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Chapter 1

PSYC51H3 Chapter Notes - Chapter 1: Photopic Vision, Abstract Type, Cortical Column


Department
Psychology
Course Code
PSYC51H3
Professor
Jonathan Cant
Chapter
1

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Chapter and Lecture One
Light and the Eye
light is a stimulus for vision
-seeing involved a stimulus (light) and a mechanism (the visual system)
can regard light as both a wave and a particle
-light as a wave: reflection, refraction
-light as a particle: absorption by receptors
vision is based on visible light, a band of energy within the electromagnetic spectrum
Electromagnetic spectrum: a continuum of electromagnetic energy produced by electrial
charges that is radiated as waves
-energy within the spectrum is described by its wavelength- the distance between the
peaks of electromagnetic waves
-all electromagnetic radiation is essentially the same: physical differences between
radio waves, infrared, visible light etc are only due to frequencies
Visible Light
-we can only see a small part of the electromagnetic spectrum
-can detect light between 400- 700 nm
-when light travels through transparent substance it is slowed down to a velocity which
depends on the refractive index (roughly the density) of the medium it is passing
though
-this slowing down is extremely important: causes prisms to bend light and lenses to
form images.
Newtons Experiment
-Newton discovered that when light is bent by a prism, each wavelength (or frequency)
bends at a slightly different angle, so the original beam of white light comes out (or is
refracted) with the 7 spectral colors fanned out (ROYGBIV)
-this paved the way to understanding of color vision: light itself is not colored, but
instead elicits colors (the colors being produced by specialized cells in the brain)
-bees see the ultraviolet portion of the spectrum
Early Vision
Non-optical components of the eye
sclera: tough, dense outer fibrous coat
choroid: contains network of blood vessels
cornea: transparent- allows light to enter eye. main focusing element
anterior chamber: tranports O2 glucose. carries wastes
iris/ pupil: pupil size regulates amount of light entering eye. Pupil size depends on light
level and autonomic nervous system: fear, excitement etc.
Optical Components
cornea(refract light rays onto the cornea), anterior chamber, iris/pupil
lens: varies its flatness, changing optical power. provides fine focus. Refract light rays
onto the retina
vitreous body: contains vitreous humour: the thick fluid that fills the eye, can contain
floaters/ debris.
Neural Components
-retina: direct extension of the CNS
-optic nerve: ganglion cell axons

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The Eye and Camera
-both are optical devices designed to record visual images on light sensitive material.
camera: light goes through aperture, focused by lens and focused on film
eye: light goes through pupil, focused by cornea and lens and focused on
photoreceptors of the retina
-eye does additional processing on the image
-extracts biologically meaningful information
-transmits to the brain for interpretation and reaction
-you need a brain to understand an image
-light is focused by lens to form image on retina
Human Retina
-optic disk, fovea, blood vessels
Photoreceptors
graded potentials: variable change in amplitude of membrane potential and variable
time course.
action potentials: fixed change in membrane potential and fixed time course
-photopigments transduce light(photons) into neural signals
-hyperpolarize to light (interior voltage of cell becomes more negative) in turn firing less
glutamate (actually more active when not stimulated by light)
-dont fire action potentials, instead fire graded potentials
Bipolar Cells
-fire graded potentials, not action potentials
-can hyperpolarize or depolarize to light
- visual system partitioned image into two: cells that favor sensitivity and cells that favor
resolution
Rods: high sensitivity, low resolution. Persist under conditions of low illumination- work
in the dark.
Cones: high resolution, low sensitivity. Must have sufficient light to function- colorblind in
the dark. Three types of cones, absorb different pigments.
-strength of cone proportional to amount of light energy absorbed by its pigment.
blue: 445 nm green: 535 nm red: 559 nm
-perceptual quality of color corresponds to physical property of wavelength.
Three Cone Types
-3 kinds (short, medium and long wavelength cones) in people with normal color vision,
each most sensitive to a different color or range of wavelengths
-Rods most sensitive to light of about 505 nm
Retinal Ganglion Cells
-fire action potentials
-axons form optic nerve
-two types: M cells (parasol) and P cells (midget)-absolute levels of illumination are
replaced with retinotopic map of difference: points where light region borders a dark
one.
Receptive Fields
-receptive fields are regions of the retina where stimulation by light will influence the
response of a single neuron in the visual pathway
-although the receptive field is on the retina, it belongs to the neuron that is affected
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