PSYC 2410 DE S12 Textbook Notes Chapter 6.pdf

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19 Aug 2012
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Chapter 6: The Visual System
The visual system does not create an exactly accurate recreation of the external world for us to
interpret
The information projected onto the visual receptors is distorted, upside-down and two-
dimensional.
From this information the brain creates a representation of what is 'out there'
Two types of research inform us about our visual system:
Research that explores the visual system with neuroanatomical, neurochemical, and
neurophysiological techniques
Research that focuses on the assessment of what we see
6.1: Light Enters the Eye and Reaches the Retina
Some animals have adaptations that allow them to see in very dim light, but no visual system is
able to work in total darkness
Light: Often defined as waves of electromagnetic energy that are between 380 and 760
nanometers in length.
Light has some properties of waves and some properties of particles.
This range is the spectrum we are able to perceive with our eyes, however some animals are
capable of perceiving wavelengths we cannot.
Two properties of light are of particular interest:
Wavelength: Used interchangeably with colour.
Intensite: Used interchangeably with brightness.
The Pupil and the Lense:
How much light reaches the retinas is regulated by a donut-shaped band of contractile tissue
called the iris.
Light enters the eye through the pupil, the hole in the center of the iris.
Sensitivity: The ability to detect the presence of dimly lit objects
Acuity: The ability to see details of objects
Pupil size is adjusted in response to light in order to balance sensitivity and acuity.
High illumination (ie. Sensitivity is not important) leads to constricting of the pupils to
allows for the greatest acuity.
Low illumination (ie. Sensitivity is important) leads to dilation of the pupils to allow for
greater sensitivity at the expense of acuity.
Constriction of the pupils allows a greater depth of focus, meaning a greater range of depth
differences are able to be kept in focus simultaneously.
Behind each pupil is a lens which focuses incoming light onto the retina.
Accomodation: The process by which the configuration of the lenses of the eyes bring
images into focus.
Ciliary Muscles: Muscles which hold the lens of the eye in place and adjust its tension
to allow us to focus either nearby or far away.
When we observe something near, the lens assumes its natural cylindrical shape,
allowing the lens to refract the light and bring close objects into focus.
When we observe something far away, the lens is flattened which reduces refraction and
brings distant objects into focus.
Eye Position and Binocular Disparity
One reasons why vertebrates have two eyes is because they have two sides
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By having an eye on each side of the head – the most common vertebrate occular
arrangement – animals can see in almost every direction without moving their heads.
Having both eyes on the front side of the head sacrifices being able to see what is going on
behind us for being able to see what is going on in front of us much more clearly.
In general, predators tend to have front-facing eyes because it enables them to accurately
perceive how far away their prey are.
Prey animals tend to have side facing eyes because this arrangement gives them a larger
field of vision and the ability to perceive oncoming predators from any direction.
The movements of your eyes are coordinated so that each point in your visual world is
projected onto corresponding points on each of the two retinas.
Because both eyes must see corresponding images, your eyes must turn in slightly.
Because your eyes are in two different places, you cannot actually have the same image
on both eyes at the same time.
Binocular Disparity: Difference in the position of the same image on the two retinas.
Binocular disparity is larger for close objects than for faraway objects.
The brain uses binocular disparity to create a three dimensional representation of our
world based on two dimensional images captured by the retinas.
6.2: The Retina and Translation of Light into Neural Signals
The retina converts light to neural signals, conducts them towards the CNS, and participates in the
processing of the signals.
Receptors: Cells that are specialized to receive chemical, mechanical or rediant signals from the
environment; Also proteins that contain binding sites for particular neurotransmitters.
oFifth and final layer of ocular cells
Horizontal Cells: Type of retinal neurons whose specialized function is lateral communication.
oSpecialized for lateral communication (Communication across the major channels of sensory
input)
oFourth layer of ocular cells
Bipolar Cells: Bipolar neurons that form he middle layer of the retina.
oThird layar of ocular cells
Amacrine Cells: A type of retinal neuron whose specialized function is lateral communication.
oSpecialized for lateral communication (Communication across the major channels of sensory
input)
oSecond layar of ocular cells
Retinal Ganglion Cells: Retinal neurons whose axons leave the eyeball and form the optic nerve.
oFirst layar of ocular cells
oProject across the inside of the retina before gathering together in a bundle and exiting the eye-
ball.
Each of these cell types comes in a variety of subtypes. (> 50 unique cell types have been identified
in the retina!)
Retinal neurons communicate both chemically via synapses and electrically via gap junctions.
Light reaches the receptor layer only after passing through the other four layers.
Once the receptors have been activated, the signal is transmitted back out through the retinal layers
to the retinal ganglion cells.
This arrangement of signal reception and transfer results in two problems:
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oIncoming light is distorted by the retinal tissue because it must pass through layars of tissue
before it reaches the receptors.
oIn order for the bundle of retinal nerves to leave the eye, there must be a gap in the receptor
layar.
oBlind Spot: The gap in the receptor layar where the bundle of retinal nerves exits the eye.
Fovea: Indentation about 0.33cm in diameter and the center of the retina.
oSpecialized for high-acuity vision (ie. the seeing of fine details)
oThe thinning of the retinal ganglion cell layer at the fovea reduces the distortion of incoming
light.
Completion: The visual system;s automatic use of information obtained from receptors around the
blind spot, or scotoma, to create a perception of the missing portion of the retinal image.
oWhen the visual system detects a straight bar going into one side of the blind spot and another
straight bar leaving the other side, it fills in the missing bit for you, regardless of what is
actually there.
oCompletion phenomenon is evidence that the visual system does more than create a faithful
copy of the external world.
oCompletion is not a reaction to retinal blind spots, it occurs in all areas of vision.
oOur eyes detect key features which are transmitted to the brain (most importantly edges and
location) where a recreation of the expected object is generated based on that partial
information.
oSurface Interpolation: The process by which we perceive surfaces; The visual system extracts
information about edges and from it infers the appearance of large surfaces.
Cone and Rod Vision:
oTwo different types of receptors in the the human retina
Cones: Cells in the human retina that are responsible for photopic vision.
Species active only during the day tend to have cone-only retinas.
Short and stocky appearing cells
In the center of the fovea there are exclusively cones
Cone density decreases as you look closer to the perimiter of the foveal indentation.
Rods: Cells in the human retina that are responsible for scotopic vision.
Species active only at night tend to have rod-only retinas
Long and 'fibrous' appearing cells
There are no rods in the fovea
Around the outside of the foveal indentation there are more rods and significantly less
cones
Highest concentration of rods is located at 20 degrees from the center of the fovea.
There are more rods in the nasal hemiretina (the half of the retina closer to the nose)
than in the temporal hemiretina( the half of the retina closer to the temples)
oDuplexity Theory: The theory that cones and rods mediate different kinds of vision.
oPhotopic Vision: Predominates in good lighting and provides good acuity. Allows for detailed
and coloured perceptions of the world.
In a photopic vision system, the output of only one or two cones converge are on a single
retinal ganglion cell.
It takes more light to trigger a reaction from a cone, but when it does react there is less
ambiguity than in scotopic vision about where the stimulus which triggered the reaction.
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