Cellular Model of Biological Visual Processing:
Image Data (Photons)
Captured by Receptors (Rods & Cones) (Retina)
Pass Thru layers of cells (Preprocessing of the Data) (Retina)
Transmitted over distance (Ganglia (optic nerve))
Higher – level processing (Visual Cortex)
http://www.cis.rit.edu/mcsl/faq/faq1.shtml (good link on questions of vision and color.
http://www.redherring.com/mag/issue86/mag-mutant-86.html (interesting link on tetrachromats)
Retina is sufficiently complex to be a small brain
(not a photographic plate, rather operates on contrast and differences)
Continual movement in a sequence of saccades
Each takes about 30 msec to jump to a new point of regard
Dwells there for 230 [70~700] msec
Low resolution, Peripheral vision ~ 180 deg.
High resolution, Fovea ~ 2 deg.
5 classes of neurons (fig 27-6) organized in 3 physical layers producing 2 distinct synaptic layers:
Rods & Cones (Photoreceptors, ~108)
Horizontal cells (couple many rods & cones) [inner layer]
Bipolar (flat cone & rod varieties)
Amacrine (couple many Bipolars to a single ganglia)
Ganglion (off-center and on-center varieties, ~106]
Outer nuclear layer: Rods, Cones
Inner layer: Horizontal, Bipolar, Amacrine, (compute differences in “signals”)
Ganglia layer: Ganglion
Outer plexiform layer: receptors, bipolar, horizontal
Inner plexiform layer: Bipolar, Amacrine, ganglia
2 major information flow pathways:
direct route: receptor, bipolar, ganglion
integration route: horizontal cells integrate and transfer info from distant receptors to bipolar-ganglion cell pathway
Rods & Cones:
Outer layer!! Light must pass through other cellular layers to reach them
Fovea: 2 deg. Fovia; intervening neural layers are pealed back
Blind spot: nasal to fovea, where optic nerve fibers leave retina: no photoreceptor
Cones: detect color. 3 varieties: RGB, not very sensitive, day vision
Rods: low light gray-scale receptors. 10 times as many as cones, (108)
Cones: less sensitive, faster response, directionally sensitive, connected to more individualized neural channels leads to higher acuity.
Rods: longer photoreceptors allow them to capture more light (more sensitive), are affected by more scattered light and many rods synapse on the same bipolar cell, thus, they have poor spatial resolution; “firing rate” changed upon achieving a threshold of stimulus.
They transduce light into electrical signals through synaptic activity:
Surprisingly: light inhibits and darkness excites photo-receptor cells!
Most cells in visual system show continued synaptic activity (discharge) even in the absence of illumination. Nominal synaptic rate of .001 sec.
Appropriate stimuli modulates background synaptic activity, (increasing or decreasing)
Rods & cones make direct synaptic contact with bipolar cells. Horizontal and amacrine cells mediate between lateral interactions between receptors and ganglion. Ganglion cells project to the lateral geniculate nucleus and the superior colliculus in the visual cortex.
A single cone synapses on two separate bipolar cell channels: one is excited by light activation (on-center (depolarizing)) and the other is inhibited by light activation (off-center (hyper-polarizing)).
Horizontal Cells: deliver to the ganglion response of more distant photoreceptors than those directly connected to the bipolar.
This leads to Center-Surround Antagonism; Direct light on the center of a receptive field is antagonized by direct light on the surround of its receptive field. This is important in determining borders (27-7)
Ganglion cells : on-center and off-center variety
Also have morphologically and functionally different subsets that serve the same photo receptors in parallel
X ganglion: medium sized cell bodies, narrow dendritic fields: high visual acuity (28-7) (80%)
Y ganglion: largest cell bodies, large dendritic arborization: respond to large targets, perform initial crude evaluation (10%)
W ganglion: small cell bodies, large arborization, project to the superior colliculus: are involved in head and eye movement (10%)
See Fig 28-1 for visual fields
Lens inverts the image on the retina (28-2),
Nasal portion of the visual field projects on the temporal portion of the retina, etc.
Left optic track contains complete representation of right hemifield of vision. (28-3)
Injury to portions of brain lead to defects in visual fields (28-10)
Processing form and movement:
Retina is mapped in the lateral geniculate nucleus and the visual cortex.(29-1, 29-4)
Lateral geniculate nucleus enhances the antagonism between the center and its surround. (29-5)
Receptive field of neurons in the visual cortex have the following 3 features:
1. correspond to a specific retinal position
2. have discrete excitatory and inhibitory zones
3. have specific axis of orientation (see fig. 29-7)
Appearance of an object depends not on the intensity of the light source but on the contrast between the object and its surround (29-6)
Fig 29-8 clearly describes the orientation of the receptive field of a simple cell in the primary visual cortex.
Views of Viewing: Computational Modeling Vision and Visual
Cognition - A. Kornhauser
· Objective of low-level: Segmentation and decomposition of a scene into constituent independent parts/objects.
Motion in space (Relative motion train moving or station moving?)
· Seems to be executed independent of domain and task
Given a scene on two different occasions with different tasks/goals, same low-level process applied
· Process applied is mostly independent of specific object knowledge
Motion perception uses assumption about the continuity and rigidity of objects but not that it is a pen or a hat. The use of rigidity in the perception of motion is present at the age of 5 months.
· Done in parallel / simultaneously in large portions of the visual field
· Process can be modeled as sub-processes (Marr):
3 stage process:
a. Primal sketch (edge detection) (input)
b. Modular processes of Shape extraction, Texture, Color, Binocular, Motion
c. 2.5 D sketch (view-centered representation)
Given: Array of intensity values.
1st step is to normalize the values (eye works on relative intensity)
next steps (going on in parallel if possible, somewhat independent)
· Edge detection:
find zero crossing of 2nd derivative of image intensity,
need convolution of filter to “smooth out noise”
· Texture segmentation:
Can be viewed either as
Statistical property (variation of spatial intensity levels), or
A class of primitive elements (template matching): textons
· Binocular Stereo:
Must solve the correspondence problem: what matches with what?
· If corresponding primitives can be found: triangularization computes depth
· Epipolar line constraints: A primitive in the left eye can only be matched with primitives lying on the (epipolar) line on the right eye. (fig 1.6b)
· Direction of Gaze (relative orientation of the eyes)can also compute depth
· Changes in Direction of Gaze:
· changes in direction of gaze changes the complexity of correspondence! Correspondence is simplest if eyes are fixating directly on object. (fig 1.9)
Color constancy under varying illumination levels:
Perceived color of a patch is largely independent of illumination (intensity), but does depend on the color of neighboring patches. (Red London bus shade or sun)
Implications on shape, depth
Solve the correspondence problem over a temporal sequence of images
1. Motion Measurement: construct a Velocity Field, 2. Recovery of Structure
Long Range motion correspondence: Minimal mapping: finds the correspondence that minimizes a distance function while ensuring that there will be at least one match.
Short Range motion correspondence:
Match neighboring point: “easy”
Match neighboring contours: Aperture problem:
get good information on velocity field normal to the contour,
get bad/no information on velocity field tangential to the contour (1.14)
(good for collision avoidance)
Pattern Recognition: problems
Independent of scale
Independent of orientation
Nonaccidental properties: 1.Smooth Continuation, 2.Cotermination, 3. Parallelism, 4. Symmetry
Complex entities invite decomposition into simple parts.
Recognition-by-components / Geons
24 geons, 4 attribute x-sections,
108 viewpoint invariant relations
3 relative aspect ratios (larger, smaller, equal)
Then: 3 geons give 109 combinations
Extension of scene perception, Transversality principle: matching vertices
Parallel distributed processing