Correlation of brain rhythms between frontal and left temporal (Wernicke's) cortical areas during verbal thinking.

The aim of this study was to investigate an interaction between frontal and left temporo-parietal cortices in tasks requiring word association. A new method was used to examine averaged event-related potentials in different frequency bands by calculating correlation coefficients between wavelet curves in distant cortical areas. This method was applied to previous event-related potentials recordings which found successive activation of frontal and left posterior areas [1]. Correlated activity at 17 Hz was observed between frontal and left temporal (Wernicke's) areas prior to full activation of Wernicke's area.

Depth electrodes in clinical neurophysiology: neuronal activity and human cognitive function.

Depth intracranial electrodes are used in medicine for the diagnosis and treatment of certain medically intractable disorders including epilepsy and parkinsonism. Electrophysiological studies including the recording of neuronal activity via depth electrodes during cognitive tasks allow the mapping of the functional significance of various brain regions for certain cognitive functions. Knowledge accumulated throughout the last several decades allows us to draw some theoretical inferences about neuronal mechanisms of human cognition. The present paper provides a selective review of studies of neuronal activity of the human brain in relation to language, verbal memory and error detection.

Neuroanatomy, circuitry and plasticity of word reading.

The use of neuroimaging has provided a basis for suggesting the brain areas active during reading of words and sentences. When combined with high density electrical recording from the scalp, it is possible to obtain information on the time course of activation of these brain areas and compare them with the temporal structure of reading from studies of eye movements. The paper summarizes results in these areas and suggests how acquisition and practice of the skill might alter the circuitry involved.

Neuronal activity of human caudate nucleus and prefrontal cortex in cognitive tasks.

Lesions of the caudate nucleus and prefrontal cortex may display similar cognitive deficits. Recent advances in cognitive neuroscience have clarified the functional role of prefrontal cortical areas in certain cognitive operations involved in simple language tasks. We have addressed the role of the caudate nucleus in tasks of lexical decision, semantic categorization, recognition memory, reading aloud and object naming by recording neuronal activity in patients with depth electrodes. During visual processing of words, caudate cells exhibited excitatory responses related to both semantic and phonological-articulatory encoding with non-overlapping time courses. The firing rate of the cells was increased when the semantic processing was required. This occurred within 400-600 ms after the stimulus onset, or within the first 200-300 ms of the delay period. The increased firing within 1000-1200 ms after the stimulus onset was related to the phonological processing. These responses turned out to be strikingly similar to those in Broca's area. Both reading aloud and explicit memory retrieval tasks elicited a sustained inhibition of firing of the same cells with a greater onset latency. Chronometric comparison of prefrontal, temporo-parietal and caudate activities in similar tasks relates the time course of these activations to the fronto-caudate anatomical loops and helps further understanding of the anatomy and circuitry involved in human cognition.

Event-related brain potential imaging of semantic encoding during processing single words.

Functional brain imaging studies with positron emission tomography (PET) have identified blood flow changes in widely separated areas of brain during the performance of word processing tasks. In the present study we have utilized event-related brain potentials (ERPs) to investigate the temporal relationships among cortical areas previously identified by PET to be differentially activated when performing semantic tasks with visual words. ERPs revealed task-related differences over the central and left inferior frontal regions around 170 and 220 ms, respectively, over a left occipital region around 200 ms, over a large left parietotemporal region around 600 ms, and finally over the right temporal lobe around 800 ms after the word presentation. Analysis of topographic maps and dipole sources as well as PET data allowed relating frontal midline positivity around 170 ms to the anterior cingulate activation, and left inferior frontal positivity around 220 ms to the PET activation of the left inferior prefrontal cortex. The left parieto-temporal positivity around 600 ms seems to reflect the activity of Wernicke's area. The right anterior temporal negativity beginning around 800 ms and peaking around 1100 ms may reflect the activity of the right insula. The left occipital negativity around 200 ms is likely to reflect activation of a visual word-form area in the left occipital lobe. These results provide the time course for parts of the circuitry involved in semantic processing of words and also demonstrate how combining the spatial localization of PET with the temporal resolution of ERPs helps to understand the brain mechanisms involved in human cognition.

Cognitive operations in the human caudate nucleus.

We have addressed the role of the head of caudate nucleus in taks of lexical decision, semantic categorization, reading aloud, memory retrieval and object naming by recording neuronal activity from depth electrodes in parkinsonism patients. The firing rate of cells was increased within 400-600 ms when semantic processing was required. Phonological differences increased firing from 1000 to 1200 ms. These responses were similar to those described earlier in Broca's area. Both reading aloud and explicit memory retrieval tasks elicited an inhibition in firing of the same cells with an onset around 800 ms. The results confirm earlier lesion and stimulation studies, and provide a basis for relating cellular activity to studies of blood flow and scalp electrical activity in similar cognitive tasks thus helping us to understanding the cellular mechanisms involved in human cognition.

Scalp electrical potentials reflect regional cerebral blood flow responses during processing of written words.

Functional brain imaging studies with positron emission tomography (PET) have identified blood flow changes in widely separated areas of the brain during the performance of word-related tasks. In the present study, we have utilized event-related electrical potentials (ERPs) to investigate the temporal relationships among cortical areas previously identified by PET to be differentially activated when performing a task involving generating the uses of visually presented nouns versus reading aloud. ERPs showed strong task-related differences over left and middle inferior frontal and left parietotemporal regions. Frontal and left parietotemporal channels revealed these differences around 200 and 700 msec, respectively, after word presentation. These results provide the time course for parts of the anatomical circuit involved in generating the meaning of a word. Our results also demonstrate how combining the spatial localization of PET with the temporal resolution of ERPs greatly enhances the capacity to understand the mechanisms involved in human cognition.

Neuronal correlate of the higher-order semantic code in human prefrontal cortex in language tasks.

Recently, Posner et al. (Science, 1988, 240: 1627-1631) have shown by means of positron-emission tomography (PET) that the highest activation related to the higher-order semantic encoding in language tasks is revealed in the anterior inferior region of the left prefrontal cortex. We had a case where we explored the neuronal activity in this cerebral area in a patient with diagnostic intracerebral electrodes. Neurons of the left cortical areas 10 and 46 exhibited responses related to the processing of semantic and grammatic signs of the presented phrases. Specific responses in neuronal activity of the same cells in other higher-order language tasks, in discrimination of concrete and abstract words, and the absence of significant responses in other relatively simpler language tasks, such as object naming and lexical decision, supports the PET finding on the selective participation of this cerebral area in the higher-order semantic encoding and discovers new neuronal mechanisms of this encoding.

How to select epochs of cognitive event-related potentials for brain mapping?

Event-related potentials were recorded in healthy volunteers while they were performing cognitive tasks. The new parameter developed for evaluation of how ERPs from different electrodes are not like each other allowed to study the mental chronometry as well as the localization of specific cortical areas. Occipital lobes were most active in passive visual word perception, the left anterior frontal lobe was activated in semantic association. The developed approach allows the combination of imaging and chronometric studies.

Effect of phenytoin on the human neuronal activity and depth event-related potential in emotional task.

Neuronal activity and depth ERP were recorded from cortical areas and subcortical nuclei of three patients with diagnostic and/or therapeutic intracerebral electrodes while they were performing emotional task before and after phenytoin administration. Phenytoin acted as a bioelectrical modulator, in some cases increasing and in some cases decreasing the mean background neuronal firing level. In some few cases the more specific task-related effect of phenytoin was found as an inversion of the inhibitory response into the excitatory one in trials evoking negative emotions.

Properties of neuronal activity in cortex and subcortical nuclei of the human brain during single-word processing.

Neuronal impulse activity (NIA) in different cortical areas, subcortical nuclei of the thalamus and striopallidum was recorded via depth electrodes in human patients as they performed a visual word recognition task. The properties of neuronal responsiveness were compared across regions. During single-word processing approximately 75-80% of the responsive cortical neurons were characterized by poststimulus inhibition of firing rate; an increased firing rate (i.e., excitatory response) was observed in about 20-25% of the responsive cortical neurons. Among the subcortical nuclei, mainly in the thalamus and striopallidum, more than 90% of the responsive neurons were characterized by poststimulus excitation; less than 10% exhibited poststimulus inhibition of firing rate.

Neuronal activity in frontal speech area 44 of the human cerebral cortex during word recognition.

The activity of neurons was recorded from cortical speech area 44 of human patients with diagnostic and/or therapeutical depth electrodes while they were performing word recognition and object naming tasks. Some cells responded selectively to either semantic or phonological aspects of word processing with the same latency, others responded consecutively to both semantic and phonological features. The present finding provides the first demonstration of neuronal mechanisms in frontal language cortex and evidences that both consecutive and parallel processing of these speech signs occur in neuronal activity of this area.

Neural correlate of mental error detection in the human brain cortex.

The use of long-term intracerebral electrodes combined with advances in stereotaxic neurosurgery and a complex multimethodological approach to brain research has made possible a study of the neurophysiological mechanisms of higher mental functions in the course of diagnosis and/or treatment of some chronic cerebral diseases. Investigations in which both local oxygen tension and neuronal impulse activity were recorded while the patients were performing psychological tasks have revealed selective responses in some neuronal populations of the thalamus and of the striopallidal complex, which occur only during erroneous task performance. So-called 'error detector' neurons were also found in the perirolandic and parietotemporal regions of the human brain cortex.

Cerebral lateralization for language revealed in neuronal responses of the human brain cortex.

Neuronal impulse activity (NIA) of the cortex and subcortical nuclei in the thalamus and striopallidar complex was studied during performance of the word recognition test in patients with long-term intracerebral electrodes implanted for diagnostics and therapy. In NIA of the left hemisphere cortex, endogenous responses were found to prevail in the interval between expositions of two stimuli, whereas NIA of the right cortex exhibited more responses during perception of stimuli. Asymmetry in neuronal responses of subcortical nuclei in the thalamus and striopallidar complex appeared to differ from that observed in the cerebral cortex.

Visual properties of neurons in the thalamus and striopallidar complex of the human brain.

In parkinsonian patients diagnosed and treated with the aid of long-term intracerebral electrodes, neuronal impulse activity (NIA) was recorded in different nuclei of the thalamus and striopallidar complex during visual testing for orientation sensitivity. Neuronal populations responding only to the presented stimuli with specific angular orientations have been revealed. Orientation-selective neurons have been found with significant responses in NIA observed to the same angular orientations of stimuli during 90 degrees head tilts. It is hypothesized that the visual perception constancy can be maintained with the participation of cerebral nuclei containing such neurons on the basis of convergence of motor, somatosensory and visual information.

Psychophysiological micromapping of the human brain.

Neuronal impulse activity (NIA) of cortex and subcortical nuclei of the brain was studied in patients diagnosed and treated with the aid of long-term intracerebral electrodes. A special battery of psychophysiological tests was constructed for study of reflexion of different mental functions in NIA, such as various speech functions, short-term and long-term memory, count and arithmetic operations, emotions. Neuronal populations with the significant responses have been revealed during these testings. First results of psychophysiological mapping of the human brain obtained from NIA data have been presented.