Have you ever noticed that even detailed, sophisticated virtual reality experiences don’t feel completely “real”?

It all comes down to your inner ear - and a study published earlier this month using rats may help explain why this is the case.

Researchers from University of California, Los Angeles, let rats run along a virtual narrow hall and measured their brain activity, and compared these virtual-world rats with rats running along a real hall (real-world rats).

Even when the rats could move in a virtual world, their sense of space was less than fully normal, at least as far as their brain activities - namely the firing of their “place cells,” explained below - showed.

Locating place cells

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The researchers measured activity in a much-studied part of the brain known to play a crucial role in spatial cognition and memory in general: a seahorse-shaped structure called the hippocampus (named after the genus name of seahorses).

The hippocampus contains a population of cells known as place cells, first discovered in the late 1960s in behavioural neuroscientist John O’Keefe’s lab.

Place cells have place-specific firing properties: they fire a lot only when the animal is at a particular place in space.

Place cells have been found in humans as well as rats, along with other mammals and birds.

The study’s authors wanted to find out if hippocampal cells with place cell properties were as abundant in virtual-world rats as in real-world rats.

A virtual rat reality

Unlike a console with buttons for video-game aficionados, the researchers’ virtual rat world was far more realistic.

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Their rats got to move, at least on the spot, on a big ball (see panel A above).

The ball rotated under them as they walked, so that they never get anywhere in real space.

But in the virtual world, the visual input moved as it should when a rat actually moves through the space: the projected visual world was cleverly linked to the ball movement via a computer program (panels B and C).

The virtual-world rats thus had visual cues as well a bunch of bodily cues, those stemming from its limbs, as it moved in the virtual world.

Basically, all that was missing were cues, stemming from the vestibular apparatus in the inner ear, which told the animals that they were actually accelerating (or not).

Inner workings

We have vestibular apparatuses in our inner ears as well, and they contain sensory hairs in fluid-filled chambers.

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When a rat (as with humans) moves its head, the fluids (cupola) slosh and displace the sensory hairs, causing them to fire signals to the brain.

The pattern of firing tells us how the head is moving.

The vestibular apparatuses of virtual-world rats sensed little displacement as the rats ran on the track ball, because the rats were strapped in place.

With only the vestibular apparatus not functioning normally in the virtual world, the authors nevertheless discovered hippocampal place cells were much harder to find in those rats compared with real-world rats.

The place cells in virtual-world rats also had wider fields, meaning they were less precise in defining a place.

The sense of space, as philosopher Rene Descartes proposed in the 17th century, seems to be a multimodal sense, and it is only complete when all the senses — visual, bodily, vestibular, and probably olfactory and auditory as well — deliver their spatial information.

For rat neuroscience, it means that this beautiful and elegant virtual world has its limits in probing spatial cognition.

It would be wonderful to probe a rat’s brain as it virtually travelled its natural scale of hundreds or thousands of metres, rather than in the one- to two-metre experimental arenas typically foisted on lab rats.

But this research shows that virtual-world rats’ place cells would not “behave” as they do in the real world.

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From rats to humans

These results may explain how virtual reality scenarios, such as stationary flight simulators, don’t completely fool our brains into thinking we’re in a different world.

But what about games in head-mounted virtual reality?

The head-mounted system projects a virtual scene to our visual system via goggles, and as we actually move — hopefully in a huge space without obstructions — the visual world changes accordingly.

With appropriate smells and sounds added, this would get all our senses orchestrated.

It’s quite the challenge for developers, but ah - such is the stuff as dreams are made on.