent mechanism. Our method also permits direct quantification of the correlation shift predictors, assessing precise time locking of spikes to the stimulus. Only 1 of 125 cross-correlation shift predictors had a significant central peak, meaning that most of the CCH features were not related to cell discharges time-locked to the stimulus presentation. Further investigation using a burst-jittering method showed that synchrony in the superior colliculus is attributable to precise synchronization of short bursts of spikes. Such synchrony could be related to the network dynamics and the common inhibitory feedback from local interneurons, which would act as temporal selectors of the cells with greatest or fastest response.

Action Potentials↗
PubMed2001

Synchronization in monkey motor cortex during a precision grip task. I. Task-dependent modulation in single-unit synchrony.

Neural synchronization in the cortex, and its potential role in information coding, has attracted much recent attention. In this study, we have recorded long spike trains (mean, 33,000 spikes) simultaneously from multiple single neurons in the primary motor cortex (M1) of two conscious macaque monkeys performing a precision grip task. The task required the monkey to use its index finger and thumb to move two spring-loaded levers into a target, hold them there for 1 s, and release for a food reward. Synchrony was analyzed using a time-resolved cross-correlation method, normalized using an estimate of the instantaneous firing rate of the cell. This was shown to be more reliable than methods using trial-averaged firing rate. A total of 375 neurons was recorded from the M1 hand area; 235 were identified as pyramidal tract neurons. Synchrony was weak [mean k' = 1.05 +/- 0.04 (SD)] but widespread among pairs of M1 neurons (218/1359 pairs with above-chance synchrony), including output neurons. Synchrony usually took the form of a broad central peak [average width, 18.7 +/- 8.7 (SD) ms]. There were marked changes during different phases of the task. As a population, synchrony was greatest during the steady hold period in striking contrast to the averaged cell firing rate, which was maximal when the animal was moving the levers into target. However, the modulation of synchrony during task performance showed considerable variation across individual cell pairs. Two types of synchrony were identified: oscillatory (with periodic side lobes in the cross-correlation) and nonoscillatory. Their relative contributions were quantified by filtering the cross-correlations to exclude either frequencies from 18 to 37 Hz or all higher and lower frequencies. At the peak of population synchrony during the hold period, about half (51.7% in one monkey, 56.2% in the other) of the synchronization was within this oscillatory bandwidth. This study provides strong support for assemblies of neurons being synchronized during specific phases of a complex task with potentially important consequences for both information processing within M1 and for the impact of M1 commands on target motoneurons.

Action Potentials↗
PubMed2001

Investigation into non-monosynaptic corticospinal excitation of macaque upper limb single motor units.

There has been considerable recent debate as to relative importance, in the primate, of propriospinal transmission of corticospinal excitation to upper limb motoneurons. Previous studies in the anesthetized macaque monkey suggested that, compared with the cat, the transmission of such excitation via a system of C3-C4 propriospinal neurons may be relatively weak. However, it is possible that in the anesthetized preparation, propriospinal transmission of cortical inputs to motoneurons may be depressed. To address this issue, the current study investigated the responses of single motor units (SMUs) to corticospinal inputs in either awake (n = 1) or lightly sedated (n = 3) macaque monkeys. Recordings in the awake state were made during performance of a precision grip task. The responses of spontaneously discharging SMUs to electrical stimulation of the pyramidal tract (PT) via chronically implanted electrodes were examined for evidence of non-monosynaptic, presumed propriospinal, effects. Single PT stimuli (up to 250 microA; duration, 0.2 ms, 2 Hz) were delivered during steady discharge of the SMU (10-30 imp/s). SMUs were recorded from muscles acting on the thumb (adductor pollicis and abductor pollicis brevis, n = 18), wrist (extensor carpi radialis, n = 29) and elbow (biceps, n = 9). In all SMUs, the poststimulus time histograms to PT stimulation consisted of a single peak at a fixed latency and with a brief duration [0.74 +/- 0.25 (SD) ms, n = 56], consistent with the responses being mediated by monosynaptic action of cortico-motoneuronal (CM) impulses. Later peaks, indicating non-monosynaptic action, were not present even when the probability of the initial peak response was low and when there was no evidence for suppression of ongoing SMU activity following this peak (n = 20 SMUs). Even when repetitive (double-pulse) PT stimuli were used to facilitate transmission through oligosynaptic linkages, no later peaks were observed (16 SMUs). In some thumb muscle SMUs (n = 8), responses to PT stimulation were compared with those evoked by transcranial magnetic stimulation, using a figure-eight coil held over the motor cortex. Responses varied according the orientation of the coil: in the latero-medial position, single peak responses similar to those from the PT were obtained; their latencies confirmed direct excitation of CM cells, and there were no later peaks. In the posterio-anterior orientation, responses had longer latencies and consisted of two to three subpeaks. At least under the conditions that we have tested, the results provide no positive evidence for transmission of cortical excitation to upper limb motoneurons by non-monosynaptic pathways in the macaque monkey.

Anesthesia↗
PubMed2001

Determination of response latency and its application to normalization of cross-correlation measures.

It is often of interest experimentally to assess how synchronization between two neurons changes following a stimulus or other behaviorally relevant marker. The joint peristimulus time histogram (JPSTH) achieves this, but assumes that changes in the cells' firing rate following the stimulus are stereotyped from one sweep to the next. Erroneous results can be generated if this is not the case. We here present a method to assess whether there are variations in response latency or amplitude from sweep to sweep. We then describe how the effects of response latency variation can be mitigated by realigning sweeps to their individual latencies. Three methods of detecting response latency are presented and their performance compared on simulated data. Finally, the effect on the JPSTH of sweep realignment using detected latencies is illustrated.

Animals↗
PubMed2000

Human cortical muscle coherence is directly related to specific motor parameters.

Cortical oscillations have been the target of many recent investigations, because it has been proposed that they could function to solve the "binding" problem. In the motor cortex, oscillatory activity has been reported at a variety of frequencies between approximately 4 and approximately 60 Hz. Previous research has shown that 15-30 Hz oscillatory activity in the primary motor cortex is coherent or phase locked to activity in contralateral hand and forearm muscles during isometric contractions. However, the function of this oscillatory activity remains unclear. Is it simply an epiphenomenon or is it related to specific motor parameters? In this study, we investigated task-dependent modulation in coherence between motor cortex and hand muscles during precision grip tasks. Twelve right-handed subjects used index finger and thumb to grip two levers that were under robotic control. Each lever was fitted with a sensitive force gauge. Subjects received visual feedback of lever force levels and were instructed to keep them within target boxes throughout each trial. Surface EMGs were recorded from four hand and forearm muscles, and magnetoencephalography (MEG) was recorded using a 306 channel neuromagnetometer. All subjects showed significant levels of coherence (0.086-0.599) between MEG and muscle in the 15-30 Hz range. Coherence was significantly smaller when the task was performed under an isometric condition (levers fixed) compared with a compliant condition in which subjects moved the levers against a spring-like load. Furthermore, there was a positive, significant relationship between the level of coherence and the degree of lever compliance. These results argue in favor of coherence between cortex and muscle being related to specific parameters of hand motor function.

Adult↗
PubMed2000

Precise spatiotemporal repeating patterns in monkey primary and supplementary motor areas occur at chance levels.

Precise spatiotemporal patterns in neural discharge are a possible mechanism for information encoding in the brain. Previous studies have found that such patterns repeat and appear to relate to key behavioral events. Whether these patterns occur above chance levels remains controversial. To address this question, we have made simultaneous recordings from between two and nine neurons in the primary motor cortex and supplementary motor area of three monkeys while they performed a precision grip task. Out of a total of 67 neurons, 46 were antidromically identified as pyramidal tract neurons. Sections of recordings 60 s long were searched for patterns involving three or more spikes that repeated at least twice. The allowed jitter for pattern repetition was 3 ms, and the pattern length was limited to 192 ms. In all 11 recordings analyzed, large numbers of repeating patterns were found. To assess the expected chance level of patterns, "surrogate" datasets were generated. These had the same moment-by-moment modulation in firing rate as the experimental spike trains, and matched their interspike interval distribution, but did not preserve the precise timing of individual spikes. The number of repeating patterns in 10 randomly generated surrogates was used to form 99% confidence limits on the repeating pattern count expected by chance. There was close agreement between these confidence limits and the number of patterns seen in the experimental data. Analysis of high complexity patterns was carried out in four long recordings (mean duration 23.2 min, mean number of neurons simultaneously recorded 7.5). This analysis logged only patterns composed of a larger number (7-11) of spikes. The number of patterns seen in the surrogate datasets showed a small but significant excess over those seen in the original experimental data; this is discussed in the context of surrogate generation. The occurrence of repeating patterns in the experimental data were strongly associated with particular phases of the precision grip task; however, a similar task dependence was seen for the surrogate data. When a repeating pattern was used as a template to find inexact matches, in which up to half of the component spikes could be missing, similar numbers of matches were found in experimental and surrogate data, and the time of occurrence of such matches showed the same task dependence. We conclude that the existence of precise repeating patterns in our data are not due to cortical mechanisms that favor this form of coding, since as many, if not more, patterns are produced by spike trains constructed only to modulate their firing rate in the same way as the experimental data, and to match the interspike interval histograms. The task dependence of pattern occurrence is explicable as an artifact of the modulation of neural firing rate. The consequences for theories of temporal coding in the cortex are discussed.

Action Potentials↗
PubMed2000

Improvements to the sensitivity of gravitational clustering for multiple neuron recordings.

We outline two improvements to the technique of gravitational clustering for detection of neuronal synchrony, which are capable of improving the method's detection of weak synchrony with limited data. The advantages of the enhancements are illustrated using data with known levels of synchrony and different interspike interval distributions. The novel simulation method described can easily generate such test data. An important dependence of the sensitivity of gravitational clustering to the interspike interval distribution of the analysed spike trains is described.

Action Potentials↗
PubMed2000

An accurate measure of the instantaneous discharge probability, with application to unitary joint-even analysis.

We present an estimate for the instantaneous discharge probability of a neurone, based on single-trial spike-train analysis. By detecting points where the neurone abruptly changes its firing rate and treating them specially, the method is able to achieve smooth estimates yet avoid the blurring of significant changes. This estimate of instantaneous discharge probability is then applied to the method of unitary event analysis. We show that the unitary event analysis as originally conceived is highly sensitive to firing-rate nonstationarities and covariations, but that it can be considerably improved if calculations of statistical significance use an instantaneous discharge probability instead of a firing-rate estimate based on averaging across multiple trials.

Action Potentials↗
PubMed1999

Multiple single unit recording in the cortex of monkeys using independently moveable microelectrodes.

Simultaneous recording from multiple single neurones presents many technical difficulties. However, obtaining such data has many advantages, which make it highly worthwhile to overcome the technical problems. This report describes methods which we have developed to permit recordings in awake behaving monkeys using the 'Eckhorn' 16 electrode microdrive. Structural magnetic resonance images are collected to guide electrode placement. Head fixation is achieved using a specially designed headpiece, modified for the multiple electrode approach, and access to the cortex is provided via a novel recording chamber. Growth of scar tissue over the exposed dura mater is reduced using an anti-mitotic compound. Control of the microdrive is achieved by a computerised system which permits several experimenters to move different electrodes simultaneously, considerably reducing the load on an individual operator. Neurones are identified as pyramidal tract neurones by antidromic stimulation through chronically implanted electrodes; stimulus control is integrated into the computerised system. Finally, analysis of multiple single unit recordings requires accurate methods to correct for non-stationarity in unit firing. A novel technique for such correction is discussed.

Animals↗
PubMed1999

Task-dependent modulation of 15-30 Hz coherence between rectified EMGs from human hand and forearm muscles.

1. Recent reports have shown task-related changes in oscillatory activity in the 15-30 Hz range in the sensorimotor cortex of human subjects and monkeys during skilled hand movements. In the monkey these oscillations have been shown to be coherent with oscillatory activity in the electromyographic activity of hand and forearm muscles. 2. In this study we investigated the modulation of oscillations in the electromyogram (EMG) of human volunteers during tasks requiring precision grip of two spring-loaded levers. 3. Two tasks were investigated: in the 'hold' task, subjects were required to maintain a steady grip force (ca 2.1 N or 2.6 N) for 8 s. In the 'ramp' task, there was an initial hold period for 3 s (force ca 2.1 N) followed by a linear increase in grip force over a 2 s period. The task ended with a further steady hold for 3 s at the higher force level (ca 2.6 N). 4. Surface EMGs were recorded from five hand and forearm muscles in 12 subjects. The coherence of oscillatory activity was calculated between each muscle pair. Frequencies between 1 and 100 Hz were analysed. 5. Each subject showed a peak in the coherence spectra in the 15-30 Hz bandwidth during the hold task. This coherence was absent during the initial movement of the levers. During the ramp task the coherence in the 15-30 Hz range was also significantly reduced during the movement phase, and significantly increased during the second hold period, relative to the initial hold. 6. There was coherence between the simultaneously recorded magnetoencephalogram (MEG) and EMG during steady grip in the hold task; this coherence disappeared during the initial lever movement. Using a single equivalent current dipole source model, the coherent cortical activity was localized to the hand region of the contralateral motor cortex. This suggests that the EMG-EMG coherence was, therefore, at least in part, of cortical origin. 7. The results are discussed in terms of a possible role for synchrony in the efficient recruitment of motor units during maintained grip.

Adult↗
PubMed1999

The role of synchrony and oscillations in the motor output.

There is currently much interest in the synchronisation of neural discharge and the potential role it may play in information coding within the nervous system. We describe some recent results from investigations of synchronisation within the motor system. Local field potentials (LFPs) and identified pyramidal tract neurones (PTNs) were recorded from the primary motor cortex of monkeys trained to perform a precision grip task. The LFPs showed bursts of oscillatory activity at 20-30 Hz, which were coherent with the rectified electromyographs (EMG) of contralateral hand and forearm muscles. This oscillatory synchronisation showed a highly specific task dependence, being present only during the part of the task when the animal maintained a steady grip and not during the movement phases before or after it. PTNs were phase-locked to LFP oscillations, implying that at least part of the coherence between cortical activity and EMG was mediated by corticospinal fibres. The phase locking of the PTNs to LFP oscillations produced task-dependent oscillatory synchronisation between PTN pairs, as assessed by the single-unit cross-correlation histogram. Recordings were also made from normal human subjects performing a precision grip similar to that used in the monkey recordings. Pairs of EMGs recorded from intrinsic hand and forearm muscles showed 20-30 Hz coherence, which modulated during task performance, being present only during periods of steady contraction. We suggest that these changes in EMG-EMG synchronisation reflect changing levels of synchronous drive from the corticospinal system. The generation of oscillations in the cortex is discussed in the light of results from a model of local cortical circuits. Other modelling work has shown that synchrony in the corticospinal inputs could act to recruit motoneurones more efficiently, producing more output force from a muscle than asynchronous inputs firing at the same mean rate. A speculative hypothesis is presented on the role of synchronous oscillations in the motor system, which is consistent with experimental observations to date.

Animals↗
PubMed1999

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.

Computer Systems↗
PubMed1998

The importance of the cortico-motoneuronal system for control of grasp.

Our recent work has revealed new evidence of the importance of direct cortico-motoneuronal (CM) connections for voluntary control of the hand. Most of these connections are derived from corticospinal neurons located in the M1 hand area, although there are some much smaller contributions from other secondary motor areas, such as the supplementary motor area (SMA). Intracellular recordings show that 75% of upper limb motoneurons in the chloralose-anaesthetized macaque monkey receive a monosynaptic projection from the corticospinal tract; evidence for non-monosynaptic, propriospinal excitatory influences from the corticospinal tract was conspicuously lacking in these anaesthetized preparations. Moreover, in the conscious monkey, hand and arm muscle motor unit responses to corticospinal tract input are dominated by single, brief peaks compatible with monosynaptic excitation. CM excitatory post-synaptic potentials, recorded from a comparable sample of hand and arm motoneurons in anaesthetized macaque and squirrel monkeys, were found to be larger and faster rising in the macaque, which is by far the more dexterous of the two species. CM cells facilitating a given muscle in the conscious macaque are distributed over a wide region of M1 cortex, and each contributes a particular pattern of discharge during a skilled task. In addition to their direct effects on target muscles there may be weaker but potentially important effects that derive from the synchronous binding of assemblies of output neurons. Synchronous oscillations between these neurons are particularly prevalent during steady grip, but disappear during digit movement.

Animals↗
PubMed1998

An investigation of the intrinsic circuitry of the motor cortex of the monkey using intra-cortical microstimulation.

The motor cortex contains a distributed map of muscles, with a single muscle represented over a wide cortical area. We have searched for inter-connections between distant sites projecting to common muscles by delivering pairs of 20-microA single-pulse intracortical microstimuli (ICMS) to sites separated by 1.5-2 mm in the hand-area primary motor cortex of two macaque monkeys performing a precision grip task. The facilitation of hand- and forearm-muscle rectified EMG was measured. When stimuli were delivered simultaneously, responses were quantified using a technique to correct for non-linearities inherent in the use of averaged, rectified EMG. A spatial facilitation was seen for such simultaneous stimuli; however, it was of the same magnitude as that occurring when ICMS was paired with stimulation of corticospinal axons in the pyramidal tract (PT), so that it was likely to be spinal in origin. When two such distant sites were stimulated separated by a 10- or 20-ms delay, the second response scaled with the level of background EMG in the same way as a response to the PT stimulus. By contrast, when the same site was stimulated twice with these delays, the second response showed a facilitation compared with a similarly timed PT response. There would therefore appear to be a local facilitation of the cortical output at these intervals, which is not seen between distant sites. Antidromically identified pyramidal-tract neurones (PTNs) were recorded whilst stimuli were delivered to a cortical site, with a distance between stimulating and recording electrodes of also 1.5-2 mm. The most common response was a facilitation followed by a suppression. Six of eleven PTNs showed a facilitation in their discharge following this stimulation (maximum connection strength s=0.19), 8/11 showed a suppression (maximum s=0.16). It is concluded that powerful inter-connections do exist between distributed parts of the motor output and that there is widespread cortical activation after even a single ICMS pulse. However, these inter-connections do not lead to interactions between cortical outputs following stimulation, as assessed from the EMG. It is proposed that this is likely to reflect differences in the summation of output cells to local versus remote stimulation.

Animals↗
PubMed1998

Computer simulation of post-spike facilitation in spike-triggered averages of rectified EMG.

When the spikes of a motor cortical cell are used to compile a spike-triggered average (STA) of rectified electromyographic (EMG) activity, a post-spike facilitation (PSF) is sometimes seen. This is generally thought to be indicative of direct corticomotoneuronal (CM) connections. However, it has been claimed that a PSF could be caused by synchronization between CM and non-CM cells. This study investigates the generation of PSF using a computer model. A population of cortical cells was simulated, some of which made CM connections to a pool of 103 motoneurons. Motoneurons were simulated using a biophysically realistic model. A subpopulation of the cortical cells was synchronized together. After a motoneuron discharge, a motor unit action potential was generated; these were summed to produce an EMG output. Realistic values were used for the corticospinal and peripheral nerve conduction velocity distribution, for slowing of impulse conduction in CM terminal axons, and for the amount of cortical synchrony. STA of the rectified EMG from all cortical neurons showed PSF; however, these were qualitatively different for CM versus non-CM cells. Using an epoch analysis to determine reliability in a quantitative manner, it was shown that the onset latency of PSF did not distinguish the two classes of cells after 10,000 spikes because of high noise in the averages. The time of the PSF peak and the peak width at half-maximum (PWHM) could separate CM from synchrony effects. However, only PWHM was robust against changes in motor unit action-potential shape and duration and against changes in the width of cortical synchrony. The amplitude of PSF from a CM cell could be doubled by the presence of synchrony. It is proposed that, if a PSF has PWHM < 7 ms, this reliably indicates that the trigger is a CM cell projecting to the muscle whose EMG is averaged. In an analysis of experimental data where macaque motor cortical cells facilitated hand and forearm muscle EMG, 74% of PSFs fulfilled this criterion. The PWHM criterion could be applied to other STA studies in which it is important to exclude the effects of synchrony.

Action Potentials↗
PubMed1997

Coherent oscillations in monkey motor cortex and hand muscle EMG show task-dependent modulation.

1. Recordings were made of local field potential (slow waves) and pyramidal tract neurone (PTN) discharge from pairs of sites separated by a horizontal distance of up to 1.5 mm in the primary motor cortex of two conscious macaque monkeys performing a precision grip task. 2. In both monkeys, the slow wave recordings showed bursts of oscillations in the 20-30 Hz range. Spectral analysis revealed that the oscillations were coherent between the two simultaneously recorded cortical sites. In the monkey from which most data were recorded, the mean frequency of peak coherence was 23.4 Hz. 3. Coherence in this frequency range was also seen between cortical slow wave recordings and rectified EMG of hand and forearm muscles active during the task, and between pairs of rectified EMGs. 4. The dynamics of the coherence were investigated by analysing short, quasi-stationary data segments aligned relative to task performance. This revealed that the 20-30 Hz coherent oscillations were present mainly during the hold phase of the precision grip task. 5. The spikes of identified PTNs were used to compile spike-triggered averages of the slow wave recordings. Oscillations were seen in 11/17 averages of the slow wave recorded on the same electrode as the triggering spike, and 11/17 averages of the slow wave recorded on the distant electrode. The mean period of these oscillations was 45.8 ms. 6. It is concluded that oscillations in the range 20-30 Hz are present in monkey motor cortex, are coherent between spatially separated cortical sites, and encompass the pyramidal tract output neurones. They are discernable in the EMG of active muscles, and show a consistent task-dependent modulation.

Animals↗
PubMed1997

Direct and indirect corticospinal control of arm and hand motoneurons in the squirrel monkey (Saimiri sciureus).

Anatomic evidence suggests that direct corticomotoneuronal (CM) projections to hand motoneurons in the New World squirrel monkey (Saimiri sciureus) are weak or absent, but electrophysiological evidence is lacking. The nature of the corticospinal linkage to these motoneurons was therefore investigated first with the use of transcranial magnetic stimulation (TMS) of the motor cortex under ketamine sedation in five monkeys. TMS produced early responses in hand muscle electromyogram, but thresholds were high (compared with macaque monkey) and the onset latency was variable. Second, stimulation of the pyramidal tract (PT) was carried out with the use of chronically implanted electrodes in ketamine-sedated monkeys; this produced more robust responses that were markedly facilitated by repetitive stimulation, with little decrease in latency on the third compared with the first shock. Finally, postsynaptic potentials were recorded intracellularly from 93 arm and hand motoneurons in five monkeys under general chloralose anesthesia. After a single PT stimulus, the most common response was a small, slowly rising excitatory postsynaptic potential (EPSP), either alone (35 of 93 motoneurons) or followed by an inhibitory postsynaptic potential (39 of 93). The segmental delay of the early EPSPs was within the monosynaptic range (mean 0.85 ms); however, the rise time of these EPSPs was slow (mean 1.3 ms) and their amplitude was small (mean 0.74 mV). These values are significantly slower and smaller than EPSPs in a comparable sample of Old World macaque monkey motoneurons. The results show that CM connections do exist in the squirrel monkey but that they are weak and possibly located on the remote dendrites of the motoneurons. The findings are consistent with earlier anatomic studies. Repetitive PT stimulation produced large, late EPSPs in some motoneurons, suggesting that, in this species, there are relatively strong nonmonosynaptic pathways linking the corticospinal tract to hand motoneurons.

Animals↗