The clinical and functional measurement of cortical (in)activity in the visual brain, with special reference to the two subdivisions (V4 and V4 alpha) of the human colour centre.
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We argue below that, at least in studying the visual brain, the old and simple methods of detailed clinical assessment and perimetric measurement still yield important insights into the organization of the visual brain as a whole, as well as the organization of the individual areas within it. To demonstrate our point, we rely especially on the motion and colour systems, emphasizing in particular how clinical observations predicted an important feature of the organization of the colour centre in the human brain. With the use of data from functional magnetic resonance imaging analysed by statistical parametric mapping and independent component analysis, we show that the colour centre is composed of two subdivisions, V4 and V4 alpha the two together constituting the V4 complex of the human brain. These two subdivisions are intimately linked anatomically and act cooperatively. The new evidence about the architecture of the colour centre might help to explain why the syndrome, cerebral achromatopsia, produced by lesions in it is so variable. (+info)
The association of visual field deficits and visuo-spatial neglect in acute right-hemisphere stroke patients.
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BACKGROUND: Visuo-spatial neglect (VSN) after stroke is associated with a poor prognosis for rehabilitation. The co-existence of a visual field deficit (VFD) with VSN may be associated with impaired visuo-spatial functioning and thereby poor functional outcome. OBJECTIVE: To determine whether the presence of a VFD (i) exacerbates VSN and (ii) influences recovery of VSN. METHODS: A prospective study of consecutive acute (<7 days), right-hemisphere stroke patients who were able to undergo detailed assessment of visuo-spatial functioning and visual fields. Clinical assessment and a standardized neuropsychological test was administered by one observer, followed by independent assessment of visual fields by a second observer. Patients were followed up for 12 weeks with 4-weekly re-assessments. RESULTS: 44 consecutive patients (23 women) with a first in a lifetime, acute hemisphere stroke were recruited. Twenty had VSN and VFD, seven VSN only, one VFD only and 17 had normal visual fields and no neglect. The finding of a VFD was significantly associated with the presence of VSN (P<0.0001). Patients with both VFD and VSN had a significantly lower score on the behavioural inattention test. One month post-stroke, this difference was no longer significant. Recovery of VSN and VFD was maximal in the first month, however VSN recovery continued for up to 12 weeks. Patients with VSN and a VFD on admission had a greater mortality at 1 and 3 months. CONCLUSION: The presence of a VFD does appear to exacerbate neglect in the acute stroke patient; this effect is no longer seen after 1 month. Recovery of VSN continues independent of a VFD. Patients with neglect and a VFD have an increased mortality, probably because of greater neurological impairment. (+info)
Ganglion cell losses underlying visual field defects from experimental glaucoma.
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PURPOSE: To investigate the relationship between ganglion cell losses and visual field defects caused by glaucoma. METHODS: Behavioral perimetry and histology data were obtained from 10 rhesus monkeys with unilateral experimental glaucoma that was induced by argon laser treatments to their trabecular meshwork. After significant visual field defects had developed, the retinas were collected for histologic analysis. The ganglion cells were counted by light microscopy in cresyl violet-stained retina sections, and the percentage of ganglion cell loss (treated to control eye counts) was compared with the depth of visual field defect (treated to control eye thresholds) at corresponding retinal and perimetry test locations. Sensitivity losses as a function of ganglion cell losses were analyzed for Goldmann III, white and Goldmann V, and short- and long-wavelength perimetry test stimuli. RESULTS: The relationship between the proportional losses of ganglion cells and visual sensitivity, measured with either white or colored stimuli, was nonlinear. With white stimuli, the visual sensitivity losses were relatively constant (approximately 6 dB) for ganglion cell losses of less than 30% to 50%, and then with greater amounts of cell loss the visual defects were more systematically related to ganglion cell loss (approximately 0.42 dB/percent cell loss). The forms of the neural-sensitivity relationships for visual defects measured with short- or long-wavelength perimetry stimuli were similar when the visual thresholds were normalized to compensate for differences in expected normal thresholds for white and colored perimetry stimuli. CONCLUSIONS: Current perimetry regimens with either white or monochromatic stimuli do not provide a useful estimate of ganglion cell loss until a substantial proportion have died. The variance in ganglion cell loss is large for mild defects that would be diagnostic of early glaucoma and for visual field locations near the fovea where sensitivity losses occur relatively late in the disease process. The neural-sensitivity relationships were essentially identical for both white and monochromatic test stimuli, and it therefore seems unlikely that the higher sensitivity for detecting glaucoma with monochromatic stimuli is based on the size-dependent susceptibility of ganglion cells to injury from glaucoma. (+info)
Risk factors for late presentation in chronic glaucoma.
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PURPOSE: To identify the risk factors for having advanced glaucomatous visual field loss on the first visit at three hospital eye services. METHODS: This was a hospital-based, case-control study involving patients newly diagnosed with glaucoma at first visit to one of three ophthalmic departments in the United Kingdom. Patients with a previous history of ocular hypertension or any documented suspicion of glaucoma (within the hospital eye service) were excluded. RESULTS: Occupational group, initial intraocular pressure (IOP), family history of glaucoma, method of referral to hospital, and the number of years since the last visit to an optometrist were found to be independently associated with late presentation. A linear trend of increasing odds of late attendance was associated with increasing Standard Occupational Classification. Those in managerial (category II) and skilled (category III) groups estimated (95% confidence intervals) to be, respectively. 0.2 (0.00, 0.16) and 0.27 (0.1, 0.8) as likely to attend with advanced glaucomatous field loss as unskilled (category V) people with similar initial IOP, family history, referral route, and time since last optometrist visit. The data strongly suggest an association between IOP and advanced field loss at initial hospital examination. There was a 1.2 (1.12, 1.28) increase in the OR of late presentation per unit increase in millimeters of mercury after adjustment for the other mentioned factors. People with a family history of glaucoma were estimated to be almost one third (adjusted OR, 0.29 [0.12, 0.74]) as likely to have advanced field loss as those with no family history. People referred by any source other than an optometrist who has made the correct diagnosis of glaucoma were 4.5 times (adjusted OR, 4.53 [1.52, 13.48]) more likely to be late attenders than patients so referred but similar in other mentioned factors. These data also provide strong evidence that the more years since the last visit to an optometrist, the greater the likelihood of having advanced glaucomatous visual field loss on the first visit to the eye service (adjusted OR per year, 1.25 [1.10, 1.42]). CONCLUSIONS: These data strongly suggest that certain subgroups of people with glaucoma were at greater risk of having advanced and irremediable field loss on first visiting the eye services studied. (+info)
Asymmetric responses in cortical visually evoked potentials to motion are not derived from eye movements.
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PURPOSE: Normal neonates and many adults after abnormal visual development have directional preferences for visual stimulus motions; i.e., they give better responses for optokinetic nystagmus (OKN) and visually evoked potentials (VEPs) in one direction than to those in the opposite direction. The authors tested whether the VEP responses were asymmetrical because of abnormal eye movements. METHODS: VEPs were recorded from the visual cortices of five macaque monkeys: one normal, one neonate, and three reared with alternating monocular occlusion (AMO). They were lightly anesthetized, followed by paralysis to prevent eye movements. They then had "jittered" vertical grating patterns presented in their visual fields. The steady state VEPs were analyzed with discrete Fourier transforms to obtain the amplitudes and phases of the asymmetries. RESULTS: The normal, control monkey had small, insignificant amplitudes of its asymmetrical Fourier component and random phases that were not 180 degrees out of phase across the left and right eyes. The neonatal monkey and the AMO monkeys all had large, significant asymmetries that were approximately 180 degrees out of phase between the left and right eyes. CONCLUSIONS: The neonate and abnormally reared monkeys continued to have asymmetrical responses even after their eyes were paralyzed. Therefore, eye movements cannot be the source of the asymmetrical amplitudes of the VEPs, and the visual cortex is at least one source responsible for asymmetries observed in neonates and adults reared under abnormal visual inputs. (+info)
The nondiscriminating zone of directionally selective retinal ganglion cells: comparison with dendritic structure and implications for mechanism.
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We have studied, at high resolution, the sizes and pattern of dendrites of directionally selective retinal ganglion cells in the rabbit. The dendrites had a distinctive pattern of branching. The major dendritic trunks were relatively thick, beginning at approximately 1 micrometer and tapering to approximately 0.5 micrometer in diameter. Higher order dendrites exiting from them generally stepped abruptly to a diameter of 0.4-0.6 micrometer, which they maintained throughout their length. Recording confirmed the existence of a zone within the receptive field, usually occupying 20-25% of its area, where direction of movement was only weakly discriminated. The dendritic arbors of cells, injected with Lucifer yellow after recording, revealed no difference in dendritic structure between the discriminating and nondiscriminating zones. The nondiscriminating zone was located on the preferred side of the receptive field (the side from which movement in the preferred direction originates). This is consistent with a mechanism of direction selectivity based on inhibition generated by movement in the null direction but not with feedforward excitation, as occurs in flies and is postulated in some models of mammalian direction selectivity. (+info)
Effects of attention on the reliability of individual neurons in monkey visual cortex.
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To determine the physiological mechanisms underlying the enhancement of performance by attention, we examined how attention affects the ability of isolated neurons to discriminate orientation by investigating the reliability of responses with and without attention. Recording from 262 neurons in cortical area V4 while two rhesus macaques did a delayed match-to-sample task with oriented stimuli, we found that attention did not produce detectable changes in the variability of neuronal responses but did improve the orientation discriminability of the neurons. We also found that attention did not change the relationship between burst rate and response rate. Our results are consistent with the idea that attention selects groups of neurons for a multiplicative enhancement in response strength. (+info)
Cavernous angioma of the optic chiasm--case report.
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A 31-year-old female presented with cavernous angioma originating from the optic chiasm manifesting as sudden onset of right retroorbital pain and right visual disturbance. She had a psychomotor seizure 10 years ago. Cavernous angioma at the right basal ganglia had been partially removed at that time. After the operation, the patient had left hemiparesis, but gradually improved. Neurological examination revealed decreased right visual acuity, left homonymous hemianopsia, and left hemiparesis. Magnetic resonance imaging revealed a mixed signal intensity mass at the right optic nerve to the optic chiasm with a low signal intensity rim on T2-weighted imaging, situated at the right basal ganglia where the cavernous angioma had been partially resected. Right frontotemporal craniotomy was performed by the pterional approach. A subpial hematoma was situated at the right optic nerve to the optic chiasm. The hematoma with an angiomatous component was completely resected from the surrounding structure. Histological examination of the specimens confirmed cavernous angioma. Postoperatively, her right visual acuity was slightly improved, but the visual field defect was unchanged. We emphasize the importance of correct diagnosis by magnetic resonance imaging and subsequent resection for preserving and improving the visual function of patients with cavernous angiomas of the optic chiasm. (+info)