1/92. Effects of temporal cueing on residual visual discrimination in blindsight. We tested the ability of a blindsight patient, GY, to identify in which of two locations a target was presented in a spatial two-alternative forced choice paradigm (spatial 2AFC). On each trial the subject was asked to make a second manual response indicating whether he had had any awareness of an event occurring during the trial. A cue, presented at the fixation location, could signal the 0.4 s period over which the target appeared within the 10 s duration of each trial. Targets of three contrasts, 93, 43 and 22% were used. We found that GY's ability to discriminate the location of targets in his blind field remained significantly above chance, with and without cueing, for each contrast. Cueing, did, however, significantly improve his performance for low contrast targets. When he performed a similar task with near threshold contrast targets in his spared visual field his discrimination was at chance unless the presentation of targets was cued, despite his reporting more awareness for these stimuli than he did for low-contrast stimuli in his blind field. These results are compared with those previously reported in monkeys who received lesions to their visual cortices as infants or adults. We conclude that (1) GY's blindsight is qualitatively different from near-threshold normal vision. (2) In common with infant-lesioned monkeys his blindsight remains even in the absence of temporal cues. (3) Residual vision is subject to modulation by attentional processes, or arousal, associated with temporal cueing. ( info) |
| A 54-year-old woman with a myelodysplastic syndrome treated with high-dose chemotherapy and an allogenic bone marrow transplant developed acute cortical blindness while receiving tacrolimus (FK506). MRI showed white matter abnormalities. After discontinuation of FK506, the patient's vision returned within 8 days. FK506 neurotoxicity is similar to cyclosporine neurotoxicity and can occur in allogenic bone marrow transplant patients treated with FK506. ( info) |
3/92. visual perception of motion, luminance and colour in a human hemianope. Human patients rendered cortically blind by lesions to V1 can nevertheless discriminate between visual stimuli presented to their blind fields. Experimental evidence suggests that two response modes are involved. patients are either unaware or aware of the visual stimuli, which they are able to discriminate. However, under both conditions patients insist that they do not see. We investigate the fundamental difference between percepts derived for the normal and affected hemifield in a human hemianope with visual stimuli of which he was aware. The psychophysical experiments we employed required the patient, GY, to make comparisons between stimuli presented in his affected and normal hemifields. The subject discriminated between, and was allowed to match, the stimuli. Our study reveals that the stimulus parameters of colour and motion can be discriminated and matched between the normal and blind hemifields, whereas brightness cannot. We provide evidence for associations between the percepts of colour and motion, but a dissociation between the percepts of brightness, derived from the normal and hemianopic fields. Our results are consistent with the proposal that the perception of different stimulus attributes is expressed in activity of functionally segregated visual areas of the brain. We also believe our results explain the patient's insistence that he does not see stimuli, but can discriminate between them with awareness. ( info) |
4/92. Positron emission tomography scan in cortical visual loss in patients with organophosphate intoxication. OBJECTIVE: To determine the cerebral metabolism of patients with cortical visual loss. DESIGN: Two observational case studies. TESTING: Two patients who survived acute organophosphate poisoning with respiratory failure experienced severe visual loss despite relatively normal ophthalmic examination results. magnetic resonance imaging of the brain revealed no abnormality of the visual system in either patient. Positron emission tomography (PET) was performed in these 2 patients and in 12 normal subjects with fluorine-18 fluorodeoxyglucose (FDG) as a tracer to measure cerebral glucose metabolism for the estimation of neurologic deficit in the visual cortex. MAIN OUTCOME MEASURES: The FDG uptake values were measured as nanoCurie per cubic centimeters of tissue (nCi/cc). The relative uptake index in visual cortex was computed as the ratio of uptake of FDG in each region of visual cortex to that of cerebellum (regional visual cortex/cerebellum). RESULTS: Hypometabolism was observed in the visual cortex of both patients. The relative uptake index of FDG in visual cortex (visual cortex/cerebellum) was significantly decreased in those patients compared with normal subjects. CONCLUSIONS: In patients with cortical visual loss, conventional neuroimaging techniques can fail to visualize damage that can be detected by PET scanning, and PET analysis may be helpful in estimating the metabolic deficit of visual cortex and in establishing the organic nature of cortical visual loss in these patients. ( info) |
| The coincidence of hellp syndrome and cortical blindness is an uncommon but very dramatic event, for the patient as well as the obstetrician. This report describes the first case of HELLP-syndrome-associated cortical blindness occuring suddenly in the third stage of labour. There were only modest correlates of cortical blindness in cerebral CT, MRI and angiography findings, but no signs of a posterior leucoencephalopathy syndrome. Mother and baby were discharged from hospital to outpatient care in good health on the 12th day. ( info) |
6/92. An unusual case of cortical blindness associated with aortography--a case report. Transient cortical blindness is a rare but well-recognized benign complication of angiography that is due to neurotoxicity of the contrast agent. The blindness completely resolves in a few days. This report describes a patient suffering an unusual course of cortical blindness following aortography resulting in permanent, partial blindness. It was found that blindness was the consequence of bilateral occipital embolisms. This case emphasizes that cortical blindness associated with angiography does not always have a favorable outcome, since it might be, in rare cases, due to isolated occipital embolisms, which initially produce a clinical picture identical to that of the neurotoxic effect of contrast agents. ( info) |
7/92. Visual hallucinations in recovery from cortical blindness: imaging correlates. OBJECTIVE: To investigate the cerebral metabolic and functional patterns during recovery from cortical blindness. DESIGN: Follow-up study with serial clinical, metabolic, and functional imaging and visual evoked potentials. CASE PRESENTATION: A 24-year-old woman suffered from cortical blindness after cardiac arrest and recovered over a 6-month period. During recovery, she experienced complex visual hallucinations that could be initiated by visual imagery. RESULTS: Initially, the regional cerebral metabolic rate of glucose was severely reduced in the visual and parieto-occipital cortex bilaterally but recovered almost completely. Visual hallucinations led to significant increases of the regional cerebral blood flow in the initially severely hypometabolic parieto-occipital and temporo-lateral cortex. CONCLUSIONS: Recovery of vision was related to normalization of the postlesionally dysfunctional cortex. Visual hallucinations appeared as the clinical correlate of the electrophysiological hyperexcitability of the recovering partially damaged visual cortex. ( info) |
| A case of presumed embolic transient ischemic episodes and multifocal infarcts to the occipital and parietal cortices and the cerebellum of a young woman with ulcerative colitis is reported. These episodes were manifested by multifocal neurologic deficits including cortical blindness, visual hallucinations, and homonymous hemianopsia. They correlated with parenteral nutrition via a central line, presumed venous, but found to be in the subclavian artery. The complications of central venous lines are reviewed. The need for attention to neighborhood structures and unexpected symptoms, in view of the less well-recognized arterial embolic complications is emphasized. ( info) |
| BACKGROUND: Cortical blindness is a complication of severe preeclampsia, but it is unclear whether it results from cerebral vasospasm and ischemic injury or vasogenic (hydrostatic) edema due to increased capillary permeability. CASE: Reversible cortical blindness in a 33-year-old gravida 2, para 1, with severe postpartum preeclampsia after evacuation of a partial molar pregnancy at 19 weeks' gestation is presented. Initial neuroimaging studies showed hyperperfusion on head single-photon-emission computed tomography scan, which corresponded with lesions found on head computed tomography and magnetic resonance imaging scans. Follow-up neuroimaging studies 2 weeks later, by which time the patient's visual acuity had returned to normal, showed complete resolution of radiologic abnormalities. CONCLUSION: neuroimaging studies in a woman with severe postpartum preeclampsia complicated by reversible cortical blindness showed that blindness resulted from vasogenic (hydrostatic) cerebral edema and not cerebral vasospasm. ( info) |
10/92. Intact verbal description of letters with diminished awareness of their forms. Visual processing and its conscious awareness can be dissociated. To examine the extent of dissociation between ability to read characters or words and to be consciously aware of their forms, reading ability and conscious awareness for characters were examined using a tachistoscope in an alexic patient. A right handed woman with 14 years of education presented with incomplete right hemianopia, alexia with kanji (ideogram) agraphia, anomia, and amnesia. brain MRI disclosed cerebral infarction limited to the left lower bank of the calcarine fissure, lingual and parahippocampal gyri, and an old infarction in the right medial frontal lobe. Tachistoscopic examination disclosed that she could read characters aloud in the right lower hemifield when she was not clearly aware of their forms and only noted their presence vaguely. Although her performance in reading kanji was better in the left than the right field, she could read kana (phonogram) characters and Arabic numerals equally well in both fields. By contrast, she claimed that she saw only a flash of light in 61% of trials and noticed vague forms of stimuli in 36% of trials. She never recognised a form of a letter in the right lower field precisely. She performed judgment tasks better in the left than right lower hemifield where she had to judge whether two kana characters were the same or different. Although dissociation between performance of visual recognition tasks and conscious awareness of the visual experience was found in patients with blindsight or residual vision, reading (verbal identification) of characters without clear awareness of their forms has not been reported in clinical cases. Diminished awareness of forms in our patient may reflect incomplete input to the extrastriate cortex. ( info) |