Motion in the depth direction appears faster when the target is closer to the observer

Abstract

The target velocity at the retina and the initial phase of target motion are known to affect the perceived velocity of a target in planar motion. For depth motion, however, the role of this information in velocity perception remains unclear. Therefore, the purpose of this study was to reveal the role of the angular velocity derived from the vergence angle and the initial phase of target motion on the perceived velocity for depth motion. We devised two experimental tasks with five stimuli and used a two-alternative forced-choice paradigm to investigate velocity perception. In the tasks, a target moving toward or away from the observer was used. The five stimuli in each task moved between 40 and 240 cm (standard stimulus), 20 and 240 cm, 20 and 220 cm, 40 and 260 cm, and 60 and 260 cm from the participants. In the comparison of the standard stimulus with other stimuli, the stimuli approaching or receding from a distance of 20 cm were perceived as faster than the standard stimulus approaching or receding from a distance of 40 cm. We also showed that the stimuli that receded starting from a distance of 60 cm were perceived as moving slower than the standard stimulus. Our results suggest that larger changes in angular velocity affect velocity perception for depth motion; thus, observers perceive the target velocity as faster when the target is closer to the observer.

Fig. 1

The conditions for the experimental task. A visual target moved in the depth direction

Fig. 2

Moving distances of the visual stimuli in Experiment 1. Stimuli I, II, III, IV and V are shown. The black dashed lines indicate the distances from a participant in the depth direction. The green circles indicate the visual targets. The green dashed lines indicate the pathways of the visual targets

Fig. 3

Target velocity in the real world and the angular velocity of the target derived from the vergence angles in Experiment 1. Stimulus I (240/40 cm), Stimulus II (240/20 cm), Stimulus III (220/20 cm), Stimulus IV (260/40 cm) and Stimulus V (260/60 cm) are shown. The target moves at a constant speed in the real world (physically), while it moves with acceleration in terms of the angular velocity. In the Stimulus I condition, 240/40 cm means that the target approaches from a distance of 240 cm away from the observer to a distance of 40 cm from the observer, and the same applies to the other stimuli. The red, orange, yellow, green, aqua, blue and purple lines indicate the order of speed. The black lines indicate the standard stimuli

Fig. 4

Outline of the experimental task in Experiment 1

Fig. 5

The psychometric functions for 5 types of stimuli for each participant in Experiment 1, where 001–011 represent the participants; P (“faster response”) indicates the probability that the comparison stimulus was perceived as faster than the standard stimulus

Fig. 6

The PSEs for each stimulus in Experiment 1. The error bars indicate the standard deviation

Fig. 7

Moving distances of visual stimuli in Experiment 2. Stimuli VI, VII, VIII, IX, and X are shown. The black dashed lines indicate the distances from a participant in depth. The green circles indicate the visual targets. The green dashed lines indicate the pathways of the visual targets

Fig. 8

Target velocity in the real world and the angular velocity of the target derived from the vergence angles in Experiment 2. Stimulus VI (40/240 cm), Stimulus VII (20/240 cm), Stimulus VIII (20/220 cm), Stimulus IX (40/260 cm) and Stimulus X (60/260 cm) are shown. The target moves at a constant speed in the real world (physically), whereas it moves with deceleration in terms of the angular velocity. In the Stimulus VI condition, 40/240 cm means that the target moves from a distance of 40 cm from the observer to a distance of 240 cm, and the same applies to the other stimuli. The red, orange, yellow, green, aqua, blue, and purple lines indicate the order of speed. The black lines indicate the standard stimuli

Fig. 9

Outline of the experimental task in Experiment 2

Fig. 10

Psychometric functions for 5 types of stimuli for each participant in Experiment 2. The numbers (e.g., 001, 005) at the upper part of each chart represent the participants. P (“faster response”) indicates the probability that the comparison stimulus was perceived as faster than the standard stimulus

Fig. 11

The PSEs for each stimulus in Experiment 2. The error bars indicate the standard deviation