CHAPTER 5
VISION
A. Visual Coding
The majority of information in the brain is stored in terms of which neurons are
active and how active they are right now. While some neurons' impulses indicate light,
others' indicate sound, touch, or other sensations. This realization was referred to by
Johannes Müller as the law of particular nerve energies in 1838. According to Müller,
anything that stimulates a certain nerve creates a special type of energy that is specific to
that nerve. Modern terminology states that the brain somehow interprets the action
potentials from the auditory nerve as sounds, the olfactory nerve's as scents, and so on.
That phrase, it must be said, somehow hides a complex mystery.
The pupil, a hole in the center of the iris, is where light enters the eye. It is
projected onto the retina, the back surface of the eye, which is lined with visual receptors,
after being focussed by the lens (which is adjustable) and cornea (which is not
adjustable). The right part of the retina receives light from the left side of the world, and
vice versa. The retina is split in half, with light from above striking the bottom half and
below striking the top. The neural system is unaffected by the image's inversion. Keep in
mind that the image is not duplicated by the visual system. It encodes it through a variety
of neural activities.
If this concept baffles you, picture a computer. There is no need for the chip to
reside in the upper left corner of the computer; it simply signals what should be displayed
in the upper left corner of your screen. This anatomy has the effect of allowing light to
travel through the ganglion, amacrine, and bipolar cells on its way to the receptors.
However, because of their transparency, light can pass through them without being
distorted. The blind spot is a more significant effect. The optic nerve, which leaves the
eye through the back, is made up of the combined ganglion cell axons.
Cones make up roughly 90% of the input to the brain, despite the fact that rods
outnumber cones in the human retina by a factor of about 20 to 1. Keep in mind the tiny
ganglion cells. Each cone in the fovea has a separate neural pathway to the brain. Each
receptor in the periphery (mainly rods) shares a line with tens or hundreds of other
receptors. In the visual nerve, on average, 120 million rods and 6 million cones converge
into 1 million axons.
Electromagnetic radiation having wavelengths between 400 nm (nanometer, or
10-9 m) and 700 nm or more makes up visible light. Violet, which has the shortest visible
wavelengths, is what we see. Blue, green, yellow, orange, and red are viewed as gradually
longer wavelengths. Only because the receptors in our eyes are tuned to detecting them
do we refer to these wavelengths as "light". We would define light differently if we had
different sensors. According to Stevens & Cuthill (2007), many species of birds, fish, and
insects have visual receptors that are sensitive to what is known as ultraviolet radiation.
Although we cannot know what it appears to them, ultraviolet radiation is a type of light
in their eyes. Because the male reflects more ultraviolet light than the female in some bird
species, they appear the same to humans but are different to birds.
Thomas Young (1773–1829), an incredibly busy guy, was the first to make
progress in our understanding of this issue. The Rosetta Stone's earliest translations were
made by Young. In addition, he established the present wave theory of light, modern
energy definition, annuity calculation, introduction of the coefficient of elasticity,
extensive discussion of eye anatomy, and significant contributions to other fields. In the
past, scientists believed that by comprehending the physics of light, they could explain
color. Young realized that a biological explanation was necessary for color. By comparing
the responses from a few distinct types of receptors, each of which was sensitive to a
different spectrum of wavelengths, he concluded that we sense color.
The Young-Helmholtz theory, which Hermann von Helmholtz later improved, is
today referred to as the trichromatic theory of color vision. This hypothesis states that
color perception is based on the relative rates of response by three separate types of
cones, each of which is most sensitive to a certain range of wavelengths. (Trichromatic is
short for "three colors.") How did Helmholtz choose three as the number? He discovered
that by combining the right quantities of just three wavelengths, anyone could match any
hue. Therefore, he came to the conclusion that three different receptor types—now
known as cones—were adequate to account for human color vision.
The trichromatic theory states that we distinguish between wavelengths based on
the proportion of activity among the three different types of cones. For instance, light at
550 nm excites the short-wavelength receptor almost entirely, while medium- and long-
wavelength receptors are excited somewhat equally. The proportion of responses from the
three cones impacts how yellow-green is seen. Without considerable variation in their
ratio of responses, all three cones become more active as light intensity increases. The
light appears brighter but remains the same color as a result. White or gray is what we see
when all three types of cones are equally active.
The opponent-process theory was put forth by 19th-century physiologist Ewald
Hering to explain this and other phenomena: According to Hurvich and Jameson (1957),
we see color in terms of opposites. To put it another way, the brain has a system that
perceives color on a continuum ranging from red to green, another from yellow to blue,
and another from white to black. When you spend enough time staring at one hue in one
place, your reaction becomes worn out and you start to react differently.
Color constancy—the capacity to recognize colors despite changes in lighting—
cannot be effectively explained by either the trichromatic theory or the opponent-process
theory. You would still recognize bananas as yellow, paper as white, and so on even if
you were wearing green-tinted spectacles or switching out your white light bulb for a
green one. Your brain subtracts a specific amount of green from each color when
comparing the colors of two objects.
The condition known as colorblindness, or more accurately color vision
impairment, was one of the first psychological findings. (It's uncommon to have complete
colorblindness, which perceives only black and white. Before the 1600s, people believed
that everyone saw the same way and that what we perceived was the actual object. Today,
we are aware that some people perceive color differently than others. Then researchers
showed that some individuals can perceive color, but not to the same extent as other
others. Meaning, color is not found in the light or the object itself, but rather in the brain.
Many birds, reptiles, and fish have four types of cones in contrast to our three. They
believe that all people are color-deficient.
B. How the Brain Processes Visual Information
The ganglion cells' axons constitute the optic nerve, which exits the retina and
traverses the ventral aspect of the brain. At the optic chiasm, the optic nerves originating
from both eyes converge. In humans, precisely half of the axons from each eye undergo
decussation and project to the contralateral hemisphere of the brain. The transfer of
information from the nasal portion of each eye (i.e., the side that is closer to the nose)
occurs in a contralateral manner, meaning that it crosses over to the opposite hemisphere.
The transmission of information originating from the temporal half, specifically the side
oriented towards the temporal cortex, is directed towards the hemisphere located on the
same side of the body.
The degree of genetic crossover is subject to variation across different species,
contingent upon the positioning of ocular structures. In species characterized by laterally
positioned eyes, such as rabbits and guinea pigs, the vast majority of axons undergo
contralateral crossing. The majority of axons from ganglion cells are directed towards the
lateral geniculate nucleus, which is a component of the thalamus. The etymology of the
term geniculate can be traced back to its Latin root, genu, which translates to "knee." The
act of genuflection involves the bending of the knee. The lateral geniculate nucleus
exhibits a structural resemblance to the knee joint, albeit requiring a degree of
imaginative interpretation. A reduced quantity of axons project towards the superior
colliculus and other regions, encompassing a portion of the hypothalamus that governs
the circadian rhythm. The axons from the lateral geniculate are projected to various
regions of the thalamus and the visual cortex. Axons originating from the cortex and
projecting to the thalamus have the ability to alter the activity of the thalamus.
The retina employs lateral inhibition as a mechanism to enhance the contrast of
object borders. As an analogy, let us consider the scenario where... A group of 15
individuals are arranged in a linear formation. Initially, each individual possesses a single
cookie. Subsequently, an additional five cookies are distributed to the individuals
positioned in the middle of the line. However, each of the aforementioned individuals is
required to discard one of their own cookies, as well as one cookie that is being held by
the person situated on each side of them. Assuming a desire to maximize the quantity of
cookies obtained, what location would be optimal? It is advisable to avoid being situated
in the median of the cohort that is bestowed with cookies, as upon acquiring five of them,
one would be compelled to discard a personal cookie and relinquish one to each adjacent
individual, resulting in a cumulative deficit of three cookies. In the event that an
individual is either the initial or final recipient of a cookie, they will discard one cookie
and relinquish another to a single adjacent individual, resulting in a cumulative loss of
two cookies.
The most unfavorable location to be situated in is either immediately prior to or
subsequent to the collective that is being bestowed with cookies. You would not obtain
any and forfeit the singular possession you previously held. The outcome presents a
distinct differentiation at the boundary separating individuals who have received cookies
and those who have not.
Every individual cell within the visual system of the brain possesses a receptive
field, which refers to a specific region within the visual space that can either stimulate or
suppress it. The receptive field of a rod or cone refers to the specific location in space
where the cell is impacted by incoming light. Several types of visual cells obtain their
receptive fields through the connections they receive. This notion holds significance,
therefore, let us allocate a considerable amount of time to delve into it. Assuming that one
monitors the occurrences within a single city block. The aforementioned concept can be
referred to as the individual's receptive field. The monitoring of events is distributed
among individuals, with one person assigned to observe occurrences on the adjacent
block, another individual responsible for the subsequent block, and so forth. Assuming a
scenario where each individual accountable for a block on a particular street is
answerable to a higher authority. The supervisor's receptive field encompasses the entire
street, as it incorporates feedback from every block situated on the street. The
neighborhood manager oversees multiple street supervisors, whose jurisdiction is limited
to specific streets within the neighborhood. The neighborhood manager, on the other
hand, has a broader scope of responsibility and oversees the entire neighborhood. The
hierarchical structure of the neighborhood management system entails that the
neighborhood manager is accountable to a district chief, who in turn reports to a higher
authority.
The primary visual cortex, also referred to as area V1 or the striate cortex due to
its striped appearance, receives input from the lateral geniculate nucleus of the thalamus.
According to Kosslyn and Thompson (2003) and Stokes et al. (2009), when an individual
visualizes an object with their eyes closed, there is an increase in activity in area V1 that
is comparable to the pattern observed when the same object is actually seen. According to
Sperandie, Chouinard, and Goodale (2012), the perception of an optical illusion results in
the activation of area V1 corresponding to the individual's interpretation of the visual
stimulus rather than the actual physical properties of the object. The precise function of
area V1 in relation to consciousness remains uncertain; however, it appears to be a crucial
component for its manifestation. Individuals who have incurred damage to the V1 region
exhibit a lack of conscious vision, visual imagery, and visual content in their dreams, as
per the findings of Hurovitz, Dunn, Domhoff, and Fiss (1999).
In contrast, it has been observed that adults who experience vision loss due to
ocular damage retain the ability to conjure visual imagery and experience visual dreams.
During the 1950s, David Hubel and Torsten Wiesel conducted an experiment in which
they utilized slender electrodes to document the behavior of cells in the occipital cortex
of cats and monkeys. This was done while exposing the retina to light patterns. Initially,
the researchers exhibited luminous points through the utilization of a slide projector and a
display surface. However, they observed minimal reactivity from cortical cells. The
inquiry pertained to the lack of responsiveness exhibited by cells, despite the established
significance of the occipital cortex in the process of vision. Subsequently, a significant
reaction was observed during the act of positioning a slide. The researchers expeditiously
discerned that the cellular response was elicited by the periphery of the slide. According
to Hubel and Wiesel's research in 1998, the receptive field of the cell was in the shape of
a bar, as opposed to the circular receptive field found in cells of the retina and lateral
geniculate. The research conducted by the individual in question, which earned them the
prestigious Nobel Prize, has been widely recognized as the catalyst for numerous
subsequent studies involving microelectrodes. It is likely that a large number of
microelectrodes have been launched at this point.
According to Hubel and Wiesel's research in 1977, cells that share similar
properties tend to cluster together in columns that are perpendicular to the surface of the
visual cortex. It is possible for cells situated in a particular column to exhibit exclusive
responsiveness to the left eye, exclusive responsiveness to the right eye, or nearly equal
responsiveness to both eyes. Furthermore, it has been observed that cells situated in a
particular column exhibit optimal responsiveness to lines that possess a uniform
orientation.
Considering the fact that neurons located in region V1 exhibit robust
responsiveness towards patterns that are shaped like bars or edges, it can be postulated
that the functioning of these cells signifies the recognition of a bar, line, or edge. The
aforementioned cells could potentially function as feature detectors, which are neurons
that exhibit responses indicative of the existence of a specific feature.
The notion of feature detectors is reinforced by the observation that extended
exposure to a specific visual feature results in reduced sensitivity to said feature,
suggesting a possible fatigue of the corresponding detectors. When an individual gazes at
a waterfall for a duration of one minute or more and subsequently shifts their gaze to the
adjacent rocks and trees, an optical illusion occurs wherein the objects in question appear
to be moving upwards. The aforementioned visual phenomenon of a waterfall illusion
implies that the neural receptors responsible for perceiving downward movement have
undergone a state of exhaustion, thereby allowing the receptors responsible for upward
motion to operate without opposition.
The visual system in a neonate mammal undergoes typical developmental
processes prior to birth, as reported by Lein and Shatz (2001) and Shatz (1996). During
the development of the retina, there are occurrences of spontaneous activity that result in
the synchronization of neighboring receptors. This synchronization allows for the
establishment of appropriate combinations of receptors to form connections with cells in
the brain. Upon initial eye-opening, the visual system cells of an animal exhibit activity
patterns that are characterized as being predominantly stochastic in nature. Rapidly
viewing a visual stimulus has been observed to decrease the level of background
interference.
According to Maffei, Nataraj, Nelson, and Turrigiano (2006), when only one eye
is open, the synapses originating from the open eye suppress the synapses originating
from the closed eye. In the absence of ocular dominance, there is no discernible axonal
preference. According to Wiesel (1982), the cortex of a kitten remains receptive to visual
stimuli for a minimum of three weeks. However, the majority of cells become responsive
to only one eye and not both. Prolonged closure of the eyes results in a decrease in
cortical responses and a loss of distinct receptive fields.
According to Stryker and Sherk (1975) and Stryker et al. (1978), if a kitten is
exposed to goggles with horizontal lines during its early sensitive period, the majority of
its visual cortex cells will develop a responsiveness solely to horizontal lines. Despite
several months of regular exposure, the feline fails to react to vertical lines.
C. Parallel Processing in the Visual Cortex
The transmission of visual information from the primary visual cortex (V1) to the
secondary visual cortex (area V2) is a well-established phenomenon. The processed
information is then relayed to other cortical areas. Reciprocal connections exist within the
visual cortex. An instance of information exchange occurs between V1 and V2, where V1
transmits information to V2 and V2 reciprocates by returning information to V1.
Following the V2 stage, the data diverges into multiple pathways for distinct processing
purposes.
The differentiation between the ventral and dorsal streams is a notable focus of
study among researchers. The ventral stream, which passes through the temporal cortex,
is commonly referred to as the perception pathway or the "what pathway" due to its
crucial role in the identification and recognition of objects. The pathway known as the
dorsal stream, which traverses the parietal cortex, is commonly referred to as the action
pathway or the "how" pathway due to its crucial role in visually guided movements.
The cells located in the inferior temporal cortex, undergo a process of acquiring
knowledge to identify significant objects. A sensory neuron that is stimulated by visual
input of a particular object exhibits a preference for the specific angle from which the
object is viewed. However, with repeated exposure to the object, the neuron adapts and
becomes responsive to the object regardless of the angle of presentation. The
phenomenon of object responsiveness remains consistent despite significant alterations in
the visual pattern that is detected by the retina, as noted by Murty and Arun (2015).
The study conducted by Baylis and Driver (2001) revealed that cells located in the
inferior temporal cortex of monkeys exhibited comparable responses to the mirror image
or contrast reversal of an original profile. However, these cells did not respond in the
same way to a figure-ground reversal. Regarding the specific arrangement of luminance
and obscurity, the phenomenon of figure-ground reversal bears resemblance to its initial
state. However, it is commonly perceived by the majority of individuals (and apparently
primates as well) as a white entity situated against a black backdrop, rather than a
countenance.
The ability to recognize faces holds significant importance for human beings. In
order for a society to thrive, it is imperative to possess the ability to differentiate between
individuals who are trustworthy and those who are not. This discernment necessitates the
identification of individuals who may not have been seen for extended periods of time. In
the future, it is possible that you may participate in a gathering of former high school or
college classmates, commonly known as a reunion, where you will have the opportunity
to reconnect with individuals whom you have not had contact with for a prolonged period
of time. The individuals can be identified despite their physical changes such as weight
gain, baldness, or hair dye. The challenge of developing machines capable of facial
recognition has been a significant obstacle for computer programmers, despite its
apparent simplicity for human beings.
Individuals exhibit significant variability in their capacity to identify faces, and
this phenomenon cannot be solely attributed to differences in attentiveness or interest.
Individuals who exhibit significant difficulties in recognizing faces are commonly
diagnosed with prosopagnosia. The aforementioned issue may arise due to either
impairment of the fusiform gyrus or incomplete development of the same. According to
several studies, a smaller than average right fusiform gyrus and reduced connections with
the occipital cortex have been observed in certain individuals. On the contrary,
individuals who exhibit superior facial recognition abilities may possess heightened
connectivity between the fusiform gyrus and occipital cortex, indicating an above-
average richness of neural connections.
Mobile entities frequently require prompt consideration. A mobile entity could
potentially serve as a potential partner, a prey for hunting and consumption, or a predator
seeking to consume you. To provide a response, it is necessary to specify the object, its
destination, and its velocity. The human brain is structured in a manner that facilitates
rapid and efficient computation.
The perception of motion is particularly reliant on two specific areas, namely area
MT (middle temporal cortex), which is also referred to as area V5, and a neighboring
region known as area MST (medial superior temporal cortex). The aforementioned
regions predominantly receive sensory input from the magnocellular pathway, as reported
by Nassi and Callaway (2006). This pathway is responsible for detecting global patterns,
such as motion across extensive portions of the visual space. Since the magnocellular
pathway lacks sensitivity to color, it follows that MT also lacks sensitivity to color.
The discovery of area MT from monkey research has revealed a mechanism
whereby motion blindness could potentially occur. The report about patient LM has been
deemed acceptable by researchers. One may contemplate the experience of lacking the
ability to perceive motion. Please consider observing this demonstration: Direct your
attention towards your left eye while observing your reflection in a mirror. Attempt to
establish eye contact with another individual by directing your gaze towards their eyes
while they concentrate on one of your eyes, followed by the other. It is noteworthy that
the movement of the other person's eyes is observable, despite the fact that they
underwent identical displacement and velocity as one's own. An eye movement that is
perceptible to the human eye is characterized by a size and speed that fall within a certain
range.
The reason why individuals are unable to perceive their own eye movements is
due to the decreased activity in the area MT and certain regions of the parietal cortex
during voluntary eye movements, also referred to as saccades. The level of activity
remains constant during the period when an individual's eyes are tracking a moving
object. The brain regions responsible for overseeing saccades communicate to the MT
area and the parietal cortex, instructing them to temporarily cease activity in anticipation
of impending eye muscle movement. The neural activity and blood flow in the MT and
parietal cortex regions exhibit a decrease of 75 milliseconds prior to the initiation of eye
movement and continue to remain suppressed during the movement. This phenomenon
has been reported in studies conducted by Burr, Morrone, and Ross (1994), Paus, Marrett,
Worsley, and Evans (1995), as well as Vallines and Greenlee (2006). Briefly stated,
voluntary eye movements induce a momentary state of motion blindness. It is possible
that one may now have a better comprehension of the perpetual experience of individuals
with motion blindness. The phenomenon of motion blindness has an antithetical
manifestation. Motion blindness is a condition where individuals have impaired visual
perception, limiting their ability to detect motion. Some individuals may experience this
condition while still retaining the ability to detect other visual stimuli.
In light of the fact that regions MT and MST exhibit robust activation in response
to mobile stimuli exclusively, what would be the outcome subsequent to impairment of
said regions? Motion blindness refers to the inability to perceive the direction, speed, and
presence of movement in objects, despite being able to visually detect them. Individuals
afflicted with motion blindness exhibit superior performance in reaching for a moving
object relative to their ability to verbally describe its motion, as evidenced by Schenk,
Mai, Ditterich, and Zihl's (2000) findings. However, their overall proficiency in
processing visual motion falls significantly short of that of the general population.
The occurrence of motion blindness in the absence of any other associated
dysfunction is an infrequent phenomenon. The most accurately documented instance,
referred to as "LM," stated that she experienced unease when individuals walked around
as they appeared to suddenly appear in different locations without observable movement.
Individuals would seemingly manifest or vanish abruptly, despite her efforts to monitor
their whereabouts. An individual who is in motion would be perceived by the observer as
being in a state of agitation, however, the observer would not possess knowledge
regarding the direction in which the individual is moving.
The individual would experience a sense of discomfort to the extent that they
would halt their own locomotion until the departure of the other party. The individual in
question exhibited difficulty in crossing a street unassisted due to an inability to discern
the movement and velocity of oncoming vehicles. The act of dispensing coffee became
challenging. The liquid in motion seemed to be in a solid state of matter, prompting her to
continue pouring until the cup reached its maximum capacity.
