Posterior Left Hemisphere Research Articles

The N2pc component as an indicator of attentional selectivity

Event-related potentials (ERPs) were recorded during visual discrimination tasks in which stimulus arrays were presented that contained one lateral target and 3 (experiment 1) or one (experiments 2 and 3) non-targets. In experiments 1 and 2, targets differed from non-targets with respect to their form or their color. In experiment 3, word pairs were presented, with targets differing from non-targets with respect to their content. Subjects were required to respond to the identity of the target. In all experiments, and enhanced negativity was elicited at posterior electrodes contralateral to the location of the target. In the form discrimination tasks, this effect was present in the N1, N2, and P3 time intervals. In the color discrimination tasks, it was confined to the N2 time range. In the word discrimination task (experiment 3), this effect could only be observed over the left posterior hemisphere. It is argued that these lateralized negativities reflect the N2pc component that is assumed to indicate attentional filtering processes during visual search tasks. The present results extend this assumption by showing that this component is also elicited when targets are presented together with just one non-target item. It is argued that the N2pc may reflect the attentional selection of task-relevant stimuli.

The N2pc component as an indicator of attentional selectivity

Event-related potentials (ERPs) were recorded during visual discrimination tasks in which stimulus arrays were presented that contained one lateral target and 3 (experiment 1) or one (experiments 2 and 3) non-targets. In experiments 1 and 2, targets differed from non-targets with respect to their form or their color. In experiment 3, word pairs were presented, with targets differing from non-targets with respect to their content. Subjects were required to respond to the identity of the target. In all experiments, and enhanced negativity was elicited at posterior electrodes contralateral to the location of the target. In the form discrimination tasks, this effect was present in the N1, N2, and P3 time intervals. In the color discrimination tasks, it was confined to the N2 time range. In the word discrimination task (experiment 3), this effect could only be observed over the left posterior hemisphere. It is argued that these lateralized negativities reflect the N2pc component that is assumed to indicate attentional filtering processes during visual search tasks. The present results extend this assumption by showing that this component is also elicited when targets are presented together with just one non-target item. It is argued that the N2pc may reflect the attentional selection of task-relevant stimuli.

Elevated androgen, brain development and language/learning disabilities in children with congenital adrenal hyperplasia.

Individuals with congenital adrenal hyperplasia (CAH) provide a test population for the theory that elevated testosterone levels alter prenatal brain development and increase the risk of learning disabilities. Eleven subjects with CAH, five of their non-CAH siblings and 16 matched control subjects participated in two studies. The first study documented hand preference, verbal skills and non-verbal skills. A higher prevalence of language/learning disability was found in both the CAH subjects and their families than in the control subjects. The second study examined the prevalence of atypical perisylvian asymmetries on MRI scans. These revealed an atypical pattern of asymmetry (R = L or R > L) in the majority of the subjects with CAH and in all of their siblings. One subject with CAH also showed evidence of a neuromigratory disturbance in the posterior left hemisphere. Of the control subjects, only one showed an atypical pattern of asymmetry and none showed evidence of a neuromigratory disorder. The findings indicate that an elevated familial rate for language-based learning disabilities and altered brain asymmetries co-occur in families with the gene for CAH.

Evidence for Development of Distinction of Voice Onset Time in a Child with Left-Hemisphere Lesion

Following surgery for partial removal of the posterior left hemisphere at 5 mo., voice onset time was assessed to 9; 11 yr. Left-hemisphere language function associated with voicing appeared subsumed by the right hemisphere.

The possibility of organic left posterior hemisphere dysfunction as a contributing factor in sex-offending behavior

Abstract Twenty-four individuals who were referred to inpatient psychiatric treatment for sex-offending behavior or professional sexual misconduct were tested with quantitative electroencephalogram (QEEG) to determine if there is support for the hypothesis that there is underlying neuro-organicity contributing to sexually offending behaviors. All subjects participated in a program ranging from six weeks to more than one year. Insurance coverage, personal resources, and other payment methods to cover the cost of the test were the only criteria used in determining which patient received the QEEG. The subjects covered all socioeconomic, education, and IQ levels. Significant abnormalities were found in all the QEEGs. In every case, some type of abnormality was noted in the left posterior hemisphere, compared with a normal control population. These findings are consistent with an hypothesis that sex-offending behavior results from misperception of an inappropriate sex object as appropriate by structures in the...

Special talents in Geschwind's and Galaburda's theory of cerebral lateralization: an examination in a female population.

Geschwind and Galaburda (1985) proposed that delayed growth of posterior left hemisphere sites in male dyslexics are associated with markers of anomalous dominance (AD) and special cognitive talents. We found that talent in a female population was related to the strength of nonright-handedness, and was elevated in right-handed individuals with histories of familial left-handedness. Subjects reporting a history of dyslexia tended to be musically talented. While immune disorders were not related to talent for the whole population, they did co-occur with talents in a group of 10 individuals reporting 7 or more AD markers.

The generation of visual images: a review of neuropsychological research and theory.

Since the advent of the view that mental imagery is a subdividable process, claims have been made that the generation of images is the domain of the left posterior hemisphere. This article examines theory and data about the neuropsychology of image generation by focusing on the main contributors to the debate. An attempt is made to make the theory and beliefs associated with each of the key figures explicit, because these have influenced the structure of research and the development of new theories. Support is found for the involvement of the left hemisphere, although many researchers claim that the posterior regions of both hemispheres contribute to image generation but that they do so in different ways. The nature of this difference remains the subject of ongoing research.

EEG alpha asymmetries in stutterers and non-stutterers: Effects of linguistic variables on hemispheric processing and fluency

The EEG hemispheric alpha asymmetry technique was used to gather data from anterior and posterior language areas during a resting condition and in a sentence repetition task. Subjects were nine adult stuttering males and nine adult fluent male controls. Sentences were controlled for imagery, syntactic complexity, and rate. Significant within- and between-groups differences were found for both resting and testing conditions. Posteriorly, stutterers showed no differences between resting and testing conditions, while controls showed increased left hemisphere activation. Of the linguistic variables investigated, only imagery was involved in a significant interaction. Fluent males showed greater posterior left hemispheric activation for high and low visual imagery sentences, compared to greater posterior right hemispheric activation in stuttering males. Significantly greater alpha power was found for low vs high imagery sentences in the anterior and posterior left hemisphere sites for both subject groups, and in the right hemisphere sites for the stuttering group only. Fluency data is presented. Differences between groups in alpha power changes from resting to testing conditions are discussed.

Neuropsychological and environmental characteristics of early readers

The neuropsychological functioning and home environment of early readers was investigated. To 21 early readers attending preschools matched for age and intelligence with 21 controls (nonreaders), a neuropsychological test battery was administered. Home environment was assessed by administering a brief questionnaire to parents. Although it had been hypothesized that early readers would be superior to controls on tests believed to be sensitive to left posterior hemisphere functioning, the early readers in this study demonstrated superior functioning only on the left frontal measure and on right hemisphere measures. The only environmental factor that significantly differentiated the two groups was that the early readers had been read to more frequently than the control group children. The implications of these findings and their possible educational significance are discussed.

Anatomy of posterior pathways in reading: A reassessment

Contemporary accounts of the neurology of reading stem from Dejerine's original visual-verbal disconnection hypothesis of pure alexia. Reassessment of Dejerine's traditional formulations for posterior left hemisphere pathways in reading suggests that the occipital cortex-left angular gyrus-Wernicke's area scheme is undoubtedly oversimplified. The role of left angular gyrus cortex in reading is unsettled, and although clinically undefined, more inferior portions of the left temporal lobe may also contribute to the reading process. Nevertheless, to a surprising extent, the neuroanatomic edifice erected by Dejerine remains largely intact.

Gustave Flaubert's illness: a case report in evidence against the erroneous notion of psychogenic epilepsy.

The concept of psychogenic epilepsy was recently revived by Sartre (L'Idiot de la Famille, 1971-2) in relation to Gustave Flaubert's epilepsy, which he believed was secondary to hysterical neurosis and not due to a cerebral lesion. A detailed clinical description of Gustave Flaubert's personal and medical history from birth to death is provided. The relationships between his epileptic seizures and both his personal life and the response of others to the attacks are discussed, as is the interaction between seizure occurrence and his literary work and productivity. The various diagnoses made by medical and nonmedical persons during and after Flaubert's lifetime are then reviewed. Particular emphasis is given to Sartre's purely psychogenic interpretation of the seizures, to his total disinterest in their medical aspects, and to the fact that he did not obtain any medical opinions. It is shown that a definitive diagnosis can be made for Flaubert's illness--that of epilepsy associated with a quite normal psychosocial response by current standards. Moreover, the clinical nature of the seizures and other clinical details permit the more specific diagnosis of "complex partial epilepsy of occipital-temporal origin, secondary to lesion of the left posterior hemisphere with occasional secondary generalization of seizures." The concept of the existence of "psychogenic epilepsy" is reaffirmed as erroneous, despite the occasional precipitation of seizures in some individuals by psychological factors. Finally, Gustave Flaubert's case, like Dostoevski's, points to the possible coexistence of a serious and poorly controlled form of epilepsy with true literary genius without the latter being dependent in any way on the former.

Varieties of perseveration

We propose a new taxonomy for perseveration consisting of three categories which are distinctive at the levels of clinical features, process, and neuroanatomy. Stuck-in-set perseveration, the inappripriate maintenance of a current category or framework, involves an underlying process deficit in executive functioning and is related neuroanatomically to frontal lobe damage. Recurrent perseveration, the unintentional repetition of a previous response to a subsequent stimulus, involves an abnormal post-facilitation of memory traces and is related neuroanatomically to posterior left hemisphere damage. Continuous perseveration is the inappropriate prolongation or repetition of a behavior without interruption. It involves a deficit in motor output and is most common in patients with damage to the basal ganglia.

Word and letter reading and the mechanism of the third alexia.

Four patients, two with global aphasia and two with Broca's aphasia, demonstrated the seemingly paradoxical ability to read words but not their component letters. Picture naming was only moderately impaired, and repetition of word and letter names was intact, excluding both a generalized dysnomia and an articulatory disturbance as the cause of the literal alexia. Matching tests revealed processing deficits in three of the patients, more severe for letters than for words. Oral reading of word lists showed that short, high-frequency, and picturable words were read best, whereas nonsense trigrams, which require phonetic processing, were the most difficult. The residual reading of patients with severe Broca's or global aphasia and the "third alexia" appears to involve purely visual, nonphonetic mechanisms for word recognition, using posterior left hemisphere or even minor hemisphere centers. Letter reading, by contrast, along with phonetic reading of syllables, appears to be a more specialized, anterior left hemisphere process.

Lateralized cortical potentials evoked in humans by dynamic random-dot stereograms

Dynamic random-dot stereograms of 72 × 71 dots array size forming a horizontal rectangle of 14.5° × 9° were computer-generated on-line at 100 frames/sec with special display hardware. The stimulus, a vertical rectangular area of 4.6 × 5.7° within the array with binocular disparity different from its surround, was perceived in depth when binocularly viewed left (or right) to a center fixation point for 512 msec at periods of 1024 msec. Monocularly, only “snowstorm” could be perceived. Averaged visual evoked responses (VER) were obtained from eight subjects in 2-channel recordings, and from one subject in 37-channel recordings of the VER scalp field distributions. Presentation of the disparity area to the left hemiretinas evoked an average EEG response up to 280msec latency over the left posterior hemisphere (ipsilateral to the stimulated hemiretinas); simultaneously, the right hemisphere showed a smaller evoked potential of similar waveform. Stimulation of the right hemiretinas yielded opposite localization, i.e. mirrored results. The findings indicate the presence of a major generator of the evoked potential in the input-receiving hemisphere up to 280msec after the stimulus onset. Thus, without stimulus-synchronous activation of retina and LGB units, the responses of pools of cortical binocular disparity detectors (cyclopean retina) are measurable on the human scalp, and stereopsis is not associated with preferential activity of the right hemisphere.