Mind, Brain

1
Mind, Brain Behavior

• Monday
• February 17, 2003

2
Retina

• Two kinds of photoreceptors (Table 22-1)
• Cones wavelength specific (perceive color) and
  detail, see in daylight, detect fast flicker
• 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.

3
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.

4
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.

5
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.

6
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.

7
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.

8
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.

9
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.

10
Edge Detection

• Ganglion cells respond to contrast and change in
  visual input.
• Center-surround (on-off) receptive field.
• Bipolar cells also have center-surround receptive
  fields.
• Neurons in the visual cortex have rectilinear
  receptive fields with excitatory and inhibitory
  zones.
• Complex cells provide positional invariance.

11
Complex Forms, Motion

• Processing of form occurs outside the visual
  cortex inferior temporal cortex.
• Not organized retinotopically.
• 10 selective for specific images (hands, faces).
• Processing of motion occurs in middle temporal
  area (MT or V5), then parietal lobe.
• Used for seeing moving objects, pursuit eye
  movements, guidance of bodily movement