PSYCH 202: Intro to the Hippocampus

What cognitive processes is the hippocampus associated with

• Spatial navigation

• Long-term memory (episodic + semantic)

• Future imagination

• Creativity (generating novel ideas)

• Language

• Working memory

How are memory and navigation related in the hippocampal function

The hippocampus supports navigation because it stores and uses memories of environments, allowing individuals to navigate through space

Where is the hippocampus located

Medial Temporal Lobe (MTL)

What structures make up the parahippocampus gyrus

• Amygdala

• Perirhinal cortex

• Entorhinal cortex

• Hippocampus

• Parahippocampal cortex

Etorhinal cortex function

A MTL structure that serves as a major gateway between the cortex and hippocampus

What is significant about the connectivity of the hippocampus and surrounding regions

Extra-hippocampal regions are highly interconnected and play important roles in navigation and cognition

How does the MTL compare across mammalian species

Its structure is highly conserved across mammals, suggesting conservation of function as well

Why are rodents commonly used to study hippocampal function

Because the structure of the MTL is highly conserved between rodents and humans

How does the hippocampal size differ between rodents and humans

The hippocampus occupies a relatively larger proportion of the brain in rodents, wheras humans have proportionally larger cortical regions

What characterises hippocampal subfields

• Different cell types

• Extensive interconnectivity between subfields

Describe the major flow of information into the hippocampus

Cortex —> Entorhinal Cortex —> Hippocampus

What recording technique has been critical for understanding hippocampal function in animals

Implanting electrodes that allow animals to freely explore while researchers measure

• when neurons fire

• where the animal is

• the direction the animal is facing

Place Cells

Neurons that fire when an animal occupies a specific location within the environment

John O’Keefe (1971)

Place Cells: what is a place field

The specific area of an environment where a place cell fires most strongly

Place Cells: location

hippocampal subfields

• CA1

• CA2

• CA3

• Dentate gyrus

Place Cells: what happens to place cells when an environment changes

They undergo remapping (reorganisation)

Place Cells: what is an example of place cell remapping

Changing an enviro from a white room to a black room causes place cells to develop different place fields

Place Cells: what does place cell remapping suggest

place cells encode the specific context of an enviro, not just physical location

Grid Cells

Neurons that fire in a regular, periodic pattern as an animal moves through space

Edvard Moser and May-Britt Moser in 2005.

Grid Cells: location

Entorhinal cortex

Grid Cells: what pattern do grid cells produce

A repeating hexagonal tessellation across the environment

• Structure of firing structures form hexagons

Grid Cells: How do grid cells respond to environmental changes compared to place cells?

Grid cells remain relatively stable and show little evidence of substantial remapping.

Grid Cells: What navigation function might grid cells provide?

A coordinate system for measuring position and distance within space.

Head Direction Cells

Neurons that fire when an animal points its head in a particular allocentric direction

• allocentric = world-centred; independent of the individual’s current positon

Head Direction Cells: primary function

they act as an internal compass

Head Direction Cells: location

within the MTL

• subiculum

• entorhinal cortex

Head Direction Cells: what determines whether a head direction cell fires

Head orientation, not the animal’s location

Head Direction Cells: how does a single head direction cell respond to different locations

It fires maximally at one preferred direction (e.g., North, Southeast)

Head Direction Cells: what is a tuning curve

A graded firing response that decreases as the head moves away from the preferred direction

Head Direction Cells: how is full directional coverage achieved

Different head direction cells are tuned to different angles, collectively covering 360°

Boundary Cells

Neurons that respons to environmental boundaries

Boundary Cells: location

• subiculum

• entorhinal cortex

Boundary Cells: what feature of the enviro do they encode

The presence of boundaries such as walls or edges

Cognitive Map

Allocentric representations of spatial environments that can model space in 2 different ways

• egocentric coding

• allocentric coding

What provides the neural basis for cognitive maps

The interaction between the different hippocampal cell types (place, grid, head direction, boundary) and the surrounding regions of the hippocampus

Cognitive Maps: egocentric coding

Representing locations relative to oneself

• e.g., the object is to my left

Cognitive Maps: allocentric coding

Representing relationships between objects independently of one’s own position

Which type of spatial coding is most associated with the hippocampus

Allocentric coding

How did Tolman challenge behaviourist explanations of learning

He demonstrated that rats form internal representations of environments rather than merely learning stimulus-response associations (Skinner)

Tolman’s maze study: group 1 rats

Received rewards immediately and gradually improved at finding the exit

Tolman’s maze study: group 2 rats

they explored without rewards for 10 days and then received rewards on day 11

Tolman’s maze study: what happened when group 2 rats were finally rewarded

they found the exit faster than the trained rats

Tolman’s maze study: what concept do group 2’s performance support

Latent learning and cognitive maps

Tolman’s maze study: implications

It provided strong evidence against radical behaviourist theories such as those proposed by BF Skinner

Time Cells

Neurons that fire during specific moments within a delay period

Time Cells location

• Hippocampus

• Entorhinal cortex

Time Cells linked cognitive processes

Processing event sequences, which is important for memory

Time Cells importance for memory

they help encode the temporal order of events

Time Cells what does a delay period provide in time-cell experiments

A future target in time that the animal must anticipate

Time Cells describe a typical time-cell experiment

• Train mouse to associate objects with a stimulus they would be presented with after a delay to get a treat

  • e.g. associate green item with a smell —> must perform behaviour to get reward

• Placed in waiting dock after learning rule

• Measure how certain cells are responding during delay period

  • some neurons fire early, while some fire later during this delay period

Time Cells How do different time cells behave during a delay period?

Some fire early, while others fire later in the delay.

Time Cells what pattern emerges when time cells are ordered according to firing time

Sequential activation spanning the entire delay period

Time Cells what does sequential firing represent

the passage of time during an event sequence

Concept Cells

Neurons that respond to abstract semantic concepts rather than specific sensory features

Concept Cells Why are they considered unique

Unique to human MTL (medial temporal lobe)

Concept Cells how were they discovered

Using intracranial EEG recordings from epillepsy patients

Concept Cells What does intracranial EEG measure?

neural activity through electrodes implanted in the brain, including local field potentials and spiking activity

Concept Cells location

• Hippocampus

• Entorhinal cortex

• amygdala

Concept Cells what kind of stimuli were used to identify them

• images of famous people

• written names

• spoken names

—> they respond across multiple sensory modalities

Concept Cells why are they considered abstract

They respond to the meaning of a concept rather than its visual, auditory, or sensory characteristics

Concept Cells What famous example is commonly used to demonstrate concept cells?

A neuron that responds selectively to "Yoda" whether Yoda is presented as:

• A picture

• Written text

• Spoken name

Concept Cells What do concept cells reveal about human cognition?

The brain contains highly abstract representations of people, objects, and concepts.

What is the overall conclusion about hippocampal function?

The hippocampus and surrounding medial temporal lobe structures contain specialised cells that represent:

• Space (place, grid, head direction, boundary cells)

• Time (time cells)

• Abstract concepts (concept cells)

Together, these systems help explain the hippocampus' role in navigation, memory, imagination, creativity, and other higher cognitive functions.