Chapter 3 Gabor Patches

1. Chapter 3 Gabor Patches Ryan Scott, Pete McGrath, Nate Hofman, Katy Kolstad, Maria Peterson, and Katy Jenkins

2. Points of discussion • Sine wave grating • Gaussian window • Spatial frequency • Contrast • Phase

3. What is a Gabor patch? The figure on the right is a Gabor patch. In essence, a Gabor patch is a sine wave grating, seen through a Gaussian window.

4. What are they used for? • Gabor patches are mainly used in vision laboratories because they have “characteristics that match the receptive field properties of neurons in a primary visual cortex”

5. Activity I

6. Activity II

7. Sine wave grating • “Light intensity alternates between its darkest and lightest values according to a sine function”- pg. 53 • If separate cones pick up the lightest and darkest areas, grating is seen

8. The Gaussian Window

9. Spatial Frequency • Images in specific locations on the visual field are transported to and interpreted in specific areas of the cortex. • Objects closer to the fovea are processed by neurons in a large part of the striate cortex, while images located close to the periphery are processed in a small portion of the striate cortex. • This distortion of the visual-field map on the cortex is known as cortical magnification because the cortical representation of the fovea is greatly magnified compared to the cortical representation of peripheral vision. This causes decreased visual acuity (blurry vision) father away from the fovea.

10. Spatial Frequency Cont. • High resolution sight of the entire visual field would require much larger eyes and brains to process all of the information. • Neurons in the striate cortex respond most strongly to stripes, and individual neurons respond to specific orientations of lines. More cells are responsive to horizontal and vertical orientations than to oblique lines. • Individual cells of the striate cortex respond to specific gratings of lines, allowing the brain to distinguish the thickness of lines

11. High spatial frequency High contrast Medium spatial frequency High contrast Low spatial frequency High contrast Definition: **Contrast is the difference between the lightest and darkest portions of a Gabor Patch. In a high contrast region of a Gabor Patch: the darkest areas are black and the lightest areas are white. (there is a greater difference between the two colors) In a low contrast region of a Gabor Patch: the lightest areas are grey while the darkest areas are going to be dark grey. (there is a smaller difference between the two colors) When the Gabor Patches cycle through different phases at varying frequencies; they appear to move laterally. Contrast

12. Phases • Christina Enroth-Cugell & John Robson (1984) were the first to record responses of retinal ganglion cells to sinusoidal gratings. • Cells responded to gratings of just the right size, the response also depends on the phase of the grating (its position within the receptive field).

13. Phases • When shifted through a cycle the location of the grating within the visual field shifts. The light bar is desired to be in the center and the dark bar on the surround of the center.

14. Bibliography • J. Wolfe, K. Kluender, & D. Levi. Sensation and Perception 2nd ed.Sinauer Associates, Inc. 2009 • http://www.michaelbach.de/ot/ • http://www.sinauer.com/wolfe2e/chap3/startF.htm