1/8. 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.- - - - - - - - - - ranking = 1keywords = motion (Clic here for more details about this article) |
2/8. Visual loss in pregnancy.A 36-year-old preeclamptic woman presented with bilateral visual loss upon recovery from anesthesia following cesarean section. visual acuity was hand motion OU. Pupillary responses to light were normal, without a relative afferent pupillary defect. Treatment consisted of control of blood pressure and the patient's vision improved to 20/25 OU within 3 days.- - - - - - - - - - ranking = 0.16666666666667keywords = motion (Clic here for more details about this article) |
3/8. Creutzfeldt-Jakob disease : report of 10 cases from North india.Creutzfeldt-Jakob disease (CJD) is increasingly being reported over the last three decades as a result of heightened awareness of the disease. Various studies have reported annual incidence of 0.5-1.5 cases of CJD per million of general population. In india, the disease is still under reported. Over the period spanning from 1968-1997, National Institute of mental health and neurosciences (NIMHANS), Bangalore recorded 69 cases of CJD from different parts of india in the CJD registry. This paper describes the clinical experience with cases of CJD managed at the Department of neurology, G.B. Pant Hospital, New Delhi from 1990-1998. In this series, the mean age of the patients was 53.80 ( /- 7.32) years and there were 5 females and 5 males. myoclonus was present in all the cases and abnormal behaviour with or without other features was the presenting complaint in 7 of the 10 patients, while one patient of CJD had cerebellar ataxia as the presenting feature. One patient with occipital variant of CJD presented with acute onset cortical blindness and myoclonic jerks. One of the patients had acute psychosis precipitated by emotional stress at the onset. Extrapyramidal features were noted in 7 of the 10 patients before death. The mean duration of symptoms from the onset of disease to death was 6.6 ( /- 6.11) months. Classical EEG changes were observed in all the patients, except in one possible case of occipital variant of CJD, where we did not have access to EEG record. brain biopsy could be undertaken in 3 patients, and in 2 patients the features of subacute spongiform encephalopathy (SSE) were noted.- - - - - - - - - - ranking = 0.16666666666667keywords = motion (Clic here for more details about this article) |
4/8. Direction-specific motion blindness induced by focal stimulation of human extrastriate cortex.Motion blindness (MB) or akinetopsia is the selective disturbance of visual motion perception while other features of the visual scene such as colour and shape are normally perceived. Chronic and transient forms of MB are characterized by a global deficit of direction discrimination (pandirectional), which is generally assumed to result from damage to, or interference with, the motion complex MT /V5. However, the most characteristic feature of primate MT-neurons is not their motion specificity, but their preference for one direction of motion (direction specificity). Here, we report that focal electrical stimulation in the human posterior temporal lobe selectively impaired the perception of motion in one direction while the perception of motion in other directions was completely normal (unidirectional MB). In addition, the direction of MB was found to depend on the brain area stimulated. It is argued that direction specificity for visual motion is not only represented at the single neuron level, but also in much larger cortical units.- - - - - - - - - - ranking = 1.8333333333333keywords = motion (Clic here for more details about this article) |
5/8. Affective blindsight: intact fear conditioning to a visual cue in a cortically blind patient.Blindsight refers to remarkable residual visual abilities of patients with damage to the primary visual cortex (V1). Recent studies revealed that such residual abilities do not apply only to relatively simple object discriminations, but that these patients can also differentially categorize and respond to emotionally salient stimuli. The current study reports on a case of intact fear conditioning to a visual cue in a male patient with complete bilateral cortical blindness. The patient was admitted to the stroke unit of the neurological department because of complete loss of vision. Both CT and structural MRI scans confirmed lesions in both territories of the posterior cerebral artery. No visual evoked potentials could be detected confirming complete cortical blindness. During fear conditioning, a visual cue predicted the occurrence of an aversive electric shock. Acoustic startle probes were presented during and between the conditioned stimuli. Relative to the control condition, startle reflexes were substantially potentiated when elicited in the presence of the conditioned stimuli. No such potentiation was observed prior to conditioning. These data suggest that fear learning to visual cues does not require a cortical representation of the conditioned stimulus in the primary sensory cortex and that subcortical pathways are sufficient to activate the fear module in humans.- - - - - - - - - - ranking = 0.16666666666667keywords = motion (Clic here for more details about this article) |
6/8. Conscious visual abilities in a patient with early bilateral occipital damage.A 21-year-old male presented with occipital lobes that were extensively damaged by bilateral infarcts present at birth. The absence of the striate cortex was confirmed with anatomic and functional MRI and high-resolution EEG. His cortical visual impairment was severe, but he retained a remarkable ability to see fast-moving stimuli. Horizontal optokinetic nystagmus could be elicited from either eye. Resolution acuity was close to normal providing the patient was allowed to move his head and eyes. The direction of motion in random-dot patterns could be discriminated with perfect accuracy at speeds above 2 deg/s, and the patient reported that he could 'see' the motion at fast but not at slow speeds. This conscious residual vision for motion is known as Riddoch's phenomenon, but it has never been reported in the complete absence of the striate cortex. functional neuroimaging revealed activation that was outside the motion-responsive regions of the extrastriate cortex. This case demonstrates remarkable plasticity in the human visual system and may have implications for understanding the functional organization of the motion pathways.- - - - - - - - - - ranking = 0.83333333333333keywords = motion (Clic here for more details about this article) |
7/8. Cortical blindness.A 79-year-old woman with persistent cortical blindness caused by bilateral temporo-occipital infarctions was followed for 8 months. She had no light or visual motion perception. Our patient's visual imagery was intact, which was demonstrated when drawing elementary shapes; however, her drawing of objects was poor. Optokinetic nystagmus could not be elicited, but the vestibulo-ocular reflex (VOR) seemed intact. Although she was able to suppress her VOR by 'fixating' her outstretched hand which moved in phase with her head and body, she showed saccadic tracking eye movements in an attempt to visually 'follow' the self-generated movements of her outstretched hand, while her body and head were stationary. Such saccadic tracking seemed to be different from the previously described smooth tracking of self-moved targets by patients with acquired blindness caused by anterior visual pathway dysfunction.- - - - - - - - - - ranking = 0.16666666666667keywords = motion (Clic here for more details about this article) |
8/8. Cortical color blindness is not "blindsight for color".Cortical color blindness, or cerebral achromatopsia, has been likened by some authors to "blindsight" for color or an instance of "covert" processing of color. Recently, it has been shown that, although such patients are unable to identify or discriminate hue differences, they nevertheless show a striking ability to process wavelength differences, which can result in preserved sensitivity to chromatic contrast and motion in equiluminant displays. Moreover, visually evoked cortical potentials can still be elicited in response to chromatic stimuli. We suggest that these demonstrations reveal intact residual processes rather than the operation of covert processes, where proficient performance is accompanied by a denial of phenomenal awareness. We sought evidence for such covert processes by conducting appropriate tests on achromatopsic subject M.S. An "indirect" test entailing measurement of reaction times for letter identification failed to reveal covert color processes. In contrast, in a forced choice oddity task for color, M.S. was unable to verbally indicate the position of the different color, but was surprisingly adept at making an appropriate eye movement to its location. This "direct" test thus revealed the possible covert use of chromatic differences.- - - - - - - - - - ranking = 0.16666666666667keywords = motion (Clic here for more details about this article) |