Spatial Vision(continued)

3

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

e.g. contrast sensitivity = 1/contrast threshold (CT)=100 forCT=1% difference between dark&light stripes

Visual acuity threshold = 0.017degrees Spatial frequency = 1/0.017 = 60 (limit determined by cone spacing)

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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

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

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

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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

Receptive Fields in Striate CortexHubel 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]