Seeing through the Surface: Evaluating Visualization Techniques for Depth Perception in Augmented Microscopy

The ability to see through the surface of the human body is central to medical augmented reality (AR) applications ranging from surgical planning to intraoperative guidance. However, accurately perceiving the depth of virtual structures relative to real anatomy remains a persistent challenge. This is particularly pronounced under the operating microscope, where high magnification, limited viewpoints, and tight error tolerances make depth and occlusion both essential and difficult to infer. Prior work has proposed methods for visualizing subsurface content using Video See-Through (VST) and Optical See-Through (OST) head-mounted displays (HMDs). However, the implementation of these approaches in surgical microscopes remains an open question due to the unique microscope perceptual conditions as well as the additive characteristics of microscope optics and display systems. To the best of our knowledge, this work is the first to investigate the perceptual accuracy of AR microscope visualization techniques for depicting virtual content inside the human body. We introduce a custom depth perception measurement apparatus based on a half-silvered mirror that enables alignment between a real measurement probe and subsurface virtual targets while viewing through a stereoscopic surgical microscope. We then implement a custom alpha blending method and a virtual aperture that conforms to the three-dimensional surface geometry. Using our apparatus, we conducted a 3 x 2 within-subjects user study evaluating depth estimation accuracy and confidence across visualization techniques. Results showed that both the virtual aperture and alpha blending significantly reduced absolute depth error compared to standard opaque rendering. Alpha blending without an aperture achieved localization accuracy comparable to opaque rendering with a virtual aperture while eliminating the systematic depth underestimation bias observed in traditional renderings. Furthermore, the presence of a virtual aperture increased users' confidence and was preferred over other methods. These findings provide insights for the design of AR visualization systems for microsurgical applications.