In the fall of 1997, my university built a CAVE (Cave Automatic Virtual Environment) to help scientists, artists, and archeologists embrace 3D immersion to advance the state of those fields. Ecstatic at seeing a real-life instantiation of the Metaverse, the virtual world imagined in Neal Stephenson’s Snow Crash, I donned a set of goggles and jumped inside. And then I promptly vomited.

I never managed to overcome my nausea. I couldn’t last more than a minute in that CAVE and I still can’t watch an IMAX movie. Looking around me, I started to notice something. By and large, my male friends and colleagues had no problem with these systems. My female peers, on the other hand, turned green.

What made this peculiar was that we were all computer graphics programmers. We could all render a 3D scene with ease. But when asked to do basic tasks like jump from Point A to Point B in a Nintendo 64 game, I watched my female friends fall short. What could explain this?

At the time any notion that there might be biological differences underpinning computing systems was deemed heretical. Discussions of gender and computing centered around services like Purple Moon, a software company trying to entice girls into gaming and computing. And yet, what I was seeing gnawed at me.

That’s when a friend of mine stumbled over a footnote in an esoteric army report about simulator sickness in virtual environments. Sure enough, military researchers had noticed that women seemed to get sick at higher rates in simulators than men. While they seemed to be able to eventually adjust to the simulator, they would then get sick again when switching back into reality.

Being an activist and a troublemaker, I walked straight into the office of the head CAVE researcher and declared the CAVE sexist. He turned to me and said: “Prove it.”

The gender mystery

Over the next few years, I embarked on one of the strangest cross-disciplinary projects I’ve ever worked on. I ended up in a gender clinic in Utrecht, in the Netherlands, interviewing both male-to-female and female-to-male transsexuals as they began hormone therapy. Many reported experiencing strange visual side effects. Like adolescents going through puberty, they’d reach for doors—only to miss the door knob. But unlike adolescents, the length of their arms wasn’t changing—only their hormonal composition.

Scholars in the gender clinic were doing fascinating research on tasks like spatial rotation skills. They found that people taking androgens (a steroid hormone similar to testosterone) improved at tasks that required them to rotate Tetris-like shapes in their mind to determine if one shape was simply a rotation of another shape. Meanwhile, male-to-female transsexuals saw a decline in performance during their hormone replacement therapy.

Along the way, I also learned that there are more sex hormones on the retina than in anywhere else in the body except for the gonads. Studies on macular degeneration showed that hormone levels mattered for the retina. But why? And why would people undergoing hormonal transitions struggle with basic depth-based tasks?

Two kinds of depth perception

Back in the US, I started running visual psychology experiments. I created artificial situations where different basic depth cues—the kinds of information we pick up that tell us how far away an object is—could be put into conflict. As the work proceeded, I narrowed in on two key depth cues – “motion parallax” and “shape-from-shading.”

Motion parallax has to do with the apparent size of an object. If you put a soda can in front of you and then move it closer, it will get bigger in your visual field. Your brain assumes that the can didn’t suddenly grow and concludes that it’s just got closer to you.

Shape-from-shading is a bit trickier. If you stare at a point on an object in front of you and then move your head around, you’ll notice that the shading of that point changes ever so slightly depending on the lighting around you. The funny thing is that your eyes actually flicker constantly, recalculating the tiny differences in shading, and your brain uses that information to judge how far away the object is.

In the real world, both these cues work together to give you a sense of depth. But in virtual reality systems, they’re not treated equally.

The virtual-reality shortcut

When you enter a 3D immersive environment, the computer tries to calculate where your eyes are at in order to show you how the scene should look from that position. Binocular systems calculate slightly different images for your right and left eyes. And really good systems, like good glasses, will assess not just where your eye is, but where your retina is, and make the computation more precise.

It’s super easy—if you determine the focal point and do your linear matrix transformations accurately, which for a computer is a piece of cake—to render motion parallax properly. Shape-from-shading is a different beast. Although techniques for shading 3D models have greatly improved over the last two decades—a computer can now render an object as if it were lit by a complex collection of light sources of all shapes and colors—what they they can’t do is simulate how that tiny, constant flickering of your eyes affects the shading you perceive. As a result, 3D graphics does a terrible job of truly emulating shape-from-shading.

Tricks of the light

In my experiment, I tried to trick people’s brains. I created scenarios in which motion parallax suggested an object was at one distance, and shape-from-shading suggested it was further away or closer. The idea was to see which of these conflicting depth cues the brain would prioritize. (The brain prioritizes between conflicting cues all the time; for example, if you hold out your finger and stare at it through one eye and then the other, it will appear to be in different positions, but if you look at it through both eyes, it will be on the side of your “dominant” eye.)

What I found was startling (pdf). Although there was variability across the board, biological men were significantly more likely to prioritize motion parallax. Biological women relied more heavily on shape-from-shading. In other words, men are more likely to use the cues that 3D virtual reality systems relied on.

This, if broadly true, would explain why I, being a woman, vomited in the CAVE: My brain simply wasn’t picking up on signals the system was trying to send me about where objects were, and this made me disoriented.

My guess is that this has to do with the level of hormones in my system. If that’s true, someone undergoing hormone replacement therapy, like the people in the Utrecht gender clinic, would start to prioritize a different cue as their therapy progressed.

We need more research

However, I never did go back to the clinic to find out. The problem with this type of research is that you’re never really sure of your findings until they can be reproduced. A lot more work is needed to understand what I saw in those experiments. It’s quite possible that I wasn’t accounting for other variables that could explain the differences I was seeing. And there are certainly limitations to doing vision experiments with college-aged students in a field whose foundational studies are based almost exclusively on doing studies solely with college-age males. But what I saw among my friends, what I heard from transsexual individuals, and what I observed in my simple experiment led me to believe that we need to know more about this.

I’m excited to see Facebook invest in Oculus, the maker of the Rift headset. No one is better poised to implement Stephenson’s vision. But if we’re going to see serious investments in building the Metaverse, there are questions to be asked. I’d posit that the problems of nausea and simulator sickness that many people report when using VR headsets go deeper than pixel persistence and latency rates.

What I want to know, and what I hope someone will help me discover, is whether or not biology plays a fundamental role in shaping people’s experience with immersive virtual reality. In other words, are systems like Oculus fundamentally (if inadvertently) sexist in their design?