Psy315 - Classes 5 and 6: The Visual System

(This page last updated 14 August, 2006.)

Properties of Individual Neurons

Inputs

• Inputs received at dendrites
• Can be either excitatory (positive) or inhibitory (negative)
• Inputs from many different synapses on dedrites are combined to determine output

Outputs

• All or none: fires only after input reaches threshold
• Action potential travels full length of axon without losing strength
• When it reaches end of axon, triggers release of neurostransmitters

The Eye

• Incoming light focused by lens
• Image projected onto retina
  • Photoreceptors: Neurons that respond to light by generating neural signal
    • Rods
      • Very sensitive
      • None at center of retina (the fovea); more at the edges (the periphery)
      • Provide information about brightness, but not color.
    • Cones
      • Less sensitive than rods
      • Highly concentrated in fovea; less concentrated in periphery
      • 3 types to provide color information
        • Called red, green, and blue, but each type responds to wide array of colors
      • Because color is coded by only 3 types of cones, perception of just about any color can be generated by mixing three colors in the right way.
        • additive color mixture: mixing different wavelengths of light
        • subtractive color mixture: mixing different pigments
• Output from photoreceptors feeds into layers of processing within retina
• Final layer within retina is retinal ganglion cells
  • center-surround organization
  • axons lead to lateral geniculate nucleus

Pathway from Eye to Cortex

• optic nerve
• optic chiasm
  • Info from one side of visual field to contralateral side of brain
• lateral geniculate nucleus (LGN) in the thalamus
  • input from each eye still kept separate
• on to area V1 in the occipital lobe

Primary Visual Cortex (area V1)

• Many monocular cells, but first stage to have binocular cells
• orientatin specificity
  • Each cell responds to a wide range of orientations, with strongest response for one specific orientation
• different cell types
  • simple: orientation at specific location
  • complex: orientation anywhere within receptive field
  • hypercomplex: like complex, but with end-stopping
    • edge must end within receptive field
• ocular dominance and orientation-selective columns

Cortical Visual Areas Beyond V1

• Area V2
• two main pathways
  • "What" pathway
    • down into inferotemporal cortex (lower part of temporal lobe)
    • object recognition
    • receives information about color, orientation, form
    • damage can cause visual agnosia
  • "Where" pathway
    • up into posterior parietal cortex (back part of parietal lobe)
    • locations of things
    • includes representation of speed and direction of motion
    • important for manipulating objects
    • damage can cause spatial neglect

     

Perception seems effortless to us, but our perceptual systems must work hard to impose an interpretation on the pereptual input.

Examples

• completion of missing contours
• ambiguous figures
  • Necker cube
  • Rubin's vase
  • duck/rabbit
  • etc.
• effect of context on identification of ambiguous letters
• low-quality figures that cannot be recognized at first
  • Once interpretation is known and in memory, it cannot be avoided.

All of these figures are ambiguous at first, but eventually your visual system is able to go beyond the information given to come up with a good interpretation.

 

next class: Word Recognition

Visual Illusion Web sites you may find interesting:

• Optical Illusions and Visual Phenomena
• Visual Illusion Forum
• Illusionworks
• Illusions Gallery
• Akiyoshi's Illusions
• a collection of 3-D stereoscopic images

Psych 315: Cognitive Psychology Kyle Cave Psychology Dept. U. Mass.