The Experts below are selected from a list of 1710 Experts worldwide ranked by ideXlab platform
Alan Cowey - One of the best experts on this subject based on the ideXlab platform.
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Isoluminant coloured stimuli are undetectable in Blindsight even when they move
Experimental brain research, 2012Co-Authors: Iona Alexander, Alan CoweyAbstract:Moving stimuli are the most effective of all in eliciting Blindsight. The detection of static luminance-matched coloured stimuli is negligible or even impossible in Blindsight. However, moving coloured stimuli on an achromatic background have not been tested. We therefore tested two Blindsighted hemianopes, one of them highly experienced and the other much less so, to determine whether they could perform what should be one of the simplest of all motion tasks: detecting when an array of coloured stimuli moves. On each trial, they were presented in the hemianopic field with an array of spots, all red or green or blue or achromatic, in a circular window and on a white surround. The spots moved coherently in the first or second of two short intervals. The subject had to indicate the interval in which the motion had occurred. The luminance of the spots was varied across different blocks of trials, but the background luminance remained the same throughout. For each colour, there was a ratio of luminance between the spots and the white surround at which performance was not significantly better than chance, although at other ratios, performance was good to excellent, with the exception of blue spots in one subject. We conclude that detecting global coherent motion in Blindsight is impossible when it is based on chromatic contrast alone.
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Visual system: how does Blindsight arise?
Current biology : CB, 2010Co-Authors: Alan CoweyAbstract:Some patients can discriminate unseen visual stimuli within a field defect caused by damage to the primary visual cortex. The pathways for this 'Blindsight' have never been established, but recent studies implicate hitherto overlooked cells in the thalamic LGN.
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Edges, colour and awareness in Blindsight.
Consciousness and cognition, 2010Co-Authors: Iona Alexander, Alan CoweyAbstract:It remains unclear what is being processed in Blindsight in response to faces, colours, shapes, and patterns. This was investigated in two hemianopes with chromatic and achromatic stimuli with sharp or shallow luminance or chromatic contrast boundaries or temporal onsets. Performance was excellent only when stimuli had sharp spatial boundaries. When discrimination between isoluminant coloured Gaussians was good it declined to chance levels if stimulus onset was slow. The ability to discriminate between instantaneously presented colours in the hemianopic field depended on their luminance, indicating that wavelength discrimination totally independent of other stimulus qualities is absent. When presented with narrow-band colours the hemianopes detected a stimulus maximally effective for S-cones but invisible to M- and L-cones, indicating that Blindsight is mediated not just by the mid-brain, which receives no S-cone input, or that the rods contribute to Blindsight. The results show that only simple stimulus features are processed in Blindsight.
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The Blindsight saga
Experimental Brain Research, 2010Co-Authors: Alan CoweyAbstract:Blindsight is the ability of patients with clinically blind field defects, caused by damage to the primary visual cortex V, to detect, localise and even discriminate visual stimuli that they deny seeing. Blindsight tells us much about the nature of visual processing in the absence of the primary visual cortex and is a paradigmatic example of implicit knowledge. It has attracted widespread interest and debate amongst philosophers, cognitive neuropsychologists and visual neuroscientists. Its downside is that possible artefacts abound, much more so than with examples of implicit memory or deaf hearing and numb touch. Unfortunately the artefacts are still frequently ignored, or dismissed as captious, with the result that many of the genuine qualities of Blindsight remain uncertain. Now that Blindsight in monkeys has been established the substantial literature on the effects of removing parts or all of V1 in monkeys on the residual physiological cerebral responses to visual stimuli in their field defects is at last directly relevant to human Blindsight. Whether Blindsight is, or could be, useful in everyday life is the next unsolved problem.
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The cortical basis of global motion detection in Blindsight
Experimental brain research, 2008Co-Authors: Iona Alexander, Alan CoweyAbstract:The importance of stimulus qualities such as orientation, motion and luminance in Blindsight are well known but their cortical basis has been much less explored. We therefore studied the performance of two Blindsighted hemianopic subjects (GY and MS), in a task in which the subject had to decide in which of two adjunctive intervals a pattern of global spots moved coherently, at a variety of speeds, in the hemianopic field. Their ability was compared with that of two control subjects with normal vision. Both hemianopes performed this simple discrimination well in their blind fields but their performance was impaired by repetitive transcranial magnetic stimulation (rTMS) applied over cortical area hV5/MT+ although not, or only slightly, by stimulation over the region of V3 or the vertex. The result is a direct demonstration that area hV5/MT+ is necessary for global motion detection in Blindsight.
Lawrence Weiskrantz - One of the best experts on this subject based on the ideXlab platform.
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Pupil response as a predictor of Blindsight in hemianopia
Proceedings of the National Academy of Sciences of the United States of America, 2013Co-Authors: Arash Sahraie, Ceri T. Trevethan, Mary Joan Macleod, James Urquhart, Lawrence WeiskrantzAbstract:Significantly above-chance detection of stimuli presented within the field defect of patients with postgeniculate lesions is termed "Blindsight." It has been proposed that those with Blindsight are more likely to benefit from visual rehabilitation by repeated stimulation, leading to increased visual sensitivity within their field defect. Establishing the incidence of Blindsight and developing an objective and reliable method for its detection are of great interest. Sudden onsets of a grating pattern in the absence of any change in light flux result in a transient constriction of the pupil, termed "pupil grating response." The existence of pupil grating responses for stimuli presented within the blindfield has previously been reported in a hemianopic patient and two monkeys with removal of the primary visual cortex unilaterally. Here, we have systematically investigated the presence of a spatial channel of processing at a range of spatial frequencies using a psychophysical forced-choice technique and obtained the corresponding pupil responses in the blindfield of 19 hemianopic patients. In addition, in 13 cases we determined the pupil responses in a sighted field location that matched the blindfield eccentricities. Our findings demonstrate that blindfield pupil responses are similar to those for the sighted field, but attenuated in amplitude. Pupillometry correctly characterized the presence or absence of a significant psychophysical response and thus is worth measuring in the cortically blindfields as a predictor of intact psychophysical capacity. The incidence of Blindsight where detection performance had been investigated psychophysically over a range of spatial frequencies was 70%.
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Consciousness of the first order in Blindsight
Proceedings of the National Academy of Sciences of the United States of America, 2010Co-Authors: Arash Sahraie, Ceri T. Trevethan, Paul B. Hibbard, Kay L. Ritchie, Lawrence WeiskrantzAbstract:At suprathreshold levels, detection and awareness of visual stimuli are typically synonymous in nonclinical populations. But following postgeniculate lesions, some patients may perform above chance in forced-choice detection paradigms, while reporting not to see the visual events presented within their blind field. This phenomenon, termed “Blindsight,” is intriguing because it demonstrates a dissociation between detection and perception. It is possible, however, for a Blindsight patient to have some “feeling” of the occurrence of an event without seeing per se. This is termed Blindsight type II to distinguish it from the type I, defined as discrimination capability in the total absence of any acknowledged awareness. Here we report on a well-studied patient, D.B., whose Blindsight capabilities have been previously documented. We have found that D.B. is capable of detecting visual patterns defined by changes in luminance (first-order gratings) and those defined by contrast modulation of textured patterns (textured gratings; second-order stimuli) while being aware of the former but reporting no awareness of the latter. We have systematically investigated the parameters that could lead to visual awareness of the patterns and show that mechanisms underlying the subjective reports of visual awareness rely primarily on low spatial frequency, first-order spatial components of the image.
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Is Blindsight just degraded normal vision
Experimental brain research, 2008Co-Authors: Lawrence WeiskrantzAbstract:It is a conservative and reasonable suggestion that implicit functioning, as in Blindsight, is simply a weakened, degraded form of normal functioning, especially as the parameters of vision in Blindsight are themselves weakened--e.g., acuity is reduced, detection thresholds are raised, chromatic discrimination is coarser. But does this mean that maximum performance in Blindsight is itself uniformly degraded? The answer is no, because there are many examples of very good performance in Blindsight, in the absence of acknowledged awareness, especially if the visual parameters are chosen to be within a selective range. The question then is raised whether the parameters of Blindsight vision are qualitatively similar to normal vision, or even to any manifestation of normal vision when it is uniformly degraded, for example, by threshold rises or injection of noise? Evidence is provided that Blindsight vision and normal vision, even if it were degraded, are qualitatively different in certain respects. For example, there can be selective loss of colour but not of luminance contrast; visible after-images to unseen stimuli ("prime-sight") can be observed in one subject (DB); there can be super-sensitivity in the blind hemifield that is better than that of the intact hemifield; there can be a change in S-cone retinal inputs. Even when the stimulus contrast is deliberately lowered, Blindsight performance itself does not degrade in parallel. Another meaning of "degraded" is that even when Blindsight discriminatory or detection ability is excellent, the subject's acknowledged experience is as though the stimuli are weak and degraded to the point of extinction or near-extinction. But this is akin to the very meaning of Blindsight itself, and sets the problem to be solved in neural systems terms and philosophical analysis rather than providing a conservative solution.
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Form discrimination in a case of Blindsight.
Neuropsychologia, 2007Co-Authors: Ceri T. Trevethan, Arash Sahraie, Lawrence WeiskrantzAbstract:DB, the first extensively tested Blindsight case, has demonstrated the ability to detect and discriminate a range of visual stimuli presented within his perimetrically blind visual field [Weiskrantz L. (1986). Blindsight: A case study and implications. Oxford: Oxford University Press]. After initial reports of basic form discrimination (Weiskrantz, 1986), it later emerged that this ability to discriminate single stimuli presented to the blind field was probably based on the discrimination of orientation cues [Weiskrantz, L. (1987). Residual vision in a scotoma. A follow-up study of 'form' discrimination. Brain, 110, 77-92]. Even so, DB found it impossible to make a 'same/different' discrimination for pairs of stimuli in the blind field (Weiskrantz, 1986). In the current study, DB's discrimination and identification of stimuli on the basis of their form was tested (in 2AFC and FR paradigms). We have demonstrated that DB could successfully identify outline low contrast images of objects, make successful 'same/different' discriminations for pairs of stimuli presented in his blind field and identify complex images (digital photographs) presented entirely within his cortically blind field. The results are discussed in relation to the issues of likely neuronal mediation, the potential for improvement in cases of Blindsight and the potential relevance of these findings in relation to theories of normal visual perception.
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increased sensitivity after repeated stimulation of residual spatial channels in Blindsight
Proceedings of the National Academy of Sciences of the United States of America, 2006Co-Authors: Arash Sahraie, Ceri T. Trevethan, Alison D. Murray, Mary Joan Macleod, John A Olson, Lawrence WeiskrantzAbstract:Lesions of the occipital cortex result in areas of cortical blindness affecting the corresponding regions of the patient's visual field. The traditional view is that, aside from some spontaneous recovery in the first few months after the damage, when acute effects have subsided the areas of blindness are absolute and permanent. It has been found, however, that within such field defects some residual visual capacities may persist in the absence of acknowledged awareness by the subject (Blindsight type 1) or impaired awareness (type 2). Neuronal pathways mediating Blindsight have a specific and narrow spatial and temporal bandwidth. A group of cortically blind patients (n = 12) carried out a daily detection "training" task over a 3-month period, discriminating grating visual stimuli optimally configured for Blindsight from homogeneous luminance-matched stimuli. No feedback was given during the training. Assessment of training was by psychophysical measurements carried out before and after training and included detection of a range of spatial frequencies (0.5-7 cycles per degree), contrast detection at 1 cycle per degree, clinical perimetry, and subjective estimates of visual field defect. The results show that repeated stimulation by appropriate visual stimuli can result in improvements in visual sensitivities in the very depths of the field defect.
Shoogo Ueno - One of the best experts on this subject based on the ideXlab platform.
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Functional brain measurements within the prefrontal area on pseudo-“Blindsight” induced by extremely low frequency electromagnetic stimulations
Journal of Applied Physics, 2015Co-Authors: Hidenori Nakagawa, Shoogo UenoAbstract:For evaluating the effects of phosphene as pseudo-Blindsight closely, we used functional near-infrared spectroscopy to investigate whether or not the phosphene appearance itself substantially affects the hemodynamic responses of the prefrontal area. Seven healthy volunteers ranging in age from 22 to 72 participated in the visual stimulation experiments. First, we examined the influences of electromagnetic stimulations at around the threshold (10 mT) for a Blindsight-like phosphene on the responses. According to the results of the aged volunteers, we found the possibility that the delay in the phosphene perception might be caused by aging beyond a certain age. In the results of our measurements using the stimulation of 50 mT, no significant difference in the perception delay for all the volunteers could be detected. When the field strength was decreased from 50 mT to the threshold in steps of 10 mT, the results obtained at the threshold are equivalent to that obtained at 50 mT. Our data strongly support th...
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functional brain measurements within the prefrontal area on pseudo Blindsight induced by extremely low frequency electromagnetic stimulations
Journal of Applied Physics, 2015Co-Authors: Hidenori Nakagawa, Shoogo UenoAbstract:For evaluating the effects of phosphene as pseudo-Blindsight closely, we used functional near-infrared spectroscopy to investigate whether or not the phosphene appearance itself substantially affects the hemodynamic responses of the prefrontal area. Seven healthy volunteers ranging in age from 22 to 72 participated in the visual stimulation experiments. First, we examined the influences of electromagnetic stimulations at around the threshold (10 mT) for a Blindsight-like phosphene on the responses. According to the results of the aged volunteers, we found the possibility that the delay in the phosphene perception might be caused by aging beyond a certain age. In the results of our measurements using the stimulation of 50 mT, no significant difference in the perception delay for all the volunteers could be detected. When the field strength was decreased from 50 mT to the threshold in steps of 10 mT, the results obtained at the threshold are equivalent to that obtained at 50 mT. Our data strongly support the hypothesis that pseudo-Blindsight induced by electromagnetic stimulation of above 50 mT is able to excite all the volunteers' retinal photoreceptor cells provisionally. Hence the continuous stimulations for a long period of time might gradually activate synaptic plasticity on the neural network of the retina.
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Pseudo-“Blindsight” under exposure to extremely low frequency electromagnetic fields
Journal of Applied Physics, 2014Co-Authors: Hidenori Nakagawa, Ken Masamune, Makoto Kotani, Shoogo UenoAbstract:To show distinct evidence of the pseudo-Blindsight caused by electromagnetic stimulations in order to reveal a cure for lost visual functions, it is necessary to investigate the differences from the usual optic pathway in terms of the data processing route from the electromagnetically stimulated retina. For elucidating the scheme of phosphenes like Blindsight, we designed a new stimulus coil system and measured the hemodynamic responses in the occipital regions during the stimulations, employing a functional brain-imaging technique. Results showed the possibility that the phosphene as a pseudo-Blindsight induces activation of a parietal association area by an alternate route without the primary visual cortex and can be expected as therapy for lost visual functions.
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Pseudo-“Blindsight” Induced by Extremely Low Frequency Electromagnetic Stimulations
IEEE Transactions on Magnetics, 2014Co-Authors: Hidenori Nakagawa, Shoogo UenoAbstract:We investigated electromagnetic effects of phosphene as pseudo-Blindsight under the influence of background colors. Five healthy male subjects ranging in age from 20 to 65 participated in the visual stimulation experiments. Concerning the frequency response characteristics of phosphene sensitivity at wavelengths 430, 530, and 560 nm (corresponding with short (S), medium (M), and long (L) wave-sensitive cone, respectively) with background light (40 l×), the sensitivities decreased by nearly one and a half times more than in the control experiment, and the threshold of the phosphene at the three wavelengths was ~22 mT on average. Furthermore, we found that electromagnetic fields of up to 40 mT might possibly affect the duration of complementary colors after a 20-s process of the Blindsight-like phosphene. Our results demonstrated the possibility that pseudo-Blindsight induced by electromagnetic stimulations of 40 mT at 20 Hz specifically affects retinal S-cone input, and this new knowledge may be applied as part of therapeutic eyesight exercises.
Petra Stoerig - One of the best experts on this subject based on the ideXlab platform.
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Blindsight, conscious vision, and the role of primary visual cortex.
Progress in brain research, 2006Co-Authors: Petra StoerigAbstract:What is the role the primary visual cortex (V1) in vision? Is it necessary for conscious sight, as indicated by the cortical blindness that results from V1 destruction? Is it even necessary for Blindsight, the nonreflexive visual functions that can be evoked with stimuli presented to cortically blind fields? In the context of this controversial issue, I present evidence indicating that not only is Blindsight possible, but that conscious vision may, to a varying degree, return to formerly blind fields with time and practice even in cases where functional neuroimaging reveals no V1 activation.
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no visual responses in denervated v1 high resolution functional magnetic resonance imaging of a Blindsight patient
Neuroreport, 1998Co-Authors: Petra Stoerig, Andreas Kleinschmidt, Jens FrahmAbstract:Following severe cranio-cerebral trauma that affected the optic radiation, patient FS suffers from an incomplete macula-splitting hemianopia. Within the hemianopic field, FS exhibits Blindsight, i.e. he detects and discriminates visual stimuli he cannot (consciously) see. We performed functional magnetic resonance imaging (fMRI) at high spatial resolution using a large flickering stimulus field to assess visual responsiveness of deafferented V1. Contrasting strong activation of the normal contralesional visual cortex, ipsilesional V1 displayed no stimulus-related MRI signal changes. However, activation was observed in ipsilesional extrastriate cortex. We conclude that Blindsight does not depend on functional islands of tissue preserved within the deafferented striate cortex.
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Blindsight in monkeys
Nature, 1995Co-Authors: Alan Cowey, Petra StoerigAbstract:Blindsight, the visually evoked voluntary responses of patients with striate cortical destruction that are demonstrated despite a phenomenal blindness, has attracted attention from neuroscientists and philosophers interested in problems of perceptual consciousness and its neuronal basis. It is assumed to be mediated by the numerous extra-geniculostriate cortical retinofugal pathways whose properties are studied primarily in monkeys. Like patients with Blindsight, monkeys with lesions of the primary visual cortex can learn to detect, localize and distinguish between visual stimuli presented within their visual field defects. Although the patients deny seeing the stimuli they can nevertheless respond to (by forced-choice guessing) in their phenomenally blind fields, it is not known whether the monkeys experience the same absence of phenomenal vision. To determine whether they too have Blindsight, or whether they actually see the stimuli in their field defects, monkeys who showed excellent detection in tasks where a visual stimulus was presented on every trial, albeit at different positions, were tested in a signal-detection task in which half the trials were blank trials, with no visual stimulus. They classified the visual stimuli presented in the field defect as blank trials, demonstrating, like patients, Blindsight rather than degraded real vision.
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wavelength discrimination in Blindsight
Brain, 1992Co-Authors: Petra Stoerig, Alan CoweyAbstract:In the circumscribed, long-standing, clinically absolute visual field defects of three patients with vascular lesions that involved the optic radiation and visual cortex, forced-choice discrimination between coloured stimuli was tested. Paired stimuli were matched for luminous efficiency on the basis of previous measurements of increment-threshold spectral sensitivity made in the same patients and at the same retinal positions. To different extents all patients could discriminate between narrowband wavelength stimuli. The results imply that despite the effects of retrograde degeneration on thalamic and retinal colour-processing channels, neurons which process wavelength information are still functional, although the information they transmit is not consciously perceived.
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The neurobiology of Blindsight.
Trends in neurosciences, 1991Co-Authors: Alan Cowey, Petra StoerigAbstract:Some patients can respond to visual stimuli presented within their clinically absolute visual field defects that have been caused by partial destruction of striate cortex. This puzzling phenomenon of looking, pointing, detecting and discriminating without seeing has been called Blindsight, and has fascinated philosophers and neuroscientists alike as a spotlight on the nature of unconscious or covert awareness, and the means it provides of studying the visual information carried by pathways other than the major route through the striate cortex.
Hidenori Nakagawa - One of the best experts on this subject based on the ideXlab platform.
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Functional brain measurements within the prefrontal area on pseudo-“Blindsight” induced by extremely low frequency electromagnetic stimulations
Journal of Applied Physics, 2015Co-Authors: Hidenori Nakagawa, Shoogo UenoAbstract:For evaluating the effects of phosphene as pseudo-Blindsight closely, we used functional near-infrared spectroscopy to investigate whether or not the phosphene appearance itself substantially affects the hemodynamic responses of the prefrontal area. Seven healthy volunteers ranging in age from 22 to 72 participated in the visual stimulation experiments. First, we examined the influences of electromagnetic stimulations at around the threshold (10 mT) for a Blindsight-like phosphene on the responses. According to the results of the aged volunteers, we found the possibility that the delay in the phosphene perception might be caused by aging beyond a certain age. In the results of our measurements using the stimulation of 50 mT, no significant difference in the perception delay for all the volunteers could be detected. When the field strength was decreased from 50 mT to the threshold in steps of 10 mT, the results obtained at the threshold are equivalent to that obtained at 50 mT. Our data strongly support th...
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functional brain measurements within the prefrontal area on pseudo Blindsight induced by extremely low frequency electromagnetic stimulations
Journal of Applied Physics, 2015Co-Authors: Hidenori Nakagawa, Shoogo UenoAbstract:For evaluating the effects of phosphene as pseudo-Blindsight closely, we used functional near-infrared spectroscopy to investigate whether or not the phosphene appearance itself substantially affects the hemodynamic responses of the prefrontal area. Seven healthy volunteers ranging in age from 22 to 72 participated in the visual stimulation experiments. First, we examined the influences of electromagnetic stimulations at around the threshold (10 mT) for a Blindsight-like phosphene on the responses. According to the results of the aged volunteers, we found the possibility that the delay in the phosphene perception might be caused by aging beyond a certain age. In the results of our measurements using the stimulation of 50 mT, no significant difference in the perception delay for all the volunteers could be detected. When the field strength was decreased from 50 mT to the threshold in steps of 10 mT, the results obtained at the threshold are equivalent to that obtained at 50 mT. Our data strongly support the hypothesis that pseudo-Blindsight induced by electromagnetic stimulation of above 50 mT is able to excite all the volunteers' retinal photoreceptor cells provisionally. Hence the continuous stimulations for a long period of time might gradually activate synaptic plasticity on the neural network of the retina.
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Pseudo-“Blindsight” under exposure to extremely low frequency electromagnetic fields
Journal of Applied Physics, 2014Co-Authors: Hidenori Nakagawa, Ken Masamune, Makoto Kotani, Shoogo UenoAbstract:To show distinct evidence of the pseudo-Blindsight caused by electromagnetic stimulations in order to reveal a cure for lost visual functions, it is necessary to investigate the differences from the usual optic pathway in terms of the data processing route from the electromagnetically stimulated retina. For elucidating the scheme of phosphenes like Blindsight, we designed a new stimulus coil system and measured the hemodynamic responses in the occipital regions during the stimulations, employing a functional brain-imaging technique. Results showed the possibility that the phosphene as a pseudo-Blindsight induces activation of a parietal association area by an alternate route without the primary visual cortex and can be expected as therapy for lost visual functions.
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Pseudo-“Blindsight” Induced by Extremely Low Frequency Electromagnetic Stimulations
IEEE Transactions on Magnetics, 2014Co-Authors: Hidenori Nakagawa, Shoogo UenoAbstract:We investigated electromagnetic effects of phosphene as pseudo-Blindsight under the influence of background colors. Five healthy male subjects ranging in age from 20 to 65 participated in the visual stimulation experiments. Concerning the frequency response characteristics of phosphene sensitivity at wavelengths 430, 530, and 560 nm (corresponding with short (S), medium (M), and long (L) wave-sensitive cone, respectively) with background light (40 l×), the sensitivities decreased by nearly one and a half times more than in the control experiment, and the threshold of the phosphene at the three wavelengths was ~22 mT on average. Furthermore, we found that electromagnetic fields of up to 40 mT might possibly affect the duration of complementary colors after a 20-s process of the Blindsight-like phosphene. Our results demonstrated the possibility that pseudo-Blindsight induced by electromagnetic stimulations of 40 mT at 20 Hz specifically affects retinal S-cone input, and this new knowledge may be applied as part of therapeutic eyesight exercises.