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March 18, 2021
  

[Brain Image]    

PSY 340 Brain and Behavior


Class 17: Visual Coding and Retinal Receptors (OUTLINE)

   

Introduction:  What does it mean to see something? What is vision about?
[Wrong Model of Vision]

  • a brain constructs/builds what someone sees by combining many different but separate elements of the scene in front of that person's eyes; construction is guided by the brain's predictive nature-its expectation of what it has seen before.
  •  coded by the visual system itself; and,
      
  • what we experience visually is NOT a reproduction of the world as it is, but the brain's best guess at what we are seeing.

Components of Vision: What Are You Seeing When You See Something?
 
[Illustration of Visual Deconstruction
              1]

[Illustration of Color Constancy]

Our visual system reveals its constructive nature when we encounter visual illusions



Skye Blue Café Wall Illusion

A further illustration of the importance of "top down" or predictive expectancy in visual perception comes from the way the visual system (which we will outline later in the class and in the next two classes) actually compresses the richness and detail of the visual information at every step as shown in the figure below.

Visual Information Load


A. The Eye and Its Connections to the Brain

[Diagram of Eye and Retinal Layers]

  The Retina (see diagram above and below)

Layers of the Retikna
   The Fovea & the Periphery of the Retina (see diagram above)

B. Visual Receptors: Rods and Cones

Diagram of Rods & Cones
Rods: respond to low levels of light (not to bright light) and most abundant in the periphery of the retina.

Cones
: respond to bright light, essential for color vision, and most abundant in the fovea. There is roughly 1 cone for every 20 rods (ca. 6 million cones and 120 million rods).

Both rods & cones contain photopigments: these are substances that release energy when they are struck by light (but are stable in the dark). Each photopigment is bound to proteins called opsins. Different kinds of opsins make the photopigments sensitive to different wavelengths of light.

Transduction: the process by which photopigments release their energy when struck by light which, in turn, activates the visual receptors = turning a physical stimulus into a neural signal


C. Color Vision

[Electomagnetic/Light spectrum]


Human beings see light with wavelengths between 350 nm (violet) and about 700 nm (red).

     [Visible vs. ultraviolet light]


The Three Present-Day Models of Color Vision

1. Trichromatic (Young-Helmholtz) Theory of Vision

[Rod & Cone Responsiveness to Different
            Wavelengths]


2. The Opponent-Process Theory of Vision

The trichromatic theory has difficulty in explaining both
[Negative afterimage-Flag]

(1) negative afterimages and

[Colorblindness]

(2) color-vision deficiencies
Ewald Hering (d. 1918) proposed the opponent-process theory of vision,
i.e., we perceive color as a set of three pairs of opposites (red vs. green; yellow vs. blue; white vs. black).

Opponent
          Processing in Vision


3. The Retinex Theory of Vision

[What Color?]
Dr. Edwin Land (Inventor of the Polaroid Camera)
Illusion of Red