Visual System - Joseph's University PDF

Visual system DEPARTMENT OF PHYSICAL THERAPY DPT 542 Functional Neuroscience Anne K. Galgon PT, MPT, PhD  Objectives ► Understand the basic structures and functions of the eye and retina. ► Describe the visual pathways from the retina to the visual cortex ► Describe how visua...

Visual system DEPARTMENT OF PHYSICAL THERAPY DPT 542 Functional Neuroscience Anne K. Galgon PT, MPT, PhD  Objectives ► Understand the basic structures and functions of the eye and retina. ► Describe the visual pathways from the retina to the visual cortex ► Describe how visual information is processed for location, movement, and form and figure identification. ► Understand varied disorders of vision  Outline ► Eye function and Eye Anatomy ► Accommodation and eye shape dysfunction ► Retina structure and function ► Visual pathway ► Visual field impairments ► Visual Assessment Pupillary light reflex ► Disorders of the Vision System  Functions of the eye ► Regulate amount of light reaching photosensitive surface (retina) ► Pupillary constriction and relaxation ► Pupillary Light Reflex ► Focusing on near and far objects: ► Visual Accommodation ► Maintaining a stable relationship between these two functions ► Recording the pattern of incoming light ► Retina processing ► Upside down and backwards ► center surround receptor fields ► Visual acuity ► Ability of the eye to distinguish between two nearby points ► Measured by Snellen chart ► Input for visual maps - Visual field ► Monocular visual fields ► Binocular visual field Anatomy of the eye View of eye through an ophthalmoscope Left eye What eye is it? Eyes and the retina ► Light enters eye, passes through the lens, forms image on retina that is inverted and reversed ► (up side down and backwards) Eyes and the retina ► Right visual space projects to left hemiretina and vice versa ► Central fixation point is on the fovea Blind spot Over your right eye Image Formation Refraction by Cornea Bear 9.7 Accommodation Bear fig 9.8 This occurs through contraction and relaxation of Ciliary muscle: Controlled by the autonomic system Eye shape dysfunction Eye shape dysfunction and Visual Acuity Correction Bear Box 9.3 Farsightedness Nearsightedness Astigmatism ► See blurry and distorted images ► Cornea shaped more like a football than a basketball Retinal structure and function ► Outer nuclear layer ► Deepest retinal layer ► Photoreceptors: ► Rods and cones ► Bipolar cell layer ► Bipolar cells ► Intermediate retinal layer ► Information-integrating neurons ► Ganglion cell layer ► Ganglia cells ► Axons to optic nerve  Photoreceptors Bear fig 9.13 ► Rods ► Low acuity ► Absent in fovea ► Highly convergent ► Achromatic ► Cones ► High acuity (spatial/temporal resolution) ► Concentrated in fovea ► Slight divergence ► Chromatic ► Three types of photopigments  Regional Differences Bear fig. 9.14 Receptor fields of ganglia cells Small in central retina Large in periphery (Convergence) Fovea Cross Section Bear fig. 9.15  Cone Rod Rods and cones have four major functional regions: an outer segment, inner segment, a nucleus, and a synaptic terminal. The synaptic terminal of a rod is called a spherule, while the synaptic terminal of a cone is called a pedicle.  Sensory Transduction in Rods and Cones ► Transduction of light waves to electrochemical signals occurs in the photoreceptors ► Disks in the outer segments contain visual pigment protein ► Rods contain rhodopsin ► Three different types of cones with pigment sensitive to either blue, green, or red wavelengths Color Blindness ► Red-Green (Deuteranopia/Protanopia) ► Blue-Yellow (Tritanopia) ► Achromatopsia ► Condition of no cones – only rods From: http://rit- mcsl.org/fairchild/WhyIsColor/images/ConeMosaics.jpg Color Blindness ► Ishihara Color Test Color Blindness Compensations for Color Blindness Retina ► Ganglion cells ► Inner plexiform layer ► Information-integrating neurons that exit eyeball as the optic nerve Bipolar Cells Receptor fields ► Bipolar cells receive direct input from photoreceptor within its receptor fields ► Bipolar cells receive indirect input from photoreceptors in surrounding field. ► Via horizontal cells ► Bipolar cells can be OFF cells or ON cells. On/off cell represent simple information processing in the retina  Bear Fig 9.22 Center-surround Receptive fields Biopolar cell in the retina have 1. direct center receptor fields And 2. indirect surround A receptor field is the receptor fields # of receptors a single neuron gets information from Ganglion Cells (On/ OFF center surround receptor fields) ► Example of an Off receptor field Why is this important? Pathways to Visual Cortex Transmission of vision from retina to cortex ► Cells in retina convert light into neural signals ► Signals processed in retina and conveyed to retinal output cells (ganglia cells) ► Retinal output conveyed by axons that travel in optic nerve, optic chiasm, and optic tract ► Axons synapse in lateral geniculate nucleus of the thalamus (10% do NOT) ► From thalamus neurons travel in the geniculocalcarine tract (optic radiations) within the internal capsule to the primary visual cortex ► Fibers to pretectal area (superior colliculus) are important in the pupillary light reflex  Visual pathway ► Area of projection on visual cortex is dependent on origin of signal ► Nasal retina (closest to nose) signals cross midline at the optic chiasm projecting to contralateral visual cortex. ► Temporal retina (closest to temporal bone) signals continue ipsilaterally to project to the ipsilateral cortex ► Outcome of the arrangement of projection of visual information is: ► Information from each visual field is projected to the contralateral cortex ► Right visual field information projects to left cortex ► Left visual field information projects to right cortex  Primary visual pathway: retino- geniculo-cortical pathway Text 20-18 Macula - retinal site of greatest acuity Remember upside down and backwards.  Processing of Visual Information ► Parallel and Serial processing ► Parallel Processing ► Visual field topography (Retinotopic Maps) ► Form and Figure ► Orientation and motion ► Color ► Serial processing to integrate at the level of cortex ► Parallel processing at the level of cortex: Dorsal and Ventral Streaming of Visual Information Parallel processing at the level of Lateral Geniculate Right and Left eye information Lateral geniculate cells project to Layer IV in Cortex -Satellite cells in Layer IV also have on and off center surround response fields Processing at the level of the primary visual cortex ► Stellite Cells synapse with pyramidal cells in other layers to form simple to more complex representation of visual information Bear Fig. 10.21Orientation selectivity Direction selectivity ► Receptor fields in visual cortex that are responsive to the direction a bar of light moves across the visual field. ► Important for analysis of object movement. ► Ocular dominance columns ► Orientation columns ► Parallel and Serial processing Visual Processing Pathways Schematic of visual pathway: information is transmitted via both serial and parallel pathways: the Where and What Streams of visual processing. Where (magnocellular) Stream: Location in space & movement Dorsal pathway to posterior parietal cortex What (parvocellular) Stream: object recognition Ventral pathway to inferior temporal cortex  Parallel and Serial processing Motor planning Visual motion and Orientation Form and Figure Identification location and direction of movement Distal extremity configuration  Non-Geniculate Pathway Information NOT processed consciously Pupillary light reflex Tectospinal pathway Visual Reflexes ► Pupillary size ► Constriction of pupil due to parasympathetic innervation ► Dilation of pupil due to sympathetic innervation ► Fixation reflex ► Functions to maintain the position of the eyes so that the image of the object of interest is kept on the fovea of both eyes ► Near reflex ► Occurs when gaze is shifted from a distant object to a near one  Terminal distributions of cerebral arteries Optic radiation (parietal loop) & meyer’s loop (temporal loop) shown. See how a stroke in either MCA or PCA Could affect the axons in these pathways. Middle cerebral artery branches, AC arising from the internal carotid a. A PCA Visual field impairments Visual field impairments Monocular scotoma ► Lesion of the retina ► Location size and shape is dependent on the location and extent of the lesion. ► Causes include retinal infarcts, hemorrhage, degeneration, or infection. Monocular scotoma Monocular visual loss ► If retinal lesion severe enough, entire retina may be involved leading to total loss of vision ► Lesions of optic nerve can cause monocular visual loss ► Causes: glaucoma, optic neuritis, elevated intracranial pressure, optic glioma, schwannoma, menigioma, or trauma Bitemporal hemianopia ► Damage to the optic chiasm ► Visual loss more typically asymmetric than is seen in diagram. ► Common lesions: pituitary adenoma, menigioma Bitemporal hemianopia Homonymous hemianopia ► Retrochiasmal lesions - lesions of optic tracts, LGN, optic radiations, or visual cortex cause homonymous hemianopia Contralateral superior quandrantanopia ► Caused by lesions of the temproal lobe leading to infarcts in the optic radiations ► “pie in the sky” defect Contralateral inferior quadrantanopia ► Lesion in the parietal lobe cause interruptions in the upper portions of the optic radiations ► “pie on the floor” defect Low vision impairments Cataract ► An opacity of the lens resulting in decreased acuity; vision hazy overall, particulary in glaring light ► Does not impact field of vision ► There is NO scotoma (empty, distorted or dark area) Normal vision  Macular degeneration ► The deterioration of the macula, the central area of the retina ► Most prevalent eye disease ► Central scotoma ► Peripheral or side vision remains unaffected Diabetic retinopathy ► The leaking of retinal blood vessels may occur in advanced or long-term diabetes; affects the macula or the entire retina and vitreous. ► Likelihood of retinopathy and cataracts, along with the consistency and level of blood glucose control.  Glaucoma ► Chronic elevated eye pressure causes optic nerve atrophy and loss of peripheral vision  Retinitis pigmentosa ► Congenital degeneration of the pigmented layer of the retina leads to a severe loss of peripheral vision. Disorders of the Visual System ► Optic neuritis ► Inflammatory demyelinating disorder often related to multiple sclerosis ► Symptoms of eye pain, decreased acuity, and impaired color vision ► Recovery is common ► Papilledema ► Optic disc swelling associated with elevated intracranial pressure Deficits Associated with Higher Cortical Processing ► Visual agnosia ► Can perceive, but cannot understand meaning of what they see ► Cortical blindness ► Bilateral lesion of specific area of visual cortex ► Prosopagnosia ► Face blindness ► Damage to occipitotemporal cortex Assessment of Visual System ► Acuity ► Ophthalmic inspection ► Pupillary light reflex ► Assessment of visual fields  Pupillary Light Reflex ► Afferent limb ► Axons of the retinal ganglion cells that project to the pretectal area ► Reflex center ► Interneurons of the pretectal area that terminate bilaterally in the Edinger-Westphal nuclei of the oculomotor complex ► Controls diameter of the pupil in response to intensity of light – helps adapt to level of lightness darkness. ► Anisocoria – denotes pupillary size inequality ► Direct pupillary light reflex ► Response of eye stimulated with light ► Consensual pupillary light reflex ► Response of opposite eye  Sphincter mm of iris Dilator mm of iris & ciliary body Pupillary TOP view OpNr light CilGang Parasymp OcNr reflex From Ed-W Nu Via OcNr pathway Midbr RefF Sympathetics Via blood vessels Haines Atlas LGN 8-35 P.Comm Sup. PrTecNu CerGang Thoracic cord T1-T3