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Chapter 3b - The Visual Cortex – G9 p 63Striate cortex, area V1, Occipital lobe: About 250 million neurons (out of the 15-30 billion in the cortex).
The red line is the pathway from the eye to the occipital lobe.
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Side view of the cortex
Recall: The cortex is a sheet of neurons that covers the rest of the brain.
Some of the possible sources of input are shown.
1,
It used to be thought that the cortex was made up of 6 layers of cells. We now know that what used to be
thought of as Layer 4 are actually 4A, 4B, 4Cα, and 4Cβ.
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The Cortical Map of the Visual Field (From Wolfe, Kluender, & Levi)
Stimulus being observed
Note that what is in the
center of the visual field
ultimately projects to
“outside” of the occipital
lobe.
The left and right
peripheries of the visual
field are projected to the
area between the
hemispheres.
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Receptive fields of cortical cells – G9 p 64
Layer 4
Layer 4 cells have circular receptive fields, similar to those of the LGN cells that drive them.
Other layers
In other layers, the neurons have receptive fields that are not simply circular.
From the Nobel Prize Winning research of David Hubel and Torsten Wiesel carried out in 1950s-80s.
Simple cells G9 p 64
These cortical cells respond only to bars of light or slits of darkness located in a specific place in the visual
field.
1) located in a particular place in the visual field and
2) have a particular orientation.
If the bar or slit is moved to a different location, the neuron quits firing.
If the orientation of the bar or slit is changed, ditto.
Consider the following. Each circle represents the visual field under a different stimulation. The stimulus is
represented by the red line.
Receptive
field
Receptive
field
Yippee!
Cortical neuron
Neuron responds because
the stimulus is in its
receptive field with correct
orientation
Receptive
field
Ho hum.
Cortical neuron
Neuron does not respond
because although the
stimulus has the correct
orientation, it’s not in the
neuron’s receptive field.
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Ho hum.
Cortical neuron
Neuron does not respond
because although the
stimulus is in its receptive
field, the stimulus does not
have the appropriate
orientation.
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Complex cells G9 p 66
These cells respond to bars of light or slits of darkness, as do simple cells.
But they respond best when the bar or slit moves within a certain area of the visual field.
Many respond best to a particular direction of movement.
The figure below attempts to illustrate the stimulus for a complex cell “looking” for movement of a
particularly oriented bar moving from left to right across the visual field.
// ///
End-stopped cells (hypercomplex)
These cells respond to moving lines of a specific length (hence the term, end-stopped).
Some also respond to moving corners or angles.
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Grating detectors: Cells that respond to parallel lines in a specific orientation
Evidence . . . Selective Adaptation Experiments
Participants view a specific grating stimulus continuously.
e.g.
is presented and viewed for one minute.
They are then shown a collection of grating stimuli of various orientations and the threshold for each one is
determined
The result is that the threshold for the adapted orientation is much higher than thresholds for the others.
Note that the vertical
axis is increase in
threshold.
This research provides evidence that the visual system contains neurons that respond to orientation, and that
these neurons can become “fatigued” by being continually stimulated, as in the first part of this
experiment.
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Selective Rearing evidence for the importance of orientation-detecting neurons
Kittens were raised in environments comprised of mainly vertically oriented stimuli.
Others were raised in environments comprised of mainly horizontally oriented stimuli.
Both sets of kittens were then tested by placing them in an environment with both types of orientation.
Kittens raised in vertical environments ignored the horizontally oriented parts of their environments.
Kittens raised in horizontal environments ignored the vertically oriented parts of their environments.
Researchers also recorded activity of cortical neurons in the two types of kitten.
They found
No horizontal neurons
No vertical neurons
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What’s it all mean?
Cells in the visual cortex respond to complex features – they’re feature detectors.
1) Edges are probably more important for us than homogenous fields.
So immediate processing of the incoming stream of visual information for edges seems to be a smart thing
to do.
2) So extracting edges may be the most efficient way of enabling the processing of the visual world as it
continually changes around us.
Most of the things in the world that are important for us are defined by combinations of edges.
Recording of Hubel & Wiesel’s demonstration of simple, complex and hypercomplex cells - 11:01’ total
http://www.youtube.com/watch?v=jw6nBWo21Zk
11:01 total, 8:20 excluding hypercomplex
Wiesel discussing the accidental discovery of importance of edges:
https://www.youtube.com/watch?v=IOHayh06LJ4
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Higher order neurons – beyond “stick figure physiology” represented by edges G9 p 69
We have neurons in our brains that respond to stimuli more complex than edges. Single-cell recording of
neurons in areas outside of the occipital lobe suggest that neurons in other areas respond to specific patterns
of stimulaton that are more complicated than those found in the occipital lobe.
Findings in the Inferotemporal (IT) cortex in the monkey
“Hand” neurons: Reseachers discovered neurons that responded to stimuli shaped like a hand
“Face” neurons: Other researchers discovered neurons that responded to stimuli shaped like a face, or an
actual face.
What’s most interesting about these studies is that they were published in the late 1960s and early 1970s but
were not “taken seriously” until the 1990s . (G9 p 70). So hang in there.
More on speciaized neurons in Chapter 4 and 5.
The Sensory Code G9 p 70
How are complex stimuli like a face or a chair or a tree or a house represented in the brain?
Each stimulus
activates
all neurons
Two extremes of possibility . . .
causing them to
Each unique
respond in a specific
Sparse coding.
stimulus has its
pattern a specific
own neuron.
pattern.
Specificity
Distributed
--------------------------------------------------------------------------------------------------------------------------------The leftmost extreme is call specificity coding. It assumes that for each specific external stimulus, there is
a neuron that responds to that stimulus and only to that stimulus. Otherwise the neuron does nothing.
The rightmost extreme is called distributed coding. It assumes that each different stimulus activates all
brain neurons, with a different pattern of activity for each stimlus. So all neurons are active at all times.
They just sing different songs to different objects.
It is very likely that neither extreme can be correct.
Problems with specificity coding
Problems with distributed coding
Requires too many neurons
Wasteful
Requires too much energy – they’re always singing.
Sparse coding
Each stimulus is represented by the pattern of activity of a small (1000? 100,000?) group of neurons
Individual neurons will respond to a collection of similar external stimuli but in different ways for each
specific externa stimulus.
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