why light€¦ · web viewbeyond the striate cortex - 145/31/2016 spatial organization in the...

Spatial Organization in the CortexRecall the Cortical Map of the Visual Field (From Wolfe, Kluender, & Levi)

Mapping.

There is a mapping of the retina to the cortex.

It’s sometimes called a retinotopic map.

Sometimes it’s called a topographic 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.

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.

Beyond the striate cortex - 1 5/24/2023

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Columns of neurons 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

Each different column represents a different location in the visual field – All neurons within the same column all respond the same location in the visual field.

Three randomly selected columns from V1.

Neurons within each column have the same receptive field.


Surface of

cortex

Groups of location columns.

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 location

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.


Differences between columns within the groups of neurons

Differences in ocular dominance - .

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 eyeNeurons 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.


Ocular Dominance Column

Edge view of a slice of cortex

The 6 layers of

the cortex

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Group of columns sharing thesame receptive field location

Differences in orientation within a group – 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.

Differences in wavelength sensitivity between columns within a group

It’s been discovered that there are differences between columns in processing of wavelength information. Some columns process long wavelength light best. Others process short wavelength light best.


Edge view of a slice of cortex

Group of columns sharing thesame receptive field location

Six layers of cortical cells.

Orientation column

Summary

1) Neurons are in columns and those within the same column have the same receptive field location and probably process the same type of information.

2) Adjacent columns within a group have the the same receptive field location but process different types of information of that receptive field – orientation of edges, color, movement, corners, etc.

3) Different groups of columns have different receptive field locations.

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.

Hypercolumns are kind of like pixels in a computer screen.

Each pixel presents the brightness and color of a spot on the screen.

An HD screen has 1920 pixels per line. The screen is typically comprised of 1080 lines.So a typical HD screen has 1920 x 1080 = 2,073,600 pixels, just over 2 million.


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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.

Schematic of “wiring diagram” of simple cortical cells.


Ganglion Cells in retina

Cortical cells

Retina Cortex

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 shapeThe 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 landmarkThe 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

ResultsLesions in Parietal Lobe

Able to solve the object shape task

Not able to solve the 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.

This was a major publication. But it was not specifically applicable to humans.


b. Or in the inferotemporal cortex

<-Front <-Front

Lesions in Inferotemporal Lobe

Not able to solve object shape task

Able to solve object location task

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. had suffered carbon monoxide poisoning. The primary damage was to the lateral occipital cortex. One of D.F.s symptoms was an inability to recognize objects visually (called visual agnosia).

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 – reporting orientation.

She was unable to correctly 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 – using orientation in a motor 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.


The Two Channels

The research describe above and other similar research lead to the postulation of two separate channels of processing – the “What” channel and the “How/Where” channel.


The “What” Pathway of Signals from the Midget (Parvocellular) Retinal Ganglion CellsFigures are from the Yantis text, where parvocellular cells were called midget cells.

1st Synapse: 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


Beyond V1LGN

V1

Eye

The “Where/How/Action” Pathway of Signals from the Parasol (Magni) RGCsFigures are from the Yantis text, which called magnicellular cells parasol cells.

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


Beyond V1LGN

V1

Eye

Demonstrating the difference between the two systems in humans without brain damage.

The Ganel et al (2008) Experiment. G9 p 86. 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

In the Length Estimation task, presumably mediated by the What System, they got the line lengths incorrect – the illusion occurred. They estimated Line 1 as shorter.

But in the Grasping Task, presumably mediated by the Where/How/Action system, they got them right. They grasped at Line 1 as if it were longer, and it is long!

This supports the idea that perception and action involve two separate systems.


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Modules - G9 p 87

Play Virtual Lab 4 -4 – Nancy Kanwisher – Introduces and 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.


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.


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.


Modules for 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 and neural activity of modules 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 recordedTraining – Trained in Greeble RecognitionPosttest - 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.


Class Videos . . .

* VS Ramachandran – 3 Clues TED Talk

The fusiform face area and areas of the brain to which it connects

* G9 VL 5 -16,17 Jack Gallant on Watching Movies through Area V1

The mapping of the visual world onto Area V1

* Humanoid Robots

The importance and difficulty of the work of the cerebellum


why light€¦ · web viewbeyond the striate cortex - 145/31/2016 spatial organization in the...

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