The Editor of the Psychonomic Society’s journal Learning & Behavior , Jonathon Crystal, has launched a new section of the journal that is intended to provide an outlook on the field and a venue for discussion of the most exciting current research in learning and behavior. Jonathon blogged about this new initiative here.
So what are those outlook articles, and what do they tell us about learning and behavior? I was curious and decided to look at two of them.
“How do we know where we are? How can we find the way from one place to another? And how can we store this information in such a way that we can immediately find the way the next time we trace the same path? This year's Nobel Laureates have discovered a positioning system, an "inner GPS" in the brain that makes it possible to orient ourselves in space, demonstrating a cellular basis for higher cognitive function.”
This inner GPS involves two components: The place cells discovered by O’Keefe respond whenever an animal is in a particular place in the environment. The grid cells discovered by the Mosers, by contrast, fire according to a triangular grid that maps out the environment, thereby providing the animal with a coordinate system for navigation.
How do those two types of cell coordinate? What are their respective functions?
These questions were taken up by the two outlook articles in Learning & Behavior that commented on other recent research published in the journal Nature.
Ken Cheng on the proposed function of grid cells
The article by Ken Cheng explored recent research that focused on the proposed functions of the grid cells. Grid cells have often been assumed to code places and to provide a metric for odometry—that is, a systematic grid would enable the animal to keep track of the distance it has traveled by “ticking off” each gridline it traverses, and the conjunction of cells that are activated would provide an indication of location, similar to the way in which latitude and longitude combine to point a unique location.
However, recent results reported by O’Keefe’s group cast some doubt on these hypotheses. These recent findings suggest that the internal grid in an animal’s brain is quite flexible and re-arranged itself with changes in the environment. When the environment is highly irregular—such as a trapezoid with a sharp narrow end—the grid became sparser and bent towards that sharp narrow end. It follows that the animal’s “inner GPS” relies on a satellites with a rather wobbly and changeable orbit, and if it were used for odometry this might create a few problems.
Cheng therefore suggests that the grid cells might encode direction rather than odometry or defining a specific place in space. This idea is supported by other research showing that the grid-like properties of grid cells are disrupted if the input from cells that code the head direction of an animal is cut off. The “inner GPS” seems to rely on knowledge of where the animal’s head is pointing, and if that knowledge is available, then the grid cells can assist in maintaining the correct direction of travel.
So perhaps the inner GPS is more like an inner compass that is accompanied by a rather fuzzy location device.
Brett Gibson and Robert Mair on spatial memory encoding
The other outlook article, by Gibson and Mair, was also concerned with animals’ spatial navigation ability but commented on a recent ingenious experiment by Spellman and colleagues, in which rodents were trained in spatial navigation, and the connections between two brain structures known to play a role in spatial memory—namely the ventral hippocampus and the medial prefrontal cortex—were disrupted at various points during the experiment. Notably, the disruption was achieved by a technique known as optogenetic inhibition, which is achieved by injection of a virus that is engineered to inhibit nearby neurons in response to light: whenever a light source implanted into the animals’ prefrontal cortex was turned on, the pathway from the hippocampus was demonstrably inhibited without affecting activity in the hippocampus itself. In other words, the experiment by Spellman and colleagues involved light-triggered reversible disruption of a pathway between two brain structures that are involved in spatial memory.
What would happen when the pathway was temporarily disrupted? Unsurprisingly, the disruption impaired learning of the spatial navigation task. However, intriguingly, that impairment only occurred if the pathway was disrupted during the encoding phase of the spatial task, not during maintenance or retrieval. Once the animal had learned the association between a location and a reward, disruption of the pathway no longer impaired retrieval.
Gibson and Mair suggest that this approach, which combines molecular approaches to neurosciences with behavioral experimentation, provides a pointer to the future for research on fundamental processes of learning and memory.