Topic 6b: Navigation Flashcards
What two important areas involved in spatial cognition?
Hippocampus
(object-to-object navigation)
Posterior Parietal Cortex (self-to-object)
How does Hippocampal activity increases with navigation experience?
- Strong activation of right hippocampus
- Increased posterior hippocampal volume
- Storage and use of “mental maps”
- Neurogenesis in hippocampus
Neurogenesis in hippocampus
1/3 of adult cells are renewable
700 new neurons a day
Also a gradual loss of cells that are non renewable
How do Place cells in dorsal (posterior) hippocampus work?
What happens when you stop?
Hippocampal cells demonstrate “place fields” and individual cells fire sequentially for specific places
When the rat stops, the cells rapidly fire in reverse order.
Reverse replay suggests a functional role for spontaneous (resting) activity in learning and memory consolidation.
Place fields provide an internal representation of space or ‘cognitive map’.
Place cells remap when the behavioural context changes
Grid cells in the entorhinal cortex give input to …
They fire …
They receive their information from …
Place cells receive input from grid cells.
Grid cells fire in a grid-like pattern for many locations.
Grid cells of different spatial scale combine to activate place cells for unique locations.
Grid cells receive high-level spatial information from neocortex (including post. parietal cortex).
Grid cells have different spatial scales
Grid cells of different spatial scale combine to activate place cells for unique locations. Increase in grid scale along the entorhinal dorso-ventral axis
Are there grid cells in human entorhinal cortex?
Most likely. Grid-like pattern of activity in human enthorhinal cortex during virtual reality navigation task (measured wither with intracranial recordings or with fMRI).
Summary of allocentric maps
Maps
- Various maps encode information for different modalities
- Coordinate transformation needed
Hippocampus is critical for spatial memory
- Place cells fire at specific spatial locations
- Substrate for episodic/relational memory of landmarks in an allocentric coordinate system
What is the role of the Posterior Parietal Cortex?
Encodes object position relative to the body (i.e. egocentric coordinate framework)
Apraxia and Neglect
- What are they
- Damage to where cases them
- What are the symptoms
Apraxia:
- Damage to left parietal lobe.
- Difficulty coordinating sequences of movements in the absence of paralysis or muscular weakness.
- Apraxia of speech (difficulty coordinating sequences of sounds).
Neglect:
- Damage to right parietal lobe.
- Difficulty in detecting or using information about objects in external space.
Parietal lobe & spatial processing in neglect
What do the left and right PPC code for
Neglect (following right PPC lesions)
Right PPC codes both left & right visual space (full visual field).
Left PPC codes only for right visual space.
So, patients with right parietal lesions ignore stimuli to their left (only see right visual space).
Neglected information may still be available.
When patients asked to imagine the scene (Piazza del Duomo, Milan) below they can describe both sides (Bisiach 1978).
Thus, the retrieval of “what” information from an egocentric viewpoint is impaired.
PPC & Spatial Processing from Egocentric Perspective
Ventral VIP - Multimodal encoding of space & motion around the head.
Anterior AIP - Encodes touch, grasping & tool use.
Medial MIP - Encodes reaching & pointing.
Lateral LIP - Encodes intended eye movements.
PPC performs calculations, transforming sensory signals into sensorimotor representations of the body position to create an internal representation of our position in space (egocentric).
Summary for Egocentric maps
Hippocampus is critical for spatial memory
Place cells fire at specific spatial locations
Substrate for episodic/relational memory of landmarks in an allocentric coordinate system
Posterior parietal cortex is a complex association area
Multi-modal representation of space in egocentric coordinate system
Hierarchy of representations (retinal, head centred, body centred, arm centred)
Task-dependent representations
Representations updated with action into egocentric body schema
Context-dependent maps
Humans learn to use abstract representations of latent task structure to guide sequential context-dependent decisions
Spatial maps in the hippocampus and OFC are compressed to emphasize task-relevant dimension
Suggests dual functions of place cells in coding for current and prospective locations
