Chapter 3perception.pdf

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Matthias Niemeier

Chapter 3 – Spatial Vision: From Spots to Stripes Visual Acuity: Oh Say, Can You See? • Acuity – The smallest spatial detail that can be resolved o Eye doctors use 20/20 o Scientists talk about the smallest visual angle of a cycle of the grating that we can perceive ▪ A cycle is a repetition of a black and white stripe ▪ Visual angle is the angle that would be formed by lines going from the top and bottom of a cycle on the page, through the centre of the lens, and onto the retina • The limit is determined primarily by the spacing of photoreceptors in the retina o If the receptors are spaced such that the whitest and blackest parts of the grating fall on separate cones , we should be able to make out the grating o If entire cycle falls on a single cone, then we should see nothing but a gray field o Aliasing- when we misperceive the cycles to be longer than they are • A Visit to the Eye Doctor o Herman Snellen (1834-1908) ▪ Constructed a set of block letters for which the letter as a whole was five times as large as the strokes that formed the letter ▪ Defined visual acuity: (Distance at which a person can just identify letters) (Distance at which a person with “normal” vision can just identify letters) Acuity for Low-Contrast Stripes • What happens if the contrast of the stripes is reduced? o Otto Schade ▪ Showed people sine wave gratings with different spatial frequen- cies ▪ Spatial frequency refers to the number of times a pattern • Repeats in a given unit of space • Fergus Campbell and Dan Green o Human contrast sensitivity function (CSF) is shaped like an upside-down U ▪ Obtain the units for the y-axis in this graph by taking the recipro- cal of the contrast threshold (smallest amount of contrast re- quired to detect a pattern) ▪ Contrast of 100% corresponds to a sensitivity value of 1 Why Sine Wave Gratings? • Patterns of stripes with more or less fuzzy boundaries are quite common • Visual system breaks down real-world images into a vast number of components o Each is a sine wave grating with a particular spatial frequency Retinal Ganglion Cells and Stripes • Each ganglion cell responds well to certain types of stripes or gratings • When the spatial frequency of the grating is too low, ganglion cell responds weakly o Because part of the fat, bright bar of grating lands in the inhibitory sur- round ▪ Dampens cell’s response • When the spatial frequency is too high, ganglion cell also responds weakly o Because both dark and bright stripes fall within the receptive-field cen- tre ▪ Washes out the response • Retinal ganglion cells are ‘tuned’ to a specific spatial frequency • Their responses are also dependent on the phases of the grating within the re- ceptive field o On-centre cells respond vigorously when the grating has a light bar fill- ing the receptive field centre and dark bars filling the surround o A dark bar in the centre would produce a negative response o Any intermediate shifts blind the grating The Lateral Geniculate Nucleus • Axons of the retinal ganglion cells synapse in the two lateral geniculate nuclei (LGNs) o One in each hemisphere o Layered structure (six main layers) ▪ Neurons in the bottom two layers are larger (magnocellular lay- ers) ▪ The remaining top four layers are the parvocellular layers • Magnocellular layers o Receive input from M ganglion cells in the retina (larger) o Pathway responds to large, fast-moving objects • Parvocellularlayers o Receive input from P ganglion cells (smaller) o Pathway is responsible for processing details of stationary targets • There are layers between the magno- and parvo- layers o Koniocellular cells • Layers 1, 4, 6 of the right LGN receive input from the left eye • Layers 2, 3, 5 of the left LGN receive input from the right eye • LGN neurons o Respond well to spots and gratings o Part of the “thalamus” Striate Cortex • Receiving area for LGN inputs in the cerebral cortex lies below the inion • Has several names: Primary visual cortex (V1), or area 17 • Consists of 6 major layers o Fibers from the LGN project mainly to layer 4 ▪ Magnocellular axons projecting into sublayer 4Cα ▪ Parvocellular axons projecting into sublayer 4Cβ • Cortical magnification: the distortion of the visual-field map on the cortex o Cortical representation of the fovea is greatly magnified compared to the cortical representation of the peripheral vision Topography of the Human Cortex • Most of what we know comes from physiological studies on animals • fMRI – measures the changes in blood oxygen level that reflect neural activity o reflect a range of metabolically demanding neural signals Some Perceptual Consequences of Cortical Magnification • One important consequence of cortical magnification is that visual acuity de- clines in an orderly fashion with eccentricity • High resolution requires a great number of resources: o A dense array of photoreceptors o One-to-one lines from photoreceptors to ganglion cells o A large chunk of striate cortex • Visual crowding o The deleterious effect of clutter on the peripheral object recognition ▯ Receptive Fields in the Striate Cortex • Hubel and Wiesel o Receptive fields of striate cortex neurons are not circular – elongated ▪ Respond much more vigorously to bars, lines, edges, and gratings than to round spots of light Orientation Selectivity • Important properties of the receptive fields of neurons in the striate cortex: o An individual neuron responds best when the line or edge is just at the right orientation ▪ Orientation tuning: the cell is tuned to detect lines in a specific orientation • Evidence suggests that neural interactions within the cortex play a role in these dynamics • Arrangement of the LGN inputs is crucial • The population of neurons as a whole detects all possible orientations o HOWEVER, more cells are responsive to horizontal and vertical orientations than to obliques Other Receptive-Field Properties • Cortical cells also respond well to gratings o Respond best to gratings that have just the right spatial frequency to fill the receptive-field-centre o Tuned to a particular spatial frequency ▪ Corresponds to a particular line width o Functions as a filter for the portion of the image that excites the cell o Many respond especially well to moving lines, bars, edges, and gratings ▪ Responds stronger when a l
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