## VIRTUAL HEALING

Thank you for “The Promise of Virtual Reality” [The Science of Health], Claudia Wallis’s excellent piece on the uses of the technology in medicine. I would like to insert “medical education” as another potential tool for VR. My colleagues and I have published some research on using immersive VR to teach cardiac anatomy to medical students in the March issue of Clinical Anatomy. We found that they not only scored 24 percent higher on quizzes than non-VR students but also said they had “fun”! Fun should not be a dirty word in medical education.

HILLEL S. MARESKY University of Toronto

Wallis is excited about the completion of a randomized controlled trial on the use of VR to treat post-traumatic stress disorder. The problem is that you can’t truly single-blind a VR trial and have the trial’s subjects unaware whether they are receiving the treatment or a placebo, and the bias of knowing thus cannot be filtered. Stating that VR “will help make mental health care cheaper and more accessible” risks overselling an intervention that cannot be robustly validated.

DOUGLAS BERGER Tokyo

## GEOSCIENCE GEOMETRY

“Is Antarctica Collapsing?” by Richard B. Alley, claims that a complete collapse of the Thwaites Glacier in West Antarctica could lead to a sea-level rise of 11 feet. I am puzzled because I cannot make the arithmetic work out.

Let’s call Alley’s Figure 3.5 meters. For simplicity, let’s also say a kilometer is 1/10,000 the distance from the pole to the equator, as it was originally defined. So the earth’s circumference is 40,000 km, and its radius is about 6,400 km, giving a surface area of about 510 million square km. But the ocean is only about 70 percent of the earth’s surface, or about 360 million km2. (When I had a chance, I Googled it and confirmed my number.)

From the map, the glacier appears to be approximately a right triangle with each side measuring about 600 km. So its area is about 180,000 km2. I assume only the portion above sea level matters. The article says the glacier rises up to a mile (1.6 km) above sea level, but clearly it is not that high everywhere, such as the coast, so let’s guess 1.5 km on average. My rough estimate of the volume of the glacier above sea level is therefore 270,000 km3. Ignoring the additional water necessary to shrink the coastlines as sea level rises, that is only enough water to raise sea level by, at most, about 0.7 meter.

I didn’t expect my computation to be exact, but that results appears to be off by a factor of five. Did I do something wrong?

DAN GRAIFER via e-mail

ALLEY REPLIES: Graifer’s math is pretty good. But West Antarctica is now drained by ice that flows in other directions, into the Ross and Filchner-Ronne ice shelves, as well as Thwaites, with so-called continental divides along the highest parts of the Antarctic Ice Sheet separating the different drainages. If Thwaites deglaciated to the divides and stopped there, it would leave mile-high cliffs that would not be stable. Both our physical understanding and our models show that deglaciating any of the main drainages of West Antarctica would tap into ice that now goes out the other drainages, deglaciating the marine basins of all of them. The total is usually taken to give us 3.3 meters, or 11 feet, of global sea-level rise without too much uncertainty.

## PLANETARY PARITY

“The Exoplanet Next Door,” by M. Darby Dyar, Suzanne E. Smrekar and Stephen R. Kane, states that Venus has no magnetic field and yet has an atmosphere of extreme density and depth. Mars, on the other hand, has almost no atmosphere. The explanation for the latter’s thin atmosphere that I am most familiar with is that because the planet lost a strong magnetic field, the solar wind from the sun stripped it of a previously much thicker atmosphere.

I would think that Venus, being much closer to the sun, should have experienced a much greater solar wind effect. The two planets were created around the same time, so the atmospheric difference seems to defy common sense. What am I missing here?

CHRIS SCHOLFIELD via e-mail

THE AUTHORS REPLY: The key to answering this question is that Venus has a negligible magnetic field at the present epoch. We must not fall into the trap of thinking that this situation has persisted for the past four billion years. Assuming a composition and core size similar to Earth, models of the Venusian magnetic field through time show that the planet most likely had a field comparable to Earth’s up until about one billion years ago. Crucially, this would have protected the Venusian atmosphere when the sun was younger and much more active. Additionally, Venus’s atmosphere is much thicker and has a higher mean molecular weight than Earth’s, which makes it generally more resistant to atmospheric escape degradation.

But exactly how magnetic fields shield atmospheres is apparently more complicated than once thought. For example, recent measurements suggest that oxygen is currently being lost from Earth, Mars and Venus at similar rates! This is a very puzzling result, given the idea that mass, magnetic field and distance from the sun are the main factors in atmospheric loss, as Scholfield notes.

In “Face Values,” Doris Y. Tsao describes a technique in which she and her colleagues are able to predict how neurons in certain areas of the cerebral cortex that are dedicated to facial recognition will respond to a given face by using 50 coordinates, or dimensions, for shape and appearance.

One intriguing piece of information absent from the article is the number of discernible steps along the ramps from minimal to maximal neuron cell response. To illustrate: If only nil and maximum values could be distinguished for each coordinate, then for a 50-D space, there would be about 1015 distinct facial states. This seems likely to be more than the number of individuals who would be recognizable to a single person and raises the question of why this seemingly excessive capacity is produced by evolution of the visual sense.

TERRY GOLDMAN Los Alamos National Laboratory

TSAO REPLIES: Relating the noise characteristics of face neurons to facial discrimination behavior is an interesting idea. I’m not sure there is a discrepancy between the number of neurally distinguishable states and our ability to perceive them: as the existence of the plastic surgery industry demonstrates, we can distinguish very fine differences in facial structure.

Why the brain evolved to represent faces based on these shape and appearance axes is a deep and open question. One idea is that the fundamental job of the brain is to build an efficient model of the world—rather than to accomplish ad hoc goals such as distinguishing the faces of people you know—and extracting shape and appearance parameters is the best way to do this in the realm of face modeling. What we do know is that with 50 numbers describing shape and appearance, we can re-create a face.

## ERRATUM

“Is Antarctica Collapsing?” by Richard B. Alley, should have referred to the National Science Foundation launching an effort to study the Thwaites Glacier with the U.K.’s Natural Environment Research Council, not the British Antarctic Survey.