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Chapter 4-6

PS262 Chapter Notes - Chapter 4-6: Lateral Geniculate Nucleus, Receptive Field, Two-Streams Hypothesis


Department
Psychology
Course Code
PS262
Professor
Phillip Servos
Chapter
4-6

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Sensory Process & Perception Chapter 4-6 and 8 Notes
CHAPTER 4
Light is reflected from an object into the eye
o This light is focused to form an image of that object on the retina
o Light, in a pattern that illuminates some receptors intensely and some dimly, is absorbed by the
visual pigment molecules that pack the rod and cone outer segments
o Chemical reactions in the outer segments transducer the light into electrical signals
o As these electrical signals travel through the retina, they interact, excite, and inhibit, eventually
reaching the ganglion cells, which because of this processing have center-surround receptive
fields on the retina
o After being processed by the retina these electrical signals are sent out the back of the eye in
fibres of the optic nerve
Following the Signals From Retina to Cortex
The Visual System
Figure 4.1a, which is an overview of the visual system, pictures the pathway that the neural signals
follow once they leave the retina
Most of the signals from the retina travel out of the eye in the optic nerve to the lateral geniculate
nucleus (LGN) in the thalamus
o From here, signals travel to the primary visual receiving area in the occipital lobe of the cortex
o The visual receiving area is also called the striate cortex because of the white stripes (striate =
striped) that are created within this area of cortex by nerve fibres that run through it
o From the striate cortex, signals are transmitted along two pathways, one to the temporal lobe and
the other to the parietal lobe (blow arrows)
o Visual signals also reach areas in the frontal lobe of the brain
Figure 4.1b shows the visual system as seen from the underside of the brain
o In addition to showing the pathway from eye to LGN cortex, this view also indicates the location
of the superior colliculus, an area involved in controlling eye movements and other visual
behaviours that receives about 10% of the fibres from the optic nerve
o This view also shows how signals from half of each retina cross over to the opposite side of the
brain
Processing in the Lateral Geniculate Nucleus
What happens to the information that arrives at the lateral geniculate nucleus?
Receptive Fields of LGN Neurons
Recording from neurons in the LGN shows that LGN neurons have the same center-surround
configuration as retinal ganglion cells
Thus, neurons in the LGN, like neurons in the optic nerve, respond best to small spots of light on the
retina
A major function of the LGN is apparently not to create new receptive field properties, but to regulate
neural information as it flows from the retina to the visual cortex
Information Flow in the Lateral Geniculate Nucleus
90% of the fibres in the optic nerve arrive at the LGN (the other 10% travel to the superior colliculus)
But these signals are not the only ones that arrive at the LGN

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o The LGN also receives signals from the cortex, from the brain stem, from other neurons in the
thalamus (T), and from other neurons in the LGN (L).
o Thus, the LGN receives information from many sources, including the cortex, and then sends its
output to the cortex
Figure 4.2b indicates the amount of flow between the retina, LGN, and cortex
Notice that (1) the LGN receives more input back from the cortex than it receives from the retina; and
(2) the smallest signal of all is from the LGN to the cortex
o For every 10 nerve impulses the LGN receives from the retina, it sends only 4 to the cortex
o This decrease in firing that occurs at the LGN is one reason for the suggestion that one of the
purposes of the LGN is to regulate the neural information as it flows from the retina to the cortex
But the LGN not only regulates information flowing through it; it also organizes the information
Organization by Left and Right Eyes
The lateral geniculate nucleus (LGN) is a bilateral structure, which means there is one LGN in the left
hemisphere and one in the right hemisphere
o Viewing one of these nuclei in cross section reveals six layers
o Each layer receives signals from only one eye
o Layers 2, 3, and 5 (red layers) receive signals from the ipsilateral eye, the eye on the same side
of the body as the LGN
o Layers 1, 4, and 6 (blue layers) receive signals from the contralateral eye, the eye on the
opposite side of the body from the LGN
o Thus, each eye sends half of its neurons to the LGN that is located on the left hemisphere of the
brain and half to the LGN that is located in the right hemisphere
o Because the signals from each eye are sorted into different layers, the information from the left
and right eyes is kept separated in the LGN
Organization as a Spatial Map
Figure 4.4
Retinotopic map a map in which each point on the LGN corresponds to a point on the retina
o We can determine what this map looks like by recording from neurons in the LGN
The correspondence between locations on the retina and locations on the LGN means that neurons
entering the LGN are arranged so that fibres carrying signals from the same area of the retina end up in
the same area of the LGN, each location on the LGN corresponds to a location on the retina, and
neighbouring locations on the retina
o Thus, the receptive fields of neurons that are near each other in the LGN, such as neurons A, B,
and C, in layer 6, are adjacent to each other at A’, B’ and C’ on the retina
Retinotopic maps occur not only in layer 6, but in each of the other layers as well, and the maps of each
of the layers line up with one another
One million ganglion cell fibres travel to each LGN, and on arriving there, each fibre goes to the correct
LGN layer (remember that fibres from each eye go to different layers) and finds its way to a location
next to other fibres that left from the same place on the retina
Receptive Fields of Neurons in the Striate Cortex
More than 80% of the cortex responds to visual stimuli
Using the procedure described in Chapter 2 (pg 34) in which receptive fields are determined by flashing
spots of light on the retina, Hubel and Wiesel found cells in the striate cortex with receptive fields that,
like center-surround receptive fields of neurons in the retina and LGN, have excitatory and inhibitory
areas
o However, these areas are arranged side by side rather than in the centre-surround configuration
(figure 4.6a)

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o Cells with these side by side receptive fields are called simple cortical cells
We can tell from the layout of the excitatory and inhibitory areas of the simple cell shown in figure 4.6a
that a cell with this receptive field would respond best to vertical bars
The relationship between orientation and firing is indicated by a neuron’s orientation tuning curve,
which is determined by measuring the responses of a simple cortical cell to bars with different
orientations
Although Hubel and Wiesel were able to use small spots of light to map the receptive fields of simple
cortical cells like the one in Figure 4.6, they found that many of the cells they encountered in the cortex
refused to respond to small spots of light
o As they inserted a glass slide containing a spot stimulus into their slide projector, a cortical
neuron “went off like a machine gun”
o The neuron, as it turned out, was responding not to the spot at the center of the slide that Hubel
and Wiesel had planned to use as a stimulus, but to the image of the slide’s edge moving
downward on the screen as the slide dropped into the projector
o Upon realizing this, Hubel and Wiesel changed their stimuli from small spots to moving lines
and were then able to find cells that responded to oriented moving bars
o They discovered that many cortical neurons respond best to moving barlike stimuli with specific
orientations
o Complex cells, like simple cells, respond best to bars of a particular orientation
However, unlike simple cells, which respond to small spots of light or to stationary
stimuli, most complex cells respond only when a correctly oriented bar of light moves
across the entire receptive field
Another type of cell, called end-stopped cells, fire to moving lines of a specific length or to moving
corners or angles
Hubel and Weisel’s finding that some neurons in the cortex respond only to oriented lines was an
extremely important discovery because it indicates that neurons in the cortex do not simply respond to
“light”; they respond to some patterns of light and not to others
o Their discovery that neurons respond selectively to stationary and moving lines was an important
step toward determining how neurons respond to more complex objects
Because simple, complex, and end-stopped cells fire in response to specific features of the stimulus,
such as orientation or direction of movement, they are sometimes called feature detectors
Table 4.1 Properties of Neurons in the Optic Nerve, LGN, and Cortex
Type of Cell
Characteristics of Receptive Field
Optic nerve fiber (ganglion cell)
Center-surround receptive field. Responds best to
small spots, but will also respond to other stimuli
Lateral geniculate
Center-surround receptive fields very similar to the
receptive field of a ganglion cell
Simple cortical
Excitatory and inhibitory areas arranged side by side.
Responds best to bars of a particular orientation
Complex cortical
Responds best to movement of a correctly oriented bar
across the receptive field. Many cells respond best to a
particular direction of movement
End-stopped cortical
Responds to corners, angles, or bars of a particular
length moving in a particular direction
Do Feature Detectors Play a Role in Perception?
One way to establish a link between the firing of these neurons and perception is by using a
psychophysical procedure called selective adaptation
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