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

Abstract

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.

Keywords: Aging; Depth perception; Depth thresholds; Motion parallax; Smooth pursuit eye movements.

Figure 1

Schematic of the stimulus used in the depth threshold task. In this example, background motion is leftward. A rectangle, positioned flush with the horizontal meridian, contains dot motion in the rightward direction. This rectangle appears to be nearer in depth to the observer. Note that in the stimulus, there is no shadow visible behind the rectangle in depth; the shadow in the schematic is meant to represent the fact that this rectangle is nearer in depth compared to the background.

Figure 2

Threshold dθs for younger and older observers. The minimum within-stimulus motion (dθ) in deg/sec that observers required to perceive depth is plotted against pursuit speed condition (2.3, 10.1, and 25.0 deg/sec) on the x-axis. Black squares represent younger adults (YA), and white squares represent older adults (OA).

Figure 3

Depth thresholds (dθ/dα) for older and younger observers. Depth thresholds are plotted against pursuit speed (dα: 2.3, 10.1, and 25 deg/sec) on the x-axis. The data plotted on solid lines represent obtained thresholds for younger (YA) and older adults (OA). The data plotted on the dashed line represent the depth threshold values predicted for older adults (see text).

Figure 4

Motion thresholds (d_min) for younger and older observers. Thresholds are plotted as magnitudes of displacement, in arcsecs. YA = younger adults; OA = older adults.

Figure 5

Relationship of two MP components and depth thresholds, for older adults. In both panels, M/PRs are plotted on the Y-axis. The left panel plots M/PRs as a function of motion thresholds (d_min), and in the right panel, M/PRs are plotted as a function of gain. The legend on the right gives the symbols for each of the three pursuit velocity conditions in the depth task.

Figure 6

Pursuit gains for younger and older observers. Gains (eye velocity/target velocity) are plotted against the target speed (2.1, 10.6, and 25.5 deg/sec) on the x-axis. Black squares represent younger adults (YA), and white squares represent older adults (OA). Note that a gain of 1.0 represents perfect performance (i.e., eye velocity = target velocity).