Asymmetries of saccadic eye movements in oriented-line-target search

Visual search for a line-element target differing sufficiently in orientati on from a background of line elements can be performed rapidly, effortlessl y, and without eye movements. There is, however, a response asymmetry: dete ction is better with an oblique target element in vertical or horizontal ba ckground elements than when these orientations are interchanged. If the und erlying visual mechanisms also provide an input to the oculomotor system, t hen a similar asymmetry should be observed in eye-movement behaviour. To te st this hypothesis, an experiment was undertaken in which eye movements wer e recorded while subjects searched for a line-element target in background of line elements; orientations were chosen from the range 0 degrees, 30 deg rees, 60 degrees, and 90 degrees to the vertical. Data from three subjects showed that (1) latencies for the initial saccade, (2) angular errors in in itial-saccade direction, and (3) manual response times depended similarly o n the combination of target- and background-element orientations, performan ce being better for 30 degrees or 60 degrees targets in 0 degrees or 90 deg rees backgrounds than vice-versa. The early orientation-selective mechanism s responsible for the rapid detection of oriented-line targets are probably the same as those providing signals for saccadic eye movements. (C) 1999 E lsevier Science Ltd. All rights reserved.