China
Africa
Explore chapters and articles related to this topic
Visual Fields in Neuro-Ophthalmology
Published in Vivek Lal, A Clinical Approach to Neuro-Ophthalmic Disorders, 2023
The primary visual cortex (or calcarine cortex, V1) lies within the medial occipital lobe and is divided into superior and inferior halves by the calcarine sulcus. A complete lesion of the calcarine cortex will produce a contralateral homonymous hemianopia. A complete lesion of the inferior cortex below the calcarine sulcus will cause a contralateral superior homonymous quadrantanopia and a complete lesion of the superior calcarine cortex will produce a contralateral inferior homonymous quadrantanopia.1,11,18,22 For both superior and inferior homonymous quadrantanopias, the lesion responsible is most likely to be found in the occipital lobe than the temporal or parietal lobes, since ischemic stroke accounts for over 80% of the cases, which is most likely to occur in the occipital lobe.22
The Evolution of Consciousness
Published in Max R. Bennett, The Idea of Consciousness, 2020
The primary visual cortex has been discussed in Chapter 5 as being necessary for both visual perceptions and images in consciousness. Patients can still make visual discriminations in the blind hemifield after lesions that remove V1 in one hemisphere. In this case, visual discriminations can be made for objects that are observed by that part of the retina that projects to the topographically appropriate part of V1 which is lesioned. Such objects can be visually followed but not identified. This object detection is in general not accompanied by any awareness. However, if the object is moving, patients can sometimes report, in the absence of an experience of seeing an object, a contentless kind of awareness of something happening. Weiskrantz4, who discovered blindsight, has studied patients who claim to sense ‘a definite pinpoint of light’ yet when questioned further comment that the experience does not ‘actually look like a light but nothing at all’. Blindsight offers guidance as to which regions of the brain are involved in this kind of experience.
The nervous system
Published in Laurie K. McCorry, Martin M. Zdanowicz, Cynthia Y. Gonnella, Essentials of Human Physiology and Pathophysiology for Pharmacy and Allied Health, 2019
Laurie K. McCorry, Martin M. Zdanowicz, Cynthia Y. Gonnella
Each of these primary areas is surrounded by a “higher order” sensory area or a unimodal association area that further integrates information from a single sensory modality and provides more complex aspects of the input. For example, the primary visual cortex is the first site of processing of visual information. Association tracts originating in this area then project to the surrounding unimodal association area for higher level processing of this visual input. In fact, the visual unimodal association cortex occupies the remaining portion of the occipital lobes.
Current practice and challenges in screening for visual perception deficits after stroke: a qualitative study
Published in Disability and Rehabilitation, 2022
Kathleen Vancleef, Michael J. Colwell, Olivia Hewitt, Nele Demeyere
Visual perception is the dynamic process of perceiving the environment through sensory inputs and translating the sensory input into meaningful concepts associated with visual knowledge of the environment [1]. Visual perception problems are therefore distinct from sensory visual impairments such as reduced visual acuity, visual field and eye movements [2]. Where sensory visual impairments result from damage to the eye or early visual pathways from the eye to the primary visual cortex, visual perception deficits are attributed to impaired function in later visual processing areas in the occipital, parietal and temporal cortex [3]. Examples of visual perceptual deficits include apperceptive and associative agnosia (object recognition difficulties), prosopagnosia (face recognition difficulties), akinetopsia (difficulties in perceiving motion), achromatopsia (difficulties in perceiving colour), problems in visual memory (remembering what you have seen before), and in visuospatial abilities (e.g., judging distances or spatial relations between objects) [3]. Visual inattention or hemispatial neglect is sometimes considered to be part of visual perception [1,4], though neuropsychology research attributes this to an attentional deficit [5]. In particular, the presence of preserved perception when attention is stretched to focus on the stimuli, the existence of cross-modal neglect and manipulations of stimulus density on the extent of neglect support the classification of hemispatial neglect as a disorder of attention [6,7].
A preliminary study of atypical cortical change ability of dynamic whole-brain functional connectivity in autism spectrum disorder
Published in International Journal of Neuroscience, 2022
The occipital lobe is the visual processing center. The primary visual cortex (V1) contains a low-level description of the local orientation, spatial-frequency and color properties within small receptive fields. The V1 projects to the occipital areas of the ventral stream (visual area V2 and visual area V4), and the occipital areas of the dorsal stream—visual area V3, visual area MT (V5), and the dorsomedial area (DM).The ASD participants may have the expected prefrontal and occipital asymmetry by GMV analysis [83]. Meanwhile, there was reported that male homozygotes for the risk alleles characterized by genotype at rs7794745 (susceptibility gene for ASD) showed greater reductions in gray matter of the right frontal pole and in fractional anisotropy values of the right rostral fronto-occipital fasciculus [84].
Advances in Neuroscience, Not Devices, Will Determine the Effectiveness of Visual Prostheses
Published in Seminars in Ophthalmology, 2021
Bardia Abbasi, Joseph F. Rizzo
Whatever the cause and onset of blindness, prolonged deprivation can promote repurposing of visual cortex to benefit auditory or tactile function,107–110 as has been demonstrated when a blind individual reads Braille111,112 or mentally processes complex sentences113 or mathematical problems.114 A de-afferented visual cortex does not lie idly, and the implicit hope of the visual prosthetic community is that visual cortical function will be mutable and capable of re-capturing its original specialization after prolonged blindness. There is, however, evidence to the contrary in cochlear implant recipients, in which cross-modal activation of the superior temporal cortex by visual stimuli in functional imaging can predict worse speech outcomes following implantation.115 Similar studies in the blind are extremely limited but at least suggest a potential for recapture of visual function by the primary visual cortex.116,117 The limited understanding of the neural basis of visual rehabilitation hinders clinical trial design and candidate selection118; in the more mature cochlear prosthetic field, a better understanding of cross-modal plasticity has guided earlier intervention in congenital deafness and has yielded improved clinical outcomes.119