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Chapter 5.2-5.3

PSYCH 261 Chapter 5.2-5.3: Chapter-5.2-5.3-notes.docx


Department
Psychology
Course Code
PSYCH261
Professor
Daniel Smilek
Chapter
5.2-5.3

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Chapter 5.2 Notes- How the Brain Processes Visual Information
1. Without vision and other senses, you would have no more mental experience than a tree does, everything in
psych starts with sensations
2. Neuroscientists have developed a detailed understanding of vision, examining mechanisms of vision gives us
some idea what it means to explain something in biological terms
Overview of the Mammalian Visual System
Rods and cones of the retina make synapses with horizontal cells and bipolar cells
oHorizontal cells- make inhibitory contact onto bipolar cells, which in turn make synapses onto amacrine
cells and ganglion cells.
All cells within the eyeball
Axons of the ganglion cells form the optic nerve
oLeaves the retina and travels along the lower surface of the brain
oThe optic nerves from the two eyes meet at the optic chiasms
Optic chiasms- half of the axons from each eye cross to the opposite side of the brain
oInformation from the nasal half of each eye crosses to the contralateral hemisphere
oInformation from the temporal half goes to the ipsilateral hemisphere
Most Ganglion cell axons go the lateral geniculate nucleus, part of thalamus
oGeniculate- root word genu, meaning “knee”, looks a little like a knee
oThe lateral geniculate sends axons to the other parts of the thalamus and visual cortex
A smaller number of axons go to the superior colliculus and other areas, including part of the hypothalamus that
controls the waking-sleeping schedule.
Processing in the Retina
Rods and cones of your two retinas combined send a quarter of a billion messages, you can’t attend to all of it
and you don’t need it, you only need to extract to the meaningful patterns.
Lateral inhibition- the reduction of activity in one neuron by activity in neighboring neurons
oRetina’s way of sharpening contrasts to emphasize the borders of objects
oLateral inhibition heightens contrast
oThe receptors send messages to excite the closest bipolar cells and also sends messages to slightly
inhibit them and the neighbors to their sides
Net result will be to heighten the contrast between an illuminated area and its darker surround
oLight striking the rods and cones decreases their spontaneous output. However they have inhibitory
synapses into the bipolar cells, and therefore, light on the rods or cones decreases their inhibitory output.
oA decrease in inhibition means net excitation, so to avoid double negatives, we’ll think of the output as
excitation of the bipolar cells
In the fovea, each cone attaches to just one bipolar cell

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Further Processing
Each cell in the visual system of the brain has a receptive field
oAn area in visual space that excites or inhibits it
oThe receptive field of a rod or cone is simply the point in space from which light strikes the cell
oOther visual cells get their receptive fields form the connections they receive
A rod or cone has a tiny receptive field in space to which it is sensitive.
Some rods or cones connect to a bipolar cell, with a receptive field that is the sum of those of the cells connected
to it (includes both excitatory and inhibitory connections)
Several bipolar cells report to a ganglion cell, which therefore has a still larger receptive field
The receptive fields of several ganglion cells converge to form the receptive field at the next level, and so on
To find a cell’s receptive field, an investigator records form the cell while shining light in various locations: if light
from a particular spot excites the neuron, then that location is part of the neuron’s excitatory receptive field. If it
inhibits activity, the location is in the inhibitory receptive field.
The receptive field might be excited by light in the center and inhibited by light in the surround, or vice versa
Primate ganglion cells fall into three categories:
oParvocellular (small celled)
Small cell bodies, small receptive fields, are mostly in or near the fovea. “parvo”- small
Well suited to detect visual details with their small receptive fields
Respond to color, each neuron being excited by some wavelengths and inhibited by others
High sensitivity to detail and color relates to the fact that Parvocellular cells are located mostly in
and near the fovea, which has many cons
oMagnocellular (large celled)
Larger cell bodies and receptive fields, distributed evenly throughout the retina. “magn”- magnify
Respond strongly to movement and large overall patterns with their large receptive fields
Do not respond to color or fine details
Found throughout the retina, including the periphery
oKoniocellular (dust celled)
Small cell bodies, similar to Parvocellular neurons, but they occur throughout the retina
Have several functions and their axons terminate in several locations
Table 5.2: Three kinds of Primate Ganglion Cells
Parvocellular Neurons Magnocellular
Neurons
Koniocellular Neurons
Cell bodies Smaller Larger Small
Receptive fields Smaller Larger Mostly small; variable
Retinal location In and near fovea Throughout the retina Throughout the retina
Color sensitive Yes No Some are
Respond to Detailed analysis of
stationary objects
Movement and broad
outlines of shape
Varied
The Primary Visual Cortex
Information form the lateral geniculate nucleus of the thalamus goes to the primary visual cortex in the occipital
cortex, also known as area V1, or the striate cortex because of its striped appearance
If you close your eyes and imagine seeing something, activity increases in area V1 in a pattern similar to what
happens when you actually see the object
If you see an illusion, the activity in area V1 corresponds to what you think you see, not what the object really is
People with damage to area V1 report no conscious vision, no visual imagery, and no visual images in their
dreams
Adults who lose vision because of eye damage continue to have visual imagery and visual dreams
Some people with a damage to area V1 shows a surprising phenomenon called blind sight
oThe ability to respond in limited ways to visual information without perceiving it consciously

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oWithin the damaged part of their visual field, they have no awareness of visual input, not even to
distinguish between bright sunshine and utter darkness
oThey might be able to point accurately to something in the area where they cannot see, or move their
eyes toward it, while insisting that they are “just guessing”
oSome blind sight patients can reach for an object they cannot see, avoiding obstacles in the way
oSome can identify an object’s color, direction of movement, and approximate shape, also while insisting
that they are just guessing
oSome can identify or copy the emotional expressions of a face that they insist they do not see
oTwo explanations for blind sight
First: Small islands of healthy tissue remain within an otherwise damaged visual cortex, not large
enough to provide conscious perception but enough to support limited visual functions
Second: The thalamus sends visual input to several other brain areas outside V1, including parts
of the temporal cortex. After V1 damage, the connections to these other areas strengthen enough
to produce certain kinds of experience despite a lack of conscious visual perception
Researched arranged an apparatus so that people saw a face or a tool for three-tenths of a second in just on eye,
while the other eye was viewing a rapidly changing display, seeing something new ten times a second
oContinuous flash suppression- a viewer is conscious of the flashing stimulus and not the steady picture
oEven though people insisted they did not see a face or tool, when they were asked to guess where it was,
they were correct almost half the time
Simple and Complex Receptive Fields
David Hubel and Torsten Wiesel (1959)
oInserted thin electrodes to record activity from cells in cats and monkeys occipital cortex while they shined
light patters on the retina
At first, they presented dots of light, using a slide projector and a screen, but they fond little
response by cortical cells
They wondered why cells were so unresponsive, when they knew the occipital cortex was
essential for vision
They noticed a big response while they were moving a slide into place. Quickly realized that the
cell was responding to the edge of the slide. It had a bar-shaped receptive field, rather than a
circular receptive field like cells in the retina and lateral geniculate
David and Torsten also distinguished several types of cells in the visual cortex
oSimple Cell: has a receptive field with fixed and excitatory and inhibitory zones.
The more light shines in the excitatory zone, the more the cell responds.
The more light shines in the inhibitory zone, the less the cell responds
Most simple cells have bar shaped or edge-shaped receptive fields
More of them respond to horizontal or vertical orientations than to diagonals.
A cell that responds to a stimulus in only one location is a simple cell
oComplex Cells: respond to a pattern of light in a particular orientation anywhere within its large receptive
field
Located in areas V1 and V2, do not respond to the exact location of a stimulus
Responds most strongly to a moving stimulus- for ex: a vertical bar moving horizontally.
A cell that responds equally throughout a large area is a complex cell
oEnd-stopped, or Hypercomplex Cell:
Resemble complex cells with one exception
Has a strong inhibitory area at one end of its bar shaped receptive field
Cell responds to a bar-shaped pattern of light anywhere in broad receptive field, provided the bar
does not extend beyond a certain point
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