Central Visual Processes

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Central Visual Pathways

I. Primary Visual CortexReceptive Field

• Columns• Hypercolumns

II. Spatial Frequency

III. Nerve or Cortical Damage

IV. Higher Visual Areas

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Occipital Lobe

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Occipital Lobe: Calcarine Sulcus -- V1 -- Striate Cortex

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Cells in V1

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Single-Cell Recording (Hubel & Weisel, 1962)

• Attempted to discover what sorts of information cells in (cat) V1 respond to

• Accidentally discovered orientation specific cells organized into columns and hypercolumns

V1

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Cells In V1

Cells in V1 receive messages from certain ganglion cells such that they respond to stimuli of a certain orientation from a small portion of the retina - Orientation Specific

~ 200 Million Cells in V1

Inputs from Ganglion Cells

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Cells In V1

• One V1 cell receives inputs from many ganglion cells

• One ganglion cell may send inputs to numerous V1 cells

• Stimuli from every possible orientation, and from every position in the visual field are detected by different cells in V1

• Simple Cells detect only orientation -- Complex Cells detect orientation and motion

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How to Make a Complex Cell

• Orientation specific inputs from ganglion cells is similar to simple cells

• However, the receptive field is much larger and is designed to respond maximally when inputs from sub-fields are sequential

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Cells in V1

Occular Dominance

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Columns in V1

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Organization of Cells in V1• Columns are sections of cortex which all respond to

the same orientation from approximately the same region of cortex

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Organization of Cells in V1• Hypercolumns are groups of columns, from both

eyes, which are influenced by the same minute portion of the visual field

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Organization of Cells in V1• What sort of information are these cells detecting? • Is the information from any single cell in V1

informative?

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Response Properties of Cells in V1The extent to which columns will respond to stimuli with no interactions from other columns

Orientation Column Position on Occular Dominance Slab

Cel

lula

r A

ctiv

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Lateral InhibitionThe column with the strongest response to a given stimuli will suppress the respondse of neighboring columns

+

-

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Response Properties of Cells in V1The extent to which columns will respond to stimuli with lateral inhibition from other columns

Orientation Column Position on Occular Dominance Slab

Cel

lula

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ctiv

ity

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Processing at V1 Is Edge Detection

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Edge Detection

• While lateral inhibition normally improves the accuracy of edge detection, in this case it creates the “Deli Wall Illusion”

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Understanding Acuity: Spatial Frequency Analysis

• Measuring visual acuity:

– Eye doctors use distance (e.g., 20/20)

– Vision scientists use visual angle

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Understanding Acuity: Spatial Frequency

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Understanding Acuity: Spatial Frequency

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Describing Processes in V1: Spatial Frequency Analysis

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Describing Processes in V1: Spatial Frequency Analysis (cont.)

• Orientation

• Frequency

• Contrast

Orientation

Decreasing Contrast

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Spatial Frequency Analysis (cont.)

• Fourier - French mathematician, came up with theory that one can create any complex wave through a summation of Sinusoids (or sub-parts, sub-waves)

• Fourier Analysis divides all orientation specific cells in V1 according to the width of their receptive fields or Spatial Frequency

• 1) Low

• 2) Medium

• 3) High

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Spatial Frequency Analysis (cont.)• Neurons can then be viewed as Spatial Filters which

separately analyze differing levels of detail or scale

• Any scene can then be decomposed into images with varying spatial frequencies - low frequency images are blurry and only the most prominent features are represented - high frequency images exaggerate the fine details

• Construing form vision in terms of an emergent property of these different scales of receptors is referred to as the Multichannel Model

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Spatial frequency Analysis (cont.)

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Spatial frequency Analysis (cont.)

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Spatial frequency Analysis (cont.)

• Once divided by width, cells can further be grouped according to their orientation specificity

• This allows a vastly simplified organization of neural activity - 3 major variables - Spatial Frequency, Orientation & Contrast

• Additionally, Fourier analysis helps explain how individual cells may contribute information to the aggregate

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Spatial frequency Analysis (cont.)

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Spatial frequency Analysis (cont.)

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Spatial Frequency Analysis (cont.)

• 1f gives the fundamental waveform

• 2f ... xf : are called harmonics - increasing details

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Spatial Frequency Analysis (cont.)

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Spatial Frequency Illusions

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Spatial Frequency Illusions

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Spatial Frequency Illusions

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Spatial Frequency Illusions

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Color at V1

• Among cells selective for orientation are patches of cells selective for color (and not orientation), which are known as Blobs.

• Other cell (orientation specific cells) regions are known as interblobs.

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Organization of V2

• Thin Stripes receive information from Blobs and pass it to V4

• Thick Stripes recieve information from complex cells and send it to V5 and V3

• Interstripes recieve information from simple cells and send it to V3 and V4

• Information at V2 is 3-D

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Nerve or Cortical Damage1) Retina / Optic Nerve2) Optic Chiasm3) Optic Tract4) V1/V2

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Nerve or Cortical DamageRetina/Optic Nerve: Monocular blindness

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Nerve or Cortical DamageOptic Chiasm: Nasal field blindness

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Nerve or Cortical DamageOptic Chiasm: Bitemporal field blindness

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Nerve or Cortical Damage

Optic Tract/LGN/Radiations: Homonymous Blindness

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Nerve or Cortical DamageV1: Quadrantic blindness

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Nerve or Cortical Damage

V1/V2:

• Scotoma

• Complete blindness

• case of Blindsight

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Higher Visual Areas

• V3: Form & Dynamic Form

• V4: Color

• V5: Motion

• IT: What System: Object Recognition– Lingual Gyrus of IT: Face Recognition

• PP: Where System: Object Location and Navigation

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Simplified Functional Visual Anatomy

Type Lobe Area Function

Primary Occipital V1 (17) Initial Processing

Secondary Occipital V2 (18) 3-D Form

Secondary Occipital V3 (19) Dynamic Form

Secondary Occipital V4 (19) Color & Form

Secondary Occipital V5/MT (37) Motion

Tertiary Temporal IT (20, 21, 22) "What"

Tertiary Parietal PP (7) "Where"

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Simplified Flow Diagram of the Visual System

LGN

Parvo

Magno

Thalamus Occipital Lobe

Temporal Lobe

Parietal Lobe

V1 V2

V3

V4

V5 or MT IT

Optic Nerve

PP

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Summary

Color Form Motion

Eye / LGN P P M

V1 Blobs Simple Complex

V2 Thin Stripes Interstripes Thick Stripes

V4Form &Color

V3Dynamic

Form

V5Motion

} } }

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