Understanding Spatial Thinking

May 7th, 2013

A new study co-authored by CAS Professor of Psychology Michael Hasselmo investigates the function of grid cells by comparing the resonance properties of neurons in rats and bats. A grid cell is a type of neuron that has been found in the brains of rats, mice, bats, and monkeys and is likely to exist in other animals, including humans. Grid cells are believed to help compute self-position based on continuously updated information about position and direction. The new study appears in the April 19 issue of the journal Science.

The new study, co-authored by Hasselmo and colleagues including James G. Heys of the Graduate Program for Neuroscience in BU’s Center for Memory and Brain, is titled “Bat and Rat Neurons Differ in Theta-Frequency Reonance Despite Similar Coding of Space” (Science 19 April 2013: Vol. 340 no. 6130 pp. 363-367, DOI: 10.1126/science.1233831). Grid cells are unique because the regularity in grid spacing does not derive from any regularity in the environment or in the sensory input available to an animal: Grid cells appear to encode a type of abstract spatial structure that is constructed inside the brain and imposed on the environment by the brain without requiring regular properties of sensory features of the environment.

Both bats and rats exhibit grid cells in the medial entorhinal cortex that fire as they visit a regular array of spatial locations. (The entorhinal cortex is an area of the brain located in the medial temporal lobe and functions as a hub in a widespread network for memory and navigation.) The new study was inspired by recordings in bat entorhinal cortex showing grid cells that respond when the bat visits a periodic array of spatial locations in the environment, but without continuous theta frequency (4-10 Hz) oscillations in the EEG. By comparing the rat entorhinal cortex with the resonance properties in bat entorhinal cortex, the data showed that bat neurons do not have the same theta frequency resonance as rats, suggesting either that continuous theta is not necessary or that the different species have different, convergent mechanisms for generating grid cell firing patterns.

These results can be interpreted in two ways. They could indicate that theta frequency oscillations are not essential to the formation of grid cells and to spatial memory function, in contrast with many previous studies suggesting an important role for theta rhythm oscillations in spatial memory function. Alternately, they could indicate that grid cells are a product of convergent evolution in different species. Theta rhythm oscillations may be necessary to generation of grid cell firing patterns in rats and mice, but may be replaced by other mechanisms for generation of grid cell firing patterns in bats.

The study’s co-authors are James G. Heys, Graduate Program for Neuroscience, Center for Memory and Brain, Boston University; Katrina M. MacLeod, Department of Biology, University of Maryland; Cynthia F. Moss, Department of Psychology, University of Maryland; and Michael E. Hasselmo, Department of Psychology, Center for Memory and Brain, Boston University.

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