Mind, Brain

1
Mind, Brain Behavior

• Wednesday
• February 12, 2003

2
How is Information Encoded?

• Patterns of discharge of action potentials
• Frequency code -- strength is conveyed by
  frequency (faster rates of firing)
• Population code strength is also conveyed by
  how many neurons are responding within a group
• Slow vs fast adapting receptors differentiate
  duration of a stimulus.
• Labeled line code -- modality is encoded by the
  particular receptor type.

3
Sharpening of Stimuli

• Lateral inhibition a mechanism that combines
  excitatory and inhibitory components to enhance a
  signal.
• The difference between stimulation of one part of
  a receptive field and another is strengthened.
• Reduces the likelihood that a stimulus at the
  edges of a field will activate a relay neuron.
• Enhances the acuity of the sensory system.

4
Modality-Specific Mechanisms

• In addition to these shared properties, different
  sensory modalities perform different functions
  and have mechanisms for doing so.
• Specific receptors in somatic sensory system are
  specialized for different kinds of input.
• Population codes are most important to taste and
  smell.
• Timing and intensity of signals is important to
  localizing sounds in space.

5
Construction of the Visual Image

• Chapter 21

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Gestalt Principles of Organization

• Visual perception is a creative process.
• Gestalt laws describe how the brain builds
  percepts from details of an image
• Similarity
• Proximity
• Figure-ground (winner-take-all strategy)
  reversible figures
• Good continuation (creative filling in)
  triangle illusion

7
Other Illusions

• Illusions are misreadings of visual information
  that reveal how the brain applies assumptions to
  the information it receives.
• Muller-Lyer illusion
• Size constancy illusion
• Ames Room

8
The Visual Pathway

• Information goes from the retina to the LGN
  (thalamus) to the visual cortex (striate cortex).
• From the striate cortex information goes forward
  to adjacent areas of the cortex (extrastriate
  cortex) where motion, depth, color, form and
  pattern are processed.
• Pathways to the striate cortex are sequential.
• Pathways past the striate cortex are parallel.

9
Retina

• Two kinds of photoreceptor cells
• Cones wavelength specific (perceive color) and
  detail
• Rods perceive motion, require less illumination
  and see in black and white
• Cones are concentrated in the fovea.
• Rods are concentrated on the periphery.
• Cones and rods send axons to ganglion cells.

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Types of Ganglion Cells

• Magnocellular (M cells) large cells that
  receive input from rods.
• Parvocellular (P cells) small cells that
  receive input from cones.
• Blob pathway concerned with color perception.
• Interblob pathway concerned with shape/form.
• Both types synapse on layers within the LGN
  (lateral geniculate nucleus) of the thalamus.

11
Mapping Within the LGN

• Optic nerve carries information from ganglia to
  LGN. Crosses at optic chiasm.
• Separate layers are maintained for each eye and
  for each type of cell (M and P).
• Interneurons project from areas of the LGN to
  striate cortex.

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Mapping in the Striate Cortex

• Separate layers from LGN to striate cortex are
  maintained in ocular dominance columns.
• M and P cells enter the cortex at different
  levels of layer 4 of the visual cortex.
• Information is combined by pyramidal cells that
  synapse at higher levels in the striate cortex.
• Input from both eyes is combined at layer 3.

13
Extrastriate Pathways

• Parallel processing of visual information from
  the striate cortex.
• Three pathways
• Color processing P blob cells, goes from V1 to
  V2, then V4, then inferior temporal cortex.
• Shape processing, depth perception P interblob
  cells, goes from V1 to interior temporal cortex.
• Motion spatial relations M cells, V1 to V2,
  then MT (V5), to parietal cortex.

14
Equiluminance

• Brightness is held constant permits study of
  the contribution of color to perception.
• Results
• Brightness, not color, is important to motion
  detection, perspective, relative sizes, depth
  perception, figure-ground relations, visual
  illusions.
• Motion is a cue for distinguishing among objects.
• Things that move together belong together.

15
Visual Agnosias

• Existence of distinct agnosias for aspects of
  perception suggests that these abilities are
  localized to areas selectively damaged.
• Achromatopsia good perception of form despite
  inability to distinguish hues.
• Prosopagnosia inability to recognize faces as
  particular people (identity). Can recognize that
  it is a face, and tell the parts.

16
Binding Mechanisms

• How is information from the separate, parallel
  pathways brought together and associated?
• Treisman Julesz combination requires
  attention.
• A pre-attentive process detects the major outline
  of an object.
• An attentive process notices, selects
  highlights combinations of features.
• Maintained in separate global and detailed maps.