In 2024, Ward Pettibone received the UCLA BRI/Semel Institute Graduate Student Travel Award. He works on sleep, learning, and memory, a field that draws in part on decades of spatial learning research.

Spatial learning covers knowing the locations of things and the routes between them. It depends heavily on the hippocampus, a curved structure deep in each side of the brain.

The account below follows how scientists came to understand that link. It is a history of the field, not a summary of any one researcher’s results.

The 1950s: a surgery reveals the hippocampus

In the 1950s, surgeons removed parts of both medial temporal lobes from a patient with severe epilepsy. The operation included much of the hippocampus on each side.

Afterward, the patient could no longer form lasting new memories of events. He could hold a number in mind briefly, but lost it once his attention shifted. Older memories and many skills stayed largely intact.

Researchers studied him for decades, and he took part in memory tests for much of his life. His results showed that memory depends on specific brain structures, and that different kinds of memory rely on different systems.

The 1970s: cells that mark a location

In 1971, researchers recording from the rat hippocampus reported neurons that fired when the animal was in one particular spot. They named them place cells.

Different place cells prefer different spots. Together, their activity forms a map of the space the animal is exploring.

Place cells can remap when an environment changes. A new room or a rearranged one can prompt a fresh pattern of firing.

The idea of a cognitive map in the hippocampus was laid out in a 1978 book. It framed the structure as the brain’s system for representing space.

The term cognitive map itself dates to the 1940s. A psychologist had used it in 1948 to describe rats that took shortcuts in a maze.

The 1980s and 1990s: a pool test, a compass and replay

In 1981, a water maze was introduced as a test of spatial learning in rats. Animals use cues around the room to find a platform hidden just below the surface.

Over repeated trials, healthy animals swim more directly to the platform. Researchers measure the time and path length on each trial to track learning.

Removing the platform for a final trial tests memory alone. Animals that learned the location spend more time searching where it used to be.

Later work with the maze found that hippocampal damage impairs this kind of learning. Animals with such damage have trouble finding the hidden platform.

Head-direction cells were described between 1984 and 1990. They fire when an animal faces a certain way, acting like an internal compass.

Sleep enters the story

In 1994, researchers reported that place cells active together during exploration were also active together during later sleep. Later studies showed this replay often follows the original order of firing.

This replay happens mostly during brief high-frequency events in non-REM sleep. Replay runs much faster than the original experience.

That work tied spatial memory to sleep directly, through events that can be measured cell by cell. It helps explain why sleep research and spatial learning research now overlap so often.

The 2000s: a grid behind the map

In 2005, researchers described grid cells in the entorhinal cortex, a region that feeds into the hippocampus. Each grid cell fires at many points that form a triangular pattern across a space.

Grid cells appear to supply a sense of distance and direction. Place cells and grid cells together help an animal keep track of its position.

In 2006, researchers reported that replay also happens during quiet waking moments. Rats pausing in a maze replayed paths they had just run, sometimes in reverse order.

Other cell types joined the list. In 2008, researchers described border cells, which respond near the edges of an enclosure.

In 2009, studies that disrupted sleep replay events in rats found impaired spatial learning.

Human studies found related signals. Recordings in patients undergoing epilepsy surgery and brain imaging in volunteers point to similar systems in people.

The discoveries of place cells and grid cells were recognized with the 2014 Nobel Prize in Physiology or Medicine.

Studies of London taxi drivers added a human angle. Drivers who had learned the city’s streets showed a larger rear hippocampus than comparison groups.

Flexible memory updating is another active subject. When a goal moves, the brain has to set aside an old location and learn a new one. Tasks that move a goal from day to day test that ability.

Pettibone’s name appears on six research publications. The 2024 travel award is one of several honors he has received at UCLA.

His work on sleep, learning, and memory takes place within UCLA’s neuroscience PhD program in Los Angeles, California.

That program followed earlier career steps. He worked as an associate researcher at Mount Sinai after earning a bachelor’s degree in neuroscience from New York University.

The history above shows how the study of spatial learning moved from one surgical patient to maps of individual neurons.

About Ward Pettibone

Ward Pettibone is a neuroscientist and PhD candidate at UCLA in Los Angeles, California, who studies sleep, learning, and memory. His training spans New York University, Mount Sinai and the UCLA neuroscience program. Background on his path appears on Pettibone’s About page at his website.