Visual Computation I. Physiological Foundations II. Elements of Visual Computation III. Synchronized Oscillation in Visual Cortex

I. Physiological Foundations Visual perception: “What you see is not what is out there in the outside world, but instead what has been created all inside of your head as the nervous system’s response to external stimuli.”

“Visible” Spectrum Not all ranges of the light spectrum is perceived. Why?

The Eye

The Retina

Steps of Visual Computation S1: Rod & Con cells (light detection) S2: Horizontal & Bipolar cells (preprocessing) S3: Ganglion cells (preprocessing) S4: Lateral geniculate nucleus (LGN) cells (relay station) S5: Visual cortex (conscious perception)

Rods and Cons Rods: Cons: - Discriminate B/W brightness in low illumination - Contribute to peripheral vision Cons: - Discriminate Colors - Contribute to central vision

Three Types of Con Cells

Each wavelength gives rise to a different combined activity pattern of the three cone types, thus giving experience of different colors. Why three cone types? Why not one or two or even several? What would be like one-cone perception? Color-misnomer phenomenon.

One Type of Con Cells

Two Types of Con Cells

II. Elements of Visual Computation 1. Receptive field (RF) 2. ON-center/OFF-surround RF 3. Edge detector 4. Orientation detector 5. Location-invariant detector 6. Ice-cube model of visual cortex 7. Color computation

1. Receptive Field The receptive field (RF) of a neuron is the area of retina cells that trigger activity of that neuron.

2. On-center/OFF-surround RF

3. Edge Detector

4. Orientation Detector (Huble & Wiesel, 1959)

Hypothetical Wiring Diagram

5. Location-invariant Detector

6. Ice-cube Model of Visual Cortex

Orientation Selectivity of Cortical Neuron

Face-detecting “Grand Mother” Neuron

7. Color Computation Transformation: (B,G,R) -> (W, B-Y, R-G) -- no information loss, then why?

Take-home Message: “What you see is not what is out there in the outside world (ie., not like “taking a picture”), but instead a result of visual computation -- only those computations that are critical for survival, shaped by the evolution.”

What about Illusions? -> “Unintended” consequences of the computation

III. Synchronized Oscillation in Visual Cortex 1. Sensory Binding Problem 2. Neural Assemblies 3. Synchronized Firing

1. Sensory Binding Problem In the brain, spatially different areas are functionally specialized for processing visual attributes such as shape, color, orientation, and direction of motion. Binding Problem: How does the brain then “bind” together the sensory attributes of an object to construct a unified perception of the object?

Binding Problem

2. Neural Assemblies Neural assembly: a group of neurons whose activities are highly correlated/inter-dependent to one another.

3. Synchronized Firing Neural synchrony: in which neurons in a neural assembly tend to fire in phase-lock fashion or in unison.

Global Synchronization via long-range excitatory connections

… Meaning that the following was the input stimulus,

Synchronized Neural Assembly Coding ->Solution to the Binding Problem Two or more assemblies, each of which is tuned to a particular feature/attribute present in a stimulus object, contribute to the representation of that object by jointly firing synchroniously and simultaneoulsy. Biological plausibility: well-documented (e.g., Gray et al, 1989).

Vision is a computation.