Operant conditioning of tonic firing patterns from precentral neurons in monkey neocortex

Evoked and psychogenic epileptic seizures. I. Precipitation

The precipitation of seizures by external stimulation (evoked seizures) is well known. Less well known is the precipitation of seizures by a change in the patient's thinking or feelings. This artick uses Lockhart's monkey model of focal epilepsy to propose that there is a close relationship between seizures and ongoing brain activity. Thus, seizures precipitated by both voluntary and spontaneous changes in behaviour and thinking must commonly occur. Clinical examples of such seizure precipitation is described.

Synchrony between cortical neurons in normal and epileptogenic cortex of monkey

A series of experiments was carried out to test the hypothesis that epileptogenic cortex contains a higher degree of synchrony between neurons than does nonepileptogenic cortex. Four hundred pairs of neurons were recorded extracellularly from precentral cortex of four normal and four epileptic monkeys. For each 5 min of recording, a cross-correlation and coefficient of synchrony were calculated for each pair of neurons. Although there was some degree of synchrony between neurons, there was no apparent increase in synchronous firing within the epileptogenic focus.

Synchrony between cortical neurons during operant conditioning

One hundred twenty five pairs of neurons were recorded simultaneously from precentral cortex of Macaca mulatta monkeys during an operant conditioning task. At the end of 5-min behavioral periods, a cross-correlation histogram was generated to look for relationships between the firing of the two cortical neurons. Eighty four (67%) unit pairs showed a significant coincidence of firing within 1 ms of each other. This relationship occurred regardless of whether the units' firing rate fluctuations were correlated or not. These data imply that in the majority of cases, the two units are probably more related than previously reported.

Interneuronal synchrony in precentral cortex of monkeys during operant conditioning

One hundred thirty-three pairs of precentral neurons were recorded simultaneously from four Macaca mulatta monkeys during participation in an operant conditioning paradigm that required one of the two units to be fired tonically within a 30- to 60-ms interspike interval (ISI) range. At the end of each 5-min behavioral period, a correlation between the two units' firing rate fluctuations was computed. These correlations were used as a measure of synchrony between units. For the majority of unit pairs, the unit for which reinforcement was contingent upon could be controlled independently from the activity of the simultaneously recorded unit. In 75% of experiments the synchrony between units decreased during the operant periods in comparison to the time-out periods. In only 5% of the experiments did unit pairs consistently exhibit increased synchrony during operant periods compared with time-out periods. If the monkey developed EEG signs of drowsiness during time-out periods, the synchrony between units became greater.

Operant conditioning of single unit activity in parietal cortex

Two rhesus monkeys were trained to control firing patterns of single neurons in parietal cortex (areas 1, 2, 3, 5, 7) using an operant task previously applied to the study of precentral units. Twenty-four of 56 (43%) postcentral cells were controlled in contrast to 71 of 136 (52%) precentral units from these and 4 other rhesus monkeys. In addition, monkeys were able to drive precentral units to more sustained tonic firing rates than they could parietal units. An analysis of interspike interval (ISI) distributions showed that, in contrast to precentral units with modal ISIs of 25-50 ms, 50% of parietal units have modal ISIs of 2 ms. Such short ISIs may account for fewer postcentral units reaching control criteria for this particular operant task. Other factors that may contribute to the reduced control of postcentral cells are discussed, particularly the more complex afferent connections to parietal units when compared to precentral pyramidal tract neurons. The data indirectly support conclusions from previous studies that imply that operant control of cortical units is peripherally mediated and does not primarily involve a 'central' or 'open loop' system.

Operant control of epileptic neurons in chronic foci of monkeys

(1) Six Macaca mulatta monkeys were rendered chronically epileptic by subpial alumina injection in left precentral cortex. The monkeys were then trained to operantly control the firing patterns of single units from the epileptic focus and contralateral homotopic cortex using a standardized paradigm. In 30 experiments, units were recorded simultaneously from both hemispheres. (2) For epileptic and normal neurons, there are no consistent relationships between cortical EEG spikes and unit firing. (3) Previous parameters for quantifying single unit epileptogenicity are not adequate for defining the complex spectrum and relative degree of unit abnormalities within the chronic focus. A primary difference between normal and epileptic neurons is that monkeys can modify the firing pattern and rate of the former but can modify, only slightly, the integrated firing rate of the latter. (4) A previously described 'non-burst' epileptiform firing pattern consisting of 8--18 msec doublets is investigated in further detail. This pattern may represent units along a continuum between normalcy and long-first-interval burst firing. (5) The majority of epileptic units' firing patterns become more normal when the monkey is reinforced to control the firing pattern of a contralateral unit. Then, if the monkey is reinforced to control the epileptic unit, subtle abnormal firing patterns become more apparent. Therefore, the firing patterns of epileptic units are effected in two ways: the non-specific effect of alerting to the operant task, and the more specific effect that accompanies the monkey's attempts to attain reinforcement when it is contingent on the behavior of the specific neuron. (6) Although epileptiform EEG spikes are projected to homotopic contralateral precentral cortex, there is no evidence to suggest that single units within such cortex fire in patterns correlative with intrinsic epileptogenic burst generating properties. Therefore, if the 'mirror focus' is defined as cortex which has become autonomously epileptogenic secondary to persistent transsynaptic activity from a primary focus, these data do not support the concept that mirror foci develop in primate precentral cortex.

Functional properties of monkey motor cortex neurones receiving afferent input from the hand and fingers

1. Records have been made from area 4 of the cerebral cortex in five conscious monkeys. The properties of 216 neurones responsive to natural stimulation of the hand and fingers have been investigated.2. 46% of these neurones responded only to cutaneous stimulation (especially light brushing across the glabrous skin) and a further 38% responded only to movement of the digits. 4% responded to brief prods of the hand. 12% of the sample responded to more than one stimulus modality.3. Many hand-input neurones, including pyramidal tract neurones, responded at short-latency (8-15 msec) to light mechanical stimulation of the hand and to weak electrical stimulation of the median nerve.4. Responsive neurones were found at all depths of the cortical grey matter. Responses of shortest latency were encountered in neurones probably located in layers IV and V.5. The behaviour of eighty hand-input neurones was analysed during a simple, stereotyped task which involved pulling a lever and collecting a food reward from a small well. For comparison, the activity of 117 neurones with inputs from the wrist, elbow or shoulder was also analysed.6. Nearly all hand-input neurones modulated their activity either before (48/80) or during (29/80) the retrieval of the reward which required precision grip between index finger and thumb. Many were silent during proximal arm movements and some displayed activity patterns independent of these movements.7. By contrast, the activity of many neurones with proximal arm (elbow, shoulder) inputs was unrelated to food retrieval and manipulation, but well related to arm movements.8. Forty-three of the eighty neurones had cutaneous input from the hand. Twenty-seven were active before hand contact. Thirty-five modulated their discharge when contact was made (twenty-one excitation, fourteen inhibition).9. Most hand-input neurones were more active during fractionated movements of the hand or fingers than during power or ball grips requiring simultaneous flexion of all digits. Neurones with glabrous inputs often showed intense activity during small, precise finger movements and during active tactile exploration without the aid of vision.10. Analysis of the discharge frequency of twenty-five hand-input neurones revealed that some (mainly non-pyramidal tract neurones) had a similar mean frequency and range of modulation during both active movement and passive stimulation. Others (mainly pyramidal tract neurones) had a greater frequency range and higher mean frequency during active than during passive movements.

Operant control of precentral neurons: bilateral single unit conditioning

Two Macaca mulatta monkeys were reinforced to operantly control a precentral neuron's firing pattern while a contralateral unit was monitored simultaneously. The results from 38 complete experiments indicate the following: (a) upon altering to the operant task, both the contingent and the non-contingent neurons changed firing patterns from preconditioning levels. However, as the monkey brought the contingent unit under operant control, there were no significant changes in the firing pattern of the non-contingent neuron; (b) when the contingencies were reversed so that the monkeys were reinforced to control the originally non-contingent neuron, the firing pattern off the originally contingent neuron returned to near baseline levels. These data indicate that although many precentral units may change firing patterns when the monkey attends to the operant task, the reinforced changes in firing pattern are not the result of a generalized phenomenon at the spinal level.

Operant control of precentral neurons: control of modal interspike intervals

The objects of these experiments were: (a) to determine modal interspike intervals (ISIs) of precentral cells involved in repetitious, gross motor movements; (b) to compare those modal ISIs to the modal ISIs of similar neurons under operant control; and (c) to determine if monkeys could change the modal ISIs of operantly controlled precentral neurons. Data were obtained from 4 monkeys conditioned to produce tonic firing of precentral neurons and one monkey trained to produce repetitious movements of the neck and contralateral limbs. Results are: (a) the modal ISIs from operantly controlled precentral units do not differ significantly from precentral neurons involved in repetitive gross motor movements; and (b) while under operant control, the monkeys cannot modify significantly the modal ISI of the majority of precentral neurons.

Operant control of precentral neurons in monkeys: evidence against open loop control

Four normal monkeys were operantly conditioned to change the firing pattern of 111 precentral neurons from phasic to tonic using an operant paradigm which quantifies the control of single neurons. Two monkeys then had their contralateral pyramidal tract (PT) sectioned and one monkey had C5-7 ventral rhizotomies. Postlesion data were: (1) contralateral C1-2PT lesions did not encumber the monkeys' control of precentral PTNs: (2) contralateral C5-7 ventral rhizotomies completely abolished accurate control of precentral neurons which received proprioceptive feedback from flaccid arm regions. These results indicate that precentral neurons are operantly controlled through proprioceptive feedback from peripheral mechanoreceptors. The output of the mechanoreceptors is probably dependent upon discrete joint angles and/or muscle tension which is maintained through non-PT pathways. These data do not support the concept that precentral neurons are operantly controlled directly from a central; 'open loop', pathway.

The influence of attending on seizure activity in epileptic monkeys

Six studies are presented on the influence of attending upon epileptic activity in the alumina-gel monkey model of focal motor and secondarily generalized tonic--clonic seizures. Seizure frequency and EEG paroxysms are reported during (a) scheduled feeding periods, (b) visual attending, and (c) three different operant tasks, including the conditioning of single neurons. An explanatory hypothesis of the cumulative data is proposed in terms of the different bursting behavior of group 1 (strongly epileptic) and group 2 (weakly epileptic) neurons of the epileptogenic focus. It is suggested that attending, or participation in operant tasks, results in a decrease in bursting of group 2 neurons and a disruption of synchrony between group 1 (pacemaker), group 2, and normal neurons. This desynchronization is said to lower the probability of an ictal event occurring either during or immediately following an operant task. Attending factors may be responsible for some of the conflicting findings in therapeutic studies of epilepsy which have not controlled for this parameter.

Effects of chronic epileptic foci on control of pyramidal tract neurons in monkeys

Five Macaca mulatta monkeys were operantly conditioned to control the firing patterns of single precentral pyramidal tract neurons. The accuracy with which the monkeys could control normal PTNs from within the focus was significantly poorer than PTNs from contralateral, homotopic cortex. In comparison to nonepileptic monkeys, there was no significant difference in the accuracy with which PTNs from cortex contralateral to interictal foci were controlled. By contrast, comparison of the time necessary to gain accurate control over individual PTNs from contralateral cortex showed the epileptic monkeys to be significantly encumbered when compared to nonepileptic monkeys. These data suggest that interictal foci produce "noise" in remote regions of brain that are involved in an operant task requiring a high degree of discrimination.

Operant control of pyramidal tract neurons: the role of spinal dorsal columns

A monkey was trained to control the firing patterns of precentral pyramidal tract neurons. The operant task was for the monkey to produce consecutive interspike intervals (ISI) within a requisite range, or target. The mean time off-target (error) is used to quantify the accuracy of control the monkey could assert over each PTN. Following partial destruction of the dorsal funiculi the number of PTNs driven by peripheral stimuli greatly decreased. Those PTNs which remained responsive to peripheral stimuli were as accurately controlled as those tested before column section, whereas, those PTNs unresponsive to peripheral stimuli were significantly less accurately controlled. The conclusion is that the monkey relies heavily upon proprioceptive feedback to operantly control precentral PTNs.

Epileptic and normal neurons in monkey neocortex: a quantitative study of degree of operant control

The object of this study was to quantify and compare the degree of control monkeys may assert over firing patterns of normal and epileptic neurons. Thirty-seven epileptic and 70 normal neurons were studied in detail. The operant task was for the monkey to generate specified consecutive interspike intervals (ISI). The monkeys demonstrated far greater accuracy in controlling consecutive ISIs of normal neurons and were only able to control the intervals between bursts from epileptic neurons. The data implies that high frequency bursts of action potentials from epileptic neurons are all-or-nothing events initiated by synaptic mechanisms. In addition, some data are from a monkey with epilepsia partialis continua; in comparison to less active foci, this focus was comprised of a higher percentage of 'pacemaker' epileptic neurons.