PSYC 2650 Chapter 3: Chapter 3

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16 Nov 2017
Chapter 3 Visual Perception
The Visual System
The Photoreceptors
The process of vision begins with light
o Produced by many objects in our surroundings (lamps, sun, candles)
which is then reflected off most other objects; the reflected light launches
the processes of visual perception
Some of this light hits the front surface of the eyeball, passes through the cornea
and the lens and then hits the retina the light sensitive tissue that lines the
back of the eyeball
The cornea and lens = focus the incoming light so that a sharp image is cast onto
the retina
o This is made possible by the relaxing and tightening of the muscle
surrounding the lens
On the retina there are two types of photoreceptors specialized neural cells that
respond to the incoming light
o Rods sensitive to very low levels of light; play an essential role in
semidarkness or a dim stimulus
They are colour blind; they can distinguish between intensities of
light (brightness) but not different hues
o Cones less sensitive than rods and need more incoming light to operate
Sensitive to colour differences; there are three different types of
cones having their own pattern of sensitivities to wavelengths
Perceive colour by comparing the outputs of these three types
Strong firing from only the cones that prefer short wavelengths
accompanied by weak (or no) firing from the other cone types,
signals purple see other colours on page 66
Enable you to distinguish fine detail
Acuity the ability to see detail (higher for cones)
We position our eyes onto an image so that it falls onto the fovea
the very center of the retina where cones far outnumber rods
This is the region of the retina with the greatest acuity
In portions of the retina more distant from the fovea the rods predominate
o Better to look not directly at a star
Lateral Inhibition
Rods and cones do not report directly to the cortex
Photoreceptors stimulate bipolar cells which excite ganglion cells (which are
spread across the retina)
o Ganglion cells axons converge to form the bundle of nerve fibers that we
call the optic nerve the nerve tract that leaves the eyeball and carries
information to sites in the brain
o Information is first sent first to a station in the thalamus called the lateral
geniculate nucleus (LGN); and is then transmitted to the primary
projection area for vision in the occipital lobe
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Lateral inhibition a pattern in which cells, when stimulated, inhibit the activity
of neighbouring cells
o Highlights surface edges because the response of cells detecting the edge
of the surface will be stronger than that of cells detecting the middle of the
surface edge enhancement
Single Neurons and Single-Cell Recording
Single cell recording procedure through which investigator scan the pattern of
electrical changes within a single neuron
Interested in the cell’s firing rate, measured in “spikes per second”
Procedure that allows us to define the cell’s receptive field the size and shape
of the area in the visual world to which that cell responds
Multiple Types of Receptive Fields
Center-surrounded cells cells that fire at their maximum rate when light is
presented in a small, roughly circular area; presentations of light just outside of
this area cause the cell to fire at less than its usual “resting” rate
o Light presented in the central region of the receptive field has one
influence, while light presented to the surrounding ring has the opposite
o If light is presented to center and the surround, the cell will fire neither
more not less than usual a strong, uniform stimulus is equivalent to no
stimulus at all
Some cells fire at their maximum only when a stimulus containing an edge of just
the right orientation appears within their receptive fields “edge detectors”
o Horizontal vs. vertical edges
Parallel Processing in the Visual System
The visual system relies on a “divide and conquer” strategy, where different types
of cells, located in different areas of the cortex, each specializing in a particular
kind of analysis
Area VI
o Site on the occipital lobe where axons from the LGN first reach the cortex
o Some cells fire horizontals and others in vertical all of these cells
provides a detector for every possible stimulus making it certain that no
matter what the input is or where it is located, some cell will respond to it
Each area seems to have its own function
o MT acutely sensitive to direction and speed of movement
o V4 fire strongest when the input is of a certain colour and certain shape
The visual system relies on parallel processing a system in which many
different steps are going on simultaneously
o Don’t have to wait until motion analysis or colour analysis is complete, all
analysis’ are completed at the same time
o Example of parallel processing is photoreceptors (rods & cones)
Within the optic nerve itself; two types of cells
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