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The Visual System

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Introduction

I. Neurons in the visual systemA. Neural processing resulting in perception

II. Parallel pathway serving conscious visual perception originate in the retina

A. Progress to lateral geniculate nucleus, primary visual cortex & higher order visual areas in temporal and parietal lobes

III. Neuronal receptive fields A. Sensitive to different facets of the visual input

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The Retinofugal ProjectionThe Optic Nerve, Optic Chiasm, Optic Tract

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The Retinofugal ProjectionThe Optic Nerve, Optic Chiasm,Optic Tract,LGN,Optic Radiations

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The Retinofugal Projection

I. Right and Left Visual Hemifields

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The Retinofugal Projection

Targets of the Optic Tract

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The Retinofugal Projection

Targets of the Optic Tract

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The Retinofugal Projection

Targets of the Optic Tract

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The Retinofugal Projection

I. Nonthalamic Targets of the Optic Tract:A. Hypothalamus: Biological rhythms,

including sleep and wakefulness

B. Pretectum: Size of the pupil; certain types of eye movement

C. Superior colliculus: Orients the eyes in response to new stimuli

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The Lateral Geniculate Nucleus (LGN)

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The Lateral Geniculate Nucleus (LGN)

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The Lateral Geniculate Nucleus (LGN)

I. The Segregation of Input by Eye and by Ganglion Cell Type

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The Lateral Geniculate Nucleus (LGN)

I. Receptive FieldsA. Receptive fields of LGN neurons: Identical

to the ganglion cells that feed them

A. Magnocellular LGN neurons: Large, monocular receptive fields with transient response

A. Parvocellular LGN cells: Small,monocular receptive fields with sustained response

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The Lateral Geniculate Nucleus (LGN)

I. Nonretinal Inputs to the LGNA. Retinal ganglion cells axons: Not the main

source of synaptic input to the LGN

B. Primary visual cortex: 80% of the synaptic inputs

C. Neurons in the brain stem: Modulatory influence on neuronal activity

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Anatomy of the Striate Cortex

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Anatomy of the Striate Cortex

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Anatomy of the Striate Cortex

I. Retinotopy A. Map of the visual field onto a target structure (retina,

LGN, superior colliculus, striate cortex) - overrepresentation of central visual field

B. Discrete point of light: Activates many cells in the target structure

C. Perception: Based on the brain’s interpretation of distributed patterns of activity

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Anatomy of the Striate Cortex

I. Retinotopy

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Anatomy of the Striate Cortex

I. Lamination of the Striate Cortex

A. Layers I - VIB. Spiny stellate cells: Spine-

covered dendrites; layer IVC

C. Pyramidal cells: Spines; thick apical dendrite;

layers III, IV, V, VIA. Inhibitory neurons: Lack

spines; All cortical layers; Forms local connections

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Anatomy of the Striate Cortex

Inputs to the Striate Cortex A. Magnocellular LGN neurons: Project to layer

IVC

B. Parvocellular LGN neurons: Project to layer IVC

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Anatomy of the Striate Cortex

I. Ocular Dominance Columns

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Anatomy of the Striate Cortex

I. Inputs to the Striate Cortex A. Layer IVC innervates superficial layers

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Anatomy of the Striate Cortex

I. Outputs of the Striate Cortex:A. Layers II, III, and IVB:

Projects to other cortical areas

B. Layer V: Projects to the superior colliculus and pons

C. Layer VI: Projects back to the LGN

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Anatomy of the Striate Cortex

I. Cytochrome Oxidase Blobs A. Cytochrome oxidase is a

mitochondrial enzyme used for cell metabolism

B. Blobs: Cytochrome oxidase staining in cross sections of the striate cortex

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Physiology of the Striate Cortex

I. Receptive Fields A. Layer IVC: Monocular; center-surroundB. Layer IVC: Insensitive to the wavelength C. Layer IVC: Center-surround color opponency

II. BinocularityA. All layers superficial to IVC: First binocular

receptive fields in the visual pathway

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Physiology of the Striate Cortex

Receptive FieldSimple CellsOrientation Selectivity

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Physiology of the Striate Cortex

Receptive Field

Simple Cells

Orientation Selectivity

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Physiology of the Striate Cortex

Receptive FieldSimple CellsOrientation Selectivity

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Physiology of the Striate Cortex

I. Receptive FieldsA. Simple cells: Binocular; Orientation-

selective; Elongated on-off region with antagonistic flanks responds to optimally oriented bar of light

B. Possibly composed of three LGN cell axons with center-surround receptive fields

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Physiology of the Striate Cortex

Receptive FieldsComplex Cells Direction Selectivity

Neuron fires action potentials in response to moving bar of light

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Physiology of the Striate Cortex

Receptive FieldsComplex Cells Direction

Selectivity

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Physiology of the Striate Cortex

I. Receptive FieldsA. Complex cells: Binocular; Orientation-

selective; ON and OFF responses to the bar of light but unlike simple cells, no distinct on-off regions

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Physiology of the Striate Cortex

I. Receptive FieldsA. Blob Receptive Fields

1. Blob cells: Wavelength-sensitive; Monocular; No orientation; direction selectivity

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Physiology of the Striate Cortex

Parallel Pathways: Magnocellular ParvocellularKoniocellular

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Physiology of the Striate Cortex

Cortical Module

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Striate Cortex: SummaryType Layer Input

I

II Koniocellular, IVCα, IVCβ

Orientation (simple)

Color (blobs)

Binocular (III)

V2, V3, V4, Pyramidal III

IVAPyramidal IVB IVCα Motion (complex) V2, V5(MT)Spiney IVCα Magnocellular IVB, II, IIISpiney IVCβ Parvocellular II, IIIPyramidal V ColliculusPyramidal VI LGN

}

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Beyond Striate Cortex

I. Dorsal stream A. Analysis of visual

motion and the visual control of action

II. Ventral streamA. Perception of the

visual world and the recognition of objects

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Beyond Striate Cortex

I. The Dorsal Stream (V1, V2, V3, MT, MST, Other dorsal areas)

A. Area MT (temporal lobe)1. Most cells: Direction-selective; Respond

more to the motion of objects than their shape

B. Beyond area MT - Three roles of cells in area MST (parietal lobe)i. Navigationii. Directing eye movementsiii. Motion perception

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Beyond Striate Cortex

I. The Ventral Stream (V1, V2, V3, V4, IT, Other ventral areas)

A. Area V41. Achromatopsia: Clinical syndrome in

humans-caused by damage to area V4; Partial or complete loss of color vision

B. Area IT1. Major output of V42. Receptive fields respond to a wide variety of

colors and abstract shapes

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From Single Neurons to Perception

I. Visual perceptionA. Identifying & assigning meaning to

objectsII. Hierarchy of complex receptive fields

A. Retinal ganglion cells: Center-surround structure, Sensitive to contrast, and wavelength of light

B. Striate cortex: Orientation selectivity, direction selectivity, and binocularity

C. Extrastriate cortical areas: Selective responsive to complex shapes; e.g., Faces

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From Single Neurons to Perception

I. From Photoreceptors to Grandmother CellsA. Grandmother cells: Face-selective neurons in

area IT?B. Probably not: Perception is not based on the

activity of individual, higher order cellsII. Parallel Processing and Perception

A. Groups of cortical areas contribute to the perception of color,motion, and identifying object meaning

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Concluding Remarks

I. VisionA. Perception combines individually identified

properties of visual objects B. Achieved by simultaneous, parallel processing

of several visual pathwaysII. Parallel processing

A. Like the sound produced by an orchestra of visual areas rather than the end product of an assembly line

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End of Presentation