Feature detection

Question 1

What can a simple cell tell you?

Answer 1

tuned to orientation but also fires to different luminance

Question 2

Distributed coding

Answer 2

a signal from a number of cells is needed in order to tell you what is happening in perception

Question 3

What does selective adaptation to gratings tell you

Answer 3

Evidence for connection between orientation tuning and perception.

Question 4

Visual angle properties

Answer 4

It maintains consistency and is the angle the image subtends on the retina. We can’t use measurements like mm because they change depending on distance. In one degree there are 60 minutes of arc and in each minute there are 60 seconds.

Question 5

Snellen chart

Answer 5

Maximum contrast because we can fix poor vision but not contrast. Detail subtends 1 min arc and complete letter subtends 5 min arc. 20/20 vision. the ability to read the 20 feet letter at a distance of 20 feet.

Question 6

20/20 vision can detect a gap of?

Answer 6

can detect a gap of 1 arc min

Question 7

The use of spatial frequency gratings

Answer 7

Snellen terms, an acuity of 20/20, which represents the ability to resolve a gap 1 arc min, is equivalent to a spatial frequency of 30 cycles per degree. Gratings give us spatial frequency measures. Sine waves can be added together. any visual scene can be broken down into constituent gratings

Question 8

Fourier analysis

Answer 8

An image can be broken down into its constituent sinusoidal components (Sf, amplitude, phase).

Question 9

How can gratings change?

Answer 9

orientation, contrast, spatial frequency and phase

Question 10

Receptive fields as frequency detectors

Answer 10

Cells filter out their frequency. cells with large RF analyse visual information at a coarse scale. cells with small RF analyse information at a fine scale. images added to give a final image

Question 11

Contrast sensitivity function

Answer 11

our sensitivity to contrast varies as a function of spatial frequency. Campbell and Robson 1968. single-cell recording experiments show that simple cell in visual cortex respond to a narrow range of spatial frequencies. If we show a grating and then change the contrast the observer’s contrast detection threshold is determined

Question 12

DeValois, R. L., Albrecht, D. G. and Thorell, L. G. (1982)

Answer 12

single-cell recording experiments show that simple cell in visual cortex respond to a narrow range of spatial frequencies. Experiment on cells in the macaque cortex. also shown for different animals, and different ages