Which end is up? Two representations of orientation in visual search

What is the orientation of an object? A simple line has an axis of orientat ion. That line, turned upside-down, is indistinguishable from the original line. Thus, the possible orientations of a line range from 0 to 180 degrees . Most objects, however, have an axis and a polarity. A polar object, turne d upside-down, looks upside-down. Accordingly, the orientations of a polar object range from 0 to 360 degrees. A series of visual search experiments w ere run to determine if preattentive processes represent orientation in a 1 80 or a 360 degrees framework. Results suggest that preattentive orientatio n is represented in 180 degrees. Experiments 1 and 4 show that search for a target rotated 90 degrees from the distracters is more efficient than sear ch for a target rotated 180 degrees from the distracters. Experiments 2, 3, and 5 use a variety of different stimuli to demonstrate that search for ta rgets rotated 180 degrees from distracters is inefficient. (C) 1999 Elsevie r Science Ltd. All rights reserved.