Neurons in the hippocampal formation fire when an animal visits a particular place defined by a small region (firing field) and are known as place cells. An individual place cell may respond to more than one location (firing field) each known as a place field. Previous research has shown that converging input from grid cells in the medial entorhinal cortex, in conjunction with feedback inhibition from inhibitory interneurons in the dentate gyrus, is sufficient to account for the place fields seen in dentate gyrus granule cells.

Grid cells fire when a freely moving animal moves through a set of small regions (firing fields) which are roughly equal in size and arranged in a periodic triangular array that covers the entire available environment. The axons of many grid cells converge on the dendrites of the granule cells of the dentate gyrus.

Recent research has shown that the firing rate of a place field may be modified (increased or decreased) based on sensory input. Not only that, but the firing rate in each of multiple place fields of a single neuron may be independently modified due to sensory input. That is, the firing rate of one place field may increase while the firing rate in another place field in the same neuron may decrease due to identical sensory input. The phenomenon is known as rate remapping.

Grid cells in the medial entorhinal cortex are insensitive to sensory input. Sensory information about the environment is brought to the hippocampal formation by input from the lateral entorhinal cortex. The paper “The Mechanism of Rate Remapping in the Dentate Gyrus” (published December 22, 2010 in Neuron) reports on computer modeling research that asks the following two questions:

• What is the mechanism of rate remapping?
• Why do different place fields of the same dentate gyrus cell display independent rate remapping?

They showed that the lateral entorhinal cortex inputs do not interfere with place cell formation in the dentate gyrus by the medial entorhinal cortex inputs but they do modulate place field responses based on sensory input. The research team identified two processes that cause independent place field rate remapping:

• Changes in sensory input results in changes in lateral entorhinal cortex cell responses. Since the rate change of lateral entorhinal cortex cells is a function of the location of the animal, the change in lateral entorhinal cortex input is independent for each place field.
• The change in the excitation of other place cells will determine which place cell is most activate at a given position through a winner-take-all inhibitory mechanism. This process is localized and is therefore independent for each place field.

In summary, the simulations show that the spatial firing pattern of dentate gyrus cells is determined primarily by the medial entorhinal cortex inputs. The role of the lateral entorhinal cortex is to determine the specific rate at which place cells fire.


Other related blog posts:

Do You Know Where You Are? Place Memory

Nature versus Nurture and Place Memory Development

Spikelets and Place Cells

Watching Brain Cells Track Location