2 Photoreceptors: rods and conesFovea is area of retina where inner layers are not present – higher acuity.Rod:Cone ratio is about 20:1, but cones predominate the macula/fovea.Rods function in low light and dominate peripheral vision.Cones dominate macula and function in color vision.
Features of Rods• Contain photopigment “rhodopsin”• Responsive to a single photon of light—1000 times more sensitive than cones• Responsible for scotopic vision—ability to see in dim light – Absolute threshold: candle at 30 miles – Sacrifice detail: 20/200 in starlight – Located away from the fovea
Features of Cones• Three types of photopigments: – Green (chlorolabe) – Blue (cyanolabe) – Red (erythrolabe)• Responsible for photopic vision—ability to see in bright light: – Color vision – High clarity – Located mostly in fovea
The Retina Is a Thin Layer Containing Interneurons and Photoreceptors• Blood vessels and the optic nerves exit the eye through the optic disk.• The macula is not covered with blood vessels.• The fovea is a pit within the macula.• Acuity is best when an image falls on the fovea and macula.• Sensitivity to dim lights is best in the periphery.
The Layers of the Retina• Ganglion layer cells give rise to the axons forming the optic nerve.• Ganglion cells communicate with amacrine and bipolar cells in the inner plexiform layer.• The cell bodies of the amacrine, horizontal and bipolar cells are located in the inner nuclear layer.
More Layers of the Retina• Bipolar cells form connections with horizontal cells and photoreceptors in the outer plexiform layer.• The cell bodies of the photoreceptors are located in the outer plexiform layer.
Transduction in Rods• Absorption of light causes rhodopsin to break into opsin and retinal.• Enzymes break down cyclic GMP.• Fewer sodium channels remain open.• Cell hyperpolarizes in light (it’s depolarized in the dark).• Photoreceptors produce graded potentials.
Visual Transduction: The Conversion of Physical Energy to Neural Energy • Photopigments are located in the membrane of the outer segment of rods and cones • Each pigment consists of an opsin (a protein) and retinal (a lipid) – In the dark, membrane NA+ channels are open -> glutamate is released which depolarizes the membrane – Light splits the opsin and retinal apart-> • Activates transducin (G protein)-> • Activates phosphodiesterase-> • Reduces cGMP -> closes NA+ channels • The net effect of light is to hyperpolarize the retinal receptor and reduce the release of glutamate
Ganglion Cells• One million ganglion cells integrate input from 130 million photoreceptors. Convergence can vary in cluster size.• Axons form the optic nerve.• Communicate via conventional action potentials.• Three types: – M Cells: respond to movement, subtle contrast – P Cells: respond to color, higher contrast – Non-P-or-M cells: similar to P Cells
The Lateral Geniculate Nucleus of the Thalamus• Receives about 90% of the optic nerve fibers.• Six layers: magnocellular and parvocellular are separate.• Koniocellular layers are found between the magnocellular and parvocellular layers.• Separation of input from each eye is maintained.• Show center-surround antagonism.• Forms synapses with primary visual cortex.
Additonal Pathways• Signals leaving the retina also go to the tectum, for which they project back to the ciliary body that controls the size of the pupil; to the hypothalamus, to control circadian rhythms; and to the colliculus to control eye and head movements (saccades).
Primary Visual (Striate) CortexThe striate cortex is thicker inlayer IV, which is the “input”layer of the cortex.
Most of cerebral cortex has 6 layers.Input to visual cortex terminates in layer 4 which is extra thick in visual cortex.In myelin stained brainsections, layer 4B has many myelinated axons and forms a stripe, the stria of Gennari, so visual cortex is often known as striate cortex.
The Cortex Is Organized in Columns• Ocular dominance columns respond to either the left or right eye, but not both.• Orientation columns respond to lines of the same angle.• Hypercolumns contain orientation columns that respond to a complete 180 degrees.• Cytochrome oxidase blobs respond to color.
The Dorsal “Where” Stream and the Ventral “What” StreamThe Dorsal Stream is the “where” path.The Ventral Stream is the “what” path.
Visual association areas V4(color) Face recognition Perceive Facial Expression
The Fusiform Face Area:Identification of Faces and Members of Categories
Axons from left hemiretinas go into the left optic tract. Axons fromright hemiretinas go into the right optic tract. Nasal retinal axonsdecussate in the optic chiasm. Lesions of optic chiasm producebitemporal hemianopia
Lesions of eye, retina, or opticnerves produce monocular visualfield defects
Lesions in the optic chiasm, optictract, lateral geniculate, optic radiations,or visual cortex typically producehomonymous visual field defects(i.e. defect occurs in the same portionof the visual field for each eye).
Depth Perception• Monocular cues require only one eye: – Perspective – Shading – Relative size – Texture• Binocular cues require two eyes: – Retinal disparity refers to the differences between images projected on each retina. – Retinal disparity increases with distance.
Retinal Disparity Allows Us to See Stereograms• Four-month old human infants, monkeys, cats and falcons can see depth in stereograms.• 5–10% of the human population is “stereoblind” Courtesy Joe DiBiasi
Theories of Color Perception • Trichromacy – Color vision is based on three photopigments – Describes processing at the retinal level • Opponent Processes – Color vision is based on red-green and blue-yellow opposition – Describes central processing
Colorblindness• Dichromacy results • Monochromacy results from one missing or when there is one or abnormal no cone photopigment. photopigments. – The red/green genes are on the X • Anomalous chromosome, so trichromats match these cases are sex- colors differently. linked. • Techromats may see – Blue/yellow colorblindness is not four primary colors. sex-linked.
Color Constancy• An object’s color does not change when the light falling on the object changes.• Adaptation: neurons respond less to unchanging stimuli.• Neurons in Area V4 respond to green squares regardless of the light used to illuminate them.
Age-related Changes in Vision• Presbyopia (old sight) occurs when the lens needs more time to accommodate to changes in focal distance.• More time is needed to adapt to changes in levels of illumination (theater to daylight).• Lens thickens and yellows.• Smaller pupils due to changes in iris muscle.• Aged visual neurons fire less selectively to line orientation or direction of movement.
Visual Disorders• Amblyopia occurs when one eye does not focus on objects.• Cataracts occur when the lens becomes clouded.
Other Visual Disorders • Blindness occurs due to damage to the eye, pathways or central visual areas. – A scotoma is an area of the visual field that can’t be seen due to central damage. – Blindsight refers to a patient’s ability to point to the source of a light in a scotoma. • In visual agnosias, a person cannot recognize a stimulus. • In prosopagnosia, a person cannot recognize faces of people he/she knows. • Simultaniagnosia