Detection of scene-relative object movement and optic flow parsing across the adult lifespan

Abstract

Moving around safely relies critically on our ability to detect object movement. This is made difficult because retinal motion can arise from object movement or our own movement. Here we investigate ability to detect scene-relative object movement using a neural mechanism called optic flow parsing. This mechanism acts to subtract retinal motion caused by self-movement. Because older observers exhibit marked changes in visual motion processing, we consider performance across a broad age range (N = 30, range: 20-76 years). In Experiment 1 we measured thresholds for reliably discriminating the scene-relative movement direction of a probe presented among three-dimensional objects moving onscreen to simulate observer movement. Performance in this task did not correlate with age, suggesting that ability to detect scene-relative object movement from retinal information is preserved in ageing. In Experiment 2 we investigated changes in the underlying optic flow parsing mechanism that supports this ability, using a well-established task that measures the magnitude of globally subtracted optic flow. We found strong evidence for a positive correlation between age and global flow subtraction. These data suggest that the ability to identify object movement during self-movement from visual information is preserved in ageing, but that there are changes in the flow parsing mechanism that underpins this ability. We suggest that these changes reflect compensatory processing required to counteract other impairments in the ageing visual system.

Figure 1.

A single frame (monocular view) of an example stimulus from Experiment 1. Stimuli comprised a stereoscopically presented three-dimensional array of red wireframe background objects with a central fixation cross and probe dot to the right or left of fixation.

Figure 2.

Simplified birds-eye view of the stimulus in Experiment 1 (not to scale). Green sphere indicates the position of the observer relative to the stimulus (not to scale). The green dotted line represents the direction of the observer's gaze to the white central fixation cross and therefore the center of the field of view. The gray arrows indicate the direction of movement of the scene toward the observer. The white sphere indicates the initial position of the probe dot at the same depth as the fixation cross. The probe moves with the scene (white arrow) but with an added rightward or leftward (depending on the trial) scene-relative motion (red line). The resulting motion of the probe (blue sphere) is indicated by the blue line.

Figure 3.

Example psychometric functions from one participant for the static and the moving conditions. Psychometric functions were fitted to the binary response data, while estimated proportion of rightward responses, aggregated over similar velocities, are also shown for illustrative purposes only (filled markers: static condition; unfilled markers: moving condition). Vertical dotted lines indicate the point of subjective equality (PSE) from the psychometric functions fitted to each condition.

Figure 4.

Mean group PSE and direction discrimination thresholds plotted for each condition; static (no fill) and moving (spotted). Error bars are 95% confidence intervals.

Figure 5.

(A) The FPI, i.e., the threshold ratio (moving/static) plotted against age, with associated regression line (r² = 0.013). (B) Discrimination thresholds in the static (r² = 0.038) and moving (r² = 0.036) conditions plotted against age, with associated regression lines.

Figure 6.

(A) Schematic of one example stimulus used in Experiment 2 with flow in the right hemifield and probe in the left hemifield. The central arrow represents the 90° condition in which the probe moved vertically upwards. (B) Schematic of the on-screen axis and paddle. Participants rotated the white line CW or ACW (between 0° & 180°) to represent the perceived trajectory of the probe. (C) Illustration of the relative tilt as the angular difference between the perceived and actual trajectories. Adapted from Warren & Rushton, 2009a.

Figure 7.

(A) Average relative tilt for the three conditions; full (unfilled), same (striped), and opposite (spotted). (B) Average local (filled) and global (spotted) relative tilts. Error bars are 95% confidence intervals.

Figure 8.

(A) Full (filled triangle; r² = 0.346), Same (square; r² = 0.367), and Opposite (filled circle; r² = 0.303) relative tilts plotted against age, with associated regression lines. (B) Global (filled circle; r² = 0.303) and Local (diamond; r² = 0.058) contributions to the relative tilt effect as a function of age, with associated regression lines.