Virtual reality is used in many situations and is often seen as equivalent to actual experience. For example, it is used in training where actual experience is too expensive or dangerous. In science, it is used in experiments with the assumption that it can be compared to reality. A recent paper (Z. Aghajan, L. Acharya, J. Moore, J. Cushman, C. Vuong, M. Mehta; Impaired spatial selectivity and intact phase precession in two-dimensional virtual reality; Nature Neuroscience 2014) shows that virtual reality and ‘real’ reality are treated differently in the hippocampus where spatial mapping occurs. ScienceDaily reports on this paper (here).
It is assumed that cognitive maps are made by the neurons of the hippocampus, computing the distances to landmarks. Of course, this is not the only way a map could be constructed: sounds and echos could give clues, smells could identify places, and so on. To test whether visual clues alone could give the information to create a map, the researchers compared the activity of neurons in the hippocampus in a virtual walk and a real walk that were visually identical. In the real set-up the rat walked across a scene while in the virtual set-up the rat walked on the treadmill while the equivalent visual ‘movie’ was projected all around the rat.
The results showed that the mapping of the two environments was different. The mapping during real experience involved more activity by more neurons and was not random. In the virtual experiment, the activity was random and more sparse. It appeared, using neuron activity, as if the rat could not map virtual reality and was somewhat lost or confused, even though they appeared to be acting normally. “Careful mathematical analysis showed that neurons in the virtual world were calculating the amount of distance the rat had walked, regardless of where he was in the virtual space.“
In the same report, other research by the same group is reported. Mehta describes the complex rhythms involved in learning and memory in the hippocampus, “The complex pattern they make defies human imagination. The neurons in this memory-making region talk to each other using two entirely different languages at the same time. One of those languages is based on rhythm; the other is based on intensity.” The two languages are used simultaneously by hippocampal neurons. “Mehta’s group reports that in the virtual world, the language based on rhythm has a similar structure to that in the real world, even though it says something entirely different in the two worlds. The language based on intensity, however, is entirely disrupted.”
As a rat hippocampus is very similar to a human one and the virtual reality set up was a very realistic one, this study throws doubt on experiments and techniques that use virtual reality with humans. It is also very interesting to notice another surprising ability of neurons, to process two types of signal at the same time.
Abstract: “During real-world (RW) exploration, rodent hippocampal activity shows robust spatial selectivity, which is hypothesized to be governed largely by distal visual cues, although other sensory-motor cues also contribute. Indeed, hippocampal spatial selectivity is weak in primate and human studies that use only visual cues. To determine the contribution of distal visual cues only, we measured hippocampal activity from body-fixed rodents exploring a two-dimensional virtual reality (VR). Compared to that in RW, spatial selectivity was markedly reduced during random foraging and goal-directed tasks in VR. Instead we found small but significant selectivity to distance traveled. Despite impaired spatial selectivity in VR, most spikes occurred within ~2-s-long hippocampal motifs in both RW and VR that had similar structure, including phase precession within motif fields. Selectivity to space and distance traveled were greatly enhanced in VR tasks with stereotypical trajectories. Thus, distal visual cues alone are insufficient to generate a robust hippocampal rate code for space but are sufficient for a temporal code.”