Elsevier

Neural Networks

Volume 16, Issues 5–6, June–July 2003, Pages 933-938
Neural Networks

2003 Special issue
Moving objects appear to slow down at low contrasts

https://doi.org/10.1016/S0893-6080(03)00111-4Get rights and content

Abstract

Moving cars give the illusion of slowing down in foggy conditions, because low contrast reduces perceived speed. A grey square that drifts horizontally across a surround of black and white vertical stripes appears to stop and start as it crosses each stripe, because its contrast keeps changing. A moving square whose vertical and horizontal edges have different contrasts will show illusory distortions in perceived direction. Contrast also affects the apparent amplitude and salience of back-and-forth apparent motion. Finally, a line of black and white dots on a grey surround moves in illusory directions, because of a mismatch in the contrasts along and across the dotted line. Thus, motion signals in the early parts of the visual system are profoundly altered by stimulus luminance and contrast. This suggests that motion is coded by the relative firing rates of neural channels tuned to fast and slow motion, with contrast-dependence being a motion analog of the Bezold–Brucke hue shift.

Introduction

Motion perception allows us to keep track not only of moving objects, but also of our own movements through space (Gibson, 1950, Nakayama, 1985). It also provides valuable raw material for neural modelers. One might guess that motion is a recent evolutionary development, but in fact, as Walls (1942) pointed out, motion perception is one of the most ancient and primitive forms of vision. A hungry frog will starve to death on a heap of plump dead flies, but if one of these flies is jerked around on a fishing line in front of the frog, it will immediately snap up the insect and eat it up. Motion plays a crucial part in the constant arms race between predators and prey. Lions and gazelles have excellent vision for motion, gazelles so that they can see the big cats creeping up on them and predators so that they can track the hasty flight of their prey. Lions will stalk their prey stealthily making minimal movements, and young gazelles will often freeze as a defensive measure, in an effort to outfox the motion perception of the other species.

The range of speeds that we can see is an impressive 1000:1. The moon's slow sail across the sky is too slow, but only just too slow, for us to see. It moves through 360° of visual angle in 24 h, or 0.25 min arc per second of time. Stated differently, it moves through its own diameter in a time of 2 min. The fastest speed we can resolve is about a thousand times faster, depending on illumination and adaptation.

We can run no faster than about 10 mph. Modern cars have increased this speed 10-fold, to a maximum of 100 mph. Nowadays we could hardly live without cars—but it is easy to die in them, since the stopping distance of a vehicle goes up with the square of the velocity, from 4 ft at 10 mph to a frightening 400 ft at 100 mph. And that is on a good dry road in ideal conditions! No wonder that car accidents are the leading cause of death for people between the ages of 5 and 44. One cause of accidents that can be avoided is driving too fast in the fog. For example, on November 4th, 2002, nearly 200 cars and big-rig trucks collided in heavy fog on the Long Beach Freeway, injuring dozens of people, including nine critically. A mangled mess of cars, vans and big-rig trucks shutdown the freeway, about 25 miles south of Los Angeles, for nearly 11 h. Authorities said some motorists were driving too fast for the foggy conditions. Estimates are that cars were moving at 25–35 mph. In the state of Wisconsin alone, about 1200 vehicle accidents occur each year when dense fog is a factor. This results in about 16 deaths and 700 injuries. Nationally, an average of 950 people die in winter-related road accidents each year. Many of these could be avoided.

What can be done? Often fog simply makes other cars invisible. Motorists do not see them and crash into them, and visual science can do nothing about this, although IQ testing might help. But at other times fog makes other cars somewhat less visible without hiding them completely, and other motorists misjudge them. Here visual science can perhaps make a useful contribution. Many anecdotes suggest that during a fog, other cars and also one's own car appear to move more slowly than their actual speeds. My recent findings attribute both phenomena to the fact that objects appear to move more slowly when they are low in contrast, as they are in a fog. (Note: ‘Contrast’ refers throughout this paper to the measurable stimulus property of differences in luminance. It does not refer to the ‘simultaneous contrast’, or illusory brightness induction, that is caused by lateral inhibition.) In a fog, other cars are reduced in contrast so they appear to be going more slowly than they really are. Also, a driver judges his own speed largely by visual cues from the landscape as it slides past him, often viewed through the side windows of the car in peripheral vision (Anstis, 1998). Fog reduces the contrast of the passing landscape, so it appears to slip by him more slowly and he believes that he himself is driving slowly.

Section snippets

Results

It is known that apparent speed varies with contrast (Stone and Thompson, 1992, Thompson, 1982, Thompson and Stone, 1997, Thompson et al., 1996). I have found some novel and direct demonstrations of these illusory changes in apparent speed. Two squares, one of them light grey and the other one dark grey, moved horizontally at constant speed across a stationary surround of vertical stripes (Fig. 1). Each square was exactly two stripe widths in diameter, so that its front and back edges always

Discussion

All these contrast-based illusions of motion are compatible with models of motion coding that use velocity-tuned neural units, each tuned to a different range of speeds. Such units have been found in primate MT (Allman et al., 1985, Maunsell and Van Essen, 1983, Mikami et al., 1986, Zeki, 1974). Let us briefly compare motion coding to color coding. The retina contains three types of cones, namely R, G and B, sensitive, respectively, to long, medium and short wavelengths. The cones have broad,

Acknowledgements

Supported by grants from the UCSD Department of Psychology and Academic Senate.

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