Representations of Space in the Brain

Today's focus: representations of space in the brain, going beyond immediate surroundings.
How we navigate the world.
Animal work (rodents) is foundational.

Key Concepts

Cognitive maps.
Grid cells.
Head direction cells.
Border/boundary cells.
Parahippocampal place area.
Distance coding in Hippocampus.
Place cells in human hippocampus.
Allocentric heading in retrosplenial complex.
Brain regions for different navigation events.
Hippocampal plasticity and expertise.

Scene Memory and Spatial Navigation

Scene memory is linked to spatial navigation.
Knowing object locations is crucial.
Supports decisions on interacting/avoiding objects.
Viewpoint dependent: direction of heading.
Understanding locations beyond immediate view is vital.
Episodic memory is involved.
Necessary for exploring and building a sense of a space.
Plays a key role in future planning (finding way back home or to food).

Cognitive Maps

Representation like a physical map for orientation.
Proposed by Tolman in the 1940s (rats in mazes).
Rats adapted routes when the preferred path was blocked.
Implies a mental representation, not just procedural memory.
Spatial knowledge allows flexible navigation.

Place Cells

Discovered by O'Keefe and Dostrovsky in 1971 in the hippocampus.
Nobel Prize for the discovery of place cells.
Neurons fire vigorously when the rat is a specific location.
Experimental setup- Landmark on the wall of a circular space. An electrode is recording signal from the rat running in the space.
Neurons respond specifically to locations.
Not based on animal's perspective (allocentric).
Coordinates are relative to physical surroundings.

Grid Cells and Head Direction Cells

Grid cells: respond throughout space in a hexagonal lattice pattern (medial entorhinal cortex).
Not a specific location, but a range of locations in space.
Head direction cells: fire based on the direction the rat is heading.
Border/boundary cells: fire at a set distance from boundaries when facing a direction.
Multiple neural representations combine for a complete spatial picture--building a sense of a space and where they are , how they're facing , and how they're moving through that space.

Human Brain Implications

Do these findings apply to humans?
Similarities in rat and human brain structures (hippocampal formation, Papez circuit).
Amygdala, hippocampus, fornix, and mammillary body are important for learning and memory.
These structures are similar to rats where place cells where found.
Damage to human brain areas equivalent to rat areas results in memory function issues.
Spatial functioning in humans may be supported by these areas.

Challenges in Human Studies

Navigation is an inherently mobile task.
Humans can't move in MRI machines.
fMRI studies use virtual navigation, imagined navigation, or spatial memory recall to simulate the process.

Parahippocampal Place Area (PPA)

Discovered by Kanwisher (fusiform face area).
Region preferentially processes scenes.
Located along the parahippocampal gyrus and collateral sulcus.
Proximity to spatial processing areas makes sense.
The PPA responds to scenes, including landscapes, cityscapes, rooms, and tabletop scenes

PPA Encoding

Encodes the global geometry/shape of space.
Sensitive to scene configuration: spatial layout determined by fixed surfaces.
The PPA exhibits the biggest response to the scene and the second biggest to the background. But if the background is removed representing the shape, there is lower response.
Anything that gives you a representation of space is activating the PPA.
Boundary junctions are important - perception of surfaces is sufficient.
The PPA's response is lower if boundaries become objects.

Natural Scenes and Object Location

The PPA encodes the global spatial layout of a scene.
Lateral occipital complex (LOC) and fusiform gyrus locate local objects.
Damage to the PPA: patients see objects but lack a sense of their spatial relations.

Distance Coding

Evidence from fMRI adaptation studies.
Repeated stimuli lead to weaker brain area response.
Subjects familiar with the University of Pennsylvania viewed landmarks.
Task: covertly identify landmarks by pressing a button
Speed and neural response were related to the proximity of the preceding landmark.
If landmarks were spatially separated, the neural response was high.
If landmarks were close, the response was lower--somehow activating the location in the cognitive map associated with the prior landmark generates a lower response because the brain area has already been activated.
fMRI activity in the hippocampus scaled with the distance between the buildings.
Closer buildings were representationally similar.
It's kind of like border boundary cells in the rat.

Landmark Identity vs. Location

Researchers looked at neural responses for specific landmarks.
Tried to determine which landmarks (neural response in the brain) were associated with location versus identity.
Hippocampus: representation related to location, not landmark identity.
Other regions(parahippocampal place area, early visual areas, lateral occipital, and retrosplenial cortex) were associated with identity.
Temporal area: neural responses about object location and object representations.

Direct Neural Recording

Performed using ECOG in pre-surgical epilepsy patients with implanted electrodes.
Recorded from 317 neurons in the medial temporal and frontal lobes.
Observed virtual town navigation and found place responsive cells in virtual shops by observing movement in the virtual space and the heat of neurons that are responding vigorously as people are running through the space. This resembles the same kind of place coding that we see in rats.
Place-responsive cells were clustered in the hippocampus.
Direct analog: place cells in the rat hippocampus and humans.

Direction Coding

Viewpoint-independent direction coding in cells, heading cells in rats.
Participants learned a virtual space with landmarks.
Judged whether the maze center was left or right of their heading.
Left/right judgment is perspective-dependent.
If the showed images where the center was in the same direction (ex: Two landmarks on the left), then people were activating the left directive twice getting a lower neural response from the fMRI activation.
FMRI adaptation shows encoding of heading direction.

Learning and Heading

Heading is modulated by learning in the retrosplenial complex (RSC).
Found in active navigation tasks and passive viewing of relevant stimuli.
Consistent stimuli with the same direction diminish neural response over time.
In Reosplenial Cortex, bigger fMRI response for different headings than same direction.

Cognitive Maps in Humans

Studies by Howard et al. (unfamiliar layout of London).
Participants explored streets through walking or studying maps.
fMRI studies on the day after learning the route.
Watched first-person view movies of novel or same routes.
Movies either required navigational decisions or didn't.
If you have to make decisions about where to go, you are having to activate the brain, presumably the map, to be able to make that decision. On the other had we wouldn't have the cognitive map region in the brain be activated.

Hippocampal Response

Hippocampus FMI response related to distance from the goal.
Two types of representation: Euclidean distance (as the crow flies) and path distance (following streets).
The frontal (anterior) hippocampus responds when there is a representation that is correlated with the actual distance that was calculated using the Euclidean metric.

Hippocampal Activation

People moving through the space. Euclidean distance changes but the distance from the target is different from the road.
The anterior hippocampus: response related to Euclidean distance.
The posterior hippocampus: response related to path distance (along streets).
Graded response in the hippocampus - spatial awareness from path to Euclidean.

Hippocampal Plasticity and Expertise

London Taxi Drivers - have expert knowledge of city layout in mind.
Hippocampal volume in taxi drivers who had acquired the knowledge: their posterior hippocampal volume was higher versus anterior hippocampal volume was lower.
More time navigating London equals to greater hippocampal volume changes.
Structural plasticity of the hippocampus may support spatial representations of complex environments.

Alternative Views Of Expertise

Hippocampus is more than this.
Maybe it's not about something about the complexity of a space and being able to navigate it.
Maybe the hippocampus is actually involved with other kinds of things.
Hippocampus may be involved with attention or motor planning.
Possibly due to the stress of driving.

Taxi vs. Bus Drivers

Compared taxi drivers to bus drivers.
Bus drivers follow constrained routes and don't need complex spatial knowledge.
Hippocampal volume differences were found between taxi and bus divers.
Bus drivers and taxi drivers, the taxi drivers gray matter volume was larger than the bus driver.
Brain regions associated with spatial information building were increased as the posterior area got bigger.

Behavioral Difference

Taxi drivers were significantly better than London at navigational drivers compared to the bus drivers.
However, the bus drivers were slightly better at recalling visuals.
Learning the knowledge is related to functional changes and changes in functional volume.

Review

The need for a cognitive map for a space and navigating.
These are made of cells with properties.
Parahippocampal area for scene recognition and representation background.
Evidence of distance and place coding.
Plasticity becoming an expertise.