Figure 1.

Experimental methods. (A) Example of eye image obtained from the cameras with the overlay of automatically detected eyelids and pupil (top) and the iris pattern extracted and optimized to calculate torsion (bottom). For more details see Otero-Millan et al. (2015). (B) Subjective visual vertical (SVV) paradigm. In each trial, subjects fixated on a red dot for 1 s before the line appeared. They had 1.5 s to respond whether the line was tilted to the left or to the right of what they perceived as upright (two alternative forced choice or 2AFC). This was done by pressing the left or right button on a controller. The line was presented within a range of possible angles (yellow shade) that varied during the experiment (see insets C,D). After pressing the button, the line disappeared and the next trial started with a new line orientation. (C) Sample time course of the first 100 trials with the subject’s responses. Each triangle represents one trial. The y-axis shows the angle of the line presented and the color indicates the subject’s response for that trial. The left tilt responses are shown in blue and the right tilt responses in red. The triangles that point up indicate the line was presented in the upper hemifield and the triangles that point down indicate the line was presented in the lower hemifield. The line angles were presented randomly within a range that started at 360° and then adjusted based on previous responses (illustrated by the top circles with yellow sectors). At the end of every 10 trials, the center of this range (yellow shade) was set as the SVV value calculated from previous 30 trials. The amplitude of the range was also adjusted every 10 trials by dividing it in half until it reached 10° (±5° around the calculated center), after which it was kept constant for the rest of the trials. (D) Sample recording of torsional eye position (bottom) and SVV (top) during an entire session. Each SVV point is calculated from a psychometric fit to the responses from 100 trials (see inset E) and each torsion point corresponds with the average ocular torsion during the same block of 100 trials. First, the head is in the upright position, then tilted and finally back again in the upright position. Every time the head changed position from upright to tilt or tilt to upright, the paradigm reset and angles were presented starting again at the full range of 360° (yellow shade). (E) Examples of psychometric fits corresponding with the first block of 100 trials (gray dot in D) and the last block of 100 trials during the head tilt (black dot in D). The SVV for each block is calculated as the center of the psychometric curve (50% right and left responses).