Christopher FiorilloBiS 527, Spring 2012

042 350 4326, fiorillo@kaist.ac.kr

Part 12: Visual SystemReading: Bear, Connors, and Paradiso, Chapters 9 and 10 (or corresponding chapters from any other neurobiology textbook).

Neurophysiology and Information:Theory of Brain Function

Assistants: 윤소라 Sora Yun, <ysl0142@kaist.ac.kr>, 이주영 Ju-Young Lee, <hohojy29@kaist.ac.kr>

Why study the visual system?• Humans and other primates have a highly

developed visual system• The statistical structure of the visual world is

relatively easily accessible to us– This is not true for motor and high-level “cognitive”

neurons”– This is not true for some senses, such as olfaction

• The vertebrate visual system, especially the retina, has been studied more than any other system in neuroscience– This means that it is a good model system for asking

new questions, because there is a strong base of knowledge upon which to build

Slide

The Early Visual SystemInformation goes from the retina to LGN in the thalamus, and then to primary visual cortex, and then to higher visual areas in cortexThe visual system in humans and other primates is very large. It might be as much as 20% of the entire brain (this is just my guess).

Slide

Diversity of Retinal CellsAbout 55 types of neurons in retinaAmacrine cells are most diverse (29 types)

Most are inhibitory (GABA)Some contain slow neuromodulators, such as dopamine

Masland, 2001

Photoreceptors

Horizontal cells

Bipolar cells

Amacrine cells

Ganglion cells

Slide

Many Features of Retina are Maintained Throughout the Visual System

Receptive Fields are Larger in Later Neurons125 million photoreceptors, 1 million RGNs, in humans

But, in humans, most RGNs get input from just one photoreceptorRFs cannot possibly become smaller in later neurons

Receptive Fields are Larger in the PeripheryCenter-Surround Receptive Field StructureON cells and OFF cellsMagnocellular versus Parvocellular RGNs

Magnocellular neurons are large and have large receptive fields. They adapt quickly, and thus are very sensitive to motion. Parvocellular neurons are small, have small receptive fields, and adapt slowly.Some cortical regions (“dorsal stream”) get information from M cells and are specialized for motion, and others get information from P cells and are specialized for high acuity, color, and object recognition.

Higher Visual Areas Do Not Appear To Be Fundamentally Different From Retina

• Principles found in the retina are also found in higher areas

• There are parallel paths for processing of different types of information

• The major distinction is between areas that process motion, and those that process color and have high spatial acuity

• This distinction begins in the retina, where there are more than 10 different types of retinal ganglion neurons

• The major feature of thalamus and cortex that is different from retina is that there are many feedback projections from higher areas.

• These supply reward information and mediate attention

• Their function is not well understood

The Visual System as a Model for Development• The optic nerves consists of the axons of reginal

ganglion neurons. It innervates the lateral geniculate nucleus of the thalamus (LGN)

• LGN innervates striate cortex (V1) in the occipital lobe of cerebral cortex

Slide

The Lateral Geniculate Nucleus (LGN)• Layers 1 and 2 are Magnocellular (large cells)• Layers 3 - 6 are Parvocellular (small cells)• General Rule:

• Throughout the nervous system, magnocellular neurons have larger receptive fields, receive more synaptic inputs, and conduct action potentials faster through their axons, compared to parvocellular neurons

Receptive Field Size• Receptive fields can be measured by stimulating the skin while

recording from a single nerve fiber• Some types of neurons have a large RF, and others have a small

RF• The size of an RF corresponds to how broadly tuned it is in space

A small RF corresponds to narrow tuning, and a large RF to broad tuning

Slide

The Lateral Geniculate Nucleus (LGN)• Layers 2, 3, and 5 receive input from the Ipsilateral eye (same side)• Layers 1, 4, and 6 receive input from the Contralateral eye (opposite side)

Slide

The Lateral Geniculate Nucleus (LGN)

The Segregation of Input by Eye and by Ganglion Cell Type

Slide

Primary Visual Cortex = V1 = Striate Cortex = Area 17LGN projects to V1. V1 is the earliest / lowest visual area in the cortex.

Slide

Retinotopy in Striate Cortex (V1)Neighboring regions of retina project to neighboring regions of LGN, which project to neighboring regions of V1Similar maps are found in superior colliculus, LGN, and other cortical areas

MapsNeurons in most visual areas are organized in maps the correspond to locations in the retina and visual space

A disproportionate number of neurons are devoted to the central visual fieldThroughout the nervous system, neurons receiving information from neighboring sensory receptor cells are found close together

Neurons with information about similar parts of the world tend to communicate with each otherBy being close together, their axons can be short. This saves energy and speeds communication.

Slide

Tonotopy• Each hair cell responds to a different tone (sound

frequency), depending on its location in the cochlea • Thus, a temporal pattern (sound frequency) in the

external world is converted to a spatial pattern in the nervous system• Throughout the auditory system, neurons are arranged

in tonotopic maps, with neurons in one region of a brain structure responding to high frequency tones, and neurons in another region responding to low frequency tones

Somatotopy• Neurons with RFs in neighboring regions of the body are next to each other in the brain

• Each somotosensory brain region has a somatotopic map The figure below shows the map in primary somatosensory cortex Some body parts are represented by many neurons and appear larger than other body parts

The Homunculus• This image shows how much

of the brain is devoted to each body part• Many neurons represent the

hands and lips and tongue Most of the neurons from these

regions have small RFs. This provides high spatial acuity. A person can discriminate stimulation at two locations on the finger that are very close together. By contrast, RFs on the back are large and there are few neurons representing the back. Thus a person cannot distinguish stimulation at two locations on the back that are close together.

The back is analogous to peripheral space in the visual system, and the finger tip is analogous to the fovea (central retina and central space)

Factors Influencing Receptive Field Size• Neurons that differ only in spatial location are said to be “in

parallel” to one another• We would expect each parallel neuron to carry about the

same amount of reward information (on average)– It is most efficient for each neuron to do the same amount of

“work,” assuming that they are equally capable• A larger receptive field will contain more reward information

than a smaller receptive field (other things being equal)• A receptive field in the center of visual gaze will contain

more reward information than a receptive field in the periphery (assuming they are the same size)

• Thus, for each neuron to have the same amount of reward information, neurons in the center should have smaller receptive fields than those in the periphery

Slide

6 Layers of Cerebral CortexAll regions of neocortex have similar features, but not identical

Nissl StainVisual

Nissl StainMotor Golgi

Slide

Physiology of the Striate CortexReceptive Fields

Layer IVC: mostly similar to LGN receptive fieldsMonocularcenter-surround

Layer IVCαInsensitive to wavelength (color)Motion sensitive, magnocellular

Layer IVCβColor opponency, parvocellular

BinocularityLayers IVB and III

First binocular receptive fields in the visual pathway

Orientation SelectivitySome cells in layer IVC, and most cells in other layersReceptive fields are elongated rather than circular

Slide

Orientation Selectivity in V1Each cell has a preferred orientation Discovered by Hubel and Wiesel in early 1960s• Nobel Prize

Slide

Physiology of the Striate CortexReceptive FieldsOrientation selectivity may be created by inputs from three or more LGN cells with ON and OFF regions in their receptive fieldsOrientation selectivity could develop from a Hebbian plasticity rule

This is because regions of contrast tend to occur in lines (edges) in the natural worldSelectivity for orientation, and other objects (e.g. faces), occurs only through experience-dependent learning

Binocular Receptive Fields

• Neurons in layer 4C of V1 are monocular (they receive input from just one eye).

• Monocular neurons innervate binocular neurons in layers 4B and 3.

Binocular Receptive Fields• How do binocular receptive fields form?• Monocular deprivation experimentsOne eye is sown shut so that it is deprived of lightInputs from deprived eye are relatively random, with

low frequency firingInputs from non-deprived eye depend on sensory

patterns• Correlated, with high frequency bursts

Result: expanded representation of non-deprived eye (more neurons receive inputs from non-deprived eye)

Conclusion: formation of receptive fields is dependent on patterns of sensory activity and experience-dependent plasticity• This could be explained by Hebbian plasticity

Why and How do Receptive Fields Form?• We have already discussed the principles underlying Hebbian

learning at the cellular level.• Neurons in V1 have receptive fields (RFs) that are binocular and

elongated (orientation selective)• These RFs form because of common patterns in the inputs• Binocular RFs Some neurons in the retina and LGN have RFs in the same region of retinotopic

space, even though one is in left retina or LGN, and one in right retinal or LGN Because the two eyes normally gaze at the same region of space in humans,

these inputs will be active at about the same time. A Hebbian rule will make them strong.

• Orientation In the natural world, contrast tends to occur in long lines, not in short lines or

points. Therefore, neighboring neurons in LGN that are arranged in a line will tend to be active at the same time, and will be selected by a Hebbian rule. All orientations are about equally common.

• Conclusion: formation of RFs is dependent on the learning of sensory patterns through Hebbian rules (Bienenstock, Cooper, and Munro, 1982)

Ocular Dominance Shift• Depriving one eye causes the

other eye to become dominantOccurs within hours reversibleAnatomical changes occur later

• Establishment of binocular receptive fields depends on correlated activity across both eyes

• This is probably mediated by Hebbian plasticity

Development of Binocular Tuning Depends on Correlated Input from Each Eye

• Strabismus is a disorder in which the eyes are not properly aligned Each eye looks at a different

location

• Binocular neurons do not develop because the inputs from each eye are not correlated

• In some animals, such as rats, the two eyes do not see the same visual field These animals do not have

binocular neurons

Mechanism of plasticity following monocular deprivation• Well correlated synaptic activity from open eye causes strong Ca2+

influx through NMDA receptors (LTP, high weight)• Uncorrelated synaptic activity from closed eye causes weak Ca2+

influx (LTD, low weight)• LTD may eventually lead to a complete loss of synapses

Slide

Physiology of the Striate Cortex A Cortical Module

Hubel and Wiesel found that a point of light activated a region of striate cortex 2 x 2 mm in areaThis region is able to analyze all aspects of that part of space

Slide

Beyond Striate CortexDorsal stream

Analysis of visual motion Ventral stream

Recognition of objects (shape, color, patterns, etc.)

Slide

Beyond Striate CortexThe Dorsal Stream (V1, V2, V3, MT, MST, LIP)

Area MT (temporal lobe)Cells are direction-selective; Respond more to the motion of objects than their shapeElectrical stimulation elicits perception of motion

Beyond area MT - Roles of cells in area MST and LIP (parietal lobe)

NavigationDirecting eye movementsMotion perceptionLIP is sensorimotor; it appears to be near the top of the hierarchy for using sensory information in order to decide where to move the eyes

Slide

Beyond Striate CortexThe Ventral Stream (V1, V2, V3, V4, IT, Other ventral areas)

Area V4Specialized for color and shape

Achromatopsia: Clinical syndrome in humans-caused by damage to area V4; Partial or complete loss of color vision

Area ITMajor target of projections from V4Receptive fields respond to a wide variety of colors and abstract shapesA portion of IT cortex is selectively responsive to faces

Slide

The Highest Levels of Visual ProcessingLateral-dorsal prefrontal cortex

Neuron activity represents whatever information is currently relevant to the animal, regardless of sensory features, including sensory modalityThese neurons may be closely tied to our conscious experience

In the experiment shown below, a monkey had to discriminate cats from dogs

Slide

Attention, Reward, and Top-down Feedback Projections in Cortex

• Many more cortical projections are “top-down” than “bottom-up”

• Higher level neurons have more information about reward and less about sensory world

• An important function of top-down projections is to transmit reward information• Top-down projections gate the bottom-up flow of

information• Attention exemplifies this process

• The ability of neurons in prefrontal cortex to have information about anything that is currently relevant is the result of this sort of selection occurring at multiple levels of cortex