Event-related potentials and the phonological processing of words and non-words

Event-related potentials (ERPs) were recorded during a task which required subjects to discriminate between rhyming and non-rhyming visually presented pairs of letter strings, consisting of equal proportions of word-word and word-non-word combinations. Pair members were presented sequentially with an interstimulus interval of 1.56 sec. As in a previous study [Rugg, M. D., Brain Lang. In press], ERPs elicited by rhyming and non-rhyming words were differentiated by a late negative component (N450) in waveforms following the non-rhyming words. This effect was greatest over the midline and the right hemisphere. The same rhyme/non-rhyme difference was also observed, to an equal extent, in ERPs elicited by non-words. It is concluded that N450, presumed to be related to the "N400" component observed under conditions of semantic incongruity [Kutas, M. and Hillyard, S.A., Science 207, 203-205, 1980] does not seem to depend on linguistic processing at the semantic level for its modulation.

Visual evoked potentials to lateralized visual stimuli and the measurement of interhemispheric transmission time

Visual evoked potentials (VEPs) to lateralized light flashes were recorded from the parietal midline, and from homologous occipital and central sites, in a GO/ NOGO reaction time task. The N160 component of the VEP was found to be larger over the hemisphere contralateral to the visual field of stimulus exposure at all pairs of lateral electrodes. At the occipital sites only, N160 latency was also shorter from the contralateral hemisphere, by an average of approximately 14 msec. This was not so centrally, where a non-significant value of approximately 4 msec was obtained. These data are considered to be consistent with Milner and Lines' hypothesis that callosal transmission occurs at different rates in different functional regions of the corpus callosum.

Further study of the electrophysiological correlates of lexical decision

Discriminations were required between words, pseudohomophones, and visually matched nonwords. Two tasks were employed, one which could be accomplished on the basis of a visual code (the REAL task, involving discrimination between words and both types of nonword) and another necessitating the use of a phonological code (the REAL/PSEUD task, words and pseudohomophones vs. non-words). ERPs were recorded from three midline sites and from left and right inferior parietal sites. Two principal results were observed, (i) the peak latency of a late positive component, P637, covaried with RT, with variations in latency of around one half the corresponding RT variations, and (ii) the peak-to-peak amplitude of N100-P187 interacted with stimulus and task, such that it was larger for nonwords in the REAL task and words in the REAL/PSEUD task. No task- or stimulus-dependent asymmetries were observed in any ERP component. The P637 latency data support a model of RT variation based on the interaction of changes in parallel response preparation and stimulus evaluation processes. The observations with respect to N100-P187 suggest that ERPs are sensitive to factors related to the early processing of words and word-like visual material.

Dissociation of alpha and theta activity as a function of verbal and visuospatial tasks

EEG was recorded from P3 and P4 in 12 male subjects during a rest condition and while they performed a verbal and a visuospatial task. Power in the alpha and theta bands was derived from EEG epochs associated with task-related cognitive activity uncontaminated by response activity. Alpha power was significantly lower during performance on both tasks compared to the rest condition, but did not differ between tasks or hemispheres. Conversely, theta power was significantly higher during task performance, and, in the right hemisphere only, was higher in epochs recorded during the visuospatial as opposed to the verbal task. This difference between the tasks in right hemisphere theta correlated significantly with level of visuospatial performance as, to a lesser extent, did differences in left hemisphere theta. These results are discussed in terms of the sensitivity of different regions of the EEG spectrum to cognitive activity underlying performance on different types of task.

EEG correlates of the acquisition of high- and low-imagery words

EEG was recorded from left and right temporal areas while subjects learnt high- and low-imagery words. Asymmetry in power in the alpha band, in the form of less power from the left hemisphere, was observed only during the learning of high-imagery words. This condition also gave rise bilaterally to more theta power. The difference between conditions in levels of inter-hemispheric coherence predicted, in both frequency bands, relative levels of recall of the two word types. It is suggested that the alpha and theta bands of the EEG may reflect different aspects of information processing in these tasks.

The specificity of intrahemispheric EEG alpha coherence asymmetry related to psychological task

Male right handed subjects performed two parallel verbal and nonverbal memory tasks, known to be associated with the anterior temporal lobe, while EEG was recorded from temporal sites referred to either FZ or to linked mastoids. Intrahemispheric coherence and power in the alpha band were calculated. Coherence effects were observed, independent of reference site, which associated the right hemisphere and nonverbal tasks with increased coherence. These effects appear to be relatively specific to more anterior temporal electrode pairs, and confirm previous findings. Power effects demonstrated the difficulty of assessing task-dependent changes independent of reference site, and suggested that specific alpha enhancement may be observed with task involvement. The contribution of coherence analysis to the study of cerebral organisation is discussed.

Late positive component correlates of verbal and visuospatial processing

Bilateral VERs were elicited by letter stimuli requiring verbal processing and patterns requiring visuospatial processing in a go/no-go reaction time paradigm. The VERS were formed separately to the targets and non-targets of each stimulus type. No VER component displayed hemisphere asymmetries which varied in a task-dependent fashion. However P150 latency was found to be shorter, and N150 amplitude larger, in VERs recorded from the right hemisphere. This is considered to reflect the superiority of the right hemisphere for the early stages of the processing of visual input. P400 latency was shorter and its amplitude larger to the pattern stimuli. In addition, subjects' reaction times were shorter to these stimuli. These results were interpreted in evidence supporting the notion that the late positive component of the ER reflects processes of stimulus evaluation which are not necessarily correlated with those of response selection and execution.

Asymmetry in EEG alpha coherence and power: effects of task and sex

Male and female subjects performed two spatial and two verbal tasks while EEG was recorded from right and left temporal and parietal sites. Measures of alpha band power asymmetry, coherence and phase angle were derived. Spatial tasks produced increased coherence within the right hemisphere. Interhemispheric coherence was higher for females, and at parietal than at temporal sites. Power asymmetry effects confirmed previous findings, reflected upon sex differences in cerebral lateralisation and upon strategies associated with functional cerebral organisation for task performance. The relation to performance studies and the value of the method in investigating dynamic functional organisation are discussed.

Interhemispheric asymmetries in the visual evoked response: effects of stimulus lateralisation and task

VER's were elicited by briefly presented lateralised letters in two experiments. In experiment 1 the subjects engaged in verbal processing of the letters and in experiment 2 they engaged in visuospatial processing. In both experiments the latencies of the first two components of the VER's were consistent with previous findings and with the anatomy of the retino-cortical visual pathways. The pattern of hemisphere asymmetries of the amplitudes of the middle and late components differed in the two experiments in that (i) in the second experiment the amplitude of the middle component from the left hemisphere did not vary with field of stimulus presentation although this was the case in the first experiment and (ii) a hemisphere asymmetry (left greater than right) was observed in the late component only in the second experiment. The results are interpreted as evidence of difference in the cerebral processing of the stimuli in respect of the task demands of the two experiments.