Okay, this is a thought experiment.

Imagine there's a brain, sitting in a jar - and it's hooked up to a computer which can perfectly simulate experiences of the outside world.

The computer inputs sensory information to the brain and the brain responds, just like a real world experience.

It responds to simulated pain, touch and smells - just like you would.

To the brain, there's no difference in it being in the jar, or in a skull.

So then, how do you know that your brain is actually in your skull, and it's not just in a jar, hooked up to a computer?

This is the "Brain in a Vat" scenario, an updated version of a problem first described by the philosopher Rene Descartes in the 1600s.

And it's probably no surprise that it was an inspiration for The Matrix.

The problem questions the nature of reality and how you know that this is real life.

Philosophers have been questioning the nature of reality for centuries and now, with the advent of virtual reality and VR headsets like these, we can find out even more about how we experience the world.

Imagine you're in the Wild West.

When you move around, your brain maps out the spatial environment so you can navigate through it now, and if you returned later.

In your brain, you have two types of cells that encode this spatial information and act like an internal GPS: place cells that mark your location, and grid cells, which as their name suggests, map out your environment.

This internal GPS was first discovered in mice.

In many studies, when mice have returned to a specific location, like a position in a maze, their place fire.

We've known about this for over three decades.

But what's new, is that this is true not just in the real world, but also in virtual reality.

In one study, mice were placed in a Virtual Experience, while they were running on a treadmill.

When they returned to specific locations in the virtual experience, researchers observed that their place cells were activated.

And when they return to virtual locations that they've been to before, their specific place and grid cells fire.

In a mouse, their brain encodes and responds spatial information from the real world and a virtual experience in a similar way.

It's difficult to do these studies with people because recording the firing of neurons requires implanting electrodes into our brain.

Though one study with people already undergoing surgical treatment for epilepsy found a group of cells in their brain keep track of their location in a virtual navigation task.

To us, VR is like a big optical illusion.

We know we're in a simulated environment, but there are many ways VR experiences trick our brains into responding like it's real life.

Your brain uses your past experiences to build a set of rules to interpret the world: the sky shows you which way is up; shadows show you where light is coming from; you have an internal model of gravity that helps you predict how this apple will fall; and you know where your arms are in 3D space.

The vestibular system in your ears helps you maintain balance; and proprioception helps you have a sense of where your body parts are.

Virtual environments are effective when they follow the same rules, allowing these systems to run smoothly: moving objects obey laws of physics, shading and texture help you figure out depth and distance, your virtual hands will be in a similar position when you reach for an object.

In addition to this, technical cues like head tracking, frame rate, latency and sound all work together for the user to achieve presence - the phenomenon of behaving and feeling as if we're actually in a virtual world.

Kind of like consciousness for virtual environments.

All of these elements add up to, essentially, fool your brain.

And these movement and sound cues have been found to be more important than screen resolution in achieving presence: if you have a multi-sensory experience, it's more likely you'll perceive it to be real.

So what the philosophy behind the "Brain in a Vat" and the technology behind Virtual Reality both question is, What are the building blocks of our perceived world?

And if you manipulate these blocks, how do we react?

If the head tracking is askew in VR, you'll probably get motion sickness; if there's an absence of sensory information in real life and it's not distracting, your brain will fill in the gaps.

In both cases, a proportion of your perception is determined by the external world, but a lot of it is made inside your brain.

Further research will help us figure out what these proportions are, and how we can better shape VR experiences for research, therapy and rehabilitation, and entertainment.

For now, remember that you can count on your brain's' internal GPS to guide you through virtual experiences.

You'd be lost without it.