Visual Pathway PDF

Visual System Eye Anatomy The eyeball - approximately spherical in shape Optic nerve – emerges near the posterior pole of the eyeball The eyeball consists of three layers of tissue (Sclera, Choroid, Retina) The outermost of which is tough, fibrous and protective Sclera - an opaq...

Visual System Eye Anatomy The eyeball - approximately spherical in shape Optic nerve – emerges near the posterior pole of the eyeball The eyeball consists of three layers of tissue (Sclera, Choroid, Retina) The outermost of which is tough, fibrous and protective Sclera - an opaque white coat over most of the eyeball; extraocular muscles are attached to this layer (for eyeball movement) Cornea – transparent layer over the anterior pole of the eyeball; light enters and pass through the eye through the cornea Eye Anatomy Iris - the most anterior portion; has a central aperture Located around the pupil (through which light is admitted to eye) Made up of two rings of muscle Muscle fibers of the iris: Under the control of the autonomic nervous system Radial fibers - innervated by sympathetic neurons (dilate the pupil) Circular fibers - innervated by parasympathetic neurons (constrict the pupil and reduce the amount of light falling upon the eye) Contains pigments Eye Anatomy Ciliary body - contains ciliary muscle; under sympathetic and parasympathetic nervous system innervation Biconvex lens – in the central aperture within the annulus of the ciliary body; focuses/refracts light upon the retina Held in place by suspensory ligaments Movement of the ciliary muscle  alters the shape of the lens Eye Anatomy The lens and suspensory ligament divide the lumen of the eyeball into an: Anterior segment (in front of the lens) contains a thin, watery fluid, aqueous humor (continuously secreted from the ciliary body) Reabsorbed and drained by a small duct (the canal of Schlemm) into the venous system Nourishes the lens and maintains pressure within the eye Posterior segment (behind the lens) contains a gelatinous material known as vitreous humor Transmits the light rays, holds the retina in place, maintains intra-ocular pressure Eye Anatomy Choroid Behind the ciliary body Lining making up the inner surface of the sclera Pigmented membrane - made up of cells containing dark pigment that absorbs light and thus reduces reflection within the eye Rich in vascular supply Absorbs any light rays entering the eye (to prevent and scattering of light) Lining the inner surface of the choroid is the photoreceptive retina Eye Anatomy Retina Consists of: A non-neural portion (represented by the pigment epithelium) Rich in melanin A single layer of light- absorbing, pigmented cells lying adjacent to the choroid A neural portion Photoreceptors, neurons, neuroglia, and a rich capillary Eye Anatomy Photoreceptors Lie deepest within the retina Retinal photoreceptors transduce light energy into electrical energy (changes in membrane potential) Two types of retinal photoreceptors: Rods - exquisitely sensitive to light; particularly important for vision in dim lighting conditions (e.g., night vision); contain rhodopsin Cones - responsible for color vision and high visual acuity; contain photopsin  Eye Anatomy Bipolar cells - lie entirely within the retina; act as relay between the photoreceptors and ganglion cells Ganglion cells - form the optic nerve Information is transferred from photoreceptors  bipolar cells  to the ganglion cells The retina also contains interneurons known as horizontal cells and amacrine cells Horizontal cells – connect the photoreceptors and bipolar cells; help with light adjustment Amacrine cells – connect and modulate Photoreceptor s Eye Anatomy Optic Nerve Optic Disc – portion where optic nerve is piercing through the back of the sclera Macula - highest concentration of the cones Fovea centralis - located in the center of the macula; responsible for high-acuity vision Fundoscopy – used to observe the back of the eye The Central Visual Pathway Visual Field The area in space perceived when the eyes are in a fixed, static position when looking straight ahead Monocular Binocular Visual Field The area of space visible to one eye (monocular visual field) The area of overlap of the visual field of one eye with that of the opposite eye (binocular visual field) Can be further subdivided into quadrants: Nasal quadrant Temporal quadrants Contains a blind spot A small area in which objects cannot be viewed Located within the temporal hemifield Visual Field The upper half of the visual field forms images upon the lower halves of the retinae The lower half of the visual field forms images upon the upper halves of the retinae Visual Field RIGHT EYE Temporal retina – receives light rays from the left visual field Nasal retina – receives light rays from the right visual field LEFT EYE Temporal retina – receives light rays from the right visual field Nasal retina – receives light rays from the left visual field  Visual Field REMEMBER: Anything from the right visual field goes to the left Anything from the left visual field goes to the right Contralateral Optic tract Ipsilateral Visual Field The crossing of light rays diverging from different points on an object at the pupil causes the images of objects in the visual field to be inverted Double refraction (at the cornea and the lens)  causes inversion of image  flipped at the cortex  The Central Visual Pathway The axons of retinal ganglion cells assemble at the optic disc and form the optic nerve  enters the cranial cavity through the optic canal The two optic nerves converge to form the optic chiasm at the base of the brain In the chiasm: Axons from the nasal halves of two retinae decussate and pass into the contralateral optic tract Axons from the temporal retinae remain ipsilateral.  The optic tracts diverge away from the The Central Visual chiasm and pass round the cerebral peduncle Pathway to terminate mainly in the lateral geniculate nucleus of the thalamus Lateral geniculate nucleus  thalamocortical neurons project through the internal capsule (optic radiation)  terminates in the primary visual cortex of the occipital lobe The primary visual cortex is located predominantly on the medial surface of the hemisphere in the region above and below the calcarine sulcus There is a precise point-to-point relationship between the retina and the visual cortex The Central Visual Pathway Fibers representing the lower part of the visual field  course superiorly to terminate in the visual cortex above the calcarine sulcus Fibers representing the upper part of the visual field  sweep into the temporal lobe (Meyer's loop) before terminating below the calcarine sulcus The Central Visual Pathway A relatively small number of fibers leave the optic nerve to terminate in two regions in the midbrain: Superior colliculus Reflexive head and eye movements in response to visual stimulus Pretectal area/nucleus Involved in mediation of the light reflexes  Pupillary Light Reflex Testing for function of the optic nerve, midbrain, and the oculomotor nerve Shine light on one eye  pupils in both eyes constrict Information reaches the superior colliculus and pretectal nucleus Pretectal nucleus sends activation to the Edinger-Westphal nucleus (CN III) Edinger-Westphal nucleus sends signals to CN III  ends in the ciliary ganglion Ciliary ganglion gives rise to ciliary nerves  innervate the circular muscles (sphincter pupillae)  Pupils  Accommodation Reflex Patients can be asked to focus on the examiner’s finger Testing the optic nerve, visual cortex (frontal and occipital lobes), and the midbrain The accommodation reflex (or near response) is a three-part reflex that brings near objects into focus through: Convergence wherein eyes move inward Constriction of the pupils Contraction of the ciliary body Accommodation Reflex Three phenomena are involved: Convergence of the eyes: The oculomotor nucleus send the impulses for contraction of both medial rectus muscles  causes the eyes to converge Pupillary constriction: The oculomotor nucleus conveys the impulse for the contraction of the sphincter pupillae  the pupil constricts. Increased convexity of the lens: The oculomotor nucleus sends information to the ciliary muscle  muscle contracts  relaxes the suspensory ligaments  lens shape is modified https://collections.lib.utah.edu/ark:/87278/ Barral et al. (2009) s6157k8t  Striate cortex Visual cortex Includes the calcarine sulcus and regions immediately around it Considered to be the primary visual cortex (V1) because: most LGN axons terminate here all V1 neurons respond to visual stimuli exclusively Extrastriate Visual Cortex Regions surrounding the primary visual cortex Consist of visual areas 2, 3, 4, and 5 (also known as V2, V3, V4, and V5 Receives information from the primary visual cortex (V1) Information relate to color, shape/form, location, and motion Secondary and tertiary visual areas V2 – Secondary visual area (also called the prestriate cortex) The second major area in the visual cortex, and the first region within the visual association area Receives input from V1 (it also sends strong feedback to V1) Sends information to V3, V4, and V5 Involved in further analysis and discrimination of visual input in terms of motion, shape (particularly complex shapes), and position Secondary and tertiary visual areas Visual association areas V3-V5 V3 - Processes information related to form, color, and motion (less well-defined) V4 - Associated with color V5 or MT (overlap with the middle temporal gyrus) - associated with motion Secondary and tertiary visual areas Fusiform face area (FFA) A part of the human visual system Specialized for facial recognition Located in the inferior temporal cortex (IT) - in the fusiform gyrus Responsible for visual object recognition and receives processed visual information Visual information from the V1 –V5 regions are radiated to the IT cortex (related to the color and form of the visual stimuli) Visual pathways: dorsal and ventral streams Dorsal stream (“vision-for-action” pathway) Consists of the parietal association cortex and superior and middle temporal visual association cortex These dorsally located visual association neurons are responsible for producing our sense of spatial orientation depth perception the location, the movement and the movement direction and velocity of objects in space The dorsal stream processes information about the “where” of the visual stimulus Damage: deficits in spatial orientation, motion detection and in guidance of visual tracking eye movements Ventral stream (“vision-for-perception” pathway) The neurons in the inferior temporal visual association cortex process information about object color and form. These ventrally located visual association neurons are responsible for processing information necessary for our abilities to: recognize objects and colors read text and learn and remember visual objects (e.g., words and their meanings) This ventral stream processes information about the “what” of the visual stimulus Damage: deficits in complex visual perception tasks, attention and learning/memory  Retinotopic map The topographic (spatial) relationships of retinal neurons are maintained throughout the visual system The retina is mapped onto the LGN and striate cortex in an organized (topographic) fashion Neighboring parts of retina project to neighboring parts of LGN and neighboring parts of LGN project to neighboring parts of the striate cortex This retinotopic organization in the visual pathway results in a spatial representation of the visual field in the LGN and visual cortex  Oculomotor System The vestibular afferents have their cell bodies in the vestibular (Scarpa’s) ganglion The vestibular portion of the VIII cranial nerve enter the brainstem through the internal auditory meatus (1st order neurons) Most of the afferents project to one of the four nearby vestibular nuclei (2nd order neurons) Many of these neurons send projections to the oculomotor nuclei. Vestibular Connections to the Oculomotor Nuclei Afferents arise from the superior and medial nuclei and ascend as the medial longitudinal fasciculus (MLF) to the oculomotor nuclei (CN III, IV, VI) Second-order afferents arise in the medial and superior vestibular nuclei Connections from the MLF to the abducens, trochlear and oculomotor nuclei coordinate eye movements in response to head movements Oculomotor System  Vestibulo-ocular Reflex (VOR) The VOR controls eye movements to stabilize images during head movements As the head moves in one direction, the eyes reflexively move in the other direction If the head is moved from side to side  the image you see is stable, despite the head movement But when the speed of head movements is increased  the VOR is no longer effective, and you will see the visual image start to shift The VOR would occur in the dark, because the eyes move due to angular acceleration of the head.  RIGHT Vestibulo-ocular Reflex (VOR) Involves CN III (oculomotor), IV (trochlear), and VI (abducens) To maintain the gaze, our eyes will move in the opposite direction to our head When the head is turned left, the semicircular canals are rotated in LEFT the opposite direction  triggers the eyes to move towards to the right L cervical rotation  contraction of the R lateral rectus and L medial rectus R cervical rotation  contraction of the L lateral rectus and R medial rectus Visual field deficits Visual field deficits Monocular blindness Loss of vision in one eye (i.e., the affected eye) Caused by: abnormalities in the eyeball (cataract, intraocular hemorrhage, retinal detachment) disease of the optic nerve (multiple sclerosis and optic nerve tumors) Visual field deficits Retinitis pigmentosa An inherited metabolic disorder of the photoreceptors and retinal pigment epithelial cells Progressive night blindness, peripheral visual field constriction, and pigmentation of the retina visible on ophthalmoscopy. Senile macular degeneration A degenerative disorder of the elderly Loss of central and color vision