Coordinates of Human Visual and Inertial Heading Perception

Psychometrics of inertial heading perception

BACKGROUND

Inertial self-motion perception is thought to depend primarily on otolith cues. Recent evidence demonstrated that vestibular perceptual thresholds (including inertial heading) are adaptable, suggesting novel clinical approaches for treating perceptual impairments resulting from vestibular disease.

OBJECTIVE

Little is known about the psychometric properties of perceptual estimates of inertial heading like test-retest reliability. Here we investigate the psychometric properties of a passive inertial heading perceptual test.

METHODS

Forty-seven healthy subjects participated across two visits, performing in an inertial heading discrimination task. The point of subjective equality (PSE) and thresholds for heading discrimination were identified for the same day and across day tests. Paired t-tests determined if the PSE or thresholds significantly changed and a mixed interclass correlation coefficient (ICC) model examined test-retest reliability. Minimum detectable change (MDC) was calculated for PSE and threshold for heading discrimination.

RESULTS

Within a testing session, the heading discrimination PSE score test-retest reliability was good (ICC = 0. 80) and did not change (t(1,36) = -1.23, p = 0.23). Heading discrimination thresholds were moderately reliable (ICC = 0.67) and also stable (t(1,36) = 0.10, p = 0.92). Across testing sessions, heading direction PSE scores were moderately correlated (ICC = 0.59) and stable (t(1,46) = -0.44, p = 0.66). Heading direction thresholds had poor reliability (ICC = 0.03) and were significantly smaller at the second visit (t(1,46) = 2.8, p = 0.008). MDC for heading direction PSE ranged from 6-9 degrees across tests.

CONCLUSION

The current results indicate moderate reliability for heading perception PSE and provide clinical context for interpreting change in inertial vestibular self-motion perception over time or after an intervention.

Diverse effects of gaze direction on heading perception in humans

Gaze change can misalign spatial reference frames encoding visual and vestibular signals in cortex, which may affect the heading discrimination. Here, by systematically manipulating the eye-in-head and head-on-body positions to change the gaze direction of subjects, the performance of heading discrimination was tested with visual, vestibular, and combined stimuli in a reaction-time task in which the reaction time is under the control of subjects. We found the gaze change induced substantial biases in perceived heading, increased the threshold of discrimination and reaction time of subjects in all stimulus conditions. For the visual stimulus, the gaze effects were induced by changing the eye-in-world position, and the perceived heading was biased in the opposite direction of gaze. In contrast, the vestibular gaze effects were induced by changing the eye-in-head position, and the perceived heading was biased in the same direction of gaze. Although the bias was reduced when the visual and vestibular stimuli were combined, integration of the 2 signals substantially deviated from predictions of an extended diffusion model that accumulates evidence optimally over time and across sensory modalities. These findings reveal diverse gaze effects on the heading discrimination and emphasize that the transformation of spatial reference frames may underlie the effects.

Common causation and offset effects in human visual-inertial heading direction integration

Movement direction can be determined from a combination of visual and inertial cues. Visual motion (optic flow) can represent self-motion through a fixed environment or environmental motion relative to an observer. Simultaneous visual and inertial heading cues present the question of whether the cues have a common cause (i.e., should be integrated) or whether they should be considered independent. This was studied in eight healthy human subjects who experienced 12 visual and inertial headings in the horizontal plane divided in 30° increments. The headings were estimated in two unisensory and six multisensory trial blocks. Each unisensory block included 72 stimulus presentations, while each multisensory block included 144 stimulus presentations, including every possible combination of visual and inertial headings in random order. After each multisensory stimulus, subjects reported their perception of visual and inertial headings as congruous (i.e., having common causation) or not. In the multisensory trial blocks, subjects also reported visual or inertial heading direction (3 trial blocks for each). For aligned visual-inertial headings, the rate of common causation was higher during alignment in cardinal than noncardinal directions. When visual and inertial stimuli were separated by 30°, the rate of reported common causation remained >50%, but it decreased to 15% or less for separation of ≥90°. The inertial heading was biased toward the visual heading by 11-20° for separations of 30-120°. Thus there was sensory integration even in conditions without reported common causation. The visual heading was minimally influenced by inertial direction. When trials with common causation perception were compared with those without, inertial heading perception had a stronger bias toward visual stimulus direction.NEW & NOTEWORTHY Optic flow ambiguously represents self-motion or environmental motion. When these are in different directions, it is uncertain whether these are integrated into a common perception or not. This study looks at that issue by determining whether the two modalities are consistent and by measuring their perceived directions to get a degree of influence. The visual stimulus can have significant influence on the inertial stimulus even when they are perceived as inconsistent.

Effect of range of heading differences on human visual-inertial heading estimation

Both visual and inertial cues are salient in heading determination. However, optic flow can ambiguously represent self-motion or environmental motion. It is unclear how visual and inertial heading cues are determined to have common cause and integrated vs perceived independently. In four experiments visual and inertial headings were presented simultaneously with ten subjects reporting visual or inertial headings in separate trial blocks. Experiment 1 examined inertial headings within 30° of straight-ahead and visual headings that were offset by up to 60°. Perception of the inertial heading was shifted in the direction of the visual stimulus by as much as 35° by the 60° offset, while perception of the visual stimulus remained largely uninfluenced. Experiment 2 used ± 140° range of inertial headings with up to 120° visual offset. This experiment found variable behavior between subjects with most perceiving the sensory stimuli to be shifted towards an intermediate heading but a few perceiving the headings independently. The visual and inertial headings influenced each other even at the largest offsets. Experiments 3 and 4 had similar inertial headings to experiments 1 and 2, respectively, except subjects reported environmental motion direction. Experiment 4 displayed similar perceptual influences as experiment 2, but in experiment 3 percepts were independent. Results suggested that perception of visual and inertial stimuli tend to be perceived as having common causation in most subjects with offsets up to 90° although with significant variation in perception between individuals. Limiting the range of inertial headings caused the visual heading to dominate the perception.

Effect of vibration during visual-inertial integration on human heading perception during eccentric gaze

Heading direction is determined from visual and inertial cues. Visual headings use retinal coordinates while inertial headings use body coordinates. Thus during eccentric gaze the same heading may be perceived differently by visual and inertial modalities. Stimulus weights depend on the relative reliability of these stimuli, but previous work suggests that the inertial heading may be given more weight than predicted. These experiments only varied the visual stimulus reliability, and it is unclear what occurs with variation in inertial reliability. Five human subjects completed a heading discrimination task using 2s of translation with a peak velocity of 16cm/s. Eye position was ±25° left/right with visual, inertial, or combined motion. The visual motion coherence was 50%. Inertial stimuli included 6 Hz vertical vibration with 0, 0.10, 0.15, or 0.20cm amplitude. Subjects reported perceived heading relative to the midline. With an inertial heading, perception was biased 3.6° towards the gaze direction. Visual headings biased perception 9.6° opposite gaze. The inertial threshold without vibration was 4.8° which increased significantly to 8.8° with vibration but the amplitude of vibration did not influence reliability. With visual-inertial headings, empirical stimulus weights were calculated from the bias and compared with the optimal weight calculated from the threshold. In 2 subjects empirical weights were near optimal while in the remaining 3 subjects the inertial stimuli were weighted greater than optimal predictions. On average the inertial stimulus was weighted greater than predicted. These results indicate multisensory integration may not be a function of stimulus reliability when inertial stimulus reliability is varied.

Distinct spatial coordinate of visual and vestibular heading signals in macaque FEFsem and MSTd

Precise heading estimate requires integration of visual optic flow and vestibular inertial motion originating from distinct spatial coordinates (eye- and head-centered, respectively). To explore whether the two heading signals may share a common reference frame along the hierarchy of cortical stages, we explored two multisensory areas in macaques: the smooth pursuit area of the frontal eye field (FEFsem) closer to the motor side, and the dorsal portion of medial superior temporal area (MSTd) closer to the sensory side. In both areas, vestibular signals are head-centered, whereas visual signals are mainly eye-centered. However, visual signals in FEFsem are more shifted towards the head coordinate compared to MSTd. These results are robust being largely independent on: (1) smooth pursuit eye movement, (2) motion parallax cue, and (3) behavioral context for active heading estimation, indicating that the visual and vestibular heading signals may be represented in distinct spatial coordinate in sensory cortices.

Heading representations in primates are compressed by saccades

Perceptual illusions help to understand how sensory signals are decoded in the brain. Here we report that the opposite approach is also applicable, i.e., results from decoding neural activity from monkey extrastriate visual cortex correctly predict a hitherto unknown perceptual illusion in humans. We record neural activity from monkey medial superior temporal (MST) and ventral intraparietal (VIP) area during presentation of self-motion stimuli and concurrent reflexive eye movements. A heading-decoder performs veridically during slow eye movements. During fast eye movements (saccades), however, the decoder erroneously reports compression of heading toward straight ahead. Functional equivalents of macaque areas MST and VIP have been identified in humans, implying a perceptual correlate (illusion) of this perisaccadic decoding error. Indeed, a behavioral experiment in humans shows that perceived heading is perisaccadically compressed toward the direction of gaze. Response properties of primate areas MST and VIP are consistent with being the substrate of the newly described visual illusion.Macaque higher visual areas MST and VIP encode heading direction based on self-motion stimuli. Here the authors show that, while making saccades, the heading direction decoded from the neural responses is compressed toward straight-ahead, and independently demonstrate a perceptual illusion in humans based on this perisaccadic decoding error.

Effect of eye position during human visual-vestibular integration of heading perception

Visual and inertial stimuli provide heading discrimination cues. Integration of these multisensory stimuli has been demonstrated to depend on their relative reliability. However, the reference frame of visual stimuli is eye centered while inertia is head centered, and it remains unclear how these are reconciled with combined stimuli. Seven human subjects completed a heading discrimination task consisting of a 2-s translation with a peak velocity of 16 cm/s. Eye position was varied between 0° and ±25° left/right. Experiments were done with inertial motion, visual motion, or a combined visual-inertial motion. Visual motion coherence varied between 35% and 100%. Subjects reported whether their perceived heading was left or right of the midline in a forced-choice task. With the inertial stimulus the eye position had an effect such that the point of subjective equality (PSE) shifted 4.6 ± 2.4° in the gaze direction. With the visual stimulus the PSE shift was 10.2 ± 2.2° opposite the gaze direction, consistent with retinotopic coordinates. Thus with eccentric eye positions the perceived inertial and visual headings were offset ~15°. During the visual-inertial conditions the PSE varied consistently with the relative reliability of these stimuli such that at low visual coherence the PSE was similar to that of the inertial stimulus and at high coherence it was closer to the visual stimulus. On average, the inertial stimulus was weighted near Bayesian ideal predictions, but there was significant deviation from ideal in individual subjects. These findings support visual and inertial cue integration occurring in independent coordinate systems.NEW & NOTEWORTHY In multiple cortical areas visual heading is represented in retinotopic coordinates while inertial heading is in body coordinates. It remains unclear whether multisensory integration occurs in a common coordinate system. The experiments address this using a multisensory integration task with eccentric gaze positions making the effect of coordinate systems clear. The results indicate that the coordinate systems remain separate to the perceptual level and that during the multisensory task the perception depends on relative stimulus reliability.