CBNS 106 Final Lecture 6 V1 and Beyond

Orientation Selectivity in V1 (LGN vs V1)

Recall LGN

a) Magno and Parvo

b) light sensitive DETECTS CONTRAST

V1

a) responds best to bars of light, edges, and specific orientation DETECTS ORIENTATION

• ex:

  • One neuron:

    • Strong response → vertical line

    • Weak response → horizontal line

    Another neuron:

    • Strong response → horizontal line

    • Weak response → vertical line

Simple vs Complex cells

Simple Cells (locational specific)

a) Respond to:

• edges

• bars

• specific orientations

• specific locations within the receptive fields

b) have distinct ON and OFF regions

Complex Cells (non locational)

a) Respond to:

• oriented edges

• specific orientations

b) Spatial invariance= doesn’t care where the stimulus is located within the receptive field, will fire if the correct edge appears anywhere within its receptive field

Comparison ex

Simple Cell

Correct orientation
+
Correct position

= Responds

Complex Cell

Correct orientation

↓

= Responds regardless of exact position

Orientation tuning

a) Orientation tuning curve= measures neuron firing rate vs. stimulus orientation

b) Emergence of orientation tuning= several LGN neurons with center-surround receptive fields project onto 1 V1 Neuron

Functional Modules in V1 Level 1(Orientation and Ocular Dominance columns)

Level 1: Cortical Columns

a) definition: a vertical stack of neurons that extends through multiple layers of V1, neurons within a column respond to similar visual features

1. Orientation Columns

a) Definition: Neurons prefer the same orientation

• ex:

  • One column:

    • Responds best to vertical lines

    Adjacent column:

    • Responds best to slightly tilted lines

    Next column:

    • Responds best to horizontal lines

b) Importance: Allows V1 to analyze every possible edge orientation in the visual scene

• ex: Your brain can detect

  • Vertical edges

  • Horizontal edges

  • Diagonal edges

all at the same time.

2. Ocular Dominance Columns

a) Definition: Columns that preferentially receive information from one eye. Although both eyes view the same scene, V1 initially keeps eye inputs somewhat separated.

• ex:

  • Left-Eye Column: Responds more strongly to left-eye input.

  • Right-Eye Column: Responds more strongly to right-eye input.

b) Importance: Helps the brain compare information from both eyes.

• essential for binocular vision and depth perception

Functional Modules in V1 (Blobs and Columns)

Level 2: Blobs

a) Definition: Specialized regions in V1 involved in color processing

b) Receive Strong Koniocellular Input

• Blob neurons:

  • Respond strongly to color

  • Show weak or absent orientation tuning

Level 3: Hypercolumn

a) Definition: A hypercolumn is the complete processing unit for one small location in visual space.

• Contains:

  • Left-eye ocular dominance columns

  • Right-eye ocular dominance columns

  • Full set of orientation columns

  • Color blobs

b) Importance:

• Orientation: Vertical, horizontal, diagonal

• Eye of Origin: left or right eye

• Color: red, green, blue

Parallel Processing Beyond V1

Three parallel Pathways

1. Motion: Magno dominated (V1-V5/MT)

2. Color: Blob/K pathway

3. Shape/Object: Parvo dominated (V1-V2-V4-IT)

IT

What is it? Major area for object recognition