Two Independent Mechanisms for Motion-In-Depth Perception: Evidence from Individual Differences

Abstract

Our forward-facing eyes allow us the advantage of binocular visual information: using the tiny differences between right and left eye views to learn about depth and location in three dimensions. Our visual systems also contain specialized mechanisms to detect motion-in-depth from binocular vision, but the nature of these mechanisms remains controversial. Binocular motion-in-depth perception could theoretically be based on first detecting binocular disparity and then monitoring how it changes over time. The alternative is to monitor the motion in the right and left eye separately and then compare these motion signals. Here we used an individual differences approach to test whether the two sources of information are processed via dissociated mechanisms, and to measure the relative importance of those mechanisms. Our results suggest the existence of two distinct mechanisms, each contributing to the perception of motion-in-depth in most observers. Additionally, for the first time, we demonstrate the relative prevalence of the two mechanisms within a normal population. In general, visual systems appear to rely mostly on the mechanism sensitive to changing binocular disparity, but perception of motion-in-depth is augmented by the presence of a less sensitive mechanism that uses interocular velocity differences. Occasionally, we find observers with the opposite pattern of sensitivity. More generally this work showcases the power of the individual differences approach in studying the functional organization of cognitive systems.

Dynamic cues to binocular depth

“…A study using similar stimuli to display similar cues found that only half of their 62 subjects provided thresholds for use in analysis. 33 The level of stereo-acuity (e.g. STATIC 185@) measured in the study sample may appear poor; this is due to the design of the stimuli used in the experiment.…”

“…[25][26][27][28] The lowest thresholds were found for the Z-LOCATION CHANGE condition (changing disparity/ fixed pattern), which is consistent with the idea that the CDOT cue alone is not solely responsible for depth detection of motion-in-depth stimuli, but that another cue, the IOVD cue, might be utilised, in line with previous reports. 33 Additional experiments have been conducted to determine whether isolation of the IOVD cue results in the perception of depth. By definition, no disparity information is available in the IOVD cue, as no spatially corresponding points exist between the two eyes; the IOVD cue signals only a change in position.…”

“…It appears also that the CDOT cue is used by most individuals in isolation whereas fewer are able to use the IOVD. 33 While these studies agree that the presence of motion in depth can demonstrate binocular function in the absence of measurable static stereo-acuity, there are a number of limitations of the methodologies employed, such as the comparison of different disparity ranges and the use of differing presentation methods (computer display vs paper-based testing) between the static and dynamic conditions. Also, previous studies have investigated the perception of motion in depth, rather than depth detection per se.…”

Perception of Relative Depth Interval: Systematic Biases in Perceived Depth

“…The other 16 produced data where we could not be sure that performance was different from chance for either the straightforward (Wilmer, 2008;Hokoda, 1985, Porcar & Martinez-Palomar, 1997, Watanabe et al, 2008, as well as our own lab where we have explored motion in depth (Nefs, O'Hare & Harris, 2010). Many studies do not report in detail how their observers were chosen, nor how many were excluded.…”

“…In a previous study, we tested a large number (62) of observers on a series of motion in depth tasks and a control dot-density task (Nefs et al, 2010). The idea of this latter task was to test 23 whether some of our naïve observers were poorly motivated.…”