Understanding Visual Acuity: From Snellen Test to Striate Cortex - Prof. Michael Thomas Al, Study notes of Cognitive Psychology

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Spatial Vision:

From Spots to Stripes

Visual Acuity: Oh Say, Can You See?

• (^) What is the path of image processing from the eyeball to the brain?  (^) Eye (vertical path)  Photoreceptors  Bipolar cells  Retinal ganglion cells  (^) Lateral geniculate nucleus  (^) Striate cortex

Figure 3.1 Cortical visual pathways (Part 2)

Figure 3.1 Cortical visual pathways (Part 3)

Visual Acuity: Oh Say, Can You See?

• (^) The Snellen E test  (^) Herman Snellen invented this method for designating visual acuity in 1862  (^) Notice that the strokes on the E form a small grating pattern

Visual Acuity: Oh Say, Can You See?

• (^) There are several ways to measure visual acuity  (^) Eye doctors use distance to characterize visual acuity, as in “20/20 vision”  Your distance/normal vision distance

Figure 3.4 Sine wave gratings

Visual Acuity: Oh Say, Can You See?

• (^) Why does an oriented grating appear to be gray if you are far enough away?  (^) This striped pattern is a “sine wave grating”  (^) The visual system “samples” the grating discretely

Visual Acuity: Oh Say, Can You See?

• (^) Why sine gratings?  (^) Patterns of stripes with fuzzy boundaries are quite common  Trees in a forest, books on a bookshelf, pencils in a cup  (^) 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  This is called “Fourier analysis,” which is also how our perceptual systems deal with sound waves

Visual Acuity: Oh Say, Can You See?

• (^) Visibility of a pattern as a function of spatial frequency and contrast  (^) Figure 3.7 shows the contrast sensitivity function for a person with normal vision  (^) Figure 3.8 shows a pictorial representation of the same data

Retinal Ganglion Cells and Stripes

• (^) The response (right) of a ganglion cell to gratings of different frequencies (left): ( a ) low, ( b ) medium, and ( c ) high
• (^) How do the center– surround receptive fields respond to sine wave patterns with different spatial frequencies?

Retinal Ganglion Cells and Stripes

• (^) Not only is the spatial frequency important, but so is the phase  (^) Phase: The phase of a grating refers to its position within a receptive field

The Lateral Geniculate Nucleus

• (^) We have two lateral geniculate nuclei (LGNs): Axons of retinal ganglion cells synapse there  (^) Ipsilateral: Referring to the same side of the body (or brain)  (^) Contralateral: Referring to the opposite side of the body (or brain)

Figure 3.12 Input from the right visual field is mapped in an orderly fashion onto the different layers of the left LGN, and input from the left visual field is mapped to the right LGN