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Carl E. Granrud, Hillary R. Haynes, Ashley L. Juhl, Chris J. Miller, Christopher D. Sandbach; Perceived size of stoplights: Further investigations into a failure of size constancy. Journal of Vision 2005;5(8):520. doi: 10.1167/5.8.520.
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© ARVO (1962-2015); The Authors (2016-present)
We reported previously that observers underestimate stoplight size at distances of 20 to 200m and that observers rely on assumed size when estimating the size of a distant stoplight (VSS, '03, '04). The stoplights in those studies were suspended above the ground. The present study investigated the perceived size of stoplights that rested on the ground. In Experiment 1 (N = 60), three groups of participants viewed a stoplight that rested on the ground at distances of 20, 120, and 200m. They estimated the size of the light's 30.5 cm lenses by selecting, from a set of nearby comparison circles, a circle that matched the size of the lenses. Size was estimated accurately at 20m but was underestimated, on average, by 17% at 120m and 30% at 200m. At 20 and 120m, size estimates are more accurate for stoplights that rest on the ground than for stoplights suspended above the ground. At 200m, size estimates are equally inaccurate for suspended lights and lights on the ground. The results indicate that the misperceived size of suspended stoplights at 20m is due to their suspension above the ground. At distances of 120m and greater, however, size constancy is not achieved for stoplights that rest on the ground. Experiment 2 (N = 80) investigated whether assumed size affects estimated size for stoplights on the ground. The experiment had two parts: a familiarization period and a test trial. During familiarization, participants studied a standard-sized or smaller-than-normal stoplight for 1 minute from a distance of less than 2m. In the test trial, participants viewed the standard-sized or smaller-than-normal stoplight from a distance of 120m. Test-trial size estimates conformed to the size of the stoplight seen during familiarization. The size of the stoplight seen during the test trial had no effect on size estimates. These results indicate that research participants rely on assumed size to make size estimates for objects on the ground as well as for suspended objects.
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