Psych 253 Lecture 6 Spring 14 Wede Lecture 6 Visual Acuity Path to the Brain Finish Lecture 5 Dark and Light Adaptation Two mechanisms for dark and light adaptation Pupil dilation Photopigments and their replacement Dark Adaptation A process resulting in an increase in the eye s sensitivity in the dark Range of Pupil Sizes The amount of photopigment available in photoreceptors changes over time More light means more photopigments used up and fewer photopigments available to process more light Less light means fewer photopigments are used up and more photopigments available to process what little light is there Enhanced by duplex retina Neural circuitry of the retina Remember the center surround receptive fields of the ganglion cells 1 Psych 253 Lecture 6 Spring 14 Wede If photoreceptors are not completely saturated ganglion cells will code dark and light areas not overall light level The visual system regulates the amount of light entering the eye and ignores whatever variation in overall light level What is the path of image processing from the eyeball to the brain is left over Lecture 6 Cortical Pathway Eye Photoreceptors Bipolar cells Retinal ganglion cells Lateral geniculate nucleus Thalamus Striate cortex Cortical visual pathways Visual Acuity Snellen E test Herman Snellen 1862 The strokes on the E form a small grating pattern There are several ways to measure visual acuity Eye doctors use distance 20 20 vision Your distance Normal vision distance Vision scientists Smallest visual angle of a cycle of grating Acuity The smallest amount of spatial detail that can be resolved V visual angle S is size of stimulus D is distance away Why does an oriented grating appear to be gray if you are far enough away Density of photoreceptors Factors affecting visual acuity Contrast Differences in luminance between light and dark areas Object and background 2 Psych 253 Lecture 6 Spring 14 Wede Differences between parts of an object Spatial frequency The number of cycles of a grating per unit of visual angle usually specified in degrees Number of times a pattern repeats per unit area a Low b Medium and c High spatial frequencies Contrast sensitivity function Why sine gratings Patterns of stripes with fuzzy boundaries are quite common The edge of any object produces a single stripe often blurred by a shadow in the retinal image The visual system breaks down images into a vast number of components each is a sine wave grating with a particular spatial frequency Retinal Ganglion Cells and Stripes How do the center surround receptive fields respond to sine wave patterns with different spatial frequencies The response right of a ganglion cell to gratings of different frequencies left a low b medium and c high Not only is the spatial frequency important but so is the phase Phase Position within a receptive field Lateral Geniculate Nucleus LGN We have two lateral geniculate nuclei LGNs 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 referred to as V1 Primary visual cortex 3 Psych 253 Lecture 6 Spring 14 Wede Area 17 VI is located at the back of the occipital lobe A major transformation of visual information takes place in striate cortex Circular receptive fields RFs in retina and LGN are replaced with elongated stripe RFs in cortex Two important features of striate cortex Topographical mapping Cortical magnification Dramatic scaling of information from different parts of visual field Fovea Periphery Visual acuity declines in an orderly fashion with eccentricity distance from the fovea Receptive Fields in Striate Cortex Cells in striate cortex respond best to bars of light rather than to spots of light Some cells prefer bars of light some prefer bars of dark simple cells phase sensitive Some cells respond to both bars of light and dark complex cells phase insenitive Orientation tuning Tendency to respond more to bars of certain orientations and others Response rate falls off with angular difference of bar from preferred orientation Orientation tuning function less to How are the circular receptive fields in the LGN transformed into the elongated receptive fields in striate cortex Hubel and Wiesel Very simple scheme to accomplish this transformation A cortical neuron might receive input from several retinal ganglion cells String several retinal ganglion cells together Hubel Wiesel s model 4 Psych 253 Lecture 6 Spring 14 Wede Lecture 7 Receptive Fields in Striate Cortex Receptive Fields in Striate Cortex Many cortical cells respond especially well to Moving lines Bars Edges Gratings Direction of motion Each LGN cell responds to one eye or the other never to both Each striate cortex cell can respond to input from both eyes Cortical neurons tend to have a preferred eye however Two flavors of simple cells 5 Spring 14 Wede Psych 253 Lecture 6 Columns and Hypercolumns Column A vertical arrangement of neurons Each hypercolumn contains cells responding to every possible orientation 0 degrees 180 degrees Within each column all neurons have the same orientation tuning One set prefers input from the left eye and one from the right eye Model of a Hypercolumn Columns and Hypercolumns Each column has a particular orientation preference Adjacent groups of columns have a particular ocular dominance a preference for input from one eye or the other Blobs are indicated as cubes embedded in the hypercolumn 6 Psych 253 Lecture 6 Spring 14 Wede Regular array of CO blobs in systematic columnar arrangement discovered by using cytochrome oxidase staining technique Lecture 8 Cortical Organization Selective Adaptation The Psychologist s Electrode Adaptation The diminishing response to a sustained stimulus Can deactivating groups of neurons without surgery If presented with a stimulus for an extended period of time the brain adapts to it and stops responding This fact can be exploited to selectively knock out groups of neurons for a short period This demonstration will allow you to experience selective adaptation for yourself Tilt aftereffect Perceptual illusion of tilt Suggests that we have individual neurons selective to different orientations Selective adaptation for spatial frequency 7 Psych 253 Lecture 6 Spring 14 Wede Evidence that human visual system contains neurons selective for spatial frequency Spatial Frequency Adaptation Adaptation experiments provide strong evidence that orientation and spatial frequency are coded