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The cerebral cortex is supplied by vascular microlobules, each comprised of a half dozen penetrating arterioles that surround a central draining venule. The surface arterioles that feed the penetrating arterioles are interconnected via an extensively anastomotic plexus. Embolic occlusion of a small surface arteriole rarely produces a local infarct, because collateral blood flow is available through the vascular reticulum. Collateral flow also protects against infarct after occlusion of a single penetrating arteriole. Cortical infarction requires blockage of a major arterial trunk, with arrest of blood flow to a relatively large vascular territory. For striate cortex, the major vessels compromised by emboli are the inferior calcarine and superior calcarine arteries, as well as the distal branches of the middle cerebral artery. Their vascular territories have a fairly consistent relationship with the retinotopic map. Consequently, occlusion by emboli results in stereotypical visual field defects. The organization of the arterial supply to the occipital lobe provides an anatomical explanation for a phenomenon that has long puzzled neuro-ophthalmologists, namely, that of the myriad potential patterns of cortical visual field loss, only a few are encountered commonly from embolic cortical stroke.

Evidence has been accumulated for over a century indicating that the visual system of humans and many animals is more sensitive to contour stimulation at vertical or horizontal orientations than oblique orientations. However, the neural basis for this orientation anisotropy is still a subject of debate. In the present study, we recorded brain activity over the parietal-occipital and frontal lobes with functional near-infrared spectroscopy (fNIRS) when human participants were presented with gratings in different orientations. The oblique gratings induced a much larger change in the oxygenated hemoglobin concentration than vertical and horizontal gratings in the left occipital lobe. However, we did not find any significant orientation anisotropy in the frontal lobe. Our study showed that different quantitative changes in the hemoglobin concentrations occurred in response to differently oriented stimuli in the visual cortex and that fNIRS could potentially be a valuable tool in the assessment of the hemodynamic responses of the visual system.

Background: Peripheral homonymous scotomas beyond 30° from fixation are rare. The paucity of publications describing such visual field defects might be attributed to various factors, including the absence of severe symptoms, routine visual field assessment restricted to the central 30° with automated perimetry, and the collateral circulation to the occipital cortex. The aim of this study was to correlate the brain lesions and perimetric findings in 2 unusual cases of peripheral homonymous scotomas, with the anatomic location of the optic radiation and primary visual cortex., Methods: Two patients with circumscribed homonymous scotomas beyond 30° related to infarcts in the intermediate area of the visual cortex are reported. We describe a new strategy, which relies on modern lesion analysis and stereotaxic probabilistic cytoarchitectonic maps, to accurately correlate the brain lesion site with the location of the peripheral homonymous visual field defects., Results: In Case 1, the posterior optic radiation was affected in its termination in the upper intermediate visual cortex. In Case 2, the lesion was located in the upper rostral portion of the primary visual cortex. In both, the most anterior part of the visual cortex and the occipital pole were intact, accounting for preservation of the central and most peripheral visual field. Additionally, correlation of the neuroimaging findings with commonly used maps of the representation of the visual field on the striate cortex suggested that our data were most consistent with the Holmes map., Conclusions: Modern lesion analysis and cytoarchitectonic maps, in combination with the existing retinotopic maps, may provide reliable clues for the localization of cerebral infarction and prognosis of homonymous visual field defects and may lead to a better understanding of the link between neuroanatomical landmarks and functional outcomes.

Although resting-state functional magnetic resonance imaging has shown altered functional connectivity between visual and other brain areas in the early blind individuals, it cannot answer which brain area's local activities are changed. In this study, regional homogeneity, a measure of the homogeneity of the local blood oxygen level-dependent signals, was used for the first time to investigate the changes in the resting-state brain activity in the early blind individuals. Compared with age-matched and sex-matched sighted individuals, the early blind individuals showed increased regional homogeneity only in the occipital areas, which might be explained by the abnormal cortical development and/or experience-dependent plasticity, resulted from an early visual deprivation.

Studies have shown that cortical area MT/V5 is activated by visual motion as well as illusory motion, implied motion in static images, or motion imagery. Cells within these areas are additionally tuned for speed and exhibit different responses depending on the actual speed of an object. In this study, we investigate the relationship between perceived speed as represented within static images and the patterns of activity observed in MT/V5. Consistent with other studies, we show robust responses in MT/V5 to implied motion. In addition, our results show an increase in the blood-oxygen level dependent response consistent with the increased speed information inherent within the images. In the absence of any real physical motion, the data illustrate the intricate manner in which top-down activity influences visual cortical activity.

Functional neuroimaging and electroencephalography reveal emotional effects in the early visual cortex. Here, we used functional near-infrared spectroscopy to examine haemodynamic responses evoked by neutral, positive and negative emotional pictures, matched for brightness, contrast, hue, saturation, spatial frequency and entropy. Emotion content modulated amplitude and latency of oxy, deoxy and total haemoglobin response peaks, and induced peripheral autonomic reactions. The processing of positive and negative pictures enhanced haemodynamic response amplitude, and this effect was paralleled by blood pressure changes. The processing of positive pictures was reflected in reduced haemodynamic response peak latency. Together these data suggest that the early visual cortex holds amplitude-dependent representation of stimulus salience and latency-dependent information regarding stimulus valence, providing new insight into affective interaction with sensory processing.

Objective: HIV enters the brain soon after infection causing neuronal damage and microglial/astrocyte dysfunction leading to neuropsychological impairment. We examined the impact of HIV on resting cerebral blood flow (rCBF) within the lenticular nuclei (LN) and visual cortex (VC)., Methods: This cross-sectional study used arterial spin labeling MRI (ASL-MRI) to measure rCBF within 33 HIV+ and 26 HIV- subjects. Nonparametric Wilcoxon rank sum test assessed rCBF differences due to HIV serostatus. Classification and regression tree (CART) analysis determined optimal rCBF cutoffs for differentiating HIV serostatus. The effects of neuropsychological impairment and infection duration on rCBF were evaluated., Results: rCBF within the LN and VC were significantly reduced for HIV+ compared to HIV- subjects. A 2-tiered CART approach using either LN rCBF < or =50.09 mL/100 mL/min or LN rCBF >50.09 mL/100 mL/min but VC rCBF < or =37.05 mL/100 mL/min yielded an 88% (29/33) sensitivity and an 88% (23/26) specificity for differentiating by HIV serostatus. HIV+ subjects, including neuropsychologically unimpaired, had reduced rCBF within the LN (p = 0.02) and VC (p = 0.001) compared to HIV- controls. A temporal progression of brain involvement occurred with LN rCBF significantly reduced for both acute/early (<1 year of seroconversion) and chronic HIV-infected subjects, whereas rCBF in the VC was diminished for only chronic HIV-infected subjects., Conclusion: Resting cerebral blood flow (rCBF) using arterial spin labeling MRI has the potential to be a noninvasive neuroimaging biomarker for assessing HIV in the brain. rCBF reductions that occur soon after seroconversion possibly reflect neuronal or vascular injury among HIV+ individuals not yet expressing neuropsychological impairment.

Background: Motion perception may be preserved after damage to striate cortex (primary visual cortex, area V1). Awareness and normal discrimination of fast-moving stimuli have been observed even in the complete absence of V1. These facts suggest that motion-sensitive cortex (the V5/MT complex or V5/MT+) may be activated by direct thalamic or collicular inputs that bypass V1. Such projections have been identified previously in monkeys but have not been shown in humans using neuroimaging techniques., Methods: We used diffusion tensor imaging (DTI) tractography to visualize white matter fiber tracts connecting with V5/MT+ in 10 healthy volunteers. V5/MT+ was localized for each subject using functional MRI (fMRI). Functional activity maps were overlaid on high-resolution anatomical images and registered with the diffusion-weighted images to define V5/MT+ as the region of interest (ROI) for DTI tractography analysis. Fibers connecting to V1 were excluded from the analysis., Results: Using conservative tractography parameters, we found connections between the V5/MT+ region and the posterior thalamus and/or superior colliculus in 4 of 10 subjects., Conclusions: Connections between the V5/MT+ region and the posterior thalamus and/or superior colliculus may explain visual motion awareness in the absence of a functioning V1.

Selective attention modulates brain responses in visual cortex. A common finding, using functional magnetic resonance imaging or event-related potentials, is that responses to attended relative to unattended stimuli are potentiated. We report an exceptional circumstance in a motion-processing paradigm. Participants viewed superimposed stationary and moving dots and were instructed to attend to one or the other subset. Changes in the direction of dot motion triggered an event-related potential over posterior scalp sites, with a prominent negative peak at 200 ms that was larger when attention was directed at the stationary dots. This effect was localized to extrastriate visual cortex and may be due to reflexive effects of attention orienting triggered by unattended peripheral motion.

We used functional magnetic resonance imaging to examine the neuroanatomical correlates of visual hallucinations in a patient with a left visual field defect who had suffered bilateral occipital infarction. By cross-correlating the functional magnetic resonance imaging data with the hallucination events, we were able to identify the cerebral activity underlying the hallucinations. Bilateral activation was observed during visual stimulation in the calcarine fissure and the same activation was found medially in the left and right occipital cortex adjacent to the infarcted areas. This pattern of perilesional visual cortex activation is consistent with the suggestion that primary sensory areas may be involved in visual hallucinations after stroke.

In this study, we ask whether or not the involuntary capture of attention by a specific information type can produce domain-specific activation. Participants indicated the presence of a flicker in rapidly alternating letter-string masks presented in the periphery. Despite letters not being relevant to the task, we found, using functional MRI and a novel task that allowed us to contrast attended and unattended processing of the same visual information, a robust modulation by attentional capture in a localized letter-processing region. This finding suggests that the involuntary capture of attention is sufficient to produce domain-specific activation in early visual processing.

We studied, with functional MRI, how neuronal processing for visual objects progresses in the human brain by measuring suppressive responses to stimulus pairs. Two concentric rectangular frames of slightly different sizes were presented such that the smaller frame was shown first, followed by the larger dominant frame, with interstimulus intervals of 0, 50 and 240-ms. The functional MRI response at 50-ms decreased along the ventral visual pathway. The response at 240-ms interstimulus interval was suppressed after a fusiform area at which the response at 50-ms interstimulus interval was highly suppressed already and the responses at 0, 50 and 240-ms interstimulus intervals showed a V-shaped dependence on those interstimulus intervals. These observations show a way of progression of the information processing along visual object recognition pathway.

This study compared retinotopic map identification in primary visual cortex (V1) using: (i) functional magnetic resonance imaging (fMRI) and (ii) visual evoked potentials (VEPs) coupled with dipole source localization (DSL). A multielectrode array was used to record VEPs while subjects viewed a flickering dartboard pattern modulated by a 16-bit m-sequence. The stimulus preferentially activates V1. Using a common time function DSL algorithm, the primary source of each stimulus patch was found independent of the fMRI. The VEP/DSL and fMRI localization data for each subject were aligned by a rigid translation and rotation. The average distance between VEP and corresponding fMRI sources was 10.8 mm +/- 3.8 mm. To assess the significance of the results, fMRI and DSL solutions were scrambled so the comparisons were no longer for corresponding patches. The average distance between the noncorresponding data sets was 17.2 mm for 50 million scrambles. The probability of the scrambled data yielding a better fit than the real data was p < 10(-7). The combination of multielectrode recording, multiinput visual stimulation and common time function DSL analysis can provide a detailed retinotopic map of visual cortex that has high correspondence with independent fMRI localization analysis on the same subject.

When blind people touch Braille characters, blood flow increases in visual areas, leading to speculation that visual circuitry assists tactile discrimination in the blind. We tested this hypothesis in a functional magnetic resonance imaging study designed to reveal activation appropriate to the nature of tactile stimulation. In late-blind individuals, hMT/V5 and fusiform face area activated during visual imagery of moving patterns or faces. When they touched a doll's face, right fusiform face area was again activated. Equally, hMT/V5 was activated when objects moved over the skin. We saw no difference in hMT/V5 or fusiform face area activity during motion or face perception in the congenitally blind. We conclude that specialized visual areas, once established through visual experience, assist equivalent tactile identification tasks years after the onset of blindness.

Patients with cortical blindness can reliably perceive some facial expressions even if they are unaware of their percept. We examined whether emotional body language may also be recognized in the absence of the primary visual cortex and without conscious stimulus perception. We presented emotional and neutral body images in the blind field of a patient with unilateral striate cortex damage. Using functional magnetic resonance imaging, we measured activation following presentation to the blind hemifield of whole body images (happy, neutral) with the face blurred. Unseen happy body images selectively activated area MT and the pulvinar nucleus of the thalamus, while unseen instrumental neutral body images activated the premotor cortex. Our results show that in the absence of the striate cortex implicit bodily emotion perception may be possible.

A 68-year-old man developed right homonymous hemianopic paracentral scotomas from acute infarction of the left extrastriate area. He was studied over the ensuing 12 months with visual fields, conventional MRI, functional MRI (fMRI), and diffusion tensor imaging (DTI). As the visual field defect became smaller, fMRI demonstrated progressively larger areas of cortical activation. DTI initially showed that the lesioned posterior optic radiations were completely interrupted. This interruption lessened in time and had disappeared by one year after onset. fMRI and DTI are innovative measures to follow functional and structural recovery in the central nervous system. This is the first reported application of these imaging techniques to acute cerebral visual field disorders.

Converging electrophysiological and hemodynamic findings indicate sensory processing of emotional pictures is preferred to that of neutral pictures. Whereas neuroimaging studies of emotional picture perception have employed stimulus durations lasting several seconds, recent electrocortical investigations report early visual cortical discrimination between emotionally arousing and neutral picture processing. Here, we use a hybrid picture presentation paradigm covering a range of rapid presentation rates (0.75-6 Hz), while visual system activity is recorded with functional magnetic resonance imaging. Results demonstrate widespread sensitivity to emotional arousal in the secondary and inferior temporal visual cortex. Furthermore, activity in the lateral inferior occipital and medial inferior temporal visual cortex revealed equivalent emotion-sensitive activation across all presentation rates. Results further support the notion that attention and perceptual processing are in part directed by underlying motivational systems.

Objective: To investigate patients with posterior cerebral artery (PCA) infarctions to learn whether hemispatial neglect is more frequent and severe after right than left PCA infarction; whether visual field defects (VFDs) influence the presence or severity of hemispatial neglect; and the anatomic loci of lesions that are associated with hemispatial neglect., Methods: The authors recruited 45 patients with PCA infarction that involved only the occipital lobe or the occipital lobe plus other areas served by the PCA. All subjects received seven neglect tests within 2 months after onset., Results: Overall, the frequency of hemispatial neglect was 42.2%. The frequency did not significantly differ between the right (48.0%) and left (35.0%) PCA groups, but the severity of hemispatial neglect was significantly greater in the right group. VFD alone did not influence the frequency or severity of neglect after controlling other variables. Isolated occipital lesions were rarely associated with hemispatial neglect, and it was only the occipital plus splenial lesion that significantly influenced the frequency and severity of neglect., Conclusions: This study suggests that after excluding such confounding factors as aphasia or hemiplegia, neglect frequency does not differ between the right and left posterior cerebral artery (PCA) groups, but the severity of neglect is greater after right PCA infarctions; even in the acute stage of PCA infarction; visual field defect from an isolated occipital lesion does not cause hemispatial neglect; and the injury to both the occipital lobe and the splenium of the corpus callosum is important for producing hemispatial neglect with PCA infarction.

Background: Patients with Parkinson disease (PD) often experience visual hallucinations (VH) with retained insight (nonpsychotic) but the precise mechanism remains unclear., Objective: To clarify which neural substrates participate in nonpsychotic VH in PD, the authors evaluated regional cerebral blood flow (rCBF) changes in patients with PD and VH., Methods: The authors compared 24 patients with PD who had nonpsychotic VH (hallucinators) and 41 patients with PD who had never experienced VH (non-hallucinators) using SPECT images with N-isopropyl-p-[(123)I]iodoamphetamine. There were no significant differences in age, sex, duration of disease, doses of PD medications, Hoehn and Yahr scale, or Mini-Mental State Examination (MMSE) scores between the two groups. The rCBF data were analyzed using statistical parametric mapping (SPM)., Results: The rCBF in the right fusiform gyrus was lower in the hallucinators than in the non-hallucinators (corrected p < 0.05 at cluster levels). The hallucinators revealed higher rCBF in the right superior and middle temporal gyri than the non-hallucinators (uncorrected p < 0.001). These significant differences were demonstrated after MMSE scores and duration of disease, which are the relevant factors associated with VH, were covariated out., Conclusions: Nonpsychotic visual hallucinations in Parkinson disease (PD) may be associated with hypoperfusion in the right fusiform gyrus and hyperperfusion in the right superior and middle temporal gyri. These temporal regions are important for visual object recognition and these regional cerebral blood flow changes are associated with inappropriate visual processing and are responsible for nonpsychotic visual hallucinations in PD.

Feature-based attention affects the processing of the selected feature throughout the visual field. Here, we show that such global attentional modulation is not restricted to the attended feature but spreads to task-irrelevant features that are bound to the attended one. Attention to a color in one of the visual hemifields affected the processing of task-irrelevant motion in the other hemifield when it was associated with a stimulus that shared the attended color. This cross-feature global attentional selection increased the duration of the motion aftereffect and the strength of functional magnetic resonance imaging responses in the motion-sensitive area MT+, evoked by the task-irrelevant motion. These findings imply that features belonging to the same object are bound and selected jointly even outside the focus of attention.

A grey surface on a bright background appears to be darker than the same surface on a dark background. We used functional magnetic resonance imaging to study this phenomenon called brightness induction. While being scanned, participants viewed centre-surround displays in which either centre or surround luminance was modulated in time. In both cases, participants perceive similar brightness changes in the central surface. In the region of the visual cortex encoding this central surface, both modulations evoked comparable functional magnetic resonance imaging responses. However, the surround modulation signal showed a considerable delay relative to the onset of the brightness percept. This suggests that, although correlated, the functional magnetic resonance imaging signals do not bear a direct relationship with perceived brightness. We conclude that retinotopically organized visual cortex does not represent brightness per se.

Cortical area MT/V5 in the human occipito-temporal cortex is activated by visual motion. In this study, we use functional imaging to demonstrate that a subregion of MT/V5 is more strongly activated by unidirectional motion with speed gradients than by other motion patterns. Our results suggest that like the monkey homolog middle temporal area (MT), human MT/V5 contains neurons selective for the processing of speed gradients. Such neurons may constitute an intermediate stage of processing between neurons selective for the average speed of unidirectional motion and neurons selective for different combinations of speed gradient and different motion directions such as expanding optical flow patterns.

A patient with unilateral focal cortical dysplasia with a significant impairment of visual motion perception within the contralateral hemifield was examined with fMRI. During hemifield visual motion stimulation primary visual cortex areas were activated contralaterally and deactivated ipsilaterally to the stimulated hemifield. Transcallosal visuo-visual interaction was further evident as bilateral activation in temporo-occipital areas that best correspond to the motion sensitive areas MT/V5. MT/V5 was displaced anteriorly, superiorly, and medially within the dysplastic hemisphere and separated into two distinct activation clusters. During visual motion stimulation the parieto-insular vestibular cortex showed signal decreases that agree with the concept of inhibitory visuo-vestibular interaction. Thus, fMRI is a suitable tool for detecting preserved function and transcallosal connections in patients with focal cortical dysplasia.

The blood oxygen level-dependent (BOLD) response to photic stimulation in the primary visual cortex (V1) reverses from positive to negative around 8 weeks of age. This phenomenon may be caused by increased oxygen consumption during stimulation as the result of a rapid increase of synaptic density at this age. To test this hypothesis, we applied existing mathematic models of BOLD signals to the experimental data from infants. When the stimulus-related increments of cerebral blood flow and cerebral blood volume were fixed at 60% and 20%, respectively, the mean estimated increment of the cerebral metabolic rate of oxygen of the V1 in the elder infant group (57.1% +/- 8.8%) was twice as large as that in the younger infant group (32.2% +/- 4.7%), which corresponds to the reported difference in synaptic density. The present data confirmed that a change in oxygen consumption could explain a transition from a positive to a negative BOLD response.

Background and Purpose: Severe atherosclerotic disease of the carotid arteries may affect perfusion in the posterior circulation according to the degree of collateral supply. The purpose of this study was to determine whether the responses of regional cerebral blood flow (rCBF) and regional cerebral metabolic rate of oxygen (rCMRO2) during neural stimulation are affected by the baseline perfusion or oxygen metabolism., Methods: We used PET to measure rCBF, rCMRO2, and regional oxygen extraction fraction (rOEF) in 13 patients with carotid artery steno-occlusive lesions at baseline and during visual stimulation. We examined whether the changes in CBF, CMRO2, and OEF during visual stimulation were correlated with the baseline values of these parameters in the visual cortex., Results: With visual stimulation, rCBF increased in all patients, whereas rCMRO2 showed variable changes. The baseline rCMRO2 value showed a positive relationship with the degree of rCBF increase and a negative relationship to the degree of rCMRO2 increase. rCMRO2 decreased in patients with relatively high baseline rCMRO2 values, resulting in dissociation of the rCMRO2 response from the rCBF response. The rCBF increase was large in the region with an increased baseline rOEF value. These variable changes in rCBF and rCMRO2 resulted in a significant correlation between the values of rCBF and rCMRO2 during visual stimulation in contrast to the lack of such a correlation at baseline., Conclusions: In patients with cerebrovascular disease, neural stimulation may induce variable changes in rCBF and rCMRO2 according to the baseline perfusion and oxygen metabolism.

Cerebral blood flow (CBF) was measured with PET during rudimentary singing of a single pitch and vowel, contrasted to passive listening to complex tones. CBF increases in cortical areas related to motor control were seen in the supplementary motor area, anterior cingulate cortex, precentral gyri, anterior insula (and the adjacent inner face of the precentral operculum) and cerebellum, replicating most previously seen during speech. Increases in auditory cortex were seen within right Heschl's gyrus, and in the posterior superior temporal plane (and the immediately overlying parietal cortex). Since cortex near right Heschl's has been linked to complex pitch perception, its asymmetric activation here may be related to analyzing the fundamental frequency of one's own voice for feedback-guided modulation.

Cerebral blood flow (CBF) was measured with PET during rudimentary singing of a single pitch and vowel, contrasted to passive listening to complex tones. CBF increases in cortical areas related to motor control were seen in the supplementary motor area, anterior cingulate cortex, precentral gyri, anterior insula (and the adjacent inner face of the precentral operculum) and cerebellum, replicating most previously seen during speech. Increases in auditory cortex were seen within right Heschl's gyrus, and in the posterior superior temporal plane (and the immediately overlying parietal cortex). Since cortex near right Heschl's has been linked to complex pitch perception, its asymmetric activation here may be related to analyzing the fundamental frequency of one's own voice for feedback-guided modulation.

We sought to investigate how individual differences in the regional patterns of cerebral blood flow (rCBF) relate to task performance during the perceptual matching of faces. We analyzed rCBF data obtained by PET and H2150 from nine young healthy, right-handed, adult males (mean age 29i3 years) using a statistical model of regional covariance, the Scaled Subprofile Model (SSM). SSM analysis performed on a voxel-basis for scan subtractions comparing face-matching and control tasks extracted two patterns whose subject expression in a multiple regression analysis was highly predictive of task accuracy (R2 = 0.87, p < 0.002). The pattern reflecting this linear combination was principally characterized by higher rCBF in regions of bilateral occipital and occipitotemporal cortex, right orbitofrontal cortex, left thalamus, basal ganglia, midbrain, and cerebellum with relatively lower rCBF in anterior cingulate, regions in bilateral prefrontal and temporal cortex, right thalamus, and right inferior parietal cortex. The results indicate that individual subject differences in face matching performance are specifically associated with the functional interaction of cortical and subcortical brain regions previously implicated in aspects of object perception and visual attentional processing.

Previous studies have shown that hemodynamic response overlap severely limits the maximum presentation rate with event-related functional MRI (fMRI) using fixed intertrial experimental designs. Here we demonstrate that the use of randomized experimental designs can largely overcome this limitation, thereby allowing for event-related fMRI experiments with extremely rapid presentation rates. In the first experiment, fMRI time courses were simulated using a fixed intertrial interval design with intervals of 16, 3, and 1 s, and using a randomized design having the same mean intertrial intervals. We found that using fixed intertrial interval designs the transient information decreased with decreasing intertrial intervals, whereas using randomized designs the transient information increased with decreasing mean intertrial intervals. In a second experiment, fMRI data were collected from two subjects using a randomized paradigm with visual hemifield stimuli presented randomly every 500 ms. Robust event-related activation maps and hemodynamic response estimates were obtained. These results demonstrate the feasibility of performing event-related fMRI experiments with rapid, randomized paradigms identical to those used in electrophysiological and behavioral studies, thereby expanding the applicability of event-related fMRI to a whole new range of cognitive neurosciences questions and paradigms.

Patients with striate cortical damage causing a hemianopic field defect can nevertheless demonstrate residual visual capacities in their blind field. Previous research investigating blindsight required patients to respond explicitly to stimuli appearing in the blind field by making forced choice judgements. We present data from a patient with a left occipital lesion resulting in a homonymous hemianopia, using the flanker task. This patient displayed a significant flanker congruency effect (FCE) for colour and letter stimuli even when they appeared in the blind field. A control patient with a lesion of the right thalamus showed no FCE in the blind field. This suggests that thalamo-extrastriate neural pathways are necessary for residual functioning in blindsight.

To elucidate cortical processing during vocalization, we used positron emission tomography to measure regional cerebral blood flow during vocalization in six Japanese subjects. During reading Japanese sentences used daily or short syllables aloud, the visual cortices, the left Heschl's gyrus, Broca's area, the primary motor area of the articulatory organs, the supplementary motor area and the cerebellum were significantly activated, compared with resting conditions. The superior temporal gyri were rarely activated. Significant activation of the cerebellum was observed by comparing cortical activity during reading sentences used daily with that during reading meaningless short syllables. These results suggest that vocalization of familiar materials is taken over by the cerebellum, rather than cortical speech areas, and without engagement of the superior temporal gyri.

Background and Purpose: We assessed the influence of different visual stimuli and the reproducibility and habituation of evoked flow responses using simultaneous two-channel transcranial Doppler monitoring and flow velocity averaging., Methods: We measured stimulus-related percentage changes in posterior cerebral, basilar, and middle cerebral artery blood flow velocities in 14 normal volunteers using stimulus-triggered velocity averaging. With a two-channel transcranial Doppler system, simultaneous measurements in two arteries (both posterior cerebral arteries and the basilar and middle cerebral artery) were taken using multiple-array light-emitting diodes applying flash stimuli. Both posterior cerebral arteries were monitored to assess reproducibility and habituation of the evoked response with repetitive measurements under unchanged conditions and to analyze the influence of different features of the visual stimulus., Results: There was a distinctive increase in velocities resulting from visual stimuli in both posterior cerebral and the basilar arteries but not in the middle cerebral artery. The responses in both posterior cerebral arteries were larger than in the basilar artery (P = .0001). Brightness (P < .0001), as well as complexity (P < .0001), of the visual stimulus had a significant influence on the response amplitude. There was a trend toward a greater right-sided activation. Amplitudes of the evoked response were very stable during repetitive testing (coefficient of variation of the difference was 0.6). There was a trend toward habituation with monotonous (flash) but not with complex visual stimuli. A "zero" stimulus produced no responses., Conclusions: The use of flow velocity averaging and two-channel simultaneous recording increases the sensitivity of transcranial Doppler monitoring to detect and correlate selective flow changes in the posterior cerebral arteries resulting from cerebral activation produced by visual stimulation.

Using positron emission tomography (PET) and [15O]butanol, regional cerebral blood flow (rCBF) was determined in six bank officials during exposure to a video of a jointly experienced armed bank robbery, and a control video. Besides elevating subjective and physiological indices of anxiety, traumatic stimulation increased rCBF bilaterally in the primary and secondary visual cortex, the posterior gyrus cinguli and in the left orbitofrontal cortex compared with that during the control stimulation. Decreased rCBF was found in Broca's area, the left angular gyrus, the left operculum and the secondary somatosensory cortex. Thus, the stress induced by visual re-experience of a robbery is associated with altered activity in paralimbic and cortical brain regions of relevance for cognition and affect.

Using path analysis to determine the systems-level neural networks mediating specific tasks from regional cerebral blood flow (rCBF) data obtained by positron emission tomography (PET), we recently found in young subjects strong functional linkages during a face matching task along a right hemisphere ventral network including occipital, temporal, and frontal regions. In this study, PET data obtained during a face matching task from mildly affected patients with dementia of the Alzheimer type (DAT) and healthy matched controls showed that (1) the neural model obtained in young subjects provides a good fit to data from old subjects; (2) although the DAT patients could perform this task with the same accuracy as controls, they did not use the same functional network.

Rationale and Objectives: Brain activation can be detected by functional magnetic resonance imaging. Termination of stimulation can cause drop of postactivation signal below preactivation baseline; the nature of this "undershoot" remains controversial. The authors investigated postactivation signal after stimulation of the visual cortex with photic stimuli of different duration., Methods: Activation of visual cortex in 11 healthy subjects was studied. The authors underwent short and long stimulation. The relation between activation and postactivation signal was investigated after both durations of stimulation., Results: Average postactivation signal after short stimulation was -1.71 +/- 2.66% and after long stimulation 0.82 +/- 1.59% (P < or = 0.01). Significant "undershoot" was detected after short stimulation in the majority of cases, although this was typically absent after long stimulation., Conclusions: The authors propose that "undershoot" is caused by decreased oxygen content in venous blood compared with the preactivation level and may reflect increased extraction of oxygen as a result of glycolytic metabolism. The absence of "undershoot" after longer stimulation suggests a gradual shift from uncoupling between regional cerebral blood flow and oxygen consumption toward a steady state.

Cognitive experiments using positron emission tomography (PET) with [15O]-H2O require multiple scans done at 10 to 12 min intervals. Momose and colleagues reported that regional cerebral blood flow changes in response to visual stimulation do not return to baseline even 15 min after the stimulation. If confirmed, this fact would have important implications for the design and interpretation of PET cognitive studies. The purpose of this study was to determine the temporal course of residual effects following visual and motor stimulation. Six healthy volunteers undertook six scans each. Two scans were done to establish a baseline, one to observe the task induced activation, and three scans to investigate the residual effects at varying intervals after the task. The data were analysed using both statistical parametric mapping and a region of interest analysis. The visual and motor task produced robust activations bilaterally in the occipital cortex and in the contralateral primary motor cortex. There was no evidence for a statistically significant residue effect at any of the three intervals studied (30 s, 3 and 6 min). Our results refute the findings of Momose and colleagues and suggest that the 10-min interscan interval is more than adequate to re-establish a resting baseline.

Positron emission tomography was used to investigate the neural substrate of stereopsis. Changes in cerebral blood flow were measured in nine volunteers while they judged the orientation of a rectangular cyclopean shape in random dot stereograms. This experimental condition was compared with two control conditions, one having a two-dimensional shape but no cyclopean stimulus, the other having neither shape nor cyclopean stimulus. Blood flow increases were observed in areas 17 and 18 in the right hemisphere when comparing experimental with control conditions; blood flow decreases occurred in the right inferotemporal cortex. Results indicate that stereopsis begins in posterior visual areas in the right cerebral hemisphere. Under ambiguous stereoscopic conditions, participation of more anterior areas in the temporal lobe may be elicited.

In order to map the anatomical structures participating in the analysis and processing of visual information related to discrimination of form and colour, we measured with positron emission tomography (PET) regional cerebral blood flow (rCBF) as an indicator of metabolic activity in ten right-handed volunteers during visual discrimination tasks, namely reference, form and colour tasks. Form discrimination specifically increased rCBF bilaterally in the inferior temporal and cingulate gyri, and in the left superior temporal, left occipital lateral, and left angular gyri, whereas colour discrimination did so in the left occipital superior and lateral, left parahippocampal, left occipito-temporal medial (lingual), and left superior parietal gyri, and the right precuneus.

The patient reported here had a bilateral inferior altitudinal hemianopia from lesions of the calcarine (striate) cortex of the occipital lobes. The only significant pathologic findings were bilateral calcarine artery occlusive disease, with infarcts of the striate cortex on both sides. The ages of the infarcts appeared compatible with the clinical development of the respective visual field defects. The rest of the visual system was anatomically intact.