Reaction Time in a Visual 4-Choice Reaction Time Task: ERP Effects of Motor Preparation and Hemispheric Involvement

Reaction time (RT), the most common measure of CNS efficiency, shows intra- and inter-individual variability. This may be accounted for by hemispheric specialization, individual neuroanatomy, and transient functional fluctuations between trials. To explore RT on these three levels, ERPs were measured in a visual 4-choice RT task with lateralized stimuli (left lateral, left middle, right middle, and right lateral) in 28 healthy right-handed subjects. We analyzed behavioral data, ERP microstates (MS), N1 and P3 components, and trial-by-trial variance. Across subjects, the N1 component was contralateral to the stimulation side. N1-MSs were stronger over the left hemisphere, and middle stimulation evoked stronger activation than lateral stimulation in both hemispheres. The P3 was larger for the right visual field stimulation. RTs were shorter for the right visual hemifield stimulation/right hand responses. Within subjects, covariance analysis of single trial ERPs with RTs showed consistent lateralized predictors of RT over the motor cortex (MC) in the 112–248 ms interval. Decreased RTs were related to negativity over the MC contralateral to the stimulation side, an effect that could be interpreted as the lateralized readiness potential (LRP), and which was strongest for right side stimulation. The covariance analysis linking individual mean RTs and individual mean ERPs showed a frontal negativity and an occipital positivity correlating with decreased RTs in the 212–232 ms interval. We concluded that a particular RT is a composite measure that depends on the appropriateness of the motor preparation to a particular response and on stimulus lateralization that selectively involves a particular hemisphere.     

Lateralization patterns of event-related potential and performance indices in schizophrenia: relationship to clinical state and neuroleptic treatment

In a previous study we assessed lateralization patterns of verbal stimuli processing, by means of behavioural and neurophysiological measures, in a sample of drug-free schizophrenics and one of normal controls. The main findings obtained were the following: (1) a right visual field (RVF) advantage on reaction time (RT) and late positive complex (LPC) peak of the ERPs in normal subjects but not in schizophrenics; (2) a left visual field (LVF) significant advantage on P360 and slow wave (SW) amplitude in schizophrenics but not in controls; (3) a significantly longer RT and smaller P360 and SW for RVF stimuli in schizophrenics as compared to normals; (4) a significant contralateral effect of visual field on N180 at both the left and the right parietal site in normal controls and only at the right parietal site in schizophrenics. As a further step of this investigation we re-tested 9 schizophrenics after 28 days of haloperidol treatment. The post-treatment lateralization pattern of verbal stimuli processing was characterized by a RVF advantage on LPC peak amplitude and no visual field effect on P360 and SW, resembling the normal group pattern. Moreover, the N180 amplitude was found to be reduced. Relationships between lateralization pattern and clinical picture changes induced by haloperidol treatment are discussed.     

Modulating lexical and semantic processing by transcranial direct current stimulation

Here we aim to evaluate the ability of transcranial direct current stimulation (tDCS), which is applied over Wernicke’s area and its right homologue, to influence lexical decisions and semantic priming and establish an involvement for temporo-parietal areas in lexical and semantic processing. Thirty-two subjects (17 women) completed a lexical decision task and a semantic priming task while receiving 20 min of bilateral tDCS stimulation (right anodal/left cathodal or left anodal/right cathodal stimulation) or sham stimulation. We hypothesized that right anodal/left cathodal stimulation over temporo-parietal areas would selectively interrupt the typical lexical processing dominance of the left hemisphere and facilitate mediated priming, while left anodal/right cathodal stimulation would selectively facilitate lexical processing and direct priming. Results showed impaired lexical processing under right anodal/left cathodal stimulation in comparison with sham and left anodal/right cathodal stimulation. Results are discussed in light of previous findings and hemispheric lateralization models.     

Stages of information processing and cerebral hemispheric differences in random shape recognition

The purpose of this experiment was to investigate the differences of cerebral hemisphere function with respect to the way visual pattern information is stored in the long-term memory. The shape recognition in the left and right visual fields was measured after the subjects were trained to associate the shapes with relevant verbal labels, irrelevant verbal labels, or no verbal labels. Subjects were given a shape recognition test by the visual half-field presentation and both free- and aided-recall tests for retention of associated verbal labels. Results indicated that accuracy of recognition of shapes in the left visual field was greater than in the right visual field. Pretraining condition produced a significant main effect but no differential field effect. While irrelevant verbal labeling and observation conditions recognized the stimulus with equal reaction time in either visual field, the latency of relevant verbal labeling condition was longer in right visual field than in left visual field. These results were discussed in terms of the differences of cerebral hemisphere function in each stage of information processing.     

Lateralization of repetition effects in event-related potentials to words in left- and right-handed women

In our previous event-related potential (ERP) study [A. Nowicka, I. Szatkowska, Memory-induced modulation of event-related potentials in frontal cortex of human subjects: a divided visual field study, Neurosci. Lett. 359 (2004) 171-174], word repetition effects in right-handed males were observed only in case of the direct stimulation of the left (competent) hemisphere. Since the left-handedness and the factor of female gender may determine the lateralization of verbal functions, the goal of the present study was to test the sensitivity of the ERP repetition effects to the visual field of word presentation in the group of left- and right-handed women. ERPs were recorded from symmetrical sites over the left and right hemisphere. Target words were presented in the left (LVF) or right (RVF) visual hemifield. Subjects' were instructed to recognize the target word on a response card. A substantial portion of words was repeated twice. Words elicited different ERPs at frontal sites when presented the first and second times: ERPs were more positive to the repeated stimulus than to its first occurrence. However, in right-handed participants, repetition effects were only observed when target words were presented to the RVF whereas in left-handed participants, repetition effects were only observed when target words were presented to the LVF. These findings are indicative of the asymmetrical involvement of the two hemispheres in memory-induced modulation of brain activity related to verbal processing in left- and right-handed women.     

Interference asymmetry involving concurrent tasks performed by native American students

It is assumed that Native Americans are right‐hemisphere dominant and therefore need special teaching techniques. This study examined the language and spatial lateralization of monolingual English‐speaking Native American students by means of the cognitive‐manual dual‐task model. The results revealed the following asymmetric interference: greater right‐hand disruption while reading, greater left‐hand disruption while solving spatial problems, fewer words read while tapping with the right hand, and fewer spatial problems solved while tapping with the left hand. However, the “good” Native American students had normal (left‐hemisphere language/right‐hemisphere spatial) lateralization, whereas the “poor” students had atypical lateralization for righthanders, primarily involving both left‐hemisphere language and left‐hemisphere spatial function. The neurological consequence of crowding language and spatial processing into the same hemisphere is discussed. Two major conclusions are that the Native American students of this study were not right‐hemisphere dominant, and that nonnormal language/special lateralization resulted in learning problems, possibly suggesting a new subclassification within the special education/handicapping condition of learning disabilities.     

Recognition of faces expressing emotions in patients with unilateral brain damage.

The experiment concerned the functional specialization of brain hemispheres in recognizing faces expressing emotions in patients with focal brain damage of the left and right hemispheres and in persons without damage of the central nervous system. Three types of faces were presented: happy i.e. expressing positive emotions, sad i.e. expressing negative emotions and neutral. The task of the subject was to recognize the test face exposed for 20 ms in the left or right visual field and point to the correct face on the response-card which contained three different faces. The errors made in recognizing of faces exposed in the right visual field (addressing the left hemisphere) and in the left one (addressing the right hemisphere) were analyzed. The patients with left hemisphere damage showed a similar pattern of hemispheric differences to that observed in the controls. In case of neutral and sad faces fewer errors were made in the left visual field exposures, in case of happy faces no significant differences between the two fields were stated. The results in patients with right hemispheric damage were different from both the other groups. In processing all the three types of faces, fewer errors were made in the left visual field presentations independently of the localization of the damage within the right hemisphere. Similar recognition level of different types of faces was also found in patients with left or right hemisphere damage as well as in controls. The above results suggest the crucial role of the right hemisphere in processing both positive and negative emotions expressed in faces.     

Hemispheric Asymmetry for Word Perception: Behavioral and ERP Evidence

Reaction times (RTs) to words or patterns presented in the left or right visual fields and averaged Event-Related Potentials (ERPs) evoked by the same stimuli presented foveally were measured separately. In both experiments, an oddball paradigm known to elicit the P3 component was used. Twenty-four subjects (12 males) were tested in each experiment. In the electrophysiological study EEG was measured differentially from five different scalp sites, each referred to linked ear lobes. There were two frontal and two parietal lateralized electrodes, and a fifth which was located at the vertex (C_z). The RTs to words were significantly delayed relative to the RTs to patterns. Furthermore, the responses were significantly shorter in the right visual field when the target was a word, yet only slightly shorter in the left visual field when the target was a nonsense pattern. The same pattern of results was found with the evoked potentials: A positive wave evoked by the target stimulus was measured at vertex with mean latency of 511.3 msec in the verbal task, and 464.4 msec in the nonverbal task. At the lateralized locations, a significant Task by Hemisphere interaction was found. When the target was a word the base to peak P3 amplitudes were higher over the left than over the right hemisphere. A reverse trend was found with nonverbal targets. The base to peak P3 amplitudes were slightly (but not significantly) higher over the right than over the left hemisphere.     

The left visual-field advantage in rapid visual presentation is amplified rather than reduced by posterior-parietal rTMS

In the present task, series of visual stimuli are rapidly presented left and right, containing two target stimuli, T1 and T2. In previous studies, T2 was better identified in the left than in the right visual field. This advantage of the left visual field might reflect dominance exerted by the right over the left hemisphere. If so, then repetitive transcranial magnetic stimulation (rTMS) to the right parietal cortex might release the left hemisphere from right-hemispheric control, thereby improving T2 identification in the right visual field. Alternatively or additionally, the asymmetry in T2 identification might reflect capacity limitations of the left hemisphere, which might be aggravated by rTMS to the left parietal cortex. Therefore, rTMS pulses were applied during each trial, beginning simultaneously with T1 presentation. rTMS was directed either to P4 or to P3 (right or left parietal cortex) either as effective or as sham stimulation. In two experiments, either one of these two factors, hemisphere and effectiveness of rTMS, was varied within or between participants. Again, T2 was much better identified in the left than in the right visual field. This advantage of the left visual field was indeed modified by rTMS, being further increased by rTMS to the left hemisphere rather than being reduced by rTMS to the right. It may be concluded that superiority of the right hemisphere in this task implies that this hemisphere is less irritable by external interference than the left hemisphere.     

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.     

Asymmetrical hemispheric EEG activation evoked by stimulus position during the Simon task

The Simon effect has been previously shown to be asymmetric at both the behavioral and electrophysiological levels. The present investigation was aimed to clarify whether, during a Simon task, hemispheric asymmetry is also observed in the early phases of stimulus processing. In a group of healthy subjects performing the Simon task, we analyzed scalp potentials evoked by the first lateralized cue (left or right), instead of the classical readiness potential preceding the motor response. ERP results showed a significant left cortical activation to stimuli presented in the right visual field at the 140–160 ms time window. Instead, left stimuli elicited a significant activation of the right versus left hemisphere starting at the next 160–180 ms time interval. We linked this asymmetry to that observed in behavioral data: the Simon effect recorded with left stimuli is smaller than the Simon effect recorded with right stimuli. Results confirm the hypothesis that in right handed subjects, left hemisphere is specialized for motor response selection and is able to process right stimuli faster than the right hemisphere does for left stimuli.     

Reduced interhemispheric transmission in schizophrenia patients: evidence from event-related potentials

Interhemispheric transfer was investigated in 14 schizophrenia patients and 15 age- and sex-matched healthy controls in a lateralized lexical decision task. Words and pseudowords were tachistoscopically presented either to the left or to the right visual hemifield. Event-related potentials were determined from a 65-channel electroencephalogram. Information transfer between hemispheres was assessed by the interhemispheric transmission time (IHTT), the N1-latency difference between ipsilateral and contralateral hemisphere.Controls, but not schizophrenia patients showed significantly faster IHTT from the right to the left hemisphere for words, while IHTT from the left to the right hemisphere did not differ between groups and stimuli. These findings are interpreted in terms of a deficit in schizophrenia to transfer verbal information from the right to the left hemisphere via the corpus callosum.     

Hemispheric lateralization of somatosensory processing

Processing of both painful and nonpainful somatosensory information is generally thought to be subserved by brain regions predominantly contralateral to the stimulated body region. However, lesions to right, but not left, posterior parietal cortex have been reported to produce a unilateral tactile neglect syndrome, suggesting that components of somatosensory information are preferentially processed in the right half of the brain. To better characterize right hemispheric lateralization of somatosensory processing, H(2)(15)O positron emission tomography (PET) of cerebral blood flow was used to map brain activation produced by contact thermal stimulation of both the left and right arms of right-handed subjects. To allow direct assessment of the lateralization of activation, left- and right-sided stimuli were delivered during separate PET scans. Both innocuous (35 degrees C) and painful (49 degrees C) stimuli were employed to determine whether lateralized processing occurred in a manner related to perceived pain intensity. Subjects were also scanned during a nonstimulated rest condition to characterize activation that was not related to perceived pain intensity. Pain intensity-dependent and -independent changes in activation were identified in separate multiple regression analyses. Regardless of the side of stimulation, pain intensity--dependent activation was localized to contralateral regions of the primary somatosensory cortex, secondary somatosensory cortex, insular cortex, and bilateral regions of the cerebellum, putamen, thalamus, anterior cingulate cortex, and frontal operculum. No hemispheric lateralization of pain intensity-dependent processing was detected. In sharp contrast, portions of the thalamus, inferior parietal cortex (BA 40), dorsolateral prefrontal cortex (BA 9/46), and dorsal frontal cortex (BA 6) exhibited right lateralized activation during both innocuous and painful stimulation, regardless of the side of stimulation. Thus components of information arising from the body surface are processed, in part, by right lateralized systems analogous to those that process auditory and visual spatial information arising from extrapersonal space. Such right lateralized processing can account for the left somatosensory neglect arising from injury to brain regions within the right cerebral hemisphere.     

Pseudoneglect and embryonic light stimulation in the avian brain

A bias to allocate attention to the left hemispace, similar to the well-known pseudoneglect phenomenon shown by humans, has been recently reported in domestic chicks and other species of birds. Asymmetrical light exposure of the embryo of the domestic chick in a critical period before hatching is known to be responsible for a structural asymmetry in the visual ascending projections of the thalamofugal pathway and for lateralization of some visual behaviors. Thus the animal model provided by the chick makes possible investigation of the prenatal factors that may influence asymmetry in spatial attention. Here chicks coming from eggs exposed to light (light incubated, Li-chicks) and chicks incubated in darkness (dark incubated, Di-chicks) were tested in a task in which they were required to explore an area in front of them and to sample grains of food. The results showed that Li-chicks attended more to target stimuli located in the left hemispace, whereas no asymmetry was shown by Di-chicks. When grains of food were presented with small novel pebbles as distractors, both Li- and Di-chicks tended to allocate attention toward the left hemispace. When, however, chicks were tested after familiarization with pebbles, no bias was shown by either Li- and Di-chicks. Hence it seems that cerebral lateralization associated with right hemispheric involvement in response to novelty, interacts with the modulatory effect of asymmetric embryonic light stimulation on preferential allocation of spatial attention in the left hemispace (right hemisphere) and right eye (left hemisphere) control of visual discrimination during feeding.     

Event-related potentials suggest early interaction between syntax and semantics during on-line sentence comprehension

Event-related potentials (ERPs) were used to investigate interaction between syntactic parsing and semantic integration processes during a visual sentence comprehension task. The linguistic stimuli were Finnish five-word sentences containing morphosyntactic and/or semantic violations. Single morphosyntactic violations evoked left anterior negativity (LAN) and P600 components. Single semantic violations elicited a robust N400 effect over the left hemisphere. A later and weaker N400-like response was also observed in the right hemisphere, left-right hemispheric latency difference being 40 ms. Combined morphosyntactic and semantic violations elicited a P600 component and a negative ERP component within the latency range of the LAN and N400 components. Further analysis of these ERP effects provided evidence for early processual interaction between syntax and semantics during on-line sentence comprehension. The hemispheric distribution of the LAN and N400 components was taken to suggest lateralization of initial morphosyntactic parsing and semantic integration processes to the left hemisphere. In contrast, the later syntax-related P600 component was observed as being more pronounced over the posterior areas of the right hemisphere.     

Effects of rearranged vision on event-related lateralizations of the EEG during pointing

We used event-related lateralizations of the EEG (ERLs) and reversed vision to study visuomotor processing with conflicting proprioceptive and visual information during pointing. Reversed vision decreased arm-related lateralization, probably reflecting the simultaneous activity of left and right arm specific neurons: neurons in the hemisphere contralateral to the observed action were probably activated by visual feedback, neurons in the hemisphere contralateral to the response side by the somatomotor feedback. Lateralization related to the target in parietal cortex increased, indicating that visual to motor transformation in parietal cortex required additional time and resources with reversed vision. A short period of adaptation to an additional lateral displacement of the visual field increased arm-contralateral activity in parietal cortex during the movement. This is in agreement with the, which showed that adaptation to a lateral displacement of the visual field is reflected in increased parietal involvement during pointing.     

Differential activation of face memory encoding tasks in alcohol-dependent patients compared to healthy subjects: An fMRI study

It has been hypothesized that the right hemisphere of the brain is more sensitive to alcohol-related damage than the left hemisphere. The present study tested this hypothesis, using functional MRI to determine whether the pattern for right hemispheric activity is different for alcohol-dependent patients, compared to normal healthy individuals. Two different types of memory encoding tasks were performed separately: word and face encoding for both alcohol-dependent patients and normal healthy volunteers. The data for the normal volunteers indicate that the left prefrontal region is more active during word encoding, whereas the right parahippocampal region is more active during face encoding. The results for the patient data, however, demonstrated left lateralization in the prefrontal area during word encoding, while right lateralization in the parahippocampal region during face encoding was not observed. Therefore, alcoholism appears to have no influence on left hemispheric activity, since the activation pattern was similar to that observed for normal healthy persons. However, the absence of right hemispheric lateralization in alcohol-dependent patients is consistent with the hypothesis that the right hemisphere is more vulnerable to alcohol-related damage than the left hemisphere.     

An fMRI Investigation of the Cortical Network Underlying Detection and Categorization Abilities in Hemianopic Patients

The current study aims to investigate visual scene perception and its neuro-anatomical correlates for stimuli presented in the central visual field of patients with homonymous hemianopia, and thereby to assess the effect of a right or a left occipital lesion on brain reorganization. Fourteen healthy participants, three left brain damaged (LBD) patients with right homonymous hemianopia and five right brain damaged (RBD) patients with left homonymous hemianopia performed a visual detection task (i.e. “Is there an image on the screen?”) and a categorization task (i.e. “Is it an image of a highway or a city?”) during a block-designed functional magnetic resonance imaging recording session. Cerebral activity analyses of the posterior areas—the occipital lobe in particular—highlighted bi-hemispheric activation during the detection task but more lateralized, left occipital lobe activation during the categorization task in healthy participants. Conversely, in patients, the same network of activity was observed in both tasks. However, LBD patients showed a predominant activation in their right hemisphere (occipital lobe and posterior temporal areas) whereas RBD patients showed a more bilateral activation (in the occipital lobes). Overall, our preliminary findings suggest a specific pattern of cerebral activation depending on the task instruction in healthy participants and cerebral reorganization of the posterior areas following brain injury in hemianopic patients which could depend upon the side of the occipital lesion.     

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 differences in the cerebral processing of the stimuli in respect of the task demands of the two experiments.     

Investigation of functional hemispheric asymmetry of language in tinnitus sufferers

The authors tested functional hemispheric asymmetry through word dichotic listening and lateralized lexical decision tasks in tinnitus patients and controls stimulated by a continuous tinnitus-like noise to test the influence of a tinnitus-like external stimulation. A classic right-ear advantage was shown in the auditory task for all but right-ear tinnitus patients, who performed as equally badly when the stimuli were presented to the right and left ears. Concerning the visual task, all participants demonstrated the expected right visual field advantage for word stimuli. Moreover, those who submitted to external stimulation demonstrated normal asymmetric patterns. These data suggest a specific effect of tinnitus on central processing and provide evidence for a functional reorganization induced by this auditory phantom perception.