Hemispheric asymetry of the evoked electrical activity of the cerebral cortex to letter and non-verbal stimuli]

Average evoked potentials (AEP) were recorded in practically healthy subjects to "meaningless" figures and letters, presented to different halves of the visual field. Analysis of the amplitudes of AEP late components to verbal and non-verbal stimuli reveals hemispheric asymmetry. A higher amplitude of the late positive evoked response (P300) to a "direct" stimulation both by verbal and non-verbal stimuli (in the contralateral field of vision) is recorded in the left hemisphere than in the right one. Similar stimulation of the right hemisphere does not reveal sucha difference. In the left hemisphere the P300 wave is of a clearly greater amplitude to a "direct" stimulation (contralateral visual field) than to an "indirect" one (ipsilateral visual field), regardless of the nature of the stimulus. No such difference is observed in the right hemisphere. The magnitude of the late negative wave (component N200) to non-verbal stimuli is greater in the right hemisphere both in response to "direct" and "indirect" stimulations. No intrahemispheric difference has been found in the amplitude of late evoked responses of the cerebral cortex to verbal and non-verbal stimuli.     

Hemispheric asymmetries in cortical electrical activity during sleep. I

The hypothesis of a predominance of the right hemisphere in stage REM as compared to NREM has been tested through a spectral analysis of the EEG recorded from left (T3) and right (T4) temporal sites in 5 young healthy right-handed male subjects. Variations in the asymmetry coefficient R - L/R + L in different sleep stages have been analyzed by one way ANOVAs and Sheffé's tests. The hypothesis of a progressive increase in left hemisphere activity throughout different REM cycles as one approaches final awakenings have been investigated by comparing variations in the asymmetry coefficient for epochs of REM and stage 2 NREM sampled in different phases of the REM cycle. EEG results do not support either the hypothesized stage dependent or cycle dependent variation in EEG activity during sleep. We question whether variations in EEG amplitude and synchronization can be used as indices of hemispheric asymmetries during sleep.     

Spatio-temporal analysis of cortical activity evoked by gustatory stimulation in humans.

Gustatory activated regions in the cerebral cortex have not been identified precisely in humans. In this study we recorded the magnetic fields from the brain in response to two tastants, 1 M NaCl and 3 mM saccharin. We estimated the location of areas activated sequentially after the onset of stimulation with magnetic source imaging. We investigated the primary gustatory area (area G) precisely, and found it at the transition between the parietal operculum and the insular cortex. The central sulcus was activated less frequently than area G but with almost the same latency in cases of NaCl stimulation. Following area G, we found activation in several cortical regions, e.g. both the frontal operculum and the anterior part of the insula, the hippocampus, the parahippocampal gyrus and the superior temporal sulcus.     

The influence of emotionally significant visual stimuli on cortical evoked potentials

Electrophysiological correlates of the perception of emotional stimuli were studied by means of recording the visual evoked potentials (EP) in 20 derivations (Fz, Cz, Pz, Oz, Fp _1/2,F _3/4,F _7/8,C _3/4,P _3/4,T _3/4,T _5/6, andO _1/2) during the emotional test performance. The performance of a special task by subjects was assessed positively or negatively (by administering emotionally positive or negative stimuli, respectively). Factor analysis revealed seven factors, which described the EP component structure. Analysis of variance demonstrated the influence of the emotional stimuli sign on the factorsP ₁₀₀,P ₁₄₀,N ₁₆₀,P ₂₂₀,P ₃₄₀, and “slow wave.” Hemispheric difference in reactions to the stimuli of a different emotional sign were recorded. During presentation of the positive and negative assessments, the amplitudes of the factorsP ₁₀₀,P ₃₄₀, and “slow wave” were maximally different in the left hemisphere, while the factorsN ₁₆₀ andP ₂₂₀ were maximally different in the right hemisphere.     

Influence of segmental and extra-segmental conditioning stimuli on cortical potentials evoked by painful electrical stimulation

This study evaluated the effects of two different types of segmental/extra-segmental conditioning stimuli (tonic muscle pain and non-painful vibration) on the subjective experience (perceived pain intensity) and on the cortical evoked potentials to standardized test stimuli (cutaneous electrical stimuli). Twelve subjects participated in two separate sessions to investigate the effects of tonic muscle pain or cutaneous vibration on experimental test stimuli. The experimental protocol contained a baseline registration (test stimuli only), a registration with the test stimuli in combination with the conditioning stimuli, followed by a registration with the test stimuli only. In addition, the effects of the conditioning stimuli were examined at two anatomically separated locations (segmental and extra-segmental). Compared with the test stimulus alone, the perceived pain intensity and peak-to-peak amplitudes of the evoked potentials were unchanged in the presence of non-painful conditioning stimuli at either location. In contrast, a significant decrease of the perceived pain intensity and peak-to-peak amplitudes was found in the presence of painful conditioning stimuli at the extra-segmental sites. Moreover, the topographic maps of the 32-channel recordings suggested that the distribution of the scalp evoked potentials was almost symmetrical around the vertex Cz in the baseline registration. The evoked potentials were generally decreased during hypertonic saline infusion at the extra-segmental sites, but the distribution of the topographic maps did not appear to change. Vibration has previously been shown to inhibit pain, but in the present study the perceived intensity of phasic painful electrical stimuli was unchanged. The reduced perceived pain intensity and the smaller peak-to-peak amplitude of the evoked potential in the presence of extra-segmental conditioning pain are in accordance with the concept of diffuse noxious inhibitory control.     

Effect of verbal reinforcement on evoked cortical activity

Adult healthy subjects did not manifest any difference in latency and amplitude of the wave P300 elicited by a positive ("good") and negative ("error") reinforcing stimuli. After the negative reinforcement, the P300 wave amplitude decreases in response to the standard stimulus (light bars) and increases to a lesser degree in response to test stimuli (the same bars but presented with different pauses). In the processes of learning to assess time microintervals in comparison with the standard, the latency of wave P300 to the test stimuli shortens. It is suggested that formation and consolidation of feedback connection elaborated with the participation of a reinforcing verbal stimulus constitute the physiological basis for learning of comparative assessment of time microintervals.     

Correlation between extracellular focal potentials and K+ potentials evoked by primary afferent activity.

There is a clear, positive correlation in amplitude between changes in potassium potentials (deltaEK) and focal potentials (deltaV) evoked by tetanic stimulation of afferent nerves in the cuneate nucleus and dorsal horn of cats under Dial anaesthesia or after decerebration. Data obtained with stimulations at various frequencies and intensities, or recording at different positions give a relatively constant slope of deltaV/deltaEK (varying between 0.2 and 0.6 in different experiments). These observations are fully consistent with the possibility that deltaV mainly reflects changes in extracellular potassium concentration caused by the release of K+ from active terminals. Differences in time course of deltaEK ANd deltaV evoked by single stimuli are a steep function of distance and therefore can be ascribed to the slowness of diffusion, without excluding the possibility of an early additional depolarizing effect by another mechanism.     

Gustatory evoked cortical activity in humans studied by simultaneous EEG and MEG recording

Evoked potentials are widely used in clinical medicine for objective evaluation of sensory disturbances. However, gustatory evoked potentials (GEPs) have not been extensively studied due to lack of agreement among investigators regarding the waveforms. In this study GEPs and gustatory magnetic fields (GEMfs) were simultaneously recorded from five subjects in response to 0.3 M NaCl in an attempt to establish GEP recording as an objective gustatometer. Each subject received a total of 240 stimulus presentations over six sessions. Three GEP components (P1, N1 and P2) were observed and correlated with their corresponding equivalent current dipoles (ECD1, ECD2 and ECD3, respectively). ECD1 was localized to area G in all subjects, P1 being the indicator of intact gustatory projection to area G. No significant GEP activity was detected during the time preceding P1, which suggests that there was no activity in cortical gyri other than that detected by magnetoencephalography. ECD2 and ECD3 were localized to various cortical structures, including the inferior insula and the superior temporal sulcus, indicating that N1 and P2 reflect higher order gustatory functions. The present results indicate that measurement of GEPs may be useful for objective evaluation of gustatory disturbance.     

Cerebral cortical respiratory-related evoked potentials elicited by inspiratory occlusion in lambs.

Respiratory-related evoked potentials (RREP) elicited by inspiratory mechanical loads have been recorded in humans. Early RREP peaks were hypothesized to be generated by activation of neurons in the somatosensory cortex. An animal model was developed to test this hypothesis in chronically instrumented, awake, spontaneously breathing lambs. Electrocorticogram (ECoG) was recorded bilaterally with ball electrodes on the dural surface over the somatosensory region. Inspiratory occlusions were presented through a face mask or endotracheal tube as interruptions of inspiration. Occlusion-elicited evoked potentials were obtained by computer-signal averaging the ECoG activity. A short-latency positive peak was observed bilaterally in the averaged occlusion-elicited evoked potentials in all animals breathing with the facemask and 5 of 8 lambs with the endotracheal tube. Postmortem identification of the electrode location demonstrated that the ECoG was recorded in the caudal-lateral portion of the somatosensory cortex. These results demonstrate that inspiratory occlusion elicits an evoked potential in the somatosensory cortical region of awake, spontaneously breathing lambs. The lamb cortical RREP is similar to human RREP.