Analysis of long cortical evoked potentials

Evoked potentials arising in the motor cortex in response to its direct stimulation (dendritic and slow negative potentials), to stimulation of the ventrolateral (primary response) and intralaminar (nonspecific response) thalamic nuclei, and to stimulation of the pyramidal tracts (antidromic response), and also postsynaptic responses of neurons corresponding to them were studied in acute experiments on curarized cats. Evoked potentials arising in response to direct cortical stimulation and also to stimulation of the specific and nonspecific thalamic nuclei and pyramidal tracts were recorded from the same point of the motor cortex, and the corresponding intracellular responses were recorded from the same neuron. Slow negative potentials arising under these conditions of stimulation and the IPSPs corresponding to them were shown to have an identical time course. The results show that slow negative potentials are a reflection of hyperpolarization of pyramidal neurons. It is suggested that the individual components of responses evoked by direct stimulation of the cortex and thalamic nuclei have a common genesis.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 14, No. 2, pp. 115–121, March–April, 1982.     

Model of visually evoked cortical potentials

The pattern-reversal (P-VEPs) and the motion-onset (M-VEPs) of visual evoked potentials were modeled by means of three damped oscillators (O1, O2, O3) of identical construction. The O1, assumed to simulate the response of primary visual area (V1), was driven by the firing density of the lateral geniculate nuclei. 01 contributed mainly to the N75 and P100 peaks of the P-VEPs. The O2, driven by the O1 output, mimics the activity of V2, V3a, and MT. It contributed to the negative peak N145 of the P-VEPs or to the N160 in the M-VEPs. The O3 was suggested to model late slow processes probably of an attentive origin. The model parameters were set by optimization to follow the P-VEPs and M-VEPs obtained as a grand average of four young volunteers (Pz - A2 lead). The evoked potentials were described with normalized root mean square error lower than 13%.     

Physiology-based modeling of cortical auditory evoked potentials

Evoked potentials are the transient electrical responses caused by changes in the brain following stimuli. This work uses a physiology-based continuum model of neuronal activity in the human brain to calculate theoretical cortical auditory evoked potentials (CAEPs) from the model’s linearized response. These are fitted to experimental data, allowing the fitted parameters to be related to brain physiology. This approach yields excellent fits to CAEP data, which can then be compared to fits of EEG spectra. It is shown that the differences between resting eyes-open EEG and standard CAEPs can be explained by changes in the physiology of populations of neurons in corticothalamic pathways, with notable similarities to certain aspects of slow-wave sleep. This pilot study demonstrates the ability of our model-based fitting method to provide information on the underlying physiology of the brain that is not available using standard methods.     

Motor potentials evoked by paired cortical stimuli

We recorded the motor evoked potentials (MEPs) from the abductor pollicis brevis muscle, after supramaximal electrical transcranial stimulation, and studied the effect of paired transcranial shocks with varying interstimulus time intervals, in 10 normal subjects, 4 patients with median nerve neuropathy and 2 patients with motoneurone disease.In relaxed muscles the amplitude of the MEP evoked by a single shock averaged 30% of the M wave. With intervals from 1 to 2.5 msec 2 shocks evoked one MEP far larger in size than the control MEP (70% of the M wave). With intervals of 10 msec and longer, the 2 shocks evoked 2 independent MEPs; the size of the MEP following the second shock (test) was inversely correlated with the size of the control MEP: the more the control MEP approached the size of the M wave, the smaller the test MEP. Single motor unit records showed that, in the normal subjects and patients with peripheral neuropathy, the same motor unit was activated either by the first or the second shock, whereas in the patients with motoneurone disease it fired twice. In active muscles, the control MEP averaged 70% of the M wave. With intervals of 10 msec and longer the test MEP was markedly suppressed; with 100 msec intervals it fully recovered. In relaxed muscles, by delivering a double shock at a 1.5 msec interval, thus evoking a large MEP, followed by a second double-shock, the test MEP was completely suppressed for a period of 20 msec; it began to recover at 50 msec intervals and fully recovered after 150 msec.These results indicate that: (1) high-threshold spinal motoneurones can profit from temporal summation if double-shocks are delivered at short time intervals; (2) the synchronous excitation of the motoneuronal pool produced by transcranial stimulation is followed by a 20 msec period of absolute inhibition, possibly through a massive activation of the Renshaw system; (3) during voluntary contraction, only a portion of the motoneuronal pool remains refractory, possibly because of the enhanced spinal excitability.     

Effect of neurotropic drugs on cortical evoked potentials

In acute experiments on unanesthetized, curarized cats and rabbits and also on animals anesthetized with chloralose, recordings were made of direct cortical and transcallosal responses, responses in the pyramids of the medulla to peripheral stimulation and stimulation of the motor cortex, primary responses in area S-I, and interzonal somatomotor responses. The effect of narcotics on these cortical responses was shown to persist under conditions partially or completely excluding effects mediated through the reticular formation and other subcortical structures (intracarotid injection of the drugs or their local application to the cortex, experiments after premesencephalic section or on the isolated cortex). Neuroleptics have only a slight effect on these cortical evoked responses, mainly due to their blocking action on the reticular formation. Tranquilizers of the benzodiazepine series are active against the cortical responses studied, and this effect is due to their direct action on the cortex.Institute of Pharmacology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 3, No. 6, pp. 582–591, November–December, 1971.     

Genesis and functional role of cortical evoked potentials

Experiments on cats with recording electrodes implanted into the cranial bone showed that the evoked potential (EP) in the auditory cortex of the intact waking cat in response to clicks consists of five components with a total duration of up to 300 msec. Neurons of two types participate in the response to clicks: those with and without background activity. The former respond to clicks by various changes in background activity, the latter by one or several action potentials. The latent period of this response varies in different neurons from 6 to 250 msec. In response to clicks, several groups of neurons participate successively in the response, accounting for its long duration. From the beginning of the response, neurons of all cortical layers take part in it. At any moment of EP development, some neurons are in a state of excitation, others in a state of inhibition. About 80% of neurons responding to clicks respond before or during the initial electropositivity, 12% during the initial electronegativity, and only 8% during the late components of the EP. The importance of these findings is discussed relative to the question of the nature of the EP and of processes taking place in the brain after the arrival of an afferent volley.A. A Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neiofiziologiya, Vol. 2, No. 4, pp. 349–359, July–August, 1970.     

Development of cortical somatosensory evoked potentials in rats.

The development of the contra- and ipsilateral cortical potential evoked by electrical sciatic nerve stimulation was studied in 77 male albino rats aged 5 to 45 days. A contralateral response was already recorded, as double negativity, in the youngest animals, while an ipsilateral evoked potential was not reliably present until the 10th day. At this time, however, both responses started with an inconstant positive wave and their shape was practically the same. During subsequent development the responses differed only in respect to their dominant component: in the contralateral response, the N1 wave had the highest amplitude for most of the time, while in the ipsilateral response the delayed N2 wave was the largest component. The latent periods of contralateral responses were somewhat shorter than those of ipsilateral evoked potentials. During development we noticed a phase of abrupt shortening of the latent period, which took place before the 15th day in the contralateral response and before the 20th day in the ipsilateral response. We also found a difference in the fatigability of the responses, which was greater in immature rats than in adult animals; in the ipsilateral evoked potential it approached adult values more slowly. The development of the ipsilateral response is thus delayed compared with the development of the contralateral response.     

Respiratory-related cortical potentials evoked by inspiratory occlusion in humans

It has long been recognized that humans can perceive respiratory loads. There have been several studies on the detection and psychophysical quantification of mechanical load perception. This investigation was designed to record cortical sensory neurogenic activity related to inspiratory mechanical loading in humans. Inspiration was periodically occluded in human subjects while the electroencephalographic (EEG) activity in the somatosensory region of the cerebral cortex was recorded. The onset of inspiratory mouth pressure (Pm) was used to initiate signal averaging of the EEG signals. Cortical evoked potentials elicited by inspiratory occlusions were observed when C3 and C alpha were referenced to CZ. This evoked potential was not observed with the control (unoccluded) breaths. There was considerable subject variability in the peak latencies that was related to the differences in the inspiratory drive, as measured by occlusion pressure (P0.1). The results of this study demonstrate that neurogenic activity can be recorded in the somatosensory region of the cortex that is related to inspiratory occlusions. The peak latencies are longer than analogous somatosensory evoked potentials elicited by stimulation of the hand and foot. It is hypothesized that a portion of this latency difference is related to the time required for the subject to generate sufficient inspiratory force to activate the afferents mediating the cortical response.     

Effect of psychotropic drugs on somatosensory evoked cortical potentials]

The effects of some typical clinically tested psychotropic drugs were studied in acute experiments on awake rats. Potentials in somatosensory cortex were evoked by peripheral stimulation. Haloperidol, diazepam, medazepam, desipramin and a new substance, the MPP-sulton, caused significant modification in latency and amplitude of some components of the potentials. Between the effects of psychotropic drugs exist differences which, in turn, were clearly distinguishable from the modification of these evoked responses by unspecific narcotic effects of hexobarbital.