Psych 3310 Midterm 2 Ch 3 and 5 Visual Acuity Chapter 3 Spatial Vision Acuity the smallest spatial detail that can be resolved Measuring visual acuity eye doctors use distance 20 20 Numerator the distance at which you can just identify the letters Denominator the distance at which a person with normal vision can just identify letters 20 20 isn t perfect but normal 20 8 is the physiological limit of human vision based on cone density 20 200 while wearing corrective lens is legally blind in the US Vision scientists use the smallest visual angle of a cycle of a grating Other countries use Landolt Rings Herman Snellen invented method for designating visual acuity in 1862 Spatial Frequency and Contrast Spatial Frequency the number of cycles of a grating per unit of visual angle usually specified in degrees Contrast the difference in illumination between a figure and its background Oriented grating appears to be gray if you re far enough away because this striped pattern is a sine wave grating the visual system samples the grating discretely Threshold and Sensitivity between either Sensitivity a value that defines the ease with which an observer can tell the difference the presence or absence of a stimulus the difference between stimulus 1 and 2 Just Noticeable Difference JND a difference threshold the smallest detectable difference between two stimuli the minimum change in a stimulus that enables it to be correctly judged as different Thresholds are inversely proportional to sensitivity Ex James Bond low pain sensitivity high pain threshold Carlton Banks high emotional sensitivity low emotional threshold Visual Acuity Cycles per Degree the number of dark and bright bars per degree of visual angle Contrast Sensitivity Function CSF a plot of the threshold contrast to detect the grating as opposed to seeing a uniform gray as a function of spatial frequency developed by Otto Schade inverse of Contrast Threshold if CT 0 01 then contrast sensitivity is 1 0 01 100 For photopic vision the CSF peaks around 2 to 4 cycles per degree note how sensitivity is reduced for mesopic or scotopic vision Photopic daylight vision Mesopic twilight vision Scotopic nighttime vision Contrast sensitivity is quite poor at birth improves gradually with development Contrast sensitivity is reduced with aging primarily for high spatial frequencies Contrast Threshold the minimum difference in contrast for you to detect a pattern Retinal Ganglion Cell Retinal cells like spots of light Low frequency yields weak responses Medium frequency yields strong responses High frequency yields weak responses Spatial frequency is important but so is the phase Phase the phase of a grating refers to its position within a receptive field Lateral Geniculate Nucleus Two Lateral Geniculate Nuclei LGNs axons of retinal ganglion cells synapse there Two types of layers in LGN Magnocellular Parvocellular M Ganglion cells Parasol cells Fast large moving objects P Ganglion cells Midget cells Details of static objects RETINAL PROJECTION The world is divided at the LGN Left side of space goes right Right side of space goes left Each layer input from ONE eye Each layer organized map of half of the visual world This is TOPOGRAPHICAL MAPPING LGN is not only a relay between eyes and visual cortex but it also receives info from a number of other brain areas functioning as a gate to the cortex Ipsilateral referring to the same side of the body or brain Contralateral referring to the opposite side of the body or brain Striate Cortex Also known as the Primary Visual Cortex area 17 or V1 Major transformation of visual info takes place in striate cortex V1 has about 200 million cells Retina Recap 100 million photoreceptors 1 1 5 million ganglion cells Two important features of striate cortex Topographical Mapping the organization of sensory surface matches the organization of the sensory world Neighboring stuff in the visual field will be processed by Cortical Magnification neighboring cells the dramatic scaling of info from different parts of visual field 1 degree of visual angle at fovea is processed by 15 times more neurons than 1 degree of visual angle just 10 degrees away from fovea Cortical Magnification Visual acuity declines in an orderly fashion with eccentricity Hubel Wiesel 1958 won a Nobel Prize Did a test putting a cat s head in a device that locked it in place They opened up the skull and put some sort of recording device into it Showed the cats eyes different lights and watched to show how the LGN s reacted The stimulus vs output from cell found systematic progressive change in preferred orientation all orientations were encountered in a dsitance of about 05 mm same orientation preference in columns perpendicular to the surface of cortex Receptive Field Selective Responsiveness orientation tuning tendency of neurons in striate cortex to respond optimally to certain orientations and less to others Many cortical cells respond especially well to moving lines bars edges gratings certain motion directions Simple Cells responds primarily to oriented edges and grating simple cells in the primary visual cortex can be formed by the linking of outputs from concentric lateral geniculate nucleus cells with adjacent receptive fields responds primarily to oriented edges and gratings however it has a degree of Complex Cell spatial invariance respond to bar regardless of exact positioning within RF Each LGN cell responds to one eye or the other but never to both Ocular Dominance in V1 each striate cortex cell can respond to input from both eyes with preference for one eye s input End Stopping process by which cells in the cortex first increase their firing rate as the bar length increases to fill up its receptive field and then decrease their firing rate as the bar is lengthened further Column a vertical arrangement of neurons Hypercolumn a 1 x 1 mm block of striate cortex containing all the machinery necessary to look after everything the striate cortex is responsible for in a certain small part of the visual world Hubel Method of Adaptation the diminishing response of a sense organ to a sustained stimulus Selective Adaptation Tilt Aftereffect perceptual illusion of tilt provided by adapting to a pattern of a given orientation supports the idea that the human visual system contains individual neurons selective for different orientation Selective Adaptation evidence that human visual system contains neurons selective for specific stimulus properties if