HUL 211 Colour in object perception and memory - Part 1

Document Sample
HUL 211 Colour in object perception and memory - Part 1 Powered By Docstoc
					Colour in object perception and
        memory – Part 1

         Snehlata Jaswal

                   Colour as a feature
Colour is one of the features of an object. Color can only exist when three
components are present: a viewer, an object, and light.

Although pure white light is perceived as colorless, it actually contains all
colors in the visible spectrum.

                          HUL 211 OBJECT PERCEPTION AND MEMORY
                     Coloured objects
When white light hits an object, it selectively blocks some colors and
reflects others; only the reflected colors contribute to the viewer's
perception of color. Objects may be classified as luminous and
illuminated. Luminous objects are themselves the source of light – such
as the sun, the stars, an incandescent bulb, etc. the amount of light
energy they give off is known as radiance. Most objects, however only
reflect light, and are therefore said to be illuminated.

It follows that the amount of incident light falling on an object or surface is
called illuminance. Further, the intensity of light reflected from an
illuminated surface is called luminance. E.g. the amount of light
emanating from the sun or the bulb in the room is radiance, the amount of
light falling on this page is illuminance, and the amount of light reflected
from the page is luminance. All these contribute to the psychological
experience of brightness. Changes in the brightness of adjacent areas in
the visual field is an important factor in the formation of contours and is
also the basis of perception of form and image formation.

                          HUL 211 OBJECT PERCEPTION AND MEMORY
Receiving information about Colour

 The human eye senses this spectrum using a
 combination of rod and cone cells for vision. Rod cells
 are better for low-light vision, but can only sense the
 intensity of light, whereas while cone cells can also
 discern color, they function best in bright light.

 Three types of cone cells exist in your eye, each being
 more sensitive to either short (S), medium (M), or long
 (L) wavelength light.

 The set of signals possible at all three cone cells
 describes the range of colors we can see with our eyes
 – the visible spectrum from ~400 nm to 700 nm.

       Transduction of radiant energy
In the initial stage of vision, radiant energy must be transduced, or
transformed, into a neural form. Physical energy acts on light-sensitive
tissue to produce impulses that convey sensory information.

The kind of tissue that is responsive to radiant energy is found even in
the simplest organisms. Some organisms, such as the single-celled
amoeba, possess no specialized light receptors; rather, the entire body
is light sensitive. However, most animals have a region on their body
that is maximally sensitive to light.

 Nevertheless, mere responding to light is quite different from actually
forming a visual image. Indeed, many of the light-sensitive structures of
lower forms of life act primarily to concentrate light on a light-sensitive
pigment. That is, they serve as light-gathering rather than image-forming
organs. It is in advanced stages of evolution that an image-forming eye
developed. The human visual system is the eye, attuned to detecting
light or radiant energy. cause it to focus an image on the retina.
                         HUL 211 OBJECT PERCEPTION AND MEMORY
                       The human eye
The vertebrate eye is built on a single basic plan: from fish to mammal, all
vertebrate eyes possess a photosensitive layer called a retina and a lens
whose optical properties cause it to focus an image on the retina

                          HUL 211 OBJECT PERCEPTION AND MEMORY
                   Sclera and Cornea
The eyeball, lying in a protective socket of the skull, is a globular
structure just under 1 inch (about 20 mm) in diameter. The outer
covering of the eyeball is a tough white, opaque coat about 1 mm
in thickness called the sclera (seen as the white of the eye).

The sclera at the front of the eye becomes the translucent
membrane called the cornea. Light rays entering the cornea are
refracted, or bent, by its surface. In fact, most of the refraction of
light rays reaching the eye is performed by the cornea. If the
cornea is misshapen, producing astigmatism, the person
experiences a blurring of some of the incoming light.

The cornea has no blood vessels because they would block the
incoming light. This is why corneas can be easily transplanted
with little fear of tissue rejection. Since there are no blood vessels
in the cornea to provide nutrition, the cornea receives nutrition
                        HUL 211 OBJECT PERCEPTION AND MEMORY
          Aqueous humor and choroid
Since there are no blood vessels in the cornea to provide nutrition, the
cornea receives nutrition from the aqueous humor, a watery liquid that
resembles the cerebrospinal fluid. The aqueous humor fills the anterior
chamber immediately in the back of the cornea. Though it is being
continuously recycled, the canal through which it leaves the anterior
chamber can become blocked, especially in the elderly, resulting in a
buildup of pressure called glaucoma.

A second layer of the eyeball, the choroid, is attached to the sclera. The
choroid is about 0.2 mm thick. It consists largely of blood vessels and
provides a major source of nutrition for the eye. In addition, the choroid is a
heavily pigmented, dark structure; this enables the absorption of most
extraneous light entering the eye, thereby reducing reflections within the
eyeball that might blur the image. However, some nocturnal animals
possess a retinal layer, called the tapetum, that reflects back some of the
light entering the eye. It is the reflection of light from the retina that
accounts for the eerie glow, or “eyeshine”, that appears occasionally from
the eyes of many nocturnal animals.
                           HUL 211 OBJECT PERCEPTION AND MEMORY
                         Iris and pupil
In front of the eye the choroid is modified to form the iris. The iris lies
between the cornea and the lens. It is a disk-like, colored membrane,
consisting of two smooth muscles. One function of the iris is to regulate
the amount of light entering the eye. The iris surrounds the pupil, which
is actually just a hole in the middle of iris through which the light passes.
When lighting conditions are poor, the iris opens (or dilates) to increase
the size of the pupil. By contrast, in bright light the iris closes,
constricting or reducing, the size of the pupil. In the human young adult,
the pupil is round and its size ranges from about 2 to 9 mm in diameter.
This enables more than a 20-fold variation in area and in the amount of
light admitted into the eye. Generally, the pupil’s reaction to the overall
conditions of lightning is reflexive. Shining a bright light in the eye
produces Whytt’s reflex (a reflex identified by the physiologist Robert
Whytt in 1751): an immediate constriction of the pupil in response to
bright light. the inability to demonstrate Whytt’s reflex may indicate
neural injury to the CNS.

                          HUL 211 OBJECT PERCEPTION AND MEMORY
The lens is a transparent spherical body located directly behind
the pupil. It is slightly yellow in color and is composed of an outer
layer (the lens capsule) that contains fibers arranged like the
layers of an onion (Koretz and Handelman, 1988).

The cornea bends light rays as they enter the eye. The lens
completes the task of bringing light waves into focus on the
photoreceptors that line the rear of the eye. Because the lens can
change its shape, it can focus light rays from both nearby and
faraway objects through a process called accommodation.

The ciliary muscle surrounds the lens and is attached to it by
means of tiny fibers called the Zonules of Zinn.

                       HUL 211 OBJECT PERCEPTION AND MEMORY
When we are looking at a distant object (20 feet away or ore), the ciliary
muscle relaxes, which causes the muscle to expand and pull on the
zonules. In this state (unaccommodated), the lens is pulled out to its
flattest shape, so it will bend the incoming light the least. When you are
looking at an object that is nearer, the ciliary muscle contracts, which
allows the lens to return to its natural shape. In this state
(accommodated), the lens will bend light the most.

As we grow older, however, the lens loses its ability to accommodate.
This condition, called presbyopia, is due to several factors. The lens
actually continues to grow throughout our lifetime, so it is thicker in older
people. To correct for this deficit, people have to wear reading glasses,
or bifocals if their vision needed correction prior to the onset of
presbyopia. It is crucial that the lens be clear so that the light can pass
through it. A cataract is a cloudy lens, which can happen as a result of
injury or disease. Alternatively, an intraocular lens, a substitute lens,
can be inserted through the pupil after the defective lens has been
removed.                    HUL 211 OBJECT PERCEPTION AND MEMORY
          Retina and vitreous humor
The various structures - from the cornea through the lens - serve
to bring images into focus on the eye’s photoreceptors. The retina
is a layer of light receptors, or photoreceptors and nerve cells at
the rear of the eye. The transduction of light energy takes place in
retina. The photoreceptors absorb light rays, and transform them
into information that can be transmitted by the neurons, or nerve

Between the lens and the retina is the posterior chamber that
contains the vitreous humor, a thick jelly like substance, the
pressure from which helps to maintain the shape of the eyeball,
despite external pressures.

                       HUL 211 OBJECT PERCEPTION AND MEMORY
             Fovea, rods, and cones
The photoreceptors are most densely packed within the fovea,
thus it is the portion of the retina that produces clearest vision.
Humans have a single fovea located in the center of the retina.
Many mammals lack the fovea, and some animals (horses and
birds) have two foveas in each eye. In horses this is a marvellous
evolutionary adaptation, as it enables the horse to see clearly the
ground at its feet and the space directly ahead of it at the same

A bundle of neurons, the optic nerve, carries information away
from the retina at the optic disc. The optic disc has no
photoreceptors, so you cannot see anything that falls on this part
of the retina. The optic disc therefore creates a blind spot.

Cones and rods are the two kinds of photoreceptors in the retina
that transduce the light information into neural information.
                       HUL 211 OBJECT PERCEPTION AND MEMORY
                          Cones vs. Rods
Characteristic             Cones                                  Rods
Kind of vision             Colour                                 Black and white
Shape                      Fat and pointed                        Thin and blunt
Number                     7 million                              125 million
Sensitivity                Poor                                   Excellent
Distribution               Throughout retina,                     Periphery of retina, not
                           concentrated in fovea                  in fovea
Dark adaption              Rapid, but high                        Slow, but low threshold
Light required for best     Well lit                              Dimly lit
Number of receptors for    Few                                    Many
one ganglion cell
Disc shedding              Evening                                Morning
Photo pigment              Three types                            Rhodopsin
Acuity                     Excellent
                           HUL 211 OBJECT PERCEPTION AND MEMORY
                Beyond rods and cones
The information from the cones and rods is transmitted through
the bipolar cells to the next level in the chain, the ganglion cells.
Ganglion cells take the information from the bipolar cells and bring
it toward the brain. Recent research has revealed three
structurally different ganglion cells:
X cells are the most common, they respond in a steady, sustained fashion
during stimulation, so they are important for picking up precise details about the
visual stimulus. They are perhaps important to see the pattern in non-moving
Y cells are probably involved in the perception of movement and stereoscopic
depth, but are insensitive to color because they cannot differentiate among the
cone inputs they receive. Both X and Y cells possess a central receptive field,
with antagonistic surrounds, and hence work best in the presence of contrast in
the stimulus.
W cells respond to more homogeneous, evenly distributed stimulation. The
most slowly conducting of the three types of ganglion cells, they seem to
respond best to moving stimuli. Less is known about them, probably because
they appear to show a wider range of attributes.
                            HUL 211 OBJECT PERCEPTION AND MEMORY
Between the fovea and ganglion cells

Bipolar, Horizontal and Amacrine cells
The chain of interconnections from the photoreceptors to the
bipolar cells to the ganglion cells is a vertical chain. However, the
information also travels horizontally across the retina through
horizontal cells and amacrine cells.

Horizontal cells allow communication among photoreceptors.
They also interconnect bipolar cells. Furthermore, horizontal cells
communicate with one another.

Amacrine cells allow communication between bipolar cells and
between ganglion cells. They are also interconnected by means
of other amacrine cells. In fact, with widely branching dendrites,
they are perhaps the most common cells in the retina.

                       HUL 211 OBJECT PERCEPTION AND MEMORY
            From the eye to the brain
The optic nerve is a bundle of ganglion cells leaving the eye.
Most of the ganglion cells in the optic nerve terminate in the lateral
geniculate nuclei. However, before they do, half of the ganglion
cells of either eye cross over to the other side of the brain at the
optic chiasm. The ganglion cells from the left half of the left eyes
do not cross over at the optic chiasm, but continue back to the left
lateral geniculate nucleus. Likewise, the ganglion cells from the
right half of the right eye remain on the right side of the brain.
However, the ganglion cells from the right half of the left eye cross
over to the right side at the optic chiasm. The ganglion cells from
the left half of the right eye cross over to the left side of the brain.
As a result, information from the left visual field of both eyes is
carried to the right visual cortex, and information from the right
visual field of both eyes is carried to the left visual cortex.
                        HUL 211 OBJECT PERCEPTION AND MEMORY
From the eye to the brain

           Optic nerve and optic tract
The bundle of ganglion cells is called the optic tract after it crosses the
optic chiasm. Groupings of ganglion cells are first called the optic
nerve. Then after passing through the optic chiasm, the cells are
regrouped and called the optic tract. No synapses are found in the optic
chiasm, so it serves no function in terms of processing the visual stimuli.
In the absence of a synapse, the information from the retina remains
intact and untransformed. So the change in name reflects the fact that
the optic tract contains information from half of each eye, but that
information is kept separate until later in the visual system. Optic
chiasm is merely the location where the portions of each optic nerve
cross over to the other side of the brain.

The first synapse of the ganglion cells in the optic nerve is either in the
superior colliculus or the lateral geniculate nucleus. So the first
opportunity for further processing of visual information occurs in these
two locations.
                          HUL 211 OBJECT PERCEPTION AND MEMORY
                  Superior colliculus
Some of the optic tract fibers go to the superior colliculus. The
Superior colliculus is a relatively primitive part of the midbrain that
in humans is important for eye movements as well as other
perceptual functions. Two superior colliculi are found in the visual
system, one for each optic tract. In humans, some of the Y and all
of the W ganglion cells go to the superior colliculus. Remember
that the Y cells are sensitive to movement rather than details
about shape.

The superior colliculus receives information from the auditory
system as well as information from the skin senses (e.g. touch).
This non-visual information is arranged to coincide spatially with
the organization of the visual input. Thus the superior colliculus
probably works toward integration of information from various
                        HUL 211 OBJECT PERCEPTION AND MEMORY
           Lateral Geniculate Nucleus
Most of the optic tract fibers go to the Lateral Geniculate Nucleus.
Lateral means “on the side”. Geniculate means “bent like a knee”.
Nucleus means “little nut”. So a lateral geniculate nucleus looks
like a little nut that is bent like a knee, located on each side of the
brain. The lateral geniculate nucleus (LGN) is part of the thalamus
where 80% of the ganglion cells that began in the retina finally
stop, transferring their information to new neurons. (Most of the
remaining 20% stop at the superior colliculus). Because the
ganglion cells entering the LGN have passed over the optic
chiams, input to the LGN comes from both eyes. However, this
input from the two eyes is kept separate in the LGN in a layered,
or laminated, fashion. In fact, the LGN contains six layers, three
from each eye.

                        HUL 211 OBJECT PERCEPTION AND MEMORY
           Lateral Geniculate Nucleus
The visual pathway appears to have three parallel information
systems, two of which are present in the LGN (Hubel,1988;
Livingstone, 1987). The parvocellular system (small cell) receives
input from the X cells. The magnocellular system (large cell)
receives input from the Y cells. The former is involved in the detailed
and colour processing of non-moving stimuli, whereas, the latter is
involved in movement and depth perception.

Tremendous differences are found in LGN from species to species. In
human beings, input to the LGN comes not only from the retina, but
also from other parts of the brain. So LGN is not simply a relay
station designed to keep the ganglion cells to a manageable length,
but a place where information is processed. The inputs from the
other areas of the brain must be important to LGN functioning, but
researchers are currently exploring the effects of these inputs.
                        HUL 211 OBJECT PERCEPTION AND MEMORY
                         Visual cortex
The visual cortex is the part of the cerebral cortex, or outer part of the
brain, concerned with vision. The visual cortex is in the rear part of the
brain (occipital lobe). The rear portion of the visual cortex has been
labeled Areas 17,18 and 19. Neurons from the lateral geniculate nuclei
terminate in Area 17 of the visual cortex, which is also referred to as the
primary visual cortex or the striate cortex. Striate means “striped”; a
microscopic investigation of this area of the cortex reveals pale

The clear majority of research on cortical processing in vision has
concentrated on the primary visual cortex (Area 17). Two other regions
at the back of the brain are also concerned with vision. These are Area
18 and Area 19, often called the secondary visual cortex (as opposed
to the primary visual cortex) or the extra striate cortex (where extra-
means “beyond” as in “extraterrestrial”). The extra striate cortex appears
to be involved in complex aspects of visual processing.
                         HUL 211 OBJECT PERCEPTION AND MEMORY
                       Visual cortex
There is an overrepresentation of information from the fovea in the
cortex. This cortical magnification helps to construct an accurate
three-dimensional representation of the object. Hubel and Wiesel,
who shared the Nobel Prize with Robert Sperry in 1981, isolated
three kinds of neurons in the visual cortex: simple, complex, and
hypercomplex, which respond to increasingly selective stimuli.
This is in consonance with the principle in visual processing that
cells become more selective at higher levels. Photoreceptors
respond when the light reaches them, ganglion cells and LGN
cells respond only if the center of the receptive field contrasts with
the surrounding area, simple cells respond to lines, complex cells
require moving lines, hypercomplex cells require moving lines of
particular dimensions.

                       HUL 211 OBJECT PERCEPTION AND MEMORY
 Three stages in cortical processing
Zeki and Marini (1998)

Used fMRI as participants viewed objects in their natural and
unnatural colours and their achromatic counter parts to chart
the stages of processing of colour in the brain.
Concluded that there are three stages in cortical colour processing:
1. Based on V1 and maybe V2 and mainly concerned with
   registering the presence, intensity, and differences of different
2. Based on V4 and concerned with automatic color constance
   operations without regard to memory, judgement, and
3. Based on inferior temporal and frontal cortex, concerned with
   ‘object colour’ involving memory and past experience

                      HUL 211 OBJECT PERCEPTION AND MEMORY
         Thank you


Shared By:
Description: Learn How the Brain works, how we take input to brain. Understand what exactly is attention and its bottle necks and get to know what is Top Down Processing and Bottom Up Processing.