Retina- your CCD

Light

~120 million rods~7 million cones

Most cones concentrated in the fovea

Signal Processing• Trace the signal through

the retina:

• The retina is a 7-layered

structure involved in signal transduction.

– Light enters from the GCL side first, and must penetrate all cell types before reaching the rods and cones.

– The outer segments of the rods and cones transduce the light and send the signal through the cell bodies of the ONL and out to their axons.

– In the OPL photoreceptor

axons contact the dendrites

of bipolar cells and horizontal

cells. Horizontal cells are

interneurons which aid in

signal processing

– The bipolar cells in the INL process input from photoreceptors and horizontal cells, and transmit the signal to their axons.

– In the IPL, bipolar axons

contact ganglion cell

dendrites and amacrine

cells, another class of interneurons.– The ganglion cells of the GCL send their axons

through the OFL to the optic disk to make up the optic nerve. They travel all the way to the lateral geniculate nucleus.

The Optic Nerve

Brightness and Lightness

• Brightness: Describes the light intensity of light sources such as the sun, a light bulb, or an overall scene– Dark, dim, bright, dazzling…– Sensation depends on adaptation. The same

source may produce different feeling at different times

• Lightness: Describes the appearance of the individual surfaces:– Black, shades of gray, and white– Does not depend on adaptation or illumination.

Lightness Constancy• The lightness is relative, does not depend

on the brightness.– During the day, a black cat is black and a white

piece of paper is white. At night, you see the same, although the light intensity has changed a lot!

• All objects appear to maintain their familiar lightness when the lighting condition changes.– The piece of paper always appears white even

though a light meter measures less light coming from it at night than from a sunlit black cat!

• WEBER'S LAW, in psychology, the name given to a principle first enunciated by the German scientist, Ernst Heinrich Weber (1795-1878), who became professor at Leipzig (of anatomy, 1818, of physiology, 1840). He was especially famous for his research into aural (hearing) and cutaneous (touch) sensations. His law, the purport of which is that the increase of stimulus necessary to produce an increase of sensation in any sense is not a fixed quantity but depends on the proportion which the increase bears to the immediately preceding stimulus, is the principal generalization of that branch of scientific investigation which has come to be known as psycho-physics.

Weber’s Law• Equal steps in lightness arises from steps

of equal ratio of light intensity (a logarithmic scale)– 1, 2, 4, 8, 16 … has equal steps in lightness– 1, 2, 3, 4, … does not have equal lightness

steps (3 is much closer to 4 than 1 is to 2.

• Limitations: – Beyond certain brightness, your visual

system no longer responds to the increased light. The same thing happens in the opposite limit.

• Why do our vision and hearing obey Weber’s law?– Allows good sensitivity to very different

signal levels. – The range of light intensity that we are

sensitive to is enormous .• Bright sunny day vs. very dim star light, the

intensity varies by billions of times!• Our hearing responds appropriately to sound

intensities varying by a factor of 1012 (a trillion)!

The assumption that what you see is pretty much what your eye gathers and reports to your brain is not true.

• In fact, your brain adds very substantially to the report it gets from your eye, so that a lot of what you see is actually "made up" by the brain.

• Perhaps even more interesting, the eye actually throws away much of the information it gets, leaving it to the brain to fill in additional information in its own way.

Receptive field(center-surround)

• Refers to a region of retina which will excite one ganglion, producing a signal to the brain, depending on the pattern of the light falling on it.

Mechanism of lightness constancy

• Lateral inhibition: – Increased illumination of one region of the

retina diminishes the signal to the brain from its neighboring region.

– When the overall light-intensity increases, lateral inhibition increases, and the increase is largely ignored by brain. (the result is Weber’s law, the overall response only increases logarithmically with increasing stimulus)

• At the first processing step, each photoreceptor generates a signal related to the intensity of light coming from a corresponding point of the observed object. Photoreceptors corresponding to brighter areas of the object (yellow) receive more light and generate larger signals than those corresponding to darker areas (black).

• Output neurons well to the right of the dark/light border are excited by an overlying photoreceptor but also inhibited by adjacent, similarly illuminated photoreceptors. The same is true far to the left of the dark/light border.

• Hence, assuming that the network is organized so that equal illumination of exciting and inhibiting photoreceptors balances out, output neurons far from the edge in either direction will have the same output signals.

• Only output neurons near the dark/light border will have different output signals. As one approaches the dark/light border from the left, the signals will decrease because inhibition from more brightly lit photoreceptors to the right will outweigh the excitation from the overlying dimly lit photoreceptors.

• As one approaches the dark/light border from the right, the signals will increase because excitation from brightly lit photoreceptors is not completely offset by inhibition from the dimly lit photoreceptors to the left.

• When there is a contrast, it gets amplified through the lateral inhibition.– The brain is made more aware of the

difference in lightness than it would be without the lateral inhibition.

– The edge where the light intensity changes rapidly from brighter to darker is made more noticeable (edge enhancement).

• Efficient in storage and transmission of information.– The brain only needs to remember the

edges. – Same lightness distributions: lightness

constancy

Simultaneous lightness contrast

• Lightness is affected by the neighboring region: this can produce illusions!

• The same gray rectangle appears darker when surrounded by white than by black.

• Hermann grid illusion.

• Apparent non-uniformity in the uniform change of the lightness.

Which one gives you the maximum response?