Inattention magnifies perceived length: the attentional receptive field hypothesis.

Five experiments demonstrated that a briefly presented vertical line is judged as longer when it is unattended relative to when it is attended. This effect was obtained in estimating the length of 1 of 5 possible lines (Experiment 1) and in matching the length of a test line to a criterion line (Experiments 3 and 4). The directional effect of attention was eliminated when participants estimated the length difference between 2 simultaneously presented lines (Experiment 2). An additional matching experiment (Experiment 5) demonstrated similar lengthening effects for unattended lines and for unattended distances separated by vertically displaced dots. It is proposed that the metric for unattended stimuli is composed of large attentional receptive fields and that the final output is mediated by rounding up processes, so that the unattended line is systematically perceived as longer than the attended one.