EEG afterdischarge patterns and performance of the avoidance response in hippocampally kindled cats

The relation between EEG limbic afterdischarge patterns and the performance of avoidance response was studied in eight hippocampally kindled cats. Five basic types of EEG afterdischarge patterns were identified in the hippocampal formation: (i) continuous 2-4/s high amplitude spikes or spike and wave complexes; (ii) continuous, 4.5-12/s high amplitude spikes or spike and wave complexes; (iii) dysrhythmic pattern: disorganized irregular spiking, mixed with slow waves at 3-20/s frequency; (iv) continuous, 20-30/s comb-like shaped spiking of moderate amplitude, and (v) trains of high amplitude and frequency irregular spikes, lasting for 0.2-1 s. followed by EEG depression lasting 0.3-1 s. The performance of conditioned response was tested during first 10 s. of afterdischarge development after kindling electrical stimulation of the hippocampus. A statistically significant relation was observed between the EEG afterdischarge patterns seen at the beginning of afterdischarge formation (basically types I, II and III) and the performance of avoidance response, with greatest probability of the response failure with type III of afterdischarge pattern. Latencies of the avoidance responses were prolonged during afterdischarges, in comparison to latencies observed before and after kindling hippocampal stimulation.     

Effect of after-discharge EEG patterns on performance and latency of the conditioned avoidance response in hippocampally kindled cats

The relation between different EEG after-discharge patterns and memory retrieval of the conditioned avoidance response was studied in 8 hippocampally kindled cats. The after-discharge patterns were classified into 5 basic EEG types. The relation was analyzed in three different situations: A, at the conditioning stimulation onset, B, at the conditioning stimulus termination, and C, when conditioning stimulus onset and termination were studied within the same type of EEG pattern of after-discharge. The moment of presentation of the conditioning stimulus was determined by the type of EEG after-discharge pattern. Significant relation was observed between reflex performance and type II and III EEG AD types. The type II (4.5-12/s spiking activity) was associated with a greater proportion of positive responses if presented on the uniform background of EEG after-discharge patterns. During the type III pattern (EEG dysrhythmia), there were more negative than positive responses in all the three experimental situations, especially if the type III EEG pattern was observed at the time of conditioned stimulus termination. The latency of reflex performance measured in the experimental condition C (the same pattern of EEG during conditioning stimulus onset and termination) was longer during the type II EEG pattern of after-discharges than the latencies found during type III EEG activity and in the control group. During type III pattern associated with greatest proportion of response failures there was, however, no increase of reflex latencies. The results suggest differential effect of different EEG after-discharge patterns on memory retrieval of the well established conditioned avoidance response in cats.     

Evolution of evoked potentials during conditioning in immobilized cats

To establish the relevancy of motor feedback in the evolution of evoked potentials during conditioning, recordings were made from the Caudate Nucleus (CN), the Lateral Geniculate Body (LGB), the Mesencephalic Reticular Formation (MRF), the Occipital Cortex (OC) and the Cerebellar Cortex (Cer. C) in immobilized and non-immobilized cats before and during classical conditioning. As a result of the conditioning we observed an enhancement of the average evoked potentials (AEP) recorded in CN and in LGB in those animals able to perform the conditioned response. In contrast, the AEP recorded in CN diminished in the immobilized animals, and no changes were observed in the recordings made from the other structures, except for the appearance of polyphasic components of small amplitude and high frequency (200 Hz) in the average evoked potentials' 2nd peak, recorded from LGB. From these results we conclude that the motor feedback produced by the execution of the response is important to establish the AEP changes observed in the CN.     

Removal of spurious correlations between spikes and local field potentials

Single neurons carry out important sensory and motor functions related to the larger networks in which they are embedded. Understanding the relationships between single-neuron spiking and network activity is therefore of great importance and the latter can be readily estimated from low-frequency brain signals known as local field potentials (LFPs). In this work we examine a number of issues related to the estimation of spike and LFP signals. We show that spike trains and individual spikes contain power at the frequencies that are typically thought to be exclusively related to LFPs, such that simple frequency-domain filtering cannot be effectively used to separate the two signals. Ground-truth simulations indicate that the commonly used method of estimating the LFP signal by low-pass filtering the raw voltage signal leads to artifactual correlations between spikes and LFPs and that these correlations exert a powerful influence on popular metrics of spike-LFP synchronization. Similar artifactual results were seen in data obtained from electrophysiological recordings in macaque visual cortex, when low-pass filtering was used to estimate LFP signals. In contrast LFP tuning curves in response to sensory stimuli do not appear to be affected by spike contamination, either in simulations or in real data. To address the issue of spike contamination, we devised a novel Bayesian spike removal algorithm and confirmed its effectiveness in simulations and by applying it to the electrophysiological data. The algorithm, based on a rigorous mathematical framework, outperforms other methods of spike removal on most metrics of spike-LFP correlations. Following application of this spike removal algorithm, many of our electrophysiological recordings continued to exhibit spike-LFP correlations, confirming previous reports that such relationships are a genuine aspect of neuronal activity. Overall, these results show that careful preprocessing is necessary to remove spikes from LFP signals, but that when effective spike removal is used, spike-LFP correlations can potentially yield novel insights about brain function.     

Evolution of average evoked potentials in cats during conditioning before and after tegmental lesions.

Sensory-specific and modality nonspecific average evoked potentials (AEPs) were recorded from the cortical areas and subcortical structures in two groups of cats: (1) normal cats which after conditioning were subjected to brainstem lesions and reconditioning; (2) cats with brainstem lesions and subsequent conditioning. A new waveshape of the visually evoked potenials developed in the visual cortex in the course of conditioning to light flashes (LF) in both groups of cats. In normal cats, a new component, with a peak latency from 80-100 msec and with reversed polarity, was observed in place of the late, longlasting, component of the preconditioning AEP. The latency of the new component is longer in cats with brainstem lesions. The first signs of waveform modification occurred relatively early in the process of learning, well before the animal learned to react consistently to the conditioned stimulus. Also, there seemed to be no clear relationship between the modified waveshapes of the AEPs and performance level during a particular session of conditioning. The modifications did not depend on habituation to the long-lasting exposure to light flashes presented alone. The new waveshape was preserved to a variable degree after brainstem lesions, that is, reconditioning never started with a potential characteristic for a naive animal. AEPs to licks in the auditory cortex, also changed during conditioning, although this modification was not so evident as in visual responses. In contrast to evident modifications of evoked responses in sensory specific structures during the process of conditiong, there were very small if any, changes in modality nonspecific structures, including brainstem reticular formation. Modality nonspecific responses were obtained from the brainstem reticular formation and motor cortex to light flashes and clicks, from the visual cortex to auditory stimuli and from the auditory cortex to light flashes. Only poorly developed evoked responses could be detected in the motor-sensory cortex during conditioning to light flashes although rhythmic EEG activity related to presentation of the conditioned stimulus (CS) was observed from this arena - thus indicating that they were not the same phenomena.     

Visually evoked cortical potentials in awake cats during saccadic eye movements

Visually evoked potentials (VEPs) measured under conditions of retinal image stabilization that minimized the influences of visual masking and smearing were averaged from electroencephalographic records measured from striate cortex of three cats. The amplitudes of the VEPs increased around saccade initiation. The grating-evoked potentials obtained at different times relative to the saccade exhibited changes in waveform shape that could be attributed to a saccade-evoked potential. The changes in the shape of the waveform were reasonably accounted for by the summation of the grating-evoked potential (produced when the cat did not make a saccade) and an appropriately timed saccade-evoked potential. The fundamental amplitudes of the residual potentials were computed and found to vary across the time course of the saccade. These observations suggest that there are other influences besides visual masking that are exerted early in the visual pathway to modulate visual processing during saccadic eye movements. A corollary discharge process is the most likely candidate to exert these influences. Received: 6 July 1997 / Accepted: 13 April 1998     

Cortical evoked potentials and extraversion.

Stimulus intensity modulation was studied in extraverts and introverts. Auditory evoked potentials were recorded in 29 subjects and visual evoked potentials in 55. In both sensory modalities, extraverts showed considerably larger amplitudes of the late components of the evoked potentials, suggesting that they were more "open" to stimuli than introverts. In this way, extraverts' responses to simple stimuli could be seen as similar to their interchange with the complex stimuli of the social world.     

Effect of atropine on intracortical evoked potentials during classical aversive conditioning in cats

In this article, intracortical evoked potentials (EPs) were recorded simultaneously from six different depths of the auditory cortex of freely moving cats. The effect of (a) different states of vigilance and that of atropine, (b) classical aversive conditioning, and (c) the effect of atropine during conditioning was studied on the intracortical EP profiles. Atropine induced EP changes that were similar to those seen in slow wave sleep. During classical aversive conditioning signal stimuli elicited a middle-latency negative EP component which was localized to the superficial cortical layers. Atropine (2 mg/kg body weight) did not abolish the appearance of this component but only increased its latency. It is proposed that the cholinergic part of the ascending activating system did not play an essential role in its generation.     

Hippocampal slices of kindled rats reveal calcium involvement in epileptogenesis

Summary Daily repeated tetanic electrical stimulation (kindling) of the brain may cause a long term enhancement of synaptic transmission and epileptiform activity of progressive severity and generalisation, eventually leading to spontaneous seizures. Evidence for a cellular mechanism underlying kindling has been obtained in vitro in slices from the hippocampus of kindled rats. A marked enhancement in extracellular calcium changes, induced by electrical stimulation or by iontophoresis of excitatory aminoacids was found in kindled tissue. This implies that changes in dendritic calcium conductances are involved in kindling epileptogenesis.     

Deficits in the visual evoked potentials of cats as a result of visual deprivation

Summary Visual evoked potentials in response to contrast reversal of grating patterns were used as a measure of visual function in normal and visually deprived cats. In cats which had been dark reared from birth (BD cats) there was a characteristic change in VEP waveform from normal, for both eyes and for all spatial frequencies of testing stimulus. In cats which had one eye sutured from the age of one week (MD cats), the VEP from the deprived eye was smaller for contrast reversal of coarse patterns. Kittens given only restricted periods of monocular exposure gave VEPs which resembled the pathological responses of the BD cats. However, the amplitudes of response were larger for the more experienced eye at higher spatial frequency. This work reinforces the idea that two factors govern cortical development: competition and experience. Lack of sufficient visual experience leads to severe intracortical pathology.     

Dendritic spikes in normal spinal motoneurons of cats

A spike of about 2 ms in duration and 3-8 mV in amplitude (D-spike) was produced by peripheral nerve (biceps semitendinosus, tibial and sciatic nerves) and dorsal root (L7 and S1) stimulation in normal spinal motoneurons of cats in a decerebrate, non-anesthetized state. The strength of the hyperpolarizing current to abolish the D-spike was always greater than that to abolish the IS-spike. The D-spike triggered the IS-spike at a level more depolarized than that at which the M-spike triggered the IS-spike. An inhibitory postsynaptic potential, which inhibited the IS-spike, could not inhibit the D-spike. Two different D-spikes, as distinguished by a difference in threshold, could occur in the same motoneuron. These results suggested that the D-spike was dendritic in origin.     

Visual evoked potentials to changes in the motion of a patterned field

Summary Human averaged visual evoked potentials (VEPs) have been recorded to the onset, reversal and offset of the motion of a visual noise pattern. The VEPs have been analysed into components, distinguishable by their different scalp distributions, and by their distinct dependences on the retinal area stimulated and the pattern velocity. The effects of varying the form and the scale of the pattern have been investigated. The VEPs were almost invariant with respect to changes in the brightness, the direction of motion, and the sharpness of the boundary of the visual field. Under some circumstances the motion-reversal VEP was very similar to the sum of the motion-onset and the motion-offset VEPs.These VEPs will qualify as a useful tool for investigating the processing of movement information only if they can be shown to be genuine responses to changes in the motion as such. Discussion of this question has been deferred to the following paper.