Spatial Vision (continued) 3 Spatial Vision (continued)

Visual Acuity: Oh Say, Can You See? Spatial frequency: The number of cycles of a grating per unit of visual angle (usually specified in degrees) Another way to think of spatial frequency is as the number of times a pattern repeats per unit area http://www.sinauer.com/wolfe/chap3/gaborF.htm

Visual Acuity: Oh Say, Can You See? 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

Figure 3.7 The contrast sensitivity function (red line): our window of visibility; and Figure 3.8 A modulated grating X X e.g. contrast sensitivity = 1/contrast threshold (CT)=100 for CT=1% difference between dark&light stripes Visual acuity threshold = 0.017degrees Spatial frequency = 1/0.017 = 60 (limit determined by cone spacing) X X

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

Figure 3.11 The primate lateral geniculate nucleus LGN: relay station between retina and cortex Six layers: looks like a stack of pancakes bent in the middle wolfe2e-fig-03-11-0.jpg

Left side of both retinas Right visual field Figure 3.12 Input from the right visual field is mapped in an orderly fashion onto the different layers of the left LGN, and vice-versa wolfe2e-fig-03-12-0.jpg Left side of both retinas Right visual field Right side of both retinas Left visual field

Striate Cortex Striate cortex: Also known as primary visual cortex, area 17, or V1 A major transformation of visual information takes place in striate cortex Circular receptive fields found in retina and LGN are replaced with elongated “stripe” receptive fields in cortex It has about 200 million cells!

Figure 3.13 Striate cortex } LGN wolfe2e-fig-03-13-0.jpg

Striate Cortex Two important features of striate cortex: Topographical mapping Cortical magnification: Dramatic scaling of information from different parts of visual field The amount of cortex devoted to processing the fovea is proportionally much more than the amount of cortex devoted to processing the periphery

Figure 3.14 The mapping of objects in space onto the visual cortex wolfe2e-fig-03-14-0.jpg

Receptive Fields in Striate Cortex Hubel and Wiesel: tried to map the receptive fields of neurons in striate cortex Used spots of light that worked as stimuli for ganglion cells No neuronal activity was observed! Where did the signals from ganglions disappear??? http://www.charlierose.com/view/interview/10727?sponsor_id=1 [14min]