PSU PSYCH 253 - Lecture 6 – Visual Acuity & Path to the Brain

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Psych 253 – Lecture 6 Spring ‘14 - WedeLecture 6 – Visual Acuity & Path to the BrainFinish 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 littlelight is there Enhanced by duplex retina - Neural circuitry of the retina  Remember the center-surround receptive fields of the ganglion cells1Psych 253 – Lecture 6 Spring ‘14 - Wede If photoreceptors are not completely saturated, ganglion cells will code dark and light areas, not overall lightlevel The visual system regulates the amount of light entering the eye, and ignores whatever variation in overall light levelis left over Lecture 6 Cortical Pathway - What is the path of image processing from the eyeball to the brain? Eye Photoreceptors  Bipolar cells  Retinal ganglion cells Lateral geniculate nucleus (Thalamus) Striate cortex - Cortical visual pathways Visual Acuity - Acuity: The smallest amount of spatial detail that can be resolved - 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 gratingV=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 background2Psych 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 ashadow, in the retinal image The visual system breaks down images into a vast number ofcomponents; each is a sine wave grating with a particular spatialfrequency  Retinal Ganglion Cells and Stripes- How do the center–surround receptive fields respond to sine wave patternswith 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 cortex3Psych 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 less to others Response rate falls off with angular difference of bar frompreferred orientation  Orientation tuning function  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 4Psych 253 – Lecture 6 Spring ‘14 - WedeLecture 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 5Psych 253 – Lecture 6 Spring ‘14 - Wede- -  Columns and Hypercolumns - Column: A vertical arrangement of neurons Within each column, all neurons have the same orientation tuning - Each hypercolumn contains cells responding to every possible orientation (0 degrees–180 degrees)- 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 6Psych 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


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PSU PSYCH 253 - Lecture 6 – Visual Acuity & Path to the Brain

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