Characteristic membrane potential trajectories in primate sensorimotor cortex neurons recorded in vivo

We examined the membrane potentials and firing properties of motor cortical neurons recorded intracellularly in awake, behaving primates. Three classes of neuron were distinguished by 1) the width of their spikes, 2) the shape of the afterhyperpolarization (AHP), and 3) the distribution of interspike intervals. Type I neurons had wide spikes, exhibited scoop-shaped AHPs, and fired irregularly. Type II neurons had narrower spikes, showed brief postspike afterdepolarizations before the AHP, and sometimes fired high-frequency doublets. Type III neurons had the narrowest spikes, showed a distinct post-AHP depolarization, or "rebound AHP" (rAHP), lasting nearly 30 ms, and tended to fire at 25-35 Hz. The evidence suggests that an intrinsic rAHP may confer on these neurons a tendency to fire at a preferred frequency governed by the duration of the rAHP and may contribute to a "pacemaking" role in generating cortical oscillations.

Recurrent neural networks of integrate-and-fire cells simulating short-term memory and wrist movement tasks derived from continuous dynamic networks

Dynamic recurrent neural networks composed of units with continuous activation functions provide a powerful tool for simulating a wide range of behaviors, since the requisite interconnections can be readily derived by gradient descent methods. However, it is not clear whether more realistic integrate-and-fire cells with comparable connection weights would perform the same functions. We therefore investigated methods to convert dynamic recurrent neural networks of continuous units into networks with integrate-and-fire cells. The transforms were tested on two recurrent networks derived by backpropagation. The first simulates a short-term memory task with units that mimic neural activity observed in cortex of monkeys performing instructed delay tasks. The network utilizes recurrent connections to generate sustained activity that codes the remembered value of a transient cue. The second network simulates patterns of neural activity observed in monkeys performing a step-tracking task with flexion/extension wrist movements. This more complicated network provides a working model of the interactions between multiple spinal and supraspinal centers controlling motoneurons. Our conversion algorithm replaced each continuous unit with multiple integrate-and-fire cells that interact through delayed "synaptic potentials". Successful transformation depends on obtaining an appropriate fit between the activation function of the continuous units and the input-output relation of the spiking cells. This fit can be achieved by adapting the parameters of the synaptic potentials to replicate the input-output behavior of a standard sigmoidal activation function (shown for the short-term memory network). Alternatively, a customized activation function can be derived from the input-output relation of the spiking cells for a chosen set of parameters (demonstrated for the wrist flexion/extension network). In both cases the resulting networks of spiking cells exhibited activity that replicated the activity of corresponding continuous units. This confirms that the network solutions obtained through backpropagation apply to spiking networks and provides a useful method for deriving recurrent spiking networks performing a wide range of functions.

Oscillatory activity in forelimb muscles of behaving monkeys evoked by microstimulation in the cerebellar nuclei

Coherent 20-35 Hz (beta) oscillations are a prominent feature of activity in primary motor cortex and muscles of monkeys and humans performing voluntary movements. We found that coherent beta oscillations are also present in the cerebellar nuclei (CN). Two monkeys were operantly conditioned to perform a wrist flexion/extension step-tracking task while we recorded neuronal activity or microstimulated in CN and recorded EMG activity from forelimb muscles. Coherent beta oscillations were found between discharges of some CN neurons and tonically active shoulder, elbow and wrist/finger flexion and extension muscles. Similarly, localized microstimulation pulses in CN evoked transient beta oscillations in widespread forelimb muscles. We conclude that coherent motor system beta oscillations are present in CN and that CN may be an important nodal point for the generation and/or propagation of beta oscillations throughout the motor system.

Sensory input to primate spinal cord is presynaptically inhibited during voluntary movement

During normal voluntary movements, re-afferent sensory input continuously converges on the spinal circuits that are activated by descending motor commands. This time-varying input must either be synergistically combined with the motor commands or be appropriately suppressed to minimize interference. The earliest suppression could be produced by presynaptic inhibition, which effectively reduces synaptic transmission at the initial synapse. Here we report evidence from awake, behaving monkeys that presynaptic inhibition decreases the ability of afferent impulses to affect postsynaptic neurons in a behaviorally dependent manner. Evidence indicates that cutaneous afferent input to spinal cord interneurons is inhibited presynaptically during active wrist movement, and this inhibition is effectively produced by descending commands. Our results further suggest that this presynaptic inhibition has appropriate functional consequences for movement generation and may underlie increases in perceptual thresholds during active movement.

Functional properties of primate spinal interneurones during voluntary hand movements

The activity of cervical spinal interneurones (INs) was recorded in monkeys performing alternating hand movements. The contribution of INs to voluntary movement was determined by their response patterns during ramp-and-hold wrist movements and their postspike effects on forelimb muscle activity. Most INs were active during both flexion and extension, in contrast to the unidirectional activity of muscles and corticomotoneuronal cells. When recorded during performance of an instructed delay task, the activity of many INs was modulated during the delay period between the instruction cue and the subsequent go signal. Thus, spinal INs, like cortical neurones, participate in earliest stages of preparation for movement. The modulation of peripheral input to spinal INs was tested during an instructed delay task. The monosynaptic responses to electrical stimulation of a cutaneous nerve decreased during active movement, probably due to presynaptic inhibition. These results provide new insights into the role of spinal INs in preparation and execution of voluntary movement.

Roles of primate spinal interneurons in preparation and execution of voluntary hand movement

To study the contribution of primate cervical interneurons (INs) to preparation and execution of normal voluntary hand movement we investigated their activity and correlational linkages to muscles in monkeys performing tracking tasks. During ramp-and-hold flexion-extension torques about the wrist most task-related spinal INs exhibited some activity during both flexion and extension, in unexpected contrast to the strictly unidirectional activity of corticomotoneuronal (CM) cells and motoneurons. Most INs increased their activity more in one of these two directions; response patterns in their preferred direction were typically tonic or phasic-tonic. Spike-triggered averages of EMG detected significant features in muscle activity for many task-related INs. Premotor INs (PreM-INs) were identified by post-spike facilitation or suppression with appropriate onset latencies after the trigger spike. Muscle fields of PreM-INs were smaller than those of supraspinal PreM cells in cortex and red nucleus, and rarely involved reciprocal effects on antagonist muscles. To investigate the relation of spinal INs to a repertoire of different muscle synergies, activity of INs was recorded from a macaque performing a multidirectional wrist task. The monkey generated isometric torques in flexion/extension, radial/ulnar deviation, pronation/supination, and executed a power grip that co-contracted wrist flexor and extensor muscles. Many INs showing task-modulated activity had preferred directions in this multidirectional space, typically with broadly tuned activation. The role of spinal INs in preparation for voluntary movement was revealed in monkeys performing instructed delay tasks. During the delay between a transient visual cue and a go signal a third of the tested INs showed significant delay modulation (SDM) of firing rate relative to the pre-cue rate. The SDM responses often differed from the INs' responses during the subsequent active torque period. In a monkey instructed by either visual or proprioceptive cues the delay period activity for many INs was similar in visual and perturbation trials, although other INs exhibited different SDM for visually and proprioceptively cued trials. These results suggest that spinal INs are involved, with cortex, in the earliest stages of movement preparation. The sensory input to INs could be identified in transient responses to the torque pulse, which showed two predominant patterns, consistent with inputs from cutaneous or proprioceptive receptors. We also investigated the task-dependent modulation of neural responses to peripheral input in a monkey performing wrist flexion/extension movements in a visually cued instructed delay task. Monosynaptic responses evoked by electrical stimulation of the superficial radial nerve through a cuff electrode were suppressed or abolished during the dynamic movement phase. Since task-related activity of the INs increased at the same time, the suppression was mediated by presynaptic rather than postsynaptic inhibition. These observations indicate that under normal behavioral conditions many spinal INs have response properties comparable to those previously documented for cortical neurons in behaving animals.

Changes in power and coherence of brain activity in human sensorimotor cortex during performance of visuomotor tasks

Electrocorticograms (ECoG) were recorded using subdural grid electrodes in forearm sensorimotor cortex of six human subjects. The subjects performed three visuomotor tasks, tracking a moving visual target with a joystick-controlled cursor; threading pieces of tubing; and pinching the fingers sequentially against the thumb. Control conditions were resting and active wrist extension. ECoGs were recorded at 14 sites in hand- and arm-sensorimotor area, functionally identified with electrical stimulation. For each behavior we computed spectral power of ECoG in each site and coherence in all pair-wise sites. In three out of six subjects, gamma-oscillations were observed when the subjects started the tasks. All subjects showed widespread power decrease in the range of 11-20 Hz and power increase in the 31-60 Hz ranges during performance of the visuomotor tasks. The changes in gamma-range power were more vigorous during the tracking and threading tasks compared with the wrist extension. Coherence analysis also showed similar task-related changes in coherence estimates. In contrast to the power changes, coherence estimates increased not only in gamma-range but also at lower frequencies during the manipulative visuomotor tasks. Paired sites with significant increases in coherence estimates were located within and between sensory and motor areas. These results support the hypothesis that coherent cortical activity may play a role in sensorimotor integration or attention.

Distributed processing in the motor system: spinal cord perspective

Recordings of spinal INs during a flexion/extension wrist task with an instructed delay period have shown directly that many spinal neurons modulate their rate during the preparatory period soon after a visual cue. The onset time and the relation between the delay period activity of spinal INs and the ensuing movement response suggest that this type of activity is not simply related to the forthcoming motor action, but rather reflects a correct match between the visual cue and the motor response. The existence of such activity further supports the notion that the motor system operates in a parallel mode of processing, so that even during early stages of motor processing multiple centers are activated regardless of their anatomical distance from muscles. The firing properties of spinal INs during the performance of the task seem to differ from the comparable properties of motor cortical cells. Spinal INs fire in a highly regular manner--their CV is substantially lower than the observed CV of cortical cells. Also, although neighboring cells tend to have similar response properties, the frequency of significant correlation is lower than for cortical cells and the anatomical extent of the correlation seems to be narrower. The similarity and differences between cortical and spinal cells in terms of response and firing properties suggests that while both type of cells are active in parallel throughout the behavioral phases of the motor task, each may operate in a different mode of information processing.

Functions of mammalian spinal interneurons during movement

The major recent advances in understanding the role of spinal neurons in generating movement include new information about the modulation of classic reflex pathways during fictive locomotion and in response to pharmacological probes. The possibility of understanding movements in terms of spinal representations of a basic set of movement primitives has been extended by the analysis of normal reflexes. Recordings of the activity of cervical interneurons in behaving monkeys has elucidated their contribution to generating voluntary movement and revealed their involvement in movement preparation.

Synaptic interactions mediating synchrony and oscillations in primate sensorimotor cortex

The appearance of oscillatory modes of 'gamma' activity in many cortical areas of different species has generated interest in understanding their underlying mechanisms and possible functions. This paper reviews evidence from studies on primate motor cortex showing that oscillatory activity entrains many neurons during periods of exploratory manipulative behavior. These oscillatory episodes synchronize widely spread neurons in sensorimotor cortex bilaterally, including descending corticospinal neurons, as evidenced by correlated modulations in EMG activity. The resulting neural synchronization involves task-related and -unrelated neurons similarly, suggesting that it is more likely to play some global role in attention than mediating any obvious interactions involved in coordinating movements. Intracellular recordings have elucidated the strength and types of synaptic interactions between motor cortical neurons that are involved in both normal and oscillatory activity. Spike-triggered averages (STAs) of intracellular membrane potentials have revealed serial connections in the form of unitary excitatory and inhibitory post-synaptic potentials (EPSPs and IPSPs). More commonly, STAs showed large synchronous excitatory or inhibitory potentials (ASEPs and ASIPs) beginning before the trigger spike and composed of multiple unitary events. ASEPs involved synchronous activity in a larger and more widespread group of presynaptic neurons than ASIPs. During oscillatory episodes synchronized excitatory and inhibitory synaptic potentials occurred in varying proportions. EPSPs evoked by stimulating neighboring cortical sites during the depolarizing phase of spontaneous oscillations showed evidence of transient potentiation. These observations are consistent with several functional hypotheses, but fit best with a possible role in attention or arousal.

Primate spinal interneurons show pre-movement instructed delay activity

Preparatory changes in neural activity before the execution of a movement have been documented in tasks that involve an instructed delay period (an interval between a transient instruction cue and a subsequently triggered movement). Such preparatory activity occurs in many motor centres in the brain, including the primary motor cortex, premotor cortex, supplementary motor area and basal ganglia. Activity during the instructed delay period reflects movement planning, as it correlates with parameters of the cue and the subsequent movement (such as direction and extent), although it occurs well before muscle activity. How such delay-period activity shapes the ensuing motor action remains unknown. Here we show that spinal interneurons also exhibit early pre-movement delay activity that often differs from their responses during the subsequent muscle activity. This delay activity resembles the set-related activity found in various supraspinal areas, indicating that movement preparation may occur simultaneously over widely distributed regions, including spinal levels. Our results also suggest that two processes occur in the spinal circuitry during this delay period: the motor network is primed with rate changes in the same direction as subsequent movement-related activity; and a superimposed global inhibition suppresses the expression of this activity in muscles.

Response properties of spinal interneurons in awake, behaving primates

This paper reviews studies on spinal interneurons in awake, behaving monkeys inspired by the work of Prof Patrick D. Wall. Early studies documented the sensory responses of spinal interneurons in unanesthetized monkeys to natural cutaneous and proprioceptive stimulation. More recently, cervical interneurons were documented in monkeys performing an active step-tracking task. During alternating wrist movements, most task-related interneurons showed bidirectional activity, firing during both flexion and extension (in surprising contrast to the unidirectional activity of muscles and corticomotoneuronal cells). Premotor interneurons were identified by post-spike effects in spike-triggered averages of forelimb muscle activity. The cells' post-spike effects were generally congruent with their activity in their preferred direction, although many fired during components of movement when their output effects would seem inappropriate. In an instructed delay period task many interneurons showed preparatory delay period activity, much like cortical neurons. Other studies tested the excitability of corticospinal axons to electrical stimulation and demonstrated both post-spike and task-related modulations in excitability. Together, these studies suggest that many behavioral functions of spinal interneurons remain to be revealed by recording their activity in awake, behaving animals.

Increased gamma-range activity in human sensorimotor cortex during performance of visuomotor tasks

OBJECTIVE

We documented changes in spectral power of human electrocorticograms (ECoG) during performance of sensorimotor tasks.

METHODS

In 6 human subjects, ECoGs were recorded simultaneously from 14 subdural cortical sites in forearm sensorimotor cortex. The subjects performed 3 visuomotor tasks: tracking a moving visual target with a joystick-controlled cursor, threading pieces of tubing, and pinching the fingers sequentially against the thumb. Control conditions consisted of passive resting and active extension of the wrist. For each site the spectral power of the ECoG during these behaviors was computed for 5 10 Hz ranges between 10 and 60 Hz.

RESULTS

All subjects showed power decreases in the range of 11-20 Hz and power increases in the 31-60 Hz range during performance of the visuomotor tasks, at sites in forearm sensorimotor cortex and adjacent areas. Simple wrist movements often produced little change in power. Three subjects showed episodes of explicit gamma oscillations during the visuomotor tasks. Different sites showed increases in gamma-range power for different tasks, indicating that the spatial distribution of the gamma activity is specific to the tasks. Cross-spectra showed that gamma activity could become synchronized between separate sites during particular tasks.

CONCLUSIONS

Synchronized gamma-range activity in human sensorimotor cortex increases with performance of manipulative visuomotor tasks, supporting the hypothesis that coherent gamma oscillations may play a role in sensorimotor integration or attention.

Activity of spinal interneurons and their effects on forearm muscles during voluntary wrist movements in the monkey

We studied the activity of 577 neurons in the C6-T1 spinal cord of three awake macaque monkeys while they generated visually guided, isometric flexion/extension torques about the wrist. Spike-triggered averaging of electromyographic activity (EMG) identified the units' correlational linkages with </=12 forearm muscles. One hundred interneurons produced changes in the level of average postspike EMG with onset latencies consistent with mono- or oligosynaptic connections to motoneurons; these were classified as premotor interneurons (PreM-INs). Most PreM-INs (82%) produced postspike facilitations in forearm muscles. Earlier spike-related features, often beginning before the trigger spike, were seen in spike-triggered averages from 72 neurons. Postspike effects were present in one muscle for 64% of the PreM-INs. Neurons with divergent linkages to larger "muscle fields" usually generated postspike effects in synergistic muscles. Fifty-eight percent of the PreM-INs had postspike effects in flexor muscles only and 29% in extensor muscles only. Postspike effects were distributed relatively evenly among the primary flexor and extensor muscles studied. The mean percent change in EMG level from baseline and the mean onset latencies for postspike facilitations and postspike suppressions were similar. PreM-INs exhibited a variety of response patterns during the generation of isometric wrist torque. The response patterns and output effects of 24% of the PreM-INs were consistent with a strict reciprocal organization of flexor and extensor muscle control. For another 60% of the PreM-INs, there was a congruent relation between activity and output effects for only one direction of torque production. These neurons were active for both flexion and extension torques, including 37 neurons that exhibited bidirectional increases in discharge rate. The relatively small number of postspike suppressions observed suggests that inhibitory interneurons were silent when their target muscles were recruited. Compared with premotor neurons in the motor cortex, the red nucleus and the C8-T1 dorsal root ganglia, spinal PreM-INs affected flexor muscles in greater proportions and had smaller muscle fields. The magnitudes of postspike facilitations were similar in all premotor populations. Bidirectional activity, common for PreM-INs, was rare for corticomotoneuronal and premotor dorsal root ganglion cells, which discharge only for torques in their preferred direction.

Response patterns and force relations of monkey spinal interneurons during active wrist movement

The activity of C6-T1 spinal cord neurons was recorded in three macaques while they generated isometric wrist flexion and extension torques in visually guided step-tracking tasks. Electromyographic activity (EMG) was recorded in </=12 independent forearm muscles. Spike-triggered averages (STAs) of rectified and unrectified EMG were used to classify neurons into four groups. Motoneurons (MNs) had a clear postspike motor unit signature in the unrectified STA of one muscle. Premotor interneurons (PreM-INs) had postspike effects in at least one muscle, with onset latencies of >/=3.5 ms from the trigger. Synchrony interneurons (Sy-INs) were non-PreM-Ins that had spike-related features with latencies <3.5 ms in at least one muscle. Unidentified interneurons (U-INs) showed no features in any of the STAs. A total of 572 task-related spinal neurons were studied; 29 cells were MNs, 97 PreM-INs, 32 Sy-INs, and 414 U-INs. MNs were activated predominantly in a tonic fashion during the ramp-and-hold torques and were active in one direction only. The most common response pattern for interneurons, irrespective of their class, was phasic-tonic activity, followed by purely tonic and purely phasic activity. Most interneurons (77%) were bidirectionally active in both flexion and extension. For all classes of interneurons, units with phasic response components tended to be activated first, before torque onset, followed by tonic units. The onset times of PreM-INs relative to onsets of their target muscles were distributed broadly, with a mean of -25 +/- 128 (SD) ms. For most neurons with tonic response components (all MNs, 71% of PreM-INs, 67% of Sy-INs, and 84% of U-INs), activity during the hold period was correlated significantly with the magnitude of static torque exerted by the monkey. The rate-torque regressions generally had positive slopes with higher mean slopes for extension than for flexion. The phasic response components were correlated significantly with rate of change of torque for a smaller percentage of tested PreM-Ins (50%), Sy-INs (83%), and U-INs (77%). In contrast to other premotor neurons [corticomotoneuronal (CM), rubromotoneuronal (RM), and dorsal root ganglion (DRG) afferents] previously characterized under similar conditions, a larger proportion of the spinal PreM-INs were activated after onset of their target muscles, probably reflecting a larger proportion of PreM-INs driven by peripheral input. The rate-torque slopes of PreM-INs tended to be less steep than those of CM and RM cells. Unlike the CM and DRG PreM afferents, which were activated unidirectionally, most spinal PreM-INs showed bidirectional activity, like RM cells.

Synaptic interactions between primate precentral cortex neurons revealed by spike-triggered averaging of intracellular membrane potentials in vivo

To document synaptic interactions between neurons in the precentral cortex of macaque monkeys, we recorded in vivo the intracellular (IC) membrane potentials of cortical neurons simultaneously with extracellular (EC) action potentials of neighboring cells. The synaptic potentials correlated with EC spikes were obtained by spike-triggered averages (STA) of the IC membrane potentials for 373 cell pairs recorded in anesthetized and awake behaving monkeys. Sixty-three STAs (17%) showed excitatory postsynaptic potentials (EPSPs), beginning after the trigger spike. Pure EPSPs had onset latencies of 0.9 +/- 0.7 msec (mean +/- SD) and amplitudes of 226 +/- 130 microV. Sixteen STAs (4%) showed postspike inhibitory postsynaptic potentials (IPSPs), with onset latencies of 0.4 +/- 0.4 msec and amplitudes of -274 +/- 188 microV. The most common waveform, observed in 82% of the STAs with features, was a broad depolarization straddling the trigger spikes, reflecting synchronized synaptic input to both IC and EC neurons. These average synchronous excitation potentials (ASEPs) began 14.3 +/- 6.6 msec before the trigger spike and had amplitudes of 1064 +/- 867 microV. Twenty-three STAs (6%) showed an average synchronous inhibitory potential (ASIP): a hyperpolarization beginning before the trigger spike and reflecting IPSPs produced by a group of local inhibitory cells synchronized with the trigger cell. ASIPs had an onset latency of -5.5 +/- 2.7 msec and amplitude of -589 +/- 502 microV. Combinations of synchronous and postspike potentials were also observed. Successive recordings provided examples of convergent and divergent connections between EC and IC cells. Neuron pairs with depolarizing postsynaptic potentials (PSPs) in the STA yielded peaks in the cross-correlograms of the IC and EC action potentials; the peak area was proportional to the amplitude of the PSP. These data suggest that a significantly larger proportion of cortical neurons interact through synchronous activity than through simple serial interactions; moreover, synchronous excitation affected more widely separated cell pairs than EPSPs and IPSPs, which were seen most often among the closest cells.

Oscillatory activity in sensorimotor cortex of awake monkeys: synchronization of local field potentials and relation to behavior

1. Oscillations of 20-40 Hz were observed in local field potentials (LFPs) and unit activity in sensorimotor cortices of three awake monkeys while the monkeys performed trained wrist movements and untrained exploratory arm movements. The mean frequency of LFP oscillations was 25.9 +/- 1.4 (SD) Hz and the number of cycles of oscillations per episode was variable, with a mean of 4.2 +/- 0.5 (mean +/- SE). 2. Oscillatory episodes occurred most often when the monkeys retrieved raisins from a Klüver board (0.59 +/- 0.23 episodes per s, mean +/- SD) or from unseen locations with the use of somatosensory feedback (0.62 +/- 0.12 episodes per s); they occurred less often when the monkeys performed repetitive wrist flexion and extension movements (0.22 +/- 0.04 episodes per s) or sat quietly at rest (0.23 +/- 0.17 episodes per s). 3. The amplitude of LFP oscillations increased with depth in cortex, reaching a maximum between 1 and 2 mm. LFP oscillations at the surface of the cortex were 180 degrees out of phase with oscillations in the deep cortical layers. The phase shift (with respect to the deep layers) decreased with depth in the cortex and disappeared at depths of > 1 mm. 4. LFPs were recorded simultaneously at multiple sites in the sensorimotor cortex when monkeys retrieved raisins from a Klüver board or from unseen locations. Cross-correlation of LFPs recorded at different sites indicated that oscillations in the 20- to 40-Hz range could become synchronized at sites separated by up to > or = 14 mm in the precentral cortex. 5. The probability of occurrence of significant correlations between LFP oscillations at paired sites and the average correlation amplitude decreased with increasing horizontal separation of sites in precentral cortex. The phase shift between LFP oscillations recorded at paired sites did not change significantly with increasing horizontal separation. 6. For paired sites in precentral cortex, the average strength of correlations and the proportion of oscillations that were significantly correlated were greater during exploratory behaviors such as retrieving raisins from slots of the Klüver board than during periods of rest or overtrained wrist movements. 7. Oscillations could become synchronized with small phase shifts (0.5 +/- 1.6 ms) between pre- and postcentral cortical sites. Average strength and probability of occurrence of significant correlations between pre- and postcentral LFPs increased during exploratory behaviors. 8. Oscillations occurred simultaneously in the left and right motor cortex and could become synchronized with negligible phase shifts when the monkey performed bimanual manipulations. However, synchronization occurred as often and as strongly for unimanual as for bimanual manipulations. 9. These results indicate that episodes of 20- to 40-Hz oscillations occur often and become synchronized over a large cortical area during exploratory forelimb movements. However, they have no reliable relation to particular components of the movement and therefore seem unlikely to be involved directly in movement execution; instead, they may represent a neural correlate of attention during demanding sensorimotor behaviors.

Synchronization of neurons during local field potential oscillations in sensorimotor cortex of awake monkeys

1. The neural activity associated with 20- to 40-Hz oscillations in sensorimotor cortex of awake monkeys was investigated by recording action potentials of single and multiple units. At a given site, activity of many units became synchronized with local field potential (LFP) oscillations. Cycle-triggered histograms (CTHs) of unit spikes aligned on cycles of LFP oscillations indicated that about two thirds of the recorded units (n = 268) were entrained with LFP oscillations. On average, units had the highest probability of spiking 2.7 ms before peak LFP negativity, corresponding to a -27.6 degrees phase shift relative to the negative peak of the LFP. 2. The average relative modulation amplitude (RMA), defined as the ratio of amplitude of oscillatory component of CTH and the baseline multiplied by 100, was 45 +/- 27% (mean +/- SD). The RMAs of single units did not differ significantly from those of multiple units. 3. Phase shifts and RMAs did not vary systematically with the cortical depth of recorded units. 4. Autocorrelation histograms (ACHs) of entrained units exhibited clear 20- to 40-Hz periodicity if they were compiled with spikes that occurred during oscillatory episodes in LFPs. ACHs of spikes outside oscillatory episodes usually did not show periodicity. Global ACHs of all spikes typically showed weak or no evidence of periodic activity. 5. Cross-correlation histograms (CCHs) between pairs of units complied with all spikes, whether they occurred during or outside LFP oscillations, seldom revealed significant features (19 of 134 pairs or 14%). However, CCHs compiled with spikes that occurred during oscillatory episodes (OS-CCHs) had significant features in 67 of 134 pairs recorded ipsilaterally; in these 67 cases, units at both sites showed modulation in CTHs. 6. The latencies of the OS-CCH peaks (taking the medial unit as reference) were normally distributed about a mean of -0.5 +/- 13 ms. Normalized peak height of CCHs (peak/baseline x 100) was, on average, 14.3 +/- 11.2%. Peak latency and normalized peak amplitude did not change significantly with horizontal separation of recorded precentral pairs up to 14 mm. 7. Units in the left and right hemispheres could become synchronized during oscillations. Significant features in OS-CCH were detected in 22 of 42 pairs of units recorded bilaterally. The average peak latency was 0.2 +/- 8.0 ms and the average normalized peak amplitude was 10 +/- 8%. These parameters did not differ significantly from those for ipsilateral OS-CCHs. 8. Oscillations tended to affect both the temporal structure and net rate of unit firing. For each unit, the firing rate was clamped to a narrow range of frequencies during oscillatory episodes. The coefficient of variation (SD/mean) of firing rates was significantly reduced during oscillatory episodes compared with prior rates (P < 0.001, paired t-test). However, the overall mean firing rate of each unit during all oscillatory episodes did not differ from its average rate immediately before the episodes. Thus oscillatory episodes tended to clamp mean firing rates to the cells' average rates outside episodes. 9. The strength of synchronization between units during oscillatory episodes was unrelated to their involvement in the task. For pairs of precentral units recorded ipsilaterally, the probability of occurrence of significant features in the OS-CCH was slightly larger when both units of the pair were task related (33 of 56 pairs or 59%) than when only one unit was task related (20 of 39 pairs or 51%) or neither unit was task related (7 of 16 or 44%). However, these differences were not statistically significant. The magnitude of the correlation peak and the latency to peak were also not significantly different for the three cases. 10. These results suggest that units across wide regions can become transiently synchronized specifically during LFP oscillations, even if their spikes are uncorrelated during nonoscillatory periods.

Response patterns and postspike effects of premotor neurons in cervical spinal cord of behaving monkeys

Most of our information about spinal neurons has been derived from experiments with anesthetized or surgically. reduced preparations. To investigate these neurons under normal behavioral conditions, we recorded the activity of single afferent units in cervical dorsal root ganglia and of single interneurons in the cervical spinal cord of macaque monkeys, as they generated alternating flexion and extension torques about the wrist. Spike-triggered averages of rectified electromyographic activity were used to identify "premotor" (PreM) units associated with correlated postspike effects in active muscles. In addition to postspike effects, some spike-triggered averages showed early increases in average muscle activity, which were attributed to synchronous discharges in other PreM units. In recordings of peripheral afferents, 49% of the task-related dorsal root ganglia units produced postspike facilitation (PSF) of at least one forearm muscle, with a mean PSF latency of 5.8 +/- 0.3 ms (SE). The PSF amplitude was measured as the mean percent increase (MPI): the average increase of the PSF as a percentage of the prespike baseline mean. PreM afferent units produced PSF with an average MPI of 4.6 +/- 0.3%. In a study of cervical interneurons, about 13% (72/562) of the task-related cells showed postspike effects. These PreM interneurons had a mean PSF latency of 7.2 +/- 0.3 ms and a mean MPI of 4.6 +/- 0.2%. The MPI values for spinal neurons were similar to the MPIs reported for rubromotoneuronal and corticomotoneuronal cells. PreM neurons usually facilitated a subset of the coactivated muscles called the unit's "muscle field." The PreM afferents facilitated an average of 46% of the synergistically coactivated muscles, while PreM interneurons facilitated an average of 37%. These are comparable with the percentage of muscles facilitated by corticomotoneuronal (40%) and rubromotoneuronal (50%) cells. During the step-tracking task the monkeys generated ramp-and-hold torques about the wrist. The PreM afferents typically became active during either flexion or extension of the wrist, although a few were bidirectionally active. The most common response pattern in PreM afferents was a tonic discharge, followed by phasic and phasic-tonic discharge. The most common patterns exhibited by PreM interneurons were tonic and phasic-tonic responses. PreM afferent units began to discharge on average 51 +/- 13 ms before activation of their target muscle. This early onset supports our hypothesis that these PreM afferents arose from muscle spindles, which is also consistent with their short-latency PSF and their responses to perturbations that stretched their target muscles. The results reveal some salient differences between the discharge properties of dorsal root ganglia neurons, spinal interneurons, and supraspinal PreM cells in the motor cortex and red nucleus. All four PreM populations include tonic, phasic-tonic, and phasic cells, but in significantly different proportions. Most PreM afferents resembled corticomotoneuronal cells in being active only with their target muscles, unlike rubromotoneuronal cells and spinal PreM interneurons, which tended to exhibit more bidirectional discharges.

Effects of Input Synchrony on the Firing Rate of a Three-Conductance Cortical Neuron Model

For a model cortical neuron with three active conductances, we studied the dependence of the firing rate on the degree of synchrony in its synaptic inputs. The effect of synchrony was determined as a function of three parameters: number of inputs, average input frequency, and the synaptic strength (maximal unitary conductance change). Synchrony alone could increase the cell's firing rate when the product of these three parameters was below a critical value. But for higher values of the three parameters, synchrony could reduce firing rate. Instantaneous responses to time-varying input firing rates were close to predictions from steady-state responses when input synchrony was high, but fell below steady-state responses when input synchrony was low. Effectiveness of synaptic transmission, measured by the peak area of cross-correlations between input and output spikes, increased with increasing synchrony.

Integration and Differentiation in Dynamic Recurrent Neural Networks

Dynamic neural networks with recurrent connections were trained by backpropagation to generate the differential or the leaky integral of a nonrepeating frequency-modulated sinusoidal signal. The trained networks performed these operations on arbitrary input waveforms. Reducing the network size by deleting ineffective hidden units and combining redundant units, and then retraining the network produced a minimal network that computed the same function and revealed the underlying computational algorithm. Networks could also be trained to compute simultaneously the differential and integral of the input on two outputs; the two operations were performed in distributed overlapping fashion, and the activations of the hidden units were dominated by the integral. Incorporating units with time constants into model networks generally enhanced their performance as integrators and interfered with their ability to differentiate.