Lecture 14 - Hippocampus and Place cells

What are the main anatomical subdivisions of the hippocampus?

The hippocampus includes CA1, CA2, CA3 (cornu ammonis subfields) and the dentate gyrus (DG)

What is the trisynaptic circuit of the hippocampus?

Input arrives from the entorhinal cortex via the perforant path to DG granule cells → mossy fibers to CA3 pyramidal cells → Schaffer collaterals to CA1 pyramidal cells

What is the role of hippocampal layers (e.g., stratum radiatum, pyramidale, oriens, alveus)?

They organize neuronal architecture: dendrites, somas, and axons occupy different layers, helping structure connectivity and information flow

How do neuroscientists determine if neurons “code” for information?

By correlating neuronal spike trains with behavioral variables (location, speed, sensory input) across time

What are place cells and who discovered them?

Place cells are hippocampal pyramidal neurons that fire when an animal is in a specific location. Discovered by John O’Keefe in 1971, who won the 2014 Nobel Prize for this work

How do place cells challenge prior assumptions about neuron activity?

They showed that single neurons can directly represent behavior (location), disproving the idea that neural activity was too distributed to map onto behavior

How do place cells demonstrate context dependence?

Their firing patterns change when an environment’s shape or features change (e.g., circle vs. square arena)

What are splitter cells and what do they represent?

Splitter cells are hippocampal neurons that fire differently depending on an upcoming choice (e.g., left vs. right turn in a T-maze), showing coding of future routes

What are time cells and where were they discovered?

Time cells are hippocampal neurons that fire sequentially during delays, creating “time fields” that tile time across seconds. Discovered in CA1 recordings

How do time cells encode longer timescales?

They can maintain place fields but change firing rates based on context duration, such as morning vs. evening differences (Mankin et al., 2012)

What does Mankin et al. (2012) show about hippocampal coding of time?

Hippocampal neurons adjust firing rates depending on how long since an animal entered a context, not simply the time of day, revealing flexible temporal coding

How do place cells represent “what” in addition to “where”?

They increase firing in task-relevant zones (e.g., goal areas) and can encode objects or conspecifics in the environment

How do hippocampal neurons encode sensory features beyond space and time?

For example, in rats trained to press a lever to alter tone frequency, hippocampal ensembles tiled auditory frequency space, but only during active task engagement

Why is the hippocampus considered a multimodal coding structure?

Its pyramidal cells represent “what,” “where,” and “when,” providing the foundation for episodic memory

How does hippocampal coding relate to imagining the future?

Patients with hippocampal damage cannot form new episodic memories and also struggle to imagine future events, suggesting hippocampal coding underlies both memory and future simulation

What recognition did John O’Keefe receive for discovering place cells?

He won the 2014 Nobel Prize in Physiology or Medicine (shared with May-Britt and Edvard Moser for grid cells)

How do place cells demonstrate flexibility across contexts?

Their firing patterns remap when an environment’s features (e.g., shape or cues) change, showing context-sensitive coding

What are splitter cells and what do they reveal?

Splitter cells fire differently depending on an animal’s intended route (e.g., left vs. right turn), encoding future trajectory as well as present location

What are time cells in the hippocampus?

Neurons that fire sequentially during temporal delays, tiling intervals of time like place cells tile physical space

Where were hippocampal time cells discovered?

In CA1 recordings, during tasks with delays where animals waited for rewards

What key finding did Mankin et al. (2012) contribute about hippocampal time coding?

Hippocampal neurons change firing depending on elapsed time within a context, not just external cues (e.g., morning vs. evening), suggesting internal time coding

How do hippocampal neurons encode auditory information?

When rats pressed levers to alter tone frequencies, hippocampal ensembles represented auditory frequency space, but only when task-relevant

Why is hippocampal coding considered multimodal?

Because it integrates what (content), where (location), and when (time), the three core components of episodic memory

How does hippocampal damage affect memory and imagination?

Patients cannot form new episodic memories and also cannot imagine future events, suggesting the hippocampus supports both recall and simulation

label this diagram

1. ramified - homeostatic surveillance, monitoring

2. bushy - neurodegeneration, inflammation, toxicity

3. amoeboid - phagocytosis, inflammation, motility

Label this diagram:

1. MOL = Molecular layer- Dendrites of granule cells

2. G = Granule layer - Granule cell bodies

3. LUC = Stratum Lucidum

4. LAC-MOL = Stratum Lacunosum Moleculare - Fibers of the perforant pathway

5. RAD = Stratum Radiatum - Apical dendrites of pyramidal cells

6. PYR = Stratum Pyramidale - Cell bodies of pyramidal cells

7. OR = Stratum Oriens - Interneurons and dendrites of pyramidal cells

8. ALV = Alveus axons