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Perceived Speed of Motion in Depth Is Reduced in the Periphery

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Perceived Speed of Motion in Depth Is Reduced in the Periphery Vision Research 40 (2000) 3507–3516 www.elsevier.com/locate/visres Perceived speed of motion in depth is reduced in the periphery K. Brooks *, G. Mather Experimental Psychology, Biology School, Uni6ersity of Sussex, Brighton BN19QG, UK Received 24 February 1999; received in revised form 30 November 1999 Abstract The perceived speed of motion in depth (MID) for a monocularly visible target was measured in central and peripheral vision using a 2AFC speed discrimination task. Only binocular cues to MID were available: changing disparity and interocular velocity difference (IOVD). Perceived speed for monocular lateral motion and perceived depth for static disparity were also assessed, again in both central and peripheral vision. The purpose of the experiment was to assess the relative contributions of changing disparity and IOVD cues to the perceived speed of stereomotion. Although peripheral stimuli appeared to lie at approximately the same depth as their central counterparts, their apparent speed was reduced. Monocular/lateral and binocular/MID speeds were reduced to a similar extent. It seems that reduced apparent monocular speed leads to reduced perceived MID speed, despite the fact that the disparity system appears to be unaffected. These results suggest that the IOVD cue makes a significant contribution to MID speed perception. © 2000 Published by Elsevier Science Ltd. Keywords: Motion in depth; Speed discrimination; Peripheral vision 1. Introduction motion in depth (MID) could be encoded by the mo- tion system, without the need for disparity sensitive Consider the motion of an object approaching an mechanisms. A unit capable of assessing the difference observer along the median plane. Three major cues to in velocity of each monocular motion signal could be the speed of the object are available. (1) Monocularly, used to encode the speed of MID. Such a system will be the object’s retinal image increases in size as it ap- referred to as an inter-ocular velocity difference (IOVD) proaches, and the velocity of approach could be derived system. from the rate of change. However, it has been shown Disparity change and IOVD are always present in that for small, rapidly moving objects, monocular cues natural examples of MID, but psychophysicists have should be relatively ineffective compared to binocular attempted to isolate them in the laboratory. There have cues (Regan & Beverley, 1979). (2) Binocularly, the been repeated demonstrations of MID perception in depth of the object compared to its surroundings at any dynamic random dot stereograms (DRDS) which lack one time is signalled by the relative binocular disparity any monocular motion cues (Julesz, 1971; Norcia & of its retinal images. As the object approaches, disparity Tyler, 1984; Regan, 1993; Cumming & Parker, 1994), changes and the rate of this change offers a potential suggesting that an IOVD is not critical to MID detec- cue to the rate of its motion. A unit sensitive to the tion. Cumming and Parker (1994) and Gray and Regan changing output of a population of static disparity (1996) have demonstrated that detection thresholds for detectors tuned to different depths could encode this DRDS stimuli are at least as good as those for an motion neurally. Such a system will be referred to as a equivalent random dot stereogram containing monocu- ‘changing disparity’ system. (3) As depth changes, the lar motion cues (RDS). The converse of a DRDS two monocular images drift in opposite directions (in stimulus has recently been employed, having monocular this instance their speeds are equal and both images motion signals, but no coherent disparity information. move in a temporal direction). This cue to the speed of This was achieved by making the RDS patterns corre- lated in time, but not between stereo half-images. This * Corresponding author. stimulus is capable of supporting MID perception in 0042-6989/00/$ - see front matter © 2000 Published by Elsevier Science Ltd. PII: S0042-6989(00)00095-X 3508 K. Brooks, G. Mather / Vision Research 40 (2000) 3507–3516 appropriate conditions (Shioiri, Saisho & Yaguchi, Schlykowa, Ehrenstein, Cavonius & Arnold, 1993). If 1998; Howard, Allison & Howard, 1998; Allison, the speed of MID is derived exclusively through IOVD, Howard & Howard, 1998). It would seem that changing then monocular speed signals will be reduced for a disparity is not crucial for MID detection either. stimulus approaching in the periphery, leading to a It has been suggested that IOVD plays a pivotal role predictable reduction in the apparent rate of MID. in the encoding of MID speed, since RDS speed dis- However, if we assume that there is no systematic effect crimination thresholds are lower than those for DRDS of eccentricity on perceived depth, speed perception (Harris & Watamaniuk, 1995). However, the generality based on changing disparity alone should be veridical. of this result has been challenged by others (Portfors- An intermediate perceived velocity might reflect an Yeomans & Regan, 1996; Portfors & Regan, 1997), interaction between these two systems. who find equivalent performance for cyclopean (DRDS) and monocularly visible stimuli. Portfors- Yeomans and Regan attribute the difference to the fact 2. Experiment 1 that Harris and Watamaniuk’s stimulus passed through the fixation plane, and hence the DRDS was momen- 2.1. Methods tarily undetectable. When assessed either in front of, or behind the horopter, DRDS speed discrimination per- 2.1.1. Apparatus and stimuli formance was very much improved. However, the A PC-compatible computer equipped with a super- ‘monocularly visible’ targets in these experiments were VGA display card was used to generate the left and not RDSs. Instead they were DRDSs targets moving in right halves of each stereo image side-by-side on a NEC depth on either a blank or a static noise background. Multisync Plus colour monitor. Subjects viewed the two This mode of presentation meant that individual dots in images through a mirror stereoscope (adjusted to give the display did not carry IOVD information, and the convergence appropriate for the viewing distance of 1.8 stimulus patch as a whole carried either a reduced m, whilst maintaining the line of sight as normal to the contrast first order monocular motion signal, or a display surface, to avoid unwanted disparities). A parti- second order motion signal only. It is possible that with tion was placed in the median plane between the stereo- a conventional RDS stimulus superior speed discrimi- scope and the screen, to ensure that each eye saw only nation performance may have been apparent. the appropriate monocular image. The mean luminance Another study by the referent authors attempted to of the screen was 50 cd/m2, and all tests took place in tease apart the two cues in a stimulus in which both are a darkened room. Responses were recorded from a available. Portfors-Yeomans and Regan (1997) demon- two-button response box connected to the computer’s strated that speed discrimination performance is equiv- game port. Subjects wore their best optical corrections alent for oblique MID stimuli with trajectories within for all conditions. the horizontal meridian and in the vertical meridian. The background pattern comprised 50% density This, they argue, implies the use of changing disparity bright/dark dots at a Michelson contrast of 80%, each only. For a stimulus with an oblique trajectory in the subtending 3.62 (H)×4.16 (V) min arc. Each dot field horizontal meridian, monocular velocity signals will was presented in a ‘viewport’ measuring 2.66 (H)×1.24 both be horizontal, but unequal. For a stimulus with an (V) deg arc displayed at screen mean luminance imme- oblique trajectory in the vertical meridian the visual diately above a small high contrast fixation cross, lo- system does not have immediate access to any IOVD cated in a rectangle also at mean luminance. These were information, since monocular motion in both eyes is in identical positions in each stereo half image, and oblique. However, it is not difficult to imagine a system hence were located binocularly in the fixation plane (see which is capable of resolving an oblique motion into its Fig. 1). Nonius lines were also provided on each side of horizontal and vertical components, before feeding the the cross as a fixation aid and a vergence control. The former into a MID stage. A system capable of such an targets themselves were random dot patterns (same size, operation would also predict the results obtained by density and contrast as the background) subtending these authors. 1.93 (H)×1.08 (V) deg arc. These could be displayed at This study aims to discover whether IOVD influences various positions within each viewport to simulate vari- the perception of MID speed in a stimulus containing ous speeds, disparities and binocular directions without both monocular motion signals and continuously overlapping any part of the background dot pattern. changing disparity, by comparing the perceived speed of MID in central vision and in the periphery. It has 2.1.2. Subjects long been known that the perceived rate of frontoparal- There were five subjects: three women and two men lel motion is reduced in the peripheral visual field between the ages of 20 and 30. All had normal or (Lichtenstein, 1963; Campbell & Maffei, 1979, 1981; corrected to normal vision, and though two of the Tynan & Sekuler, 1982; Johnston & Wright, 1986; subjects were experienced psychophysical observers, K. Brooks, G. Mather / Vision Research 40 (2000) 3507–3516 3509 they were all naı¨ve as to the purposes of the experi- by pressing buttons on the response box corresponding ment. All subjects received payment at an hourly rate. to ‘near’ or ‘far’. Similarly, in the stereomotion blind- ness test subjects viewed either an approaching or a 2.1.3. Design and procedure receding MID stimulus and were required to respond A two-factor repeated measures design was used in appropriately.
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