THE VISUAL SYSTEM: EYE TO CORTEX Outline 1. The Eyes

a. Structure b. Accommodation c. Binocular Disparity

2. The Retina a. Structure b. Completion c. Cone & Rod Vision d. Eye Movements

3. Visual Transduction by Rhodopsin 4. From Retina to Primary Visual Cortex

THE PERCEPTION OF CONTRAST AND COLOR Outline 1. Contrast: The Perception of Edges

a. Lateral Inhibition: The Physiological Basis of Contrast Enhancement

2. Brightness-Contrast Detectors in the Mammalian Visual System

a. Mapping Receptive Fields b. Receptive Fields of Neurons in the Retina-Geniculate-Striate

Pathway c. Simple Cortical Cells d. Complex Cortical Cells e. Hubel and Wiesel's Model of Striate-Cortex Organization

3. Seeing Color a. Component Theory b. Opponent Process Theory

Receptive Fields of Neurons in the Retina-Geniculate-Striate Pathway

Most retinal ganglion cells, lateral geniculate nucleus neurons, and the neurons in lower layer IV of the striate cortex have similar receptive fields

They are circular They are smaller in the fovea area (larger in the

periphery) Indicative of greater acuity in the fovea

They are monocular They have both an excitatory and an inhibitory area

separated by a circular boundary. Neurons in these regions have 2 patterns of

responding: (1) on firing (2) inhibition followed by off firing

Simple Cortical Cells

Respond best to bars or edges of light in a particular location in the receptive field and in a particular orientation (e.g., 45 degrees) All simple cells are monocular They also have on and off areas

Complex Cortical Cells Most of the cells in the striate cortex are

complex cells more numerous like simple cells in that they respond best to

straight-line stimuli in a particular orientation unlike simple cells in that the position of the

stimulus within the receptive field does not matter

the cell responds to the appropriate stimulus no matter where it is in its large receptive field

Over half of the complex cells are binocular, and about half of those that are binocular display ocular dominance

Hubel and Wiesel's Model of Striate-Cortex Organization When you record from visual cortex using

vertical electrode passes, you find (1) As the electrode moves up or down through the

layers of the striate cortex it finds neurons that respond to stimulation from about the same location on the retina

(2) simple and complex, cells that all prefer the same orientation – the cells respond to line orientations that are at the same degree

Info. flows from on and off cells in lower layer IV to simple cells and then to complex cells

(3) binocular complex cells that are all dominated by the same eye (if they display ocular dominance)

Hubel and Wiesel's Model of Striate-Cortex Organization

When you record from visual cortex using horizontal electrode passes you find (1) receptive field location shifts

slightly with each electrode advance (2) orientation preference shifts slightly

with each electrode advance (3) ocular dominance periodically shifts

to the other eye with electrode advances

CORTICAL MECHANISMS OF VISION Outline 1. The Parallel, Functionally Segregated

Hierarchical Model of Perception 2. Cortical Mechanisms of Vision

a. Scotomas and Blindsight b. Completion c. Secondary Visual Cortex and Association

Cortex d. Visual Agnosia

3. Selective Attention

The Current Parallel, Functionally Segregated Hierarchical Model of Sensory-System Organization

Parallel: sensory systems are organized so that information flows between different structures simultaneously along multiple pathways

Explicit –– a sensory system for consciously seeing things Implicit –– a sensory system that helps us interact with

objects in space. Functionally Segregated: There are divisions of labor

at the different hierarchical levels Some neurons respond to motion, some to color, and so

on. Hierarchical: information flows through brain

structures in order of their increasing neuroanatomical and functional complexity.

Primary Secondary Association

Vertemnus (1591) by Guiseppe Arcimboldo

The Vegetable Gardener (1590)

Neural Correlates? There is evidence that selective attention occurs by

strengthening the neural responses to attended-to aspects and by weakening the response to others.

Anticipation of a stimulus increases neural activity in the same circuits affected by the stimulus itself.

When attempting to recognize faces - ventral stream is activated

Does this face belong to the same person? face recognition activates the ventral visual pathway – is this face

the same person you saw before? fMRI

When attending to location - dorsal stream is activated. Is this face in the same position as the last face you saw?

face position activates the dorsal visual pathway – is this face in the same location as the face you saw before?

fMRI

Primates Recorded from neurons in the prestriate

area that is part of the ventral stream that are particular responsive to color.

Some cells responded to red; some to green Monkeys were taught to perform a task that

required attention to a red cue. The activity of red neurons was increased,

whereas the activity of green neurons was reduced