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PubMed1981

Visual evoked potentials in occipital lobe lesions.

Recording of visual evoked potentials (VEPs) to pattern reversal is considered to be a reliable diagnostic procedure for examining patients with anterior visual pathway lesions (optic nerves and chiasm). Less consistent results have been reported in studies of more posterior lesions. The VEPs were recorded in 20 patients with occipital lobe lesions. A maximal VEP response (P94) was recorded at the scalp electrodes situated over the involved occipital lobes and contralateral to the hemianoptic visual field defect, indicating a positive correlation of unilateral occipital lobe lesions, homonymous visual field loss, and the VEP abnormality.

Adult↗
PubMed1990

[The effect of visual deafferentation on changes in the cerebral blood flow during photic stimulation].

With the aid of transcranial dopplerography, linear velocity of the blood flow was recorded in major cerebral vessels in resting and in photostimulation in 18 healthy subjects and 7 patients with unilateral lesion of visual pathway and exclusion of one half of the field of vision. The photostimulation accelerated the blood flow in middle cerebral arteries by 4.3% and in posterior cerebral arteries by 26.3% in healthy subjects. No acceleration of the blood flow occurred in posterior cerebral artery on the side of unilateral lesion of visual pathways.

Adult↗
PubMed1998

Mechanisms of central color vision.

In monkey cerebral cortex, color information is processed along the ventral visual pathway. This pathway starts in the primary visual cortex and ends in area TE of the inferior temporal cortex. Recent studies indicate that the transformation of cone signals occurs early in the pathway to form neurons selective to a narrow range of hues. In addition, it has become apparent that area TE plays a vital role in color discrimination.

Animals↗
PubMed1999

Relationship between ventral stream for object vision and dorsal stream for spatial vision: an fMRI + ERP study.

Recent imaging studies indicated the existence of two visual pathways in humans: a ventral stream for object and form vision and a dorsal stream for spatial and motion vision. The present study was motivated by a stimulating question: Supposing shape and motion are processed separately in the two pathways, how do the respective cortical areas respond to the stimuli of "forms defined by motion"? fMRI and ERP recordings were combined in order to measure the spatiotemporal activation pattern in the two pathways responding to forms defined by motion, which were produced solely by coherent movement of random dots against a background of dynamic or static random dots. The fMRI data indicated that the stimuli of forms defined by motion indeed activated both dorsal MT/V5 and ventral GTi/GF. Furthermore, the RV curves resulting from fMRI-seeded dipole modeling indicated that each pair of dipoles located at MT/V5 or GTi/GF reached the same best-fit point; a single pair of free dipoles located near the fMRI foci of MT/V5 and GTi/GF could be identified at the corresponding best-fit point; and the source waveforms resulting from fixed dipole modeling also showed simultaneous activation of MT/V5 and GTi/GF dipoles in the time interval around the best-fit point. The present results, therefore, suggest that MT/V5 and GTi/GF appear to be activated in parallel and simultaneously responding to forms defined by motion. Such findings raise interesting issues about the hierarchical organization and the functional specialization in the two pathways.

Adult↗
PubMed1997

Cerebral magnetic resonance imaging and mental and motor function of very low birth weight children at six years of age.

In this follow-up study, 20 of a geographically based year cohort of 31 surviving non-disabled VLBW (birthweight < 1500 g) children were examined at six years of age. The aim of the study was to relate cerebral MRI findings to neuro-development in these non-disabled children at six years of age. All MRI scans were evaluated for myelination pattern, periventricular gliosis, ventricular dilation and cortical atrophy. The Peabody motor test and the Wechsler Preschool and Primary Scale of Intelligence (WPPSI) were used in the evaluation of motor, mental and perceptual function. A diagnosis of attention deficit disorder with hyperactivity was made based on the examiner's impression of the child during the examination and based on the parent's history. We found that ten (50%) of the children had periventricular gliosis, mainly in centrum semiovale (CS) (nine children) and in central occipital white matter (COW) (six children). Gliosis in CS was related to lower scores on the Peabody gross motor test for locomotion, indicating involvement of corticospinal tracts. Additional gliosis in COW was related to both fine motor and gross motor impairments. We speculate that this indicates damage to both motor and visual pathways, affecting eye-hand coordination and balance function. No relationship between MRI deviations at six years and mental function based on performance, verbal and total IQ scores was found. However, there was a significant relationship between periventricular gliosis in COW and C5 and low scores on the WPPSI performance subtests: Picture completion test and Block design test. This may indicate visual and spatial perception problems, caused by damage to posterior visual pathways and occipito-thalamic tracts dealing with visuo-motor integration.

Analysis of Variance↗
PubMed1988

Functional implications of the anatomical organization of the callosal projections of visual areas V1 and V2 in the macaque monkey.

The efferent and afferent connections of the V1/V2 border with the contralateral hemisphere have been examined using anatomical tracers. The V1/V2 border was found to exchange connections with the contralateral V2 area as well as a restricted strip of V1 lying adjacent to the V1/V2 border. Besides these homotopic projections, two heterotopic projections were found to V3/V3A and V5. Anterograde tracing of callosal connections showed that terminals in these heterotopic sites were focused in layer 4, the recipient layer of projections originating from the ipsilateral V1/V2 border. Bilateral injections of fluorescent dyes showed that these heterotopic targets of the V1/V2 border are connected to the homologous ipsilateral V1/V2 border region. The laminar location of callosal projecting neurons as well as their terminals were characteristic for each cortical region. The laminar pattern of callosal connectivity was found to differ markedly from that of associational visual pathways. Two principal hypotheses are suggested by these results. First, the fact that V1 in part is reciprocally callosally connected in all mammals supports the notion that this interhemispheric pathway completes long-range intrinsic cortical connections. Second, the convergence of inter- and intrahemispheric pathways could provide the anatomical basis for the modulation of the sensory processing within one hemisphere by ongoing activity in the contralateral hemisphere.

Animals↗
PubMed1978

Role of Siamese cat's crossed and uncrossed retinal fibres in pattern discrimination and interocular transfer.

Siamese and ordinary cats were monocularly tested in pattern discriminations and visual perimetry following unilateral optic tract section. While ordinary cats exhibited similar performances when tested with either eye, in Siamese cats the eye contralateral to the sectioned optic tract was practically blind. Thus, the Siamese cat's uncrossed visual pathways are not able to subserve either form or ambient vision. The nature of such a loss is discussed.

Animals↗
PubMedDate not supplied

Motion-onset visual evoked potentials improve the diagnosis of glaucoma.

Chronic glaucoma has been shown preferentially to damage larger retinal cells and optic nerve fibres that provide the input to the magnocellular visual pathway. We compared the motion-onset visual evoked potentials (primarily the magnocellular system) with those to standard pattern reversal in 20 patients with bilateral chronic glaucoma. For motion-onset visual evoked potentials, the pattern (isolated 40' checks of 10% contrast) moved in four cardinal directions (varied randomly from trial to trial) at a velocity of 10 deg/s for 20 ms, with an interstimulus interval of 1 s. In pattern-reversal stimulation, the checkerboard reversed at a rate of 2 reversals per second. In 60% of the eyes investigated, the results of both types of visual evoked potentials correlated, showing either normal (27.5%) or increased (32.5%) latencies. In the remaining 40% of the eyes, the normal pattern-reversal visual evoked potential latencies were accompanied by prolonged motion-onset visual evoked potentials. The high occurrence of delayed motion-onset visual evoked potentials in our patients confirms the primary magnocellular loss in chronic glaucoma and suggests that the motion-onset VEPs are suitable for detection of glaucomatous changes.

Adolescent↗
PubMed1996

Functional magnetic resonance imaging in homonymous hemianopsia.

PURPOSE: We performed functional magnetic resonance imaging associated with brain activity to evaluate the abnormality of the visual pathway in hemianoptic patients. METHODS: We studied five patients with homonymous hemianopsia caused by retrochiasmal lesions and five control subjects. On the basis of a blood oxygenation level-dependent contrast mechanism, magnetic resonance imaging was performed with a standard clinical 1.5-tesla system. We evaluated the asymmetrically increased signal intensities in the calcarine cortex during visual stimulation and compared them with the findings of Goldmann perimetry. RESULTS: Cortical activations with a marked interhemispheric contrast of the primary visual cortex consistent with the visual field defects were observed in three of the five patients. In two of the patients without macular field preservation in the defective hemifield, functional magnetic resonance imaging showed an absence of response in the primary visual cortex of the affected side. In the patient with superior quadrantanopsia, we could detect clear laterality by selecting the imaging plane inferior to the calcarine fissure. The other two hemianoptic patients with macular sparing showed symmetric responses. CONCLUSIONS: These findings suggest that functional magnetic resonance imaging is valuable in assessing local brain function in patients with visual deficits; therefore, it is a promising method for the objective detection of abnormalities in the afferent visual system.

Adolescent↗
PubMed2000

The primate pulvinar nuclei: vision and action.

The pulvinar nuclei of the thalamus are proportionately larger in higher mammals, particularly in primates, and account for a quarter of the total mass. Traditionally, these nuclei have been divided into oral (somatosensory), superior and inferior (both visual) and medial (visual, multi-sensory) divisions. With reciprocal connections to vast areas of cerebral cortex, and input from the colliculus and retina, they occupy an analogous position in the extra-striate visual system to the lateral geniculate nucleus in the primary visual pathway, but deal with higher-order visual and visuomotor transduction. With a renewed recent interest in this thalamic nuclear collection, and growth in our knowledge of the cortex with which it communicates, perhaps the time is right to look to new dimensions in the pulvinar code.

Animals↗
PubMed1996

[Post-actinic retrobulbar optic neuropathy].

BACKGROUND: Radiation optic neuropathy (RON) is a rare, unpredictable, late complication of radiotherapy secondary to obliterative endarteritis. Tumor recurrence has to be ruled out by a clinical and neuroradiological examination. METHODS: Five patients with RON were investigated by magnetic resonance imaging (MRI) during 1992. RESULTS: Radiation-induced lesions of the intracranial visual pathways were easily visible on MRI. Without Gadolinium, a sectorial swelling was detectable, which markedly enhanced with Gadolinium. Intracranial optic nerve was affected in 5/5 cases, optic chiasm in 3/5 cases, and optic tract in 2/5 cases. CONCLUSIONS: MRI is the examination of choice when RON is suspected: it will easily delineate the extent of the lesion, and compression/infiltration by a recurrent tumor will be formally ruled out. A segmental swelling of visual pathway with marked Gadolinium enhancement on MRI is highly suggestive of radionecrosis.

Cranial Irradiation↗
PubMed1995

[Sensitivity and specificity of masked field campimetry].

Patients with circumscribed visual field defects are able to perceive the scotomata immediately while looking at randomly distributed black and white squares (12' x 12') flickering on a VDU with a high frequency (approximately 30 Hz), resulting in a stimulus field comparable to the white-noise field on a TV screen without reception. In 368 eyes of 368 patients with varying lesions of the visual pathway the results of white-noise field campimetry were compared with those of conventional threshold-related, slightly suprathreshold automated grid perimetry. Rate of detection (sensitivity) was 84.2% in conventional perimetry and 80.7% in white-noise field campimetry, respectively. The results are not significantly different (p > 0.1; McNemar test). Examinations in 198 eyes of 198 persons without any indication of a visual pathway defect allowed a definition of the normal range of white-noise field campimetry to be made. The specificity of this new method was 82.3%. The outcome of this most extensive study on sensitivity and specificity of noise field campimetry is compared with other publications. The results presented emphasize the capabilities of this new method as a screening test. They encourage the initiation of a field study using the noise field stimulus broadcasted to home TV sets.

Adolescent↗
PubMed2005

A natural approach to studying vision.

An ultimate goal of systems neuroscience is to understand how sensory stimuli encountered in the natural environment are processed by neural circuits. Achieving this goal requires knowledge of both the characteristics of natural stimuli and the response properties of sensory neurons under natural stimulation. Most of our current notions of sensory processing have come from experiments using simple, parametric stimulus sets. However, a growing number of researchers have begun to question whether this approach alone is sufficient for understanding the real-life sensory tasks performed by the organism. Here, focusing on the early visual pathway, we argue that the use of natural stimuli is vital for advancing our understanding of sensory processing.

Action Potentials↗
PubMed1999

Fractionated radiosurgery for 9L gliosarcoma in the rat brain.

PURPOSE: Fractionated radiosurgery is being carried out in the clinic to improve the therapeutic ratio of single-dose radiosurgery using various fractionation schemes. Because there is a paucity of experimental radiobiological data in the literature on the tumor response and late-responding normal tissue of critical intracranial structures to radiosurgery, the present animal study was designed to compare the response following a single high dose of radiation with that obtained from calculated fractionated doses of radiosurgery. METHODS AND MATERIALS: Male Fischer rats with 9L gliosarcoma growing in their brains were stereotactically irradiated and assayed for the tumor control rate and brain tissue damage. The radiation dose needed for 50% tumor control (TCD50) was used as the endpoint of the efficacy of radiosurgery. Normal brain damage was measured histologically following a period of time over 270 days. Histological evaluation included hematoxylin-eosin (H & E), Luxol fast blue and periodic acid Schiff (LFB/PAS) for the presence of myelin and glial fibrillary acidic protein (GFAP) for the assessment of astrocytic re-activity. The optical density of optic nerves and chiasms staining with LFB/PAS was quantitatively measured using a computer image analysis to assess the magnitude of demyelination. RESULTS: Radiosurgery (RS) was found to be more effective in curing small tumors than large tumors. The dose required to control 50% of the tumored animals for 120 days was 24, 31, and 40 Gy for 2-, 6-, and 12-day-old tumors, respectively. Using 12-day-old brain tumors, two fractions of 23.5 Gy and three fractions of 18.5 Gy were found to be equivalent to the single dose of 35 Gy for tumor control. For normal brain damages, the visual pathways including optic nerves and chiasm were found to be highly radiosensitive structures. A single dose of 35 Gy produced 100% severe optic neuropathy. The fractionated RS regimens spared substantial optic nerve damage. CONCLUSION: The present data provide a strong radiobiological rationale for the use of fractionated RS in the treatment of tumors located near critical normal structures, including visual pathways. The sparing effect of fractionated RS is greater for late-responding tissues, relative to the rapidly proliferating tumor tissues. This report also characterizes the dose/time tolerance relationship of optic neuropathy after single and fractionated RS.

Animals↗
PubMedDate not supplied

[Permanent visual loss resulting from bilateral occipital infarction after shunt malfunction].

INTRODUCTION: The commonest cause of malfunction of shunts for cerebrospinal fluid is obstruction of the catheter, which may be of subacute or sudden onset. When onset is sudden, the complication is serious as this implies risk of death or permanent damage to vision. CLINICAL CASE: We describe the case of a 13 year old girl who, 8 days after insertion of a ventriculo-peritoneal shunt valve, complained of the sudden onset of headache, reduced level of consciousness, pupil changes and permanent loss of vision secondary to a bilateral occipital infarct. CONCLUSIONS: The visual pathways and oculomotor nerves are particularly sensitive to changes in intracranial pressure and displacement of intracranial structures. Therefore, neuro-ophthalmic signs are more sensitive than computerized tomography (CT) in detection of mechanical malfunction of the valve. Although CT shows structural anomalies, it cannot assess their function. The intracranial pressure may be high enough to cause damage to the visual pathways, although no enlargement of the ventricles can be seen on CT. A normal CT, or one similar to previous scans, does not rule out malfunction of the shunt. If clinical signs of raised intracranial pressure are observed, the possibility of mechanical malfunction of the valve should be considered and this should be explored as a neurosurgical emergency in view of the risk of death or permanent loss of vision.

Adolescent↗
PubMed1992

Fast perceptual learning in visual hyperacuity.

In many different spatial discrimination tasks, such as in determining the sign of the offset in a vernier stimulus, the human visual system exhibits hyperacuity by evaluating spatial relations with the precision of a fraction of a photoreceptor's diameter. It is proposed that this impressive performance depends in part on a fast learning process that uses relatively few examples and that occurs at an early processing stage in the visual pathway. This hypothesis is given support by the demonstration that it is possible to synthesize, from a small number of examples of a given task, a simple network that attains the required performance level. Psychophysical experiments agree with some of the key predictions of the model. In particular, fast stimulus-specific learning is found to take place in the human visual system, and this learning does not transfer between two slightly different hyperacuity tasks.

Algorithms↗
PubMed1999

Brain imaging in a patient with hemimicropsia.

Hemimicropsia is an isolated misperception of the size of objects in one hemifield (objects appear smaller) which is, as a phenomenon of central origin, very infrequently reported in literature. We present a case of hemimicropsia as a selective deficit of size and distance perception in the left hemifield without hemianopsia caused by a cavernous angioma with hemorrhage in the right occipitotemporal area. The symptom occurred only intermittently and was considered the consequence of a local irritation by the hemorrhage. Imaging data including a volume-rendering MR data set of the patient's brain were transformed to the 3-D stereotactic grid system by Talairach and warped to a novel digital 3-D brain atlas. Imaging analysis included functional MRI (fMRI) to analyse the patient's visual cortex areas (mainly V5) in relation to the localization of the hemangioma to establish physiological landmarks with respect to visual stimulation. The lesion was localized in the peripheral visual association cortex, Brodmann area (BA) 19, adjacent to BA 37, both of which are part of the occipitotemporal visual pathway. Additional psychophysical measurements revealed an elevated threshold for perceiving coherent motion, which we relate to a partial loss of function in V5, a region adjacent to the cavernoma. In our study, we localized for the first time a cerebral lesion causing micropsia by digital mapping in Talairach space using a 3-D brain atlas and topologically related it to fMRI data for visual motion. The localization of the brain lesion affecting BA 19 and the occipitotemporal visual pathway is discussed with respect to experimental and case report findings about the neural basis of object size perception.

Adult↗
PubMed1997

Emotional activation of limbic circuitry in elderly normal subjects in a PET study.

OBJECTIVE: This study was undertaken to identify brain structures associated with emotion in normal elderly subjects. METHOD: Eight normal subjects aged 55-78 years were shown film clips intended to provoke the emotions of happiness, fear, or disgust as well as a neutral state. During emotional activation, regional cerebral blood flow was measured with the use of [15O]H2O positron emission tomography imaging, and subjective emotional responses were recorded. Data were analyzed by subtracting the values during the neutral condition from the values in the various emotional activations. RESULTS: The stimuli produced a general activation in visual pathways that included the primary and secondary visual cortex, involving regions associated with object and spatial recognition. In addition, the specific emotions produced different regional limbic activations, which suggests that different pathways may be used for different types of emotional stimuli. CONCLUSIONS: Emotional activation in normal elderly subjects was associated with increases in blood flow in limbic and paralimbic brain structures. Brain activation may be specific to the emotion being elicited but probably involves complex sensory, association, and memory circuitry. Further studies are needed to identify activations that are specific for emotion.

Aged↗