Modeling depth from motion parallax with the motion/pursuit ratio

Visually fixating or tracking another person decreases balance control in young and older females walking in a real-world scenario

Balance control during overground walking was assessed in 10 young (23.6 ± 3.4) and 10 older (71.0 ± 5.5 years) healthy females during free gaze, and when fixating or tracking another person in an everyday use waiting room. Balance control was characterised by medial/lateral sacrum acceleration dispersion, and gaze fixations were simultaneously assessed with eye tracking equipment. The results showed decreased balance control when fixating a stationary (p = 0.003, g_av = 0.19) and tracking a walking (p = 0.027, g_av = 0.16) person compared to free gaze. The older adults exhibited reduced baseline stability throughout, but the decrease caused by the visual tasks was not more profound than the younger adults. The decreased balance control when fixating on or tracking the observed person was likely due to more challenging conditions for interpreting retinal flow, which facilitated less reliable estimates of self-motion through vision. The older adults either processed retinal flow during the tasks as effectively as the young adults, or they adopted a more rigid posture to facilitate visual stability, which masked any ageing effect of the visual tasks. The decrease in balance control, the first to be shown in this context, may warrant further investigation in those with ocular or vestibular dysfunction.

Aging does not affect integration times for the perception of depth from motion parallax

To successfully navigate throughout the world, observers must rapidly recover depth information. One depth cue that is especially important for a moving observer is motion parallax. To perceive unambiguous depth from motion parallax, the visual system must integrate information from two different proximal signals, retinal image motion and a pursuit eye movement. Previous research has shown that aging affects both of these necessary components for motion parallax depth perception, but no research has yet investigated how aging affects the mechanism for integrating motion and pursuit information to recover depth from motion parallax. The goal of the current experiment was to assess the integration time required by older adults to process depth information. In four psychophysical conditions, younger and older observers made motion and depth judgments about stationary or translating random-dot stimuli. Stimulus presentations in all four psychophysical conditions were followed by a high-contrast pattern mask, and minimum stimulus presentation durations (stimulus-to-mask onset asynchrony, or SOA) were measured. These SOAs reflect the minimum neural processing time required to make motion and motion parallax depth judgments. Pursuit latency was also measured. The results revealed that, after accounting for age-related delays in motion processing and pursuit onset, older and younger adults required similar temporal intervals to combine retinal image motion with an internal pursuit signal for the perception of depth. These results suggest that the mechanism for motion and pursuit integration is not affected by age.

The neural basis of depth perception from motion parallax

In addition to depth cues afforded by binocular vision, the brain processes relative motion signals to perceive depth. When an observer translates relative to their visual environment, the relative motion of objects at different distances (motion parallax) provides a powerful cue to three-dimensional scene structure. Although perception of depth based on motion parallax has been studied extensively in humans, relatively little is known regarding the neural basis of this visual capability. We review recent advances in elucidating the neural mechanisms for representing depth-sign (near versus far) from motion parallax. We examine a potential neural substrate in the middle temporal visual area for depth perception based on motion parallax, and we explore the nature of the signals that provide critical inputs for disambiguating depth-sign.This article is part of the themed issue 'Vision in our three-dimensional world'.

Gain Modulation as a Mechanism for Coding Depth from Motion Parallax in Macaque Area MT

Observer translation produces differential image motion between objects that are located at different distances from the observer's point of fixation [motion parallax (MP)]. However, MP can be ambiguous with respect to depth sign (near vs far), and this ambiguity can be resolved by combining retinal image motion with signals regarding eye movement relative to the scene. We have previously demonstrated that both extra-retinal and visual signals related to smooth eye movements can modulate the responses of neurons in area MT of macaque monkeys, and that these modulations generate neural selectivity for depth sign. However, the neural mechanisms that govern this selectivity have remained unclear. In this study, we analyze responses of MT neurons as a function of both retinal velocity and direction of eye movement, and we show that smooth eye movements modulate MT responses in a systematic, temporally precise, and directionally specific manner to generate depth-sign selectivity. We demonstrate that depth-sign selectivity is primarily generated by multiplicative modulations of the response gain of MT neurons. Through simulations, we further demonstrate that depth can be estimated reasonably well by a linear decoding of a population of MT neurons with response gains that depend on eye velocity. Together, our findings provide the first mechanistic description of how visual cortical neurons signal depth from MP.SIGNIFICANCE STATEMENT Motion parallax is a monocular cue to depth that commonly arises during observer translation. To compute from motion parallax whether an object appears nearer or farther than the point of fixation requires combining retinal image motion with signals related to eye rotation, but the neurobiological mechanisms have remained unclear. This study provides the first mechanistic account of how this interaction takes place in the responses of cortical neurons. Specifically, we show that smooth eye movements modulate the gain of responses of neurons in area MT in a directionally specific manner to generate selectivity for depth sign from motion parallax. We also show, through simulations, that depth could be estimated from a population of such gain-modulated neurons.

A Pursuit Theory Account for the Perception of Common Motion in Motion Parallax

The visual system uses an extraretinal pursuit eye movement signal to disambiguate the perception of depth from motion parallax. Visual motion in the same direction as the pursuit is perceived nearer in depth while visual motion in the opposite direction as pursuit is perceived farther in depth. This explanation of depth sign applies to either an allocentric frame of reference centered on the fixation point or an egocentric frame of reference centered on the observer. A related problem is that of depth order when two stimuli have a common direction of motion. The first psychophysical study determined whether perception of egocentric depth order is adequately explained by a model employing an allocentric framework, especially when the motion parallax stimuli have common rather than divergent motion. A second study determined whether a reversal in perceived depth order, produced by a reduction in pursuit velocity, is also explained by this model employing this allocentric framework. The results show than an allocentric model can explain both the egocentric perception of depth order with common motion and the perceptual depth order reversal created by a reduction in pursuit velocity. We conclude that an egocentric model is not the only explanation for perceived depth order in these common motion conditions.

The effects of aging on the perception of depth from motion parallax

Successful navigation in the world requires effective visuospatial processing. Unfortunately, older adults have many visuospatial deficits, which can have severe real-world consequences. Although some of these age effects are well documented, some others, such as the perception of depth from motion parallax, are poorly understood. Depth perception from motion parallax requires intact retinal image motion and pursuit eye movement processing. Decades of research have shown that both motion processing and pursuit eye movements are affected by age; it follows that older adults may also be less sensitive to depth from motion parallax. The goals of the present study were to characterize motion parallax depth thresholds in older adults, and to explain older adults' sensitivity to depth from motion parallax in terms of motion and pursuit deficits. Younger and older adults' motion thresholds and pursuit accuracy were measured. Observers' depth thresholds across several different stimulus conditions were measured, as well. Older adults had higher motion thresholds and less accurate pursuit than younger adults. They were also less sensitive to depth from motion parallax at slow and moderate pursuit speeds. Although older adults had higher motion thresholds than younger adults, they used the available motion signals optimally, and age differences in motion processing could not account for the older adults' increased depth thresholds. Rather, these age effects can be explained by changes in older adults' pursuit signals.