Spatial Organization in the Cortex
Recall the Cortical Map of the Visual Field (From Wolfe, Kluender, & Levi)
Mapping.
There is a mapping of the
retina to the cortex.
This mapping is called
topographic, because it maps
each part of the topography of
the retina to a corresponding
part of the visual cortex.
It’s sometimes called a
retinotopic map.
Cortical Magnification
Although the figure here does
not illustrate that
magnification,
Area 5 in the cortex is much
larger than Area 9 or Area 1.
5
5
Many more brain neurons are
used for processing a given
area of the fovea (area 5 in the
retina) than are used for the
same area of the periphery of
the retina (areas 9/1 in the
retina)
So the “pictures” shown in the
cortex in this figure should be
distorted, with huge amounts of space given to Area 5 and much smaller shown being given to
Area 9/1.
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Columns in V1
G9 p. 80
Neurons in V1 are organized in columns – 6 neurons per column, since there are 6 layers
of neurons in the cortex.
Neurons within each column respond to similar aspects of the visual stimulus
Location Columns – Neurons within a column all respond the same location of
stimulation on the visual field.
Surface
of
cortex
Three randomly selected columns from V1.
Neurons within each column have the same receptive field.
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Groups of location columns. Start here on 2/9/16.
Not only do neurons within a column have the same receptive field, but the columns are
grouped, with each column within a group having the same receptive field
Three randomly selected groups of cortical columns.
Within each group. all columns have the same receptive field location.
They respond to what’s happening at the same place in the visual field.
What’s going on??? Why do they all have the same receptive field? It’s because each
different neuron responds to a different characteristic of the stimulus at that location.
So:
1) Neurons up and down a column have the same receptive field location.
2) Adjacent columns within a group have the nearly the same receptive field
location.
3) Different groups of columns have different receptive field locations.
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Differences within the groups of neurons
Ocular Dominance columns.
Different eye preferences between columns within each group: Within each group of
columns sharing the same receptive field location, different columns have different
preferences for left eye or right eye.
A column of neurons all of which have the same eye preference is called an ocular
dominance column.
Neurons in some columns respond only to stimulation of the left eye
Neurons in some columns respond only to stimulation of the right eye.
Neurons in some columns respond equally to stimulation of both left and right eyes.
Group of columns sharing the
same receptive field location
Edge
view of a
slice of
cortex
The 6
layers of
the
cortex
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Ocular Dominance
Column
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Orientation columns: G9 p 81
Within each group of columns sharing the same receptive field location, there are
columns in which all neurons within the column respond to the same orientation of a
visual stimulus. (Layer 4 excluded). These are called orientation columns.
Adjacent columns have the same receptive field but respond to slightly different
orientations.
Group of columns sharing the
same receptive field location
Six layers of
cortical cells.
Edge view
of a slice of
cortex
Orientation column
Hypercolumns:
The groups of 100s of columns sharing the same receptive field location are frequently
called hypercolumns. The text calls them location columns.
Each hypercolumn occupies about a square millimeter (mm2) of V1.
L L | L
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1 millimeter
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Possible wiring input to simple cortical cells G9 p 82
Here’s how the simple cortical cell receptive fields might be created.
Summer 2015: This was covered in Chapter 3 – covered again here to keep with text.
Schematic of “wiring diagram” of simple cortical cells.
Cortical cells
Ganglion Cells in retina
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Processing of visual information after V1 G9 p 83
Major research providing evidence supporting the existence of separate brain areas for
different aspects of the visual scene
Ungerleider, L. G., & Mishkin, M. (1982). Two cortical visual systems. In D. J. Ingle, M. A.
Goodale, & R. J. W. Mansfield (Eds.), Analysis of visual behavior (pp. 549-586).
Cambridge, MA: MIT Press. Y1 – p. 105,
Monkeys were trained to get food in two different ways –
1. By object shape – the food is in the bin with a particular shape
The monkeys had to know the shape of the food’s container to get food.
2. By object location relative to a landmark – the food is in the bin closer to a landmark
The monkeys had to know where the food was to get the food..
The researchers then produced lesions in a specific location in the monkeys’ brains
a. in the parietal cortex
b. Or in the inferotemporal cortex
<-Front
<-Front
Results
Lesions in Parietal Lobe
Lesions in Inferotemporal Lobe
Able to solve the object shape task
Not able to solve object shape task
Not able to solve the object location task
Able to solve object location task
These result lead to the belief that the visual system of the brain is comprised of neurons in two
groups . . .
a. A group of neurons responsible for processing the “where” of visual stimuli in the parietal
cortex and
b. A group of neurons responsible for processing the “what” of visual stimuli in the
inferotemporal cortex.
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The Case of Patient D.F. A second major study pointing the way toward understanding the
major organization of the brain.
Goodale, M. A., Muner, A. D., Jakobson, L. S., & Carey, D. P. (1991). A neurological
dissocation between perceiving objects and grasping them. Nature, 349, 154-156.
D.F. suffered from an inability to recognize objects visually (called visual agnosia). It was due
to carbon monoxide poisoning. Primary damage was to the lateral occipital cortex.
She was unable to perceive the shapes of objects she
was looking at.
She could reach for and grasp objects even though
such actions required some degree of shape
perception.
The What Task.
She was unable to correct report and thus presumably perceive the orientation of objects.
This was demonstrated with a task in which she was asked to rotate a card held in her hand to
match the orientation of a slot in front of her.
In the illustration below, the card has been rotated so its orientation is the same as the slot. She
could not do that.
To the right is a record of her
performance on a task requiring
her to rotate the card so its
orientation matched the
orientation of a vertical slot.
The Where/Action Task.
But, when she was asked to simply put the card into the slot, she performed nearly perfectly, at
every orientation of the slot.
In this figure, the correct angle was vertical. But she did equally well at all
correct angles.
Based on these results, Milner and Goodale suggested that the pathway
involved should be called the How pathway. Some call it that. Others call
it the Action pathway.
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This research lead to the postulation of two separate channels of processing – the “What”
channel and the “How/Where” channel.
The Two Channels
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The “What” Pathway of Signals from the Midget (Parvi) RGCs
Figures are from the Yantis text
LGN
Eye
Beyond V1
V1
1st Synapse: Parvocellular
Layer 3, 4, 5, or 6 of the
LGN
2nd Synapse: Layer 4Cβ of
V1, then to blobs and
interblob regions of Layers
2 and 3 of V1
Subsequent Synapses:
Color -> Thin V2
bands; Form -> Pale V2
bands
Subsequent Synapses – V4
Ultimately, to the Inferotemporal Cortex
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The “Where/How/Action” Pathway of Signals from the Parasol (Magni) RGCs
Figures are from the Yantis text
LGN
Eye
Beyond V1
V1
1st Synapse: Magnocellular
Layer 1 or 2 of the LGN
2nd Synapse: Layer 4Cα of
V1
Subsequent Synapse(s):
Area V2 – thick bands
Subsequent Synapse(s):
Medial Temporal (MT) are
Ultimately Synapsing in the
Parietal Cortex
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Demonstrating the difference between the two systems in humans without brain damage.
The Ganel et al (2008) Experiment. G9 p 86. Shown in VL 3 for this chapter.
Which line is longer – Line 1 or Line 2?
Most people say that Line 2 is long.
Spoiler alert!!! In fact, Line 1 is longer.
(More in Chapter 10 on why you may incorrectly perceive
Line 2 as being longer.
The Ganel et al research involved two tasks.
1) The Length Estimation task – participants estimated the line lengths by spreading the thumb
and forefinger.
2) The Grasping Task – participants reached toward each line as if to grasp it.
The results
1
2
1
2
In the Length Estimation task, presumably mediated by the What System, they got the line
lengths incorrect – the illusion occurred.
But in the Grasping Task, presumably mediated by the Where/How/Action system, they got
them right.
This supports the idea that perception and action involve two separate systems.
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Modules - G9 p 87
Virtual Lab 4.4 – Nancy Kanwisher – illustrates some of these issues.
Module: A collection of neurons in the cortex (usually near each other) which processes a
specific related set of aspects of sensory information.
The “What” stream is a giant module.
The “Where/How/Action” stream is a second giant module.
But are there more specific modules? Are there specialties within the streams – streams within
streams?
Evidence for specialized modules – from the past 20 years of research.
1. Area V4: A module for processing color and curvature
Evidence from monkey and human studies that different neurons in V4 respond to different
wavelengths – some to short wavelength light, others to medium, others to long and others
to all wavelengths in between. Damage leads to achromatiopsia or cortical color blindness –
inability to perceive color even though the cones function perfectly well.
There is other evidence that different neurons in V4 respond to curvatures of edges.
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2. Modules for faces and places. G9 p 88
2a. The Fusiform Face Area (FFA) (Figure from Yantis text.)
Neurons in the FFA respond to faces.
2b. Parahippocampal Place Area (PPA)
The PPA contains neurons that are active when large-scale scenes are being viewed.
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3. The Extrastriate Body Area (EBA) G9 p 88
The EBA is an area whose neurons respond to pictures of bodies and parts of bodies.
Those same neurons to not respond to faces, however.
4. A movement module: The Middle Temporal (MT) area.
Neurons in the MT area respond to moving stimuli.
They’re tuned for the direction of movement – each responds best to movement in a particular
direction.
They’re also tuned for speed – each responds best to movement at a particular speed.
Damage to the MT area affects the ability to perceive and respond to moving stimuli.
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Memory and Visual Images G9 p 89
The Hippocampus is an area of the brain intimately involved in the memory of scenes.
Patient M.H. – Hippocampus in each hemisphere was removed to cure epileptic seizures. Lost
ability to remember more than a few minutes of experience.
There is also evidence that neurons in the hippocampus respond to specific concepts – concepts
which might be activated by many visual images of the same concept – such as the concept of
Jennifer Anniston, Sydney Opera House, Halle Barry, etc.
The role of Experience in perception G9 p 91 – The Greeble Experiments fMRI records showed changes in response of FFA neurons to presentation of Greeble faces after
experience with Greebles.
Pretest – shown Greeble and human faces and fMRI of the FFA recorded
Training – Trained in Greeble Recognition
Posttest - Shown Greeble and human faces and fMRI of the FFa recorded
Prior to training, FFA fMRI responsitity to Greebles was
minimal – they weren’t “faces” which is what FFA is all
about.
But after training, they were responded to as “faces” by
the FFA neurons.
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