Human Visual PerceptionGreg ZelinskyPsychology Neuronal Convergence Lateral InhibitionThe importance of…Human Visual PerceptionStructure of the EyeVision and the EyeStructure of the EyeVision and the Eye enclosed by three membranes fibrous tunic vascular tunic the retinaoutermost membranemiddle membraneinner membraneVision and the EyeVision and the Eye Cornea transparent front part of the eyeFibrous Tunic Sclera the “white” of the eye very tough; gives the globe its structure opaque contains the blood vessels that feed the eye heavily pigmented to absorb scattered light (pigment epithelium)Vacular Tunic (Choroid Layer)Vision and the Eye Iris: colored diaphragm muscle that determines pupil size Pupil: the aperture through which light enters the globeVacular Tunic (Choroid Layer)Vision and the EyeThe Retina (vertical organization) Photoreceptors (rods and cones)pigmented cells that produce electrical signals when struck by light Bipolar cellssynaptically connects cones and rods with ganglion cells Ganglion cellsAxons compose the optic nerve and leave the eye via the optic disc (blind spot) Vision and the Eyeoptic nervelightThe Blind SpotVision and the Eye A hole in the retina through which the ganglion cell axons leave the eye and travel to the brain We are blind at this location in our visual field due to the absence of photorecepters at the optic discThe Blind SpotVision and the EyeIf you close your right eye and look at the cross, the spot will seem to disappear when it falls on your blind spotThe Blind SpotVision and the EyeIf you close your right eye and look at the cross, the spot will seem to disappear when it falls on your blind spotInstead of seeing this…The Blind SpotVision and the EyeIf you close your right eye and look at the cross, the spot will seem to disappear when it falls on your blind spot…you see thisRather than seeing a black hole in our vision, our visual system fills this hole with the color and texture of the region surrounding the blind spot. This process is known as perceptual filling-in.The Retina (vertical organization)Vision and the Eyeoptic nervelightGanglion cellsBipolar cellsPhotoreceptorsThe Retina (horizontal organization)Vision and the Eyeoptic nervelight Horizontal cellsmodulate activity between the photo-receptors and the bipolar cells Amacrine cellsmodulate activity between the bipolar and the ganglion cellsThe Retina (backwards organization?)Vision and the EyelightphotoreceptorsLight coming through the pupil must pass, not only through the cornea, lens, and the aqueous and vitreous humors, but also through the ganglion, amacrine, bipolar, and horizontal cells before reaching the photoreceptors.The FoveaVision and the EyeThe Foveaoptical axisblind spotoptic nervefovea The part of the retina with the best visual acuity (i.e., the ability of the eye to resolve fine details)  In order to have the clearest possible view of an object, we want to image the object on our fovea Visual acuity decreases with increasing distance from the foveaVision and the EyeThe FoveaVision and the Eye(Wilson S. Geisler & Jeffrey S. Perry, University of Texas) Keep your eyes on the bee!Why is acuity best at the fovea?Vision and the Eyeoptical axis The fovea is very near the optical axis; images at the fovea therefore have the fewest lens distortionsblind spotoptic nervefoveaWhy is acuity best at the fovea?Vision and the Eye The fovea is very near the optical axis; images at the fovea therefore have the fewest lens distortions Very few cell bodies are located at the fovea. Given that these cells would normally scatter light, their absence allows a clearer imageCell bodies avoid blocking the lightfoveal pitlightWhy is acuity best at the fovea?Vision and the Eye The fovea is very near the optical axis; images at the fovea therefore have the fewest lens distortions Very few cell bodies are located at the fovea. Given that these cells would normally scatter light, their absence allows a clearer image The type and distribution of photoreceptors at the fovea affect visual acuity. The fovea contains mainly ‘cone’ photoreceptors, which are specialized for detailed pattern vision.PhotoreceptorsVision and the EyeRodConePhotoreceptorsVision and the Eyeconerods They are distributed differently on the retina. The fovea contains only cones; the peripheral retina contains rods and cones, but mainly rodsDifferences between Rods and ConesVision and the EyeThe size of the rod-free fovea is defined as the central 1-1.7 degrees of visual angle.120 million rods5-8 million conesTwo separate visual systems: central (cones) and peripheral (rods)The Retina (vertical organization)Vision and the Eye They are distributed differently on the retina. The fovea contains only cones; the peripheral retina contains rods and cones, but mainly rods Rods and cones are wired differently to the ganglion cells  Neuronal Convergence: many cells projecting to a smaller number of cells. On average… 120 rods project to 1 ganglion cell 1-5 cones project to 1 ganglion cellDifferences between Rods and ConesVision and the EyeConvergenceVision and the Eyefoveal retina peripheral retina They are distributed differently on the retina. The fovea contains only cones; the peripheral retina contains rods and cones, but mainly rods Rods and cones are wired differently to the ganglion cells  Neuronal Convergence: many cells projecting to a smaller number of cells.  Cone vision: good spatial resolution, poor luminance sensitivity Rod vision: poor spatial resolution, good luminance sensitivityDifferences between Rods and ConesVision and the EyeConvergenceVision and the EyeThe more photo-receptors converging on a ganglion cell, the greater the loss of spatial informationThis is a big reason why visual acuity is better in the fovea compared to the peripheral retina Peripheral vision ~120 million Poor spatial resolution More light sensitive Function in dim light Achromatic Central vision 5-8 million Good spatial resolution Less light sensitive Function in daylight Color visionDifferences between Rods and ConesVision and the EyeRods ConesNeuronal Computation All of our behavior, including perception, can be ultimately reduced to a complex pattern of activation within a population of neurons in the brain.  Models of