Computational Memory Lab: Spatial Cognition

To study human spatial cognition, we developed a virtual reality task called Yellow Cab (see Fig. 1), which encourages participants to find efficient paths from arbitrary spatial locations. In this game, participants act as virtual taxi cab drivers as they navigate through computer-generated towns. They alternately pick up passengers placed at random, unknown locations ("searching phase") and deliver them to learnable, fixed goal locations, occupied by stores, within the town ("goal-seeking phase"). Although no spatial learning is observed during "searching" for randomly placed passengers, as predicted, participants learn to find shorter paths to store locations during "goal-seeking". To download a sample of a YellowCab session, click here.



	Fig. 1: Example of the YellowCab task.

We examined whether the two key physiological markers of spatial navigation in rodents might have parallels in the human brain. When rodents navigate through a novel environment recordings of electrical activity from the hippocampus (and nearby brain structures) reveal a striking 4-10 Hz rhythmic oscillation known as the hippocampal theta rhythm. At the same time, certain cells in the hippocampus, termed place cells, increase their rate of activity when particular regions of the space are being traversed. These two phenomena figure prominently in animal models of learning and spatial navigation.

In a series of studies, we have documented the existence and character of the theta rhythm in the human brain as participants learned to navigate through complex virtual environments (Kahana, et al, 1999); Caplan, et al, 2001; Caplan, et al, 2003).

By recording the behavior of individual brain cells (in collaboration with U.C.L.A. Neurosurgeon Dr. Itzhak Fried), we have identified place cells in the human brain. These cells, which are found primarily in the human hippocampus, become highly active when a given spatial location is being traversed from any direction. We also identified two other cellular responses in the human brain: cells that become active in response to viewing a salient landmark (from any location) and cells that become active when searching for a particular goal location (irrespective of location or view). Finally, we found a large number of cells that represent combinations of these three features. These results were recently reported by Ekstrom, et al, 2003.

Fig. 2: Firing-rate map of a right hippocampal cell showing significant place selectivity. Lettered squares (SA,SB,SC) indicate target store locations, white boxes indicate non-target buildings, red lines indicate the subject's trajectory, and the red square indicates regions of significantly high firing rate (all examples, p < 0.01). Fig. 3: Anatomical distribution of place cells. Place-responsive cells were clustered in the hippocampus (H) compared with amygdala (A), parahippocampal region (PR) and frontal lobes (FR).