Early Processing in Biological Vision

1
Early Processing in Biological Vision

• Analyzing intensity changes
• in the retinal image

2
Human eye
3
Structure of a neuron

• Major parts of a neuron
• dendrites
• cell body
• axon
• terminals and synapses

• ? 1011 neurons in the brain

4
Rods and cones in the retina
Absorption spectra for S (short), M (medium), L
(long) wavelength cones
5
Projection from the retina
1st cortical stage of visual processing primary
visual cortex (or area V1)
6
Retinal ganglion cells
receptive fields exhibit a center-surround
structure, whose cross-section is the
difference of two Gaussians
7
Processing within the retina
8
Analyzing a 2D image
image after smoothing and second derivative
image
black negative white positive
zero-crossings
9

Detecting intensity changes at multiple scales

• human vision
• multiple receptive field sizes in the same region
  of the visual field
• receptive field sizes increase with eccentricity
  (distance from the center of the eye)

small s
large s
zero-crossings of convolutions of image with ?2G
operators
10
Mapping cortical receptive fields
11
Single-cell recording from visual cortex
Hubel Wiesel identified 3 basic cell
types simple, complex, hypercomplex cells
12
Some simple cells respond best to lines
Light Line Detector
Dark Line Detector
Firing Rate
Firing Rate
Horizontal Position
Horizontal Position
of a particular contrast sign, orientation,
position
13
Some simple cells respond best to edges
Light-to-dark Edge Detector
Dark-to-light Edge Detector
Firing Rate
Firing Rate
Horizontal Position
Horizontal Position
Again, of a particular contrast sign,
orientation, position
large receptive fields ? coarse spatial
structure small receptive fields ? fine spatial
structure
14
Detection of luminance edges



Simple cells appear to detect edges and lines



Many complex cells are also selective for stereo
disparity and motion