LECTURE 8

Question 1

The “Where” Pathway

Answer 1

(AKA Dorsal Visual Stream)

• Projects from primary visual areas to parietal regions
• We need to know about “space” for a variety of purposes
• This is accomplished with a variety of tightly interconnected dorsal processing streams

Question 2

Three pathways project from the
dorsal stream:

Answer 2

• Parietal cortex → prefrontal cortex [Supports spatial working memory.]
• Parietal cortex → premotor cortex [Supports visually guided actions]
• Parietal cortex → medial temporal cortex [Supports spatial navigation]

Question 3

Response Properties of Neurons in the Dorsal
Stream

Answer 3

• NOT very sensitive to color or form
• Cells in PPC most responsive to combination of retinal location of object & position of eyes/head
• Cells seem responsive to specific direction of motion
• Many cells in PPC are joint sensorimotor cells
• Fire for both motor movement and touch/sensation

Question 4

How does the brain process the world in 3D?

Answer 4

• Left vs. right
• Depth perception

Question 5

Distinguishing Left from Right

Answer 5

• Left and right are inherently relative terms
• Trickier than up vs. down
• Spatial coding of left vs. right can be specifically disrupted

Question 6

Depth Perception

Answer 6

• Depth perception helps us to localize items in the near-far plane
• Depth not provided directly by the retinotopic map, but is computed
  using:
• Binocular cues
• Monocular cues

Question 7

Binocular disparity

Answer 7

• discrepancy between the images seen by the two eyes
• Binocular depth perception

Question 8

Motion parallax

Answer 8

• As you move through environment, near objects are displaced more on retina than objects that are far away
• Monocular depth cue

Question 9

Parietal Damage/Lesions

Answer 9

• Results in impaired depth perception
• However, there haven’t been studies where ONLY depth perception was affected
• Suggests that maybe depth coding doesn’t rely on one single region
• Occurs throughout the dorsal stream

Question 10

Spatial Frames of Reference

Answer 10

• Neural coding of reference frames:
  1.Egocentric reference frame: describe space in relation to you
  2. Allocentric reference frame: describe space in relation to the room/environment/other object
• Dissociability of reference frames
• Categorical vs. coordinate spatial relations

Question 11

Neural coding of reference frames

Answer 11

• Spatial representations encoded in PPC go beyond eye-centered representations in primary visual cortex
• Some cells respond selectively to egocentric vs. allocentric frames of reference
• E.g. One cell might fire when an object is to your left, another might fire when an object is to
  the right of another object (even when you change your position)
• Different subpopulations of PPC cells participate in coding space for different
  functions
• E.g. eye-centered (important for navigating using your eyes) or head-centered
  representations (important for navigating using your body)
• Coding of location is a dynamic process that must constantly take into account the
  changing position of the eyes
• Always updating and integrating signals

Question 12

Categorical vs. coordinate spatial relations

Answer 12

Categorical spatial relations (dichotomous)
* Position of one location relative
to another
* above versus below
* top versus bottom
* front versus back
* left versus right
* LH specialization

Coordinate (or metric) spatial relations
* Distance between two locations
* RH specialization

Question 13

Motion Perception

Answer 13

MT (middle temporal or V5): involved in motion perception
MST (medial superior temporal): involved in complex
motion perception, like optic flow

Question 14

Motion Perception

Answer 14

• Motor regions of the brain could let the visual system know when eye
  movements are planned (corollary discharge)
• Corollary discharge is a hypothetical signal
• Sensory receptors at eye muscles could provide information about changes in
  eye position

provide the visual system with information about eye
movements to tell the difference between if the eye is moving or if its the object

Question 15

Optic Ataxia

Answer 15

• Impairment in visually guided reaching
• Linked to parietal lobe damage

Question 16

Spatial Navigation

Answer 16

Two basic strategies for spatial navigation:
* Route-based - Sequence of steps
* Cognitive-map-based - Allocentric understanding of landmarks

Question 17

Spatial Navigation and Neural Coding

Answer 17

• Parahippocampal place area (PPA): encodes landmarks
• Retrosplenial complex (RSC): determine location of self
• Medial temporal lobe (MTL): map-like represenations

Question 18

READ LEARNING THE PPA

Answer 18

• PPA strongly associated with spatial
  navigation
• Connected with abstract
  representation of landmarks

SLIDES IN CANVAS

Question 19

Retrosplenial Cortex

Answer 19

Closely connected with other spatial navigation areas (PPC & MTL)
* Damage leads to problems with spatial navigation
* May encode direction one is facing (heading)

Question 20

Medial Temporal Lobe (MTL)

Answer 20

• Closely tied to memory
• …but then again, so is spatial navigation
• Damage to hippocampus in rats causes problems with environmental navigation
• “Place” cells in the hippocampus

Question 21

Place Cells (in the hippocampus)

Answer 21

• A place cell is active when the animal is located in a particular place in its local environment
• Regardless of heading or the route the animal took to get there

Question 22

“Head direction” cells

Answer 22

Respond when animal is facing a particular direction