Do the Subjective Visual Vertical and the Subjective Haptic Vertical Probe a Common Estimate of Gravitational Upright?

In the subjective visual vertical (SVV) and the subjective haptic vertical (SHV) task, participants must judge the alignment of a probe rod with perceived gravitational vertical by sight or by touch. Previous research suggests that as the body is roll-tilted, SVV and SHV show a systematic, distinct pattern of errors, although reports vary. The purpose of this research was to identify whether SVV and SHV probe the same underlying representation of gravity. In Experiment 1, I confirmed that SVV and SHV errors are divergent by comparing the two measures in the same participants. In Experiment 2, I varied the tilt of the head and body separately and applied galvanic vestibular stimulation to introduce vestibular noise. I found these manipulations had differential effects on SVV and SHV. Experiment 3 showed that when neck afferents were stimulated, SVV and SHV integrate optimally, pointing to two distinct underlying estimates of gravity vertical.