Transcript Brightness and Lightness - UMD Space Physics Group

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?