Psychophysical Haptic Measurement of Vertical Perception: Elucidating a Hand Sensory Bias

Abstract

The primary sensory modality for probing spatial perception can vary among psychophysical paradigms. In the subjective visual vertical (SVV) task, the brain must account for the position of the eye within the orbit to generate an estimate of a visual line orientation, whereas in the subjective haptic vertical (SHV) task, the position of the hand is used to sense the orientation of a haptic bar. Here we investigated whether a hand sensory bias can affect SHV measurement. We measured SHV in 12 subjects (6 left-handed and 6 right-handed) with a forced-choice paradigm using their left and right hands separately. The SHV measurement was less accurate than the SVV measurements (-0.6 ± 0.7) and it was biased in the direction of the hand used in the task but was not affected by handedness; SHV left hand -6.8 ± 2.1° (left-handed -7.9 ± 3.6°, right-handed -5.8 ± 2.5°) and right hand 9.8 ± 1.5° (left-handed 7.4 ± 2.2°, right-handed 12.3 ± 1.8°). SHV measurement with the same hand was also affected by the haptic bar placement on the left or right side versus midline, showing a side effect (left vs midline -2.0 ± 1.3°, right vs midline 3.8 ± 1.7°). Midline SHV measures using the left and right hands were different, confirming a laterality effect (left hand -4.5 ± 1.7°, right hand 6.4 ± 2.0°). These results demonstrate a sensory bias in SHV measurement related to the effects of both hand-in-body (i.e., right vs left hand) and hand-in-space positions. Such modality-specific bias may result in disparity between SHV and SVV measurements, and therefore cannot be generalized to vertical or spatial perception.

Keywords: SHV; SVV; haptic; spatial orientation; vertical perception.

Figure 1.. The haptic stimulus in the SHV task was a bar that consisted of two pieces with parallel edges mounted on a stepper motor

(A). The SHV task was done with using the left hand and right hand separately, and with placement of the haptic bar on the same side of the hand used (B) or at midline (C). In the SVV task (D), the orientation of a visual line was reported with respect to perceived vertical. In the VH task (E), the orientation of the haptic bar was reported relative to the orientation of the visual line. In the haptic tasks, the trial started with an auditory cue for subjects to touch the haptic bar (F). Sample time course of 100 trials with responses from the paradigm used in all three tasks (G). The y-axis shows the angle of the stimulus presented and the color indicates the response for that trial (left in blue and right in red). The stimulus angles were presented randomly within a range that started at 180° and then adjusted based on previous responses (gray sectors in the top circles). An example of psychometric fit to the responses in the paradigm (H). The bias value is determined as the center of the curve (i.e., point of subjective equality), where the probabilities of right and left responses are equal.

Figure 2:

The hand bias and effects of laterality and side placement of the haptic stimulus are shown for SHV task. The negative values indicate leftward bias (L), and the positive values indicate rightward bias (R). The box plots indicate the interquartile range with the median value (white line) and the whiskers indicate the 5^th and 95th percentile range.

Figure 3:

Left: Both SHV and VH tasks show a bias towards the side of hand use (i.e., a rightward bias using the right hand and a leftward bias using the left hand). The box plots indicate the interquartile range with the median value (white line) and the whiskers indicate the 5^th and 95th percentile range. Right: Correlation of SHV and VH tasks across subjects, (r²= 0.49, p< 0.001). SHV: subjective haptic vertical, VH: visual haptic

Figure 4:

SHV and VH results based on the hand use and handedness. The box plots indicate the interquartile range with the median value (white line) and the whiskers indicate the 5^th and 95th percentile range. The + markers indicate outliers. SHV: Subjective haptic vertical, VH: Visual haptic

Figure 5:

The SHV and VH precisions (left) and their correlations (right). The box plots indicate the interquartile range with the median value (white line) and the whiskers indicate the 5^th and 95th percentile range. The + markers indicate outliers. SHV: Subjective haptic vertical, VH: Visual haptic