Outline Of Today’s Discussion 1.LGN Projections & Color Opponency 2.Primary Visual Cortex: Structure 3.Primary Visual Cortex: Individual Cells

Part 1 Lateral Geniculate Nucleus (LGN) Projections & Color Opponency

LGN Projections Let’s trace the visual information beyond the retinal ganglion cells’ …along the optic nerve (cranial nerve II).

LGN Projections 1.The temporal projections stay on “their side”, but the nasal projections criss-cross at the optic chiasm. 2.After the optic chiasm, the optic nerve is called the “optic tract”. 3.There is a left and a right optic tract, each projects to its respective Lateral Geniculate Nucleus (LGN).

Horizontal View of the Brain

Understanding The Projections

LGN Projections 1.The LGN has six layers, neatly organized. 2.For a given LGN, ipsilateral (‘same side’) inputs project to layers 2, 3, and 5. 3.For a given LGN, contralateral (‘opposite side’) inputs project to layers 1, 4, and 6.

Schematic of The Right LGN

LGN Projections 1.Neighboring areas of the LGN correspond to neighboring areas on the retina. 2.The LGN is therefore said to be “retinotopic”.

Schematic of Retinotopy in the LGN

Color Opponency in LGN 1.Many cells in the LGN exhibit center-surround antagonism based on color. 2.Some of these cells are red-green opponent, and others are blue-yellow opponent. 3.As we have seen before, other LGN cells are simply luminance opponent (not color opponent).

Red-Green Opponent Mostly “Parvo” Blue-Yellow Opponent Mostly “Parvo” Luminance Opponent Mostly “Magno” Color Opponent LGN Cells

Color Opponency in LGN 1.Color opponent LGN cells help to identify color-based edges. (Example: red berries on green leaves.) 2.Cells in the LGN project via that “Optic Radiations” to the primary visual cortex.

Horizontal View of the Brain

Part 2 Primary Visual Cortex: Structure

Primary Visual Cortex (Structure) 1.The primary visual cortex is located in the occipital lobe. (Near the back of your brain). 2.The primary visual cortex is known by several different names: Striate Cortex; “V1”“Brodmann’s Area 17”. 3.A person can be optically fine, but cortically blind.

Primary Visual Cortex (Structure) 1.V1 is organized in a topographic fashion (i.e., preserves spatial relationships). 2.The left visual field maps to the right V1: The right visual field maps to the left V1. 3.The upper visual field maps to the lower portion of V1: The lower visual field maps to the upper portion.

Primary Visual Cortex (Structure) 1.Like the LGN, area V1 over-represents the fovea. 2.This is called “cortical magnification”.

Cortical Magnification

Part 3 Primary Visual Cortex: Individual Cells

Primary Visual Cortex (Cells) 1.In V1, three classes of cells are widely recognized. 2.The types are distinguished from each other by their receptive field properties. 3.The classifications are “Simple Cell”, “Complex Cell”, and “Hyper Complex Cell”. 4.We’ll discuss each, in turn…

Primary Visual Cortex (Cells) 1.Simple cells have sub-divisions within their receptive fields (just like LGN cells). 2.The sub-divisions are such that the cells respond to stimuli in a position-sensitive manner, within their receptive fields. 3.We will soon learn that position in the space domain corresponds to “phase” in the frequency domain. 4.For this reason, simple cells are said to be phase sensitive (i.e., their response depends on where the stimulus is positioned in the receptive field).

On + Off - Off - Simple Cells Have Sub-Fields

On + Off - Off - What Stimuli Would This Cell Like?

On + Off - Off - This cell would “Love” this stimulus

On + Off - Off - This cell would “Hate” this stimulus

Primary Visual Cortex (Cells) 1.A simple cell’s response depends on the stimulus position (also called it’s “phase” in the frequency domain). 2.By contrast, a complex cell’s response is NOT position- sensitive. 3.Complex cells do NOT have distinct “On-versus-Off” regions.

On or Off Complex Cells Have Homogeneous RFs On or Off On or Off

What Stimuli Would This Cell Like? On or Off On or Off On or Off

This cell would “Love” this stimulus On or Off On or Off On or Off

But also would “Love” this stimulus On or Off On or Off On or Off

Primary Visual Cortex (Cells) 1.So, complex cells differ from simple cells in that only simple cells are position-sensitive (“phase sensitive”). 2.Simple and Complex cells are similar to each other in that they don’t “care about” (i.e., differentially respond to) stimulus length. 3.Hyper-complex cells DO “care about” (respond to) stimuli in a length-specific manner.

On + Off - Off - Some Hyper-complex Cells Have Heterogeneous RFs

On or Off Other Hyper-complex Cells have Homogeneous RFs On or Off On or Off

But All H.C. Cells are “End-Stopped”

They would respond to this stimulus On or Off On or Off On or Off The ends of the stimuli stop within the RF.

But not to this stimulus On or Off On or Off On or Off The ends of the stimuli exceed the RF.

Primary Visual Cortex (Cells) 1.That was the concept of end-stopping. 2.End-stopping (i.e., length selectivity) is the defining characteristic of Hyper-complex cells. 3.Let’s summarize the differences among the three cell types…

V1 Cell Type Summary

Primary Visual Cortex (Cells) 1.Many V1 Cells (or any type) respond to inputs from the two eyes. 2.V1 is the earliest point of “binocular convergence”. 3.This binocularity makes it possible to see stereoscopic depth (i.e., depth that arises solely from the difference between the two retinal images).

Primary Visual Cortex (Cells) 1.V1 is also the earliest area in the visual pathway that responds to orientation. 2.Orientation selectivity is a property of almost all V1 cells. 3.Direction selectivity also first appears in V1.

Many V1 Cells Are Orientation Tuned

Oblique Orientations Are Under-Represented in V1 Potential Pop Quiz Question: What is the oblique effect behaviorally, and what is its neural basis?

The Tilt Aftereffect Explained

Primary Visual Cortex (Cells) 1.Within the cortex, orientations are neatly organized into “orientation columns”. 2.Similarly, cortical cells are also neatly organized into “occular dominance” columns. (A given cell may be more influenced by, say, left-eye input than by right-eye input. 3.A complete set of orientations and occularity for a given retinal location is called a hyper-column.

Schematic of V1 Organization