1XX3 Vision Essentials

7/28/2019 1XX3 Vision Essentials

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IntroductionsOur Visual Sense

The important role of visual information is reflected in the many visual references we use in our language.When visual information is in conflict with information from another sense, you tend to bias your trusttowards your sense of vision-for most people, seeing is truly believing.

The eye is primarily an instrument to collect, focus, and senses the light stimulus. However, this is only thebeginning as the heavy duty processing occurs in the brain.

Case Study John was rendered blind an occipital lobe stroke at the age of 30. Although he is now unable to perceive the

visual world around him, he struggles to maintain his independence.

The Stimulus: LightAmplitude

Light travels as a wave and can vary in two respects: the height of each wave, called the amplitude, andthe distance between the peaks of successive waves, called the wavelengths.

Variations in amplitude affect the perception of brightness. Generally, the greater the amplitude of the lightwave, the more light is being reflected or emitted by that object, and so that object appears brighter or moreintense to us.

Wavelength

Variations in wavelength affect the perception of colour. Wavelength is measured in nanometers, or millionthof a millimetre.

o Smaller wavelengths refer to light waves with a higher frequency, because there is less distancebetween successive peaks.

o Larger wavelengths refer to light waves with a lower frequency.

Humans are only sensitive to a tiny portion of the total range of wavelengths of electromagnetic radiation.We refer to this tiny portion that we`re sensitive to as the visible spectrum.


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The shortest wavelength that we can see is around 360 nanometers, which looks violet, and the longestwavelength that we can see is around 750 nanometers, which looks red.

There are other species which can see the light outside our visible spectrum.o Insects like bees can see the wavelengths shorter than 360 nm in the ultraviolet spectrum, and

perceive differences in the colours of flowers that all look the same colour to us.o Some species like snake can see light made up of wavelengths longer than 750 nm in the infrared

spectrum, which allows them to find prey in the dark by being able to see the body heat that is

emitted by the prey.Purity

The two physical characteristics of light, amplitude and wavelength translate into our perceptions ofbrightness and colour, respectively.

A final physical characteristic of light we will consider is purity,which affects the perception of thesaturation,or richness of colours.

A light that is made up of a single wavelength is a pure light, and the perceived colour would be describedas completely saturated.

However, natural light you experience will more likely be a combination of many wavelengths. This lightwould be described as desaturated.

o Most of the colours we see in our everyday life are not pure but a mixture of wavelengths and thusare less intense than pure colours.The EyeThe Eye

Light first passes through the curved cornea,which begins the focusing process. The cornea is atransparent window at the front of the eye. The rest of the eye is covered by the white part of the eye calledsclera, a tougher membrane.

After the cornea, light passes through the pupil, which is the round window that you see as a black dot inthe middle of your eye.

The iris, or the coloured part of your eye, controls the size of the pupil. The iris consists of a band ofmuscles that is controlled by the brain; if not enough light is reaching the retina, these muscles cause thepupil to dilate into a larger opening, whereas if too much light is entering they eye then these muscles causethe pupil to constrict into a tiny opening.


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After passing through the pupil, light passes through the lens, a transparent structure that does finalfocusing of light onto the retina at the back of the eye.

The Lens

The curvature of the lens causes images to land on the retina upside-down and reversed from left to right.However, the final perceived image is a product of brain activity.

o Thus, rather than seeing everything upside-down and reversed, there is a correction that allows usto see a properly oriented image.

The lens is a flexible piece of tissue, the shape of which can be altered by surrounding muscles, allowing itto focus on objects that are close or far away.

If the object is close, the lens of your eye gets fatter or rounder to produce a clear image, but if the object isfar away, the lens of your eye gets elongated to focus the image on the back of your eye.

o This change in the shape of the lens to focus on objects that vary in distance is calledaccommodation.

The Retina After travelling through the lens, light passes through the vitreous humour,which is the clear jelly-like

substance that comprises the main chamber inside the eyeball.


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o The light finally lands on the retina,which is the neural tissue that lines the back of the eye.The Retina

Retinal Layer 1: Photoreceptors

The retina is a paper-thin sheet that covers that back of the eye, and is made up of a complex network ofneural cells arranged in three different layers.

The organization of these layers may seem counter-intuitive; the layer at the very back of the eye, farthestaway from the light is where thephotoreceptors are located.

Photoreceptors are cells in the retina that are responsible for translating the physical stimulus of light into aneural signal that the brain can understand. To reach the photoreceptors, light must pass through the other2 layers of retinal tissue which are transparent.

The reason for this inside-out arrangement in the retina has to do with where the photoreceptors get theirnutrients from, which is a layer of cells at the very back of the eye called the retinal pigment epithelium(RPE).The photoreceptors would die without access to the RPE cells, and if the photoreceptors werelocated at the front of the retina, facing the light, then they would not have access to the RPE that they needto survive.

Photoreceptors: Rods and Cones Cones are designed to operate at high light intensities and are primarily used for day vision. The cones

provide us with the sensation of colour and provide good visual acuity,or sharpness of detail. Conesbecome more concentrated towards the fovea,a tiny spot in the middle of the retina that containsexclusively cones.

Rods are designed to operate at low light intensities, and are primarily used for night vision. They provide nocolour information and offer poor visual acuity. There are no rods in the fovea itself, with an increasingconcentration in the region just surrounding the fovea.

When you`re trying to see an object in an environment that is dimly lit, youre betteroff looking slightly toone side of the object as opposed to trying to stare right at it.

o When you stare right at it, the image is focused on the cone rich fovea which doesnt work well in adimly lit environment.


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By staring at one side of the object, youll be using your rods and increasing the chance that youll see it.

Visual PathwaysIntroduction to Visual Pathways

The place where visual perception all comes together is the brain. You can think of the visual system asbeing comprised of a set of assembly lines.o Areas along the visual pathways process parts of the visual pathways process parts of the visual

input before sending those partially processed bits of information on to the next set of areas downthe line for further processing.

Main Pathway: Occipital Lobe After the optic chiasm, the information from each visual field arrives in the opposite hemisphere, at which

point the optic nerve fibres split and travel along two pathways.

Most of the retinal of ganglion cell axons travel along the main pathway and synapse in the lateralgeniculate nucleus (LGN), which is a part off the thalamus that receives visual information.

o After being processed here, the visual signals are sent to areas in the occipital lobe that make upthe primary visual cortex.

There are over 20 cortical areas that process visual information, but most of the research done on visualprocessing has concentrated on area V1 of the occipital lobe, otherwise known as the primary visual cortex.

o Collectively, the visual processing areas in the occipital lobe outside the striate cortex are knownas extrastriate cortex.

Primary Visual Cortex


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Just as many photoreceptors may synapse onto a single ganglion cell in the retina, the receptive field of theLGN is made up of many ganglion cells.

The receptive field of a single V1 cell is a combination of the receptive fields of many LGN cells. So again,we have information from many sources being processed down into a

The receptive fields from the retina are arranged in a topographical mapin the primary visual cortex.o This means that neighbouring locations in the retina project to neighbouring

locations in the visual cortex.

Dorsal and Ventral Streams of Extrastriate Cortex From the primary visual cortex, processed visual information, whether it is colour, form, or movement, is

sent on to the extrastriate cortex and gets separated into the dorsal and ventral streams. The dorsal streamis referred to as the where pathway because it processes where objects are, including

their depth and motion in the field.o The dorsal stream progresses from the extrastriate cortex to the parietal lobe.

In contrast, the ventral streamis referred to as the what pathway because it processes what the object is,including its colour and form.

o The ventral stream runs from the extrastriate cortex the temporal lobe.

Conclusion: Visual Pathway

As information is processed, it is compressed. The input from many retinal cells converges onto fewerganglion cells, which is then translated onto even fewer LGN cells, and the finally translated onto ever fewerprimary visual cortical cells.

o Although neurons may respond to more than one attribute of the visual stimulus, they generallyrespond most strongly to as specific attribute.

A neuron may be strongly tuned to colour, form, or movement. Shows us that the brain isdividing its labour, with certain regions performing rather specific tasks.

Evolution of the EyeLight Sensitive Patch


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For example, eyes could have started out as simple light-sensitive patches, like what jellyfish and wormshave today.

Curved Cup Eye Some individuals may have developed a light-sensitive patch that was formed into a slight depression.

o This is very much like the cup eyes that can be observed in clams. A slightly curved or cup eyewould allow the direction of light to be sensed, giving these individuals a survival advantage over

other that had only a flat eye patch.

Crude Lens Some individuals may later have adapted a crude lens, allowing them to process visual input at different

distances.

From here, the lens could have successively improved to allow better focusing and accommodation, such asa more transparent lens or one with better curvature.

Cumulative Selection

Eye evolution is an example ofcumulative selection,where small changes were made to the existing eye,and then new small changes were made to the modified eye, and son on, thus gradually increasing the

sophistication of the eye.The Costs and Benefits of Eyes

In order for an organ to evolve any modifications in the first place, the advantages of thismodification must outweigh the metabolic costs to the system of growing and maintaining themodification.

In the case of eyes, the survival and reproductive advantages seem obvious. In mostenvironments, vision makes it much easier for individuals to find food, shelter, a mate, and avoid predatorsand other dangers.

Different Eye Designs for Different Environments Eyes can vary a lot across different species according to what the species needs to deal with in

their daily lives such as: whether they live in an area with light or not, whether their food tends tocome from above or below, and whether the movement, colour, or shape of their prey is critical.

Compound Eyes Compound eyes are found in arthropods, such as insects and crabs. The eye of these pieces are

made up of an arrangement of individual tubular units called ommatidia that each point in a slightly differentdirection to gather the light that lays directly in front of it.

These eyes manage to form a single image by putting together many separate signals from eachommatidium. Compound eyes are very good at detecting movement, but only at close distances.

Simple Eye Simple eyes are found in vertebrates as well as

molluscs, such as octopus and squid. Theseare the types of eyes that we think of when wethink of eyes; they have an eyeball, lens and retina.

The vertebrate eye can vary quite a bit in its design according to the environment that the species lives in.


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Environmental factors play an important role in determining the shape and orientation of the pupil, the sizeof the eye, where the eyes are placed on the head, and where on the retina most of the photoreceptors arelocated.

The Size of Eyes If you break down and simplify the main functions of the eye, we can consider resolution (or acuity) and

sensitivity (or ability to get enough light). Larger eyes tend to be better at both of these functions. So basically we see bigger eyes in species that

need better eyesight; the benefit gained by having bigger eyes varies across species.

Eye Placement Vertebrate eyes can vary in where they are placed on the animals head. Eye placement on the head leads

to a trade-off between depth perception vs. the total view of the environment the animal can see.

Animals with eyes located on either side of their head have two laterally directed eyes like rabbits. Thisproduces a large total view, with the animal being able to see almost all the way around their body withoutturning their head.

However, these animals essentially have two separate fields of view, with very little binocular overlap, andso depth perception is poor.

Prey animals have adapted with the placement like this in order to continually scan their environment forpredators.


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Another design for eye placement is to have both eyes directed toward the front, which is the typical forpredators, including humans.

This produces a very narrow field of view because both eyes are basically looking at the same scene. However, this means that there is a lot more binocular overlap which results in very good depth perception,

a key asset for successful hunting.


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Development of Visual ArchitecturePrenatal Eye Development

The eyes are formed during the second moth of pregnancy, and are capable of reacting to light in the 6thmonth of pregnancy.

Some random firing of retinal cells also occurs during the prenatal period which is critical for the organizedwiring of the retinal cells, determining how neighbouring cells will be connected to each other.

Immaturities in the Eye The reason our visual system is the least developed sense at birth is because there is so much additional

work that has to be done to make vision fully functional, and a lot of that work requires that the systemreceive visual stimulation something that isnt possible before the baby is born and exposed to the light.

Within the eye itself, the lens muscles are weak, which limits how well the newborn can focus. Because thepupil doesnt react properly to changes in light, the clarity of the image is blurred.

o With environment interactions and continued exposure to light by around 3 months, the infantsability to focus has improved to a level that is almost adult-like.

The newborns retina has a much lower density of cells than an adult retina would have, and even thesecells are not yet fully developed. This is especially true for the fovea, where retinal cells dont reach adult-like levels until after 4 years of age.

The optic nerve and visual cortex also require several years to mature. By some estimates, the relevantbrain development is not fully complete until around 11 years of age.

This immaturity leads to a characteristic pattern of development for visual acuity. At birth, visual acuity isquite dismal, with the details that a newborn can see at 20 feet being about the same as what an adult cansee at 600 ft.

By 6 months of age, the babys visual acuity is already dramatically improved, as they are now able to seethe detail of something at 20 ft. that an adult can see at 100 ft. By one year of age, babies are close to adult-like levels for visual acuity, although they do not fully reach adult acuity until around 4 to 6 years of age.


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Returning to our Case Study Although John was rendered blind by a stroke to his occipital lobe, visual information continues to flood in

through his still functional eyes.

John cannot see objects but some visual processing pathways remain intact allowing him to recognizeobject locations, and coordinate with the motor cortex to avoid them while moving about in his environment.

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1XX3 Vision Essentials