Lateral Occipital Lobe Research Articles

Brain structure anomalies in autism spectrum disorder—a meta‐analysis of VBM studies using anatomic likelihood estimation

Autism spectrum disorders (ASD) are pervasive developmental disorders with characteristic core symptoms such as impairments in social interaction, deviance in communication, repetitive and stereotyped behavior, and impaired motor skills. Anomalies of brain structure have repeatedly been hypothesized to play a major role in the etiopathogenesis of the disorder. Our objective was to perform unbiased meta-analysis on brain structure changes as reported in the current ASD literature. We thus conducted a comprehensive search for morphometric studies by Pubmed query and literature review. We used a revised version of the activation likelihood estimation (ALE) approach for coordinate-based meta-analysis of neuroimaging results. Probabilistic cytoarchitectonic maps were applied to compare the localization of the obtained significant effects to histological areas. Each of the significant ALE clusters was analyzed separately for age effects on gray and white matter density changes. We found six significant clusters of convergence indicating disturbances in the brain structure of ASD patients, including the lateral occipital lobe, the pericentral region, the medial temporal lobe, the basal ganglia, and proximate to the right parietal operculum. Our study provides the first quantitative summary of brain structure changes reported in literature on autism spectrum disorders. In contrast to the rather small sample sizes of the original studies, our meta-analysis encompasses data of 277 ASD patients and 303 healthy controls. This unbiased summary provided evidence for consistent structural abnormalities in spite of heterogeneous diagnostic criteria and voxel-based morphometry (VBM) methodology, but also hinted at a dependency of VBM findings on the age of the patients.

Developmental cortical thinning in fetal alcohol spectrum disorders

Regional cortical thickness was evaluated using CIVET processing of 3D T1-weighted images (i) to compare the variation in cortical thickness between 33 participants with fetal alcohol spectrum disorders (FASD) aged 6-30 years (mean age 12.3 years) versus 33 age/sex/hand-matched controls, and (ii) to examine developmental changes in cortical thickness with age from children to young adults in both groups. Significant cortical thinning was found in the participants with FASD in large areas of the bilateral middle frontal lobe, pre- and post- central areas, lateral and inferior temporal and occipital lobes compared to controls. No significant cortical thickness increases were observed for the FASD group. Cortical thinning with age in a linear model was observed in both groups, but the locations were different for each group. FASD participants showed thinning with age in the left middle frontal, bilateral precentral, bilateral precuneus and paracingulate, left inferior occipital and bilateral fusiform gyri; while controls showed decreases with age in the bilateral middle frontal gyrus, right inferior frontal gyrus, bilateral precuneus gyrus, and bilateral occipital gyrus. A battery of cognitive assessments of memory, attention, motor, and verbal abilities was conducted with many of the FASD participants, but no significant correlations were found between these cognitive scores and regional cortical thickness. Non-invasive measurements of cortical thickness in children to young adults with FASD have identified both key regions of cortex that may be more deleteriously affected by prenatal alcohol exposure as well as cortical changes with age that differ from normal developmental thinning.

Total and Regional Brain Volumes in a Population-Based Normative Sample from 4 to 18 Years: The NIH MRI Study of Normal Brain Development

Using a population-based sampling strategy, the National Institutes of Health (NIH) Magnetic Resonance Imaging Study of Normal Brain Development compiled a longitudinal normative reference database of neuroimaging and correlated clinical/behavioral data from a demographically representative sample of healthy children and adolescents aged newborn through early adulthood. The present paper reports brain volume data for 325 children, ages 4.5–18 years, from the first cross-sectional time point. Measures included volumes of whole-brain gray matter (GM) and white matter (WM), left and right lateral ventricles, frontal, temporal, parietal and occipital lobe GM and WM, subcortical GM (thalamus, caudate, putamen, and globus pallidus), cerebellum, and brainstem. Associations with cross-sectional age, sex, family income, parental education, and body mass index (BMI) were evaluated. Key observations are: 1) age-related decreases in lobar GM most prominent in parietal and occipital cortex; 2) age-related increases in lobar WM, greatest in occipital, followed by the temporal lobe; 3) age-related trajectories predominantly curvilinear in females, but linear in males; and 4) small systematic associations of brain tissue volumes with BMI but not with IQ, family income, or parental education. These findings constitute a normative reference on regional brain volumes in children and adolescents.

Task-dependent Activation of Face-sensitive Cortex: An fMRI Adaptation Study

Face processing in the human brain recruits a widespread cortical network based mainly in the ventral and lateral temporal and occipital lobes. However, the extent to which activity within this network is driven by different face properties versus being determined by the manner in which faces are processed (as determined by task requirements) remains unclear. We combined a functional magnetic resonance adaptation paradigm with three target detection tasks, where participants had to detect a specific identity, emotional expression, or direction of gaze, while the task-irrelevant face properties varied independently. Our analysis focused on differentiating the influence of task demands and the processing of stimulus changes within the neural network underlying face processing. Results indicated that the fusiform and inferior occipital gyrus do not respond as a function of stimulus change (such as identity), but rather their activity depends on the task demands. Specifically, we hypothesize that, whether the task encourages a configural- or a featural-processing strategy determines activation. Our results for the superior temporal sulcus were even more specific in that we only found greater responses to stimulus changes that may engage featural processing. These results contribute to our understanding of the functional anatomy of face processing and provide insights into possible compensatory mechanisms in face processing.

A locus for bilateral occipital polymicrogyria maps to chromosome 6q16–q22

We describe the clinical, radiographic, and genetic features of a large consanguineous Moroccan family in which bilateral occipital polymicrogyria segregated as an autosomal recessive trait. Six affected members of the family had partial complex seizures often associated with behavioral abnormalities. On MRI, three patients had a thickened irregular cortex in the lateral occipital lobes with small gyri. A high-density genome-wide scan with 10,000 SNPs established linkage by homozygosity mapping to a 14-Mb region on chromosome 6q16-q22. Candidate genes by function (TUBE1, GRIK2, GPRC6A, GPR6, NR2E1, MICAL1, and MARCKS) in this locus were screened for mutations.

When written words become moving pictures: Complex visual hallucinations on stimulation of the lateral occipital lobe

Electrical stimulation of the human epileptic brain is used mainly for identification of eloquent cortical regions such as motor and speech areas. Other stimulation responses include the patient’s epileptic auras and seizures. In addition, experiential phenomena may be elicited. Here we describe the reproducible initiation of a structured complex visual hallucination on stimulation of the left lateral occipital cortex (superior part of Brodmann area 19, close to the angular gyrus of the parietal lobe). Our findings illustrate that stimulation of the left temporo-parieto-occipital junction may activate networks of visual perception (color, pattern, movement, rotation, shape, and memory) independent of the cortical hierarchy from elementary to complex information.

Brain Mapping of Epileptic Activity in a Case of Idiopathic Occipital Lobe Epilepsy (Panayiotopoulos Syndrome)

The Panayiotopoulos type of occipital lobe epilepsy has generated great interest, but the particular brain areas involved in the peculiar seizure manifestations have not been established. We studied a patient with the syndrome, using high-resolution EEG and simultaneous EEG and functional magnetic resonance imaging (fMRI). Resolution of the scalp EEG was improved using a realistic spline Laplacian algorithm, and produced a complex distribution of current sinks and sources over the occipital lobe. The spike-related blood oxygen level dependent (BOLD) effect was multifocal, with clusters in lateral and inferior occipital lobe and lateral and anterior temporal lobe. We also performed regional dipole seeding in BOLD clusters to determine their relative contribution to generation of scalp spikes. The integrated model of the neurophysiologic and vascular data strongly suggests that the epileptic activity originates in the lateral occipital area, spreading to the occipital pole and lateral temporal lobe.

Differentiation of Dementia with Lewy Bodies from Alzheimer’s Disease Using Brain SPECT

We compared regional cerebral blood flow (CBF) patterns in patients with dementia with Lewy bodies (DLB) and Alzheimer’s disease (AD) using single photon emission computed tomography (SPECT) and investigated the diagnostic utility of SPECT study in differentiating between DLB and AD. SPECT data on 20 patients with DLB and 75 patients with AD were analyzed using three-dimensional stereotactic surface projections. Regional CBF reduction was determined by quantitative analysis using stereotactic extraction estimation method. The DLB group showed a significant CBF reduction in the temporoparietal, frontal lobe and posterior cingulate, similar to the CBF pattern in the AD group, but regional CBF in the medial and lateral occipital lobes decreased significantly in patients with DLB compared with patients with AD. Receiver operating characteristic analysis revealed that regional CBF measurement of the medial occipital lobe, including the cuneus and lingual gyrus, yielded a sensitivity of 85% and a specificity of 85% in discriminating DLB from AD. Objective and quantitative CBF measurement in the medial occipital lobe may be useful in the clinical differentiation of DLB and AD.

Regional Brain Activity Associated with Visual Backward Masking

In visual backward masking, the visibility of a briefly presented visual target is disrupted by a mask that is presented shortly thereafter. The goal of the current study was to identify regions in the human cortex that may provide the neural basis of visual masking. We searched for areas whose activity correlated with perception as we systematically varied the strength of masking. A total of 13 subjects performed a backward masking task during functional magnetic resonance imaging. Target and mask were presented at three delay intervals (34, 68, and 102 msec) and behavioral measures confirmed that the targets were more visible at longer masking intervals. Two sets of regions of interest were identified: Distinct regions in the visual cortex (V1/V2, LO, hMT+) were segregated using scans to localize visual processing drawn from the existing literature. Additional cortical regions were selected in a data-driven approach based on their activity during the backward masking task. For each set, we determined the regions whose magnitude of activation increased at longer masking intervals. Nine of the subjects provided valid behavioral performance data on the visual masking task and imaging data from these subjects were used for subsequent analysis. The scans of visual processing areas identified four regions, including: early visual areas (V1 and V2), the motion-sensitive regions in the lateral occipital (LO) lobe (hMT+), and two components (dorsal and ventral) of the object-sensitive region, LO. Of these, the ventral and dorsal LO regions were sensitive to the strength of the mask. For the data-driven approach, six regions were identified on the basis of a difference map in which all masking intervals were contrasted with rest. These included the inferior parietal, anterior cingulate, precentral, insula, thalamic, and occipital areas. The predicted effects of more activity with weaker masking were seen in the thalamus, inferior parietal, and anterior cingulate. This study isolated three types of visual processing areas. The first included regions that subserve key stages of vision (including object and motion processing). The second type responded to the presentation of brief ly presented visual stimuli, regardless of masking interval. The third type (selected from the first two) included regions sensitive to the interval between the target and mask. These latter regions (including ventral LO, inferior parietal, anterior cingulate, and thalamus) may form the neural substrate of backward masking.

Using multiple functional criteria to define high-level human visual areas in the lateral occipital and temporal lobes.

Using multiple functional criteria to define high-level human visual areas in the lateral occipital and temporal lobes.

Retinotopy with coordinates of lateral occipital cortex in humans.

The lateral occipital cortex in humans is known as the "extrastriate visual cortex." It is, however, an unexplored field of research, and the anatomical nomenclature for its surface has still not been standardized. This study was designed to investigate whether the lateral occipital cortex in humans has retinotopic representation. Four right-handed patients with a diagnosis of intractable epilepsy from space-occupying lesions in the occipital lobe or epilepsy originating in the occipital lobe received permanently implanted subdural electrodes. Electrical cortical stimulation was applied directly applied to the brain through metal electrodes by using a biphasic stimulator. The location of each electrode was measured on a lateral skull x-ray study. Each patient considered a whiteboard with vertical and horizontal median lines. The patient was asked to look at the midpoint on the whiteboard. If a visual hallucination or illusion occurred, the patient recorded its outline, shape, color, location, and motion on white paper one tenth the size of, and with vertical and horizontal median lines similar to those on, the whiteboard. Polar angles and eccentricities of the midpoints of the phosphenes from the coordinate origin were measured on the paper. On stimulation of the lateral occipital lobe, 44 phosphenes occurred. All phosphenes were circular or dotted, with a diameter of approximately 1 cm, except one that was like a curtain in the peripheral end of the upper and lower visual fields on stimulation of the parietooccipital region. All phosphenes appeared in the visual field contralateral to the cerebral hemisphere stimulated. On stimulation of the lateral occipital lobe, 22 phosphenes moved centrifugally or toward a horizontal line. From three-dimensional scatterplots and contour maps of the polar angles and eccentricities in relation to the x-ray coordinates of the electrodes, one can infer that the lateral occipital cortex in humans has retinotopic representation. The authors found that phosphenes induced by electrical cortical stimulation of the lateral occipital cortex represent retinotopy. From these results one can assert that visual field representation with retinotopic relation exists in the extrastriate visual cortex.

Circuits neuronaux de la lecture et de récriture dans la langue japonaise (le kanji et la kana)

Japanese writing system consists of two different types of letters, kana (phonogram) and kanji (morphogram). Various types of alexia with or without agraphia in the Japanese language cause specific type of kanji/kanja dissociation. Upon the neuropsychological analysis of these dissociations, we have proposed a hypothesis of dual neuronal circuits of reading and writing process. Subsequent PET activation studies upon normal subjects confirmed the hypothesis; semantic reading pathway via area 37 on the inferior border of the left temporal lobe and phonological reading pathway via middle portion of the left lateral occipital lobe.

Activity in the Fusiform Gyrus Predicts Conscious Perception of Rubin's Vase–Face Illusion

We localized regions in the fusiform gyrus and superior temporal sulcus that were more active when subjects viewed photographs of real faces than when they viewed complex inanimate objects and other areas in the parahippocampal gyrus and the lateral occipital lobe that showed more activity during the presentation of nonface objects. Event-related functional magnetic resonance imaging was then used to monitor activity in these extrastriate visual areas while subjects viewed Rubin's vase–face stimulus and indicated switches in perception. Since the spontaneous shifts in interpretation were too rapid for direct correlation with hemodynamic responses, each reported percept (faces or vase) was prolonged by suddenly adding subtle local contrast gradients (embossing) to one side or the other of the figure–ground boundary, stabilizing the percept. Under these conditions, only face-selective areas in the fusiform gyrus responded more strongly during the perception of faces. To control for effects of the physical change to Rubin's stimulus (i.e., addition of embossing), we compared activity when the face contours were embossed after the subject had just reported the onset of perception of either faces or vase. Activity in the fusiform face area responded more strongly under the first condition, despite the fact that the physical stimulus sequences were identical. Moreover, on a trial-to-trial basis, the activity was statistically predictive of the subjects' responses, suggesting that the conscious perception of faces could be made explicit in this extrastriate visual area.

Regional N-acetylaspartate reduction in the hippocampus detected with fast proton magnetic resonance spectroscopic imaging in patients with Alzheimer disease.

To detect regional metabolic changes that resemble the expected spatial pattern of neuronal loss in patients with Alzheimer disease (AD). Thirty-four patients with AD and 22 healthy control subjects were included in the study. Single-slice fast proton spectroscopic imaging was performed in parallel angulation to the temporal lobes. Proton spectra were selected from the hippocampus, the lateral temporal lobe, and the occipital lobe of both hemispheres to determine metabolite concentration of N-acetylaspartate (NAA), total creatine (tCr), including phosphocreatine and creatine, and choline-containing compounds (Cho). The metabolic ratios of NAA/tCr and Cho/tCr were calculated and compared between patients with AD and healthy volunteers. The NAA/tCr ratios were significantly reduced in the left (F(1,1) = 4.34, P =.04) and right hippocampus (F(1,1) = 9.96, P =.003) in patients with AD. The Cho/tCr ratios remained unchanged in both hippocampi. There was no significant change of either NAA/tCr or Cho/tCr in the lateral temporal and occipital lobes of patients with AD. This study provides evidence that fast proton spectroscopic imaging may detect the regional pattern of disturbed neuronal integrity in patients with AD with high spatial resolution in a short acquisition time.

Volumetric stereotaxy and the supratentorial occipitosubtemporal approach in the resection of posterior hippocampus and parahippocampal gyrus lesions.

Resection of intracranial tumors in the posterior hippocampus and the parahippocampal gyrus can be associated with significant morbidity because of the parenchymal resection and the cortical retraction often required in gaining access to this infrequently explored region. With the use of image guidance, the occipitosubtemporal (OST) approach requires neither lateral cortical resection nor the placement of brain retractors to gain surgical access to the posterior hippocampus and the parahippocampal gyrus, and this approach is associated with a high rate of gross total tumor resection. The computer-assisted volumetric stereotactic OST approach was used to resect 40 posterior hippocampus and parahippocampal gyrus tumors in 34 consecutive patients during an 8-year period. Patient, radiographic, and surgical outcome data were collected retrospectively. The series included operations in 25 men and 15 women, and the patients' average age was 40.3 years (range, 15-69 yr). Twenty-five of the 40 procedures were performed to remove lesions in the dominant hemisphere, and previous craniotomies for resection had been performed in 12 of 40 cases. In 38 of 40 cases, histopathological analysis revealed a glial neoplasm, and 50% of these tumors were high-grade lesions. Preoperatively, 23 patients were neurologically intact before 40 procedures, whereas visual field deficits were noted in 7 patients, mild hemiparesis was documented in 4 patients, and other neurological deficits were present in 9 patients. An excellent outcome (Glasgow Outcome Scale Grade 5) was noted after 38 (95%) of the 40 computer-assisted volumetric stereotactic OST procedures. Permanent postoperative hemiparesis (Glasgow Outcome Scale Grade 4) occurred after one procedure, and a second patient, despite being neurologically unchanged postoperatively and despite having had an optimal tumor resection, died on postoperative Day 33 (Glasgow Outcome Scale Grade 1). Complete resection of the preoperatively defined tumor volume was noted on postoperative gadolinium-enhanced magnetic resonance imaging examinations after 39 (97.5%) of the 40 procedures. The average duration of clinical follow-up was 15.9 months (range, 0.5-67 mo). We think that the OST approach is well suited to the resection of tumors in the posterior hippocampus and the parahippocampal gyrus. By allowing the neurosurgeon to avoid unnecessary brain resection and retraction, this approach reduces the risk of injury to important lateral temporal and occipital lobe cortex and tracts. In addition, the resection of a posterior hippocampus or parahippocampal gyrus mass with the OST approach relieves temporal horn entrapment. Computer-assisted volumetric stereotaxy helps the neurosurgeon to maintain precise spatial and anatomic orientation and accurately delineates the margin between the tumor and the surrounding neural tissue.

Early and delayed Tc-99m ECD brain SPECT in SLE patients with CNS involvement

We compared early and delayed Tc-99m ECD SPECT scans in 32 SLE patients (Group 1, definite neuropsychiatric disorders; Group 2, minor neurologic symptoms or normal) with those of normal controls by visual inspection and semi-quantitative evaluation. With visual interpretation, 13 out of 14 patients in Group 1 (93%) and 7 out of 18 patients in Group 2 (39%) had diffuse uneven decrease in early scans. Seven patients in Group 2 (39%) who had normal early scans demonstrated focal decrease in the medial frontal lobe in delayed scans. With cerebral region to cerebellar ratios, in early scans, the medial frontal lobe in Group 1 and Group 2 was significantly lower than in normal controls, and lateral frontal lobe and occipital lobes in Group 1 were significantly lower than in normal controls. Nevertheless, in delayed scans, every cortical region except for the parietal lobe in Groups 1 and 2 was significantly lower than in normal controls. The retention rates in all regions in SLE patients were significantly lower than in normal controls. No case showed SPECT improvement on follow-up studies in either group in spite of clinical improvement. Delayed Tc-99m ECD brain SPECT of high sensitivity might be useful in detecting CNS involvement. Although the SPECT findings did not correlate with the neuropsychiatric symptoms, early and delayed Tc-99m ECD SPECT seems to provide useful objective diagnostic information in SLE patients.

Brain SPECT imaging and left hemispatial neglect covaried using partial least squares: the Sunnybrook Stroke study.

Hemispatial neglect, characterized as failure to attend to contralesional space, is hypothesized by current neuroanatomical models to result from damage to a network involving the frontal, parietal, and cingulate cortices, basal ganglia, and thalamus. This study investigated this model of neglect in 81 right hemisphere-damaged acute stroke patients using 99mTc-HMPAO single photon emission-computed tomography (SPECT). In order to exploit the inherent collinearity of SPECT regional brain ratios, a novel statistical technique, partial least squares (PLS), was utilized. It makes use of high correlations to identify biologically relevant patterns of brain activity. Averaged ipsilesional cerebellar ratios from 152 brain segments were covaried with performance on subtests of the Sunnybrook Neglect Battery. In this patient sample, the most influential region identified by PLS corresponded to the area surrounding the right temporal-parietal-occipital (TPO) junction that included the right lateral occipital, temporal, and inferior parietal lobes. Hypoperfusion in the medial frontal cortex, including the anterior cingulate, also emerged as significantly associated with more severe neglect. Thus, hypoperfusion in only two of the five hypothesized network regions emerged as significantly associated with hemispatial neglect on SPECT imaging. This work converges with structural imaging studies to suggest that damage to the TPO junction, not just the parietal lobe, may be the critical region for hemispatial neglect. Our study demonstrated the utility of PLS for analyzing functional imaging and behavioral data sets in a clinical population in relation to current neuroanatomical models of neglect.

Regional cerebral glucose metabolism in dementia with Lewy bodies and Alzheimer's disease.

To delineate the features of regional cerebral metabolic rate of glucose (CMRglc) in dementia with Lewy bodies (DLB). We compared absolute CMRglc in 12 patients with a clinical diagnosis of DLB, 12 patients with a clinical diagnosis of Alzheimer's disease (AD), and 12 normal volunteers (NC), using 18F-fluorodeoxyglucose (FDG) and PET. The three groups were matched for age and sex, and there were no differences in disease duration or severity of cognitive disturbances between the DLB and AD groups. CMRglc was significantly lower in patients with DLB than in that of NC in most parts of the brain, except the sensorimotor cortices, basal ganglia, thalamus, and pons. Between the DLB and AD groups, there were significant regional CMRglc differences in the medial and lateral occipital lobes. In DLB and AD, the CMRglc reduction patterns were similar, though the global metabolic reduction was larger in DLB, and the occipital CMRglc reduction in DLB could differentiate DLB from AD. The relative occipital CMRglc (normalized to the sensorimotor CMRglc) was a useful measure for the differential diagnosis of DLB from AD. The sensitivity and the specificity were 92% when using the minimal value of the normalized occipital CMRglc in the NC group as the cut-off point. These different regional CMRglc reductions substantiate the pathologic, neurochemical, and clinical differences between DLB and AD.

H. M.’s Medial Temporal Lobe Lesion: Findings from Magnetic Resonance Imaging

Although neuropsychological studies of the amnesic patient H. M. provide compelling evidence that normal memory function depends on the medial temporal lobe, the full extent of his surgical resection has not been elucidated. We conducted magnetic resonance imaging studies to specify precisely the extent of his bilateral resection and to document any other brain abnormalities. The MRI studies indicated that the lesion was bilaterally symmetrical and included the medial temporal polar cortex, most of the amygdaloid complex, most or all of the entorhinal cortex, and approximately half of the rostrocaudal extent of the intraventricular portion of the hippocampal formation (dentate gyrus, hippocampus, and subicular complex). The collateral sulcus was visible throughout much of the temporal lobe, indicating that portions of the ventral perirhinal cortex, located on the banks of the sulcus, were spared; the parahippocampal cortex (areas TF and TH) was largely intact. The rostrocaudal extent of the ablation was approximately 5.4 cm (left) and 5.1 cm (right). The caudal 2 cm, approximately, of the hippocampus body (normal length, approximately 4 cm) was intact, although atrophic. The temporal stem was intact. Outside the temporal lobes, the cerebellum demonstrated marked atrophy, and the mammillary nuclei were shrunken. The lateral temporal, frontal, parietal, and occipital lobe cortices appeared normal for age 66 years. The mediodorsal thalamic nuclei showed no obvious radiological changes. These findings reinforce the view that lesions of the hippocampal formation and adjacent cortical structures can produce global and enduring amnesia and can exacerbate amnesia beyond that seen after more selective hippocampal lesions.

Neuronal generators of visual evoked potentials in humans: visual processing in the human cortex.

We wished to define the localization of cortical generators of visual (pattern) evoked potentials (VEP) and the temporal sequence of activation in the occipital region. In 4 candidates for epilepsy surgery, a large array of subdural electrodes was placed over occipital areas. Checkerboard pattern reversal stimuli were generated and the epileptogenic focus was localized and functionally mapped. Magnetic resonance imaging did not show any occipital lesions in any of the 4 patients. The area first activated was the lingual gyrus in the mesial occipital lobe (negative potential peaks at approximately 70 ms), followed by an area superior to the calcarine fissure (negative peaks at approximately 80 ms). Later (starting at approximately 90 ms), there were positive potentials over the occipital pole and lingual gyrus, followed by potentials at the lateral occipital lobe. These data support the idea that VEP are generated in the mesial and lateral occipital cortex by different circumscribed neuronal generators with different latencies of activation. The scalp-recorded N1 and P1 potential peaks most likely derive from the progressive activation of neuronal masses in different regions of the occipital lobe.