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

Visual search for a line-element target differing sufficiently in orientation from a background of line elements can be performed rapidly, effortlessly, and without eye movements. There is, however, a response asymmetry: detection is better with an oblique target element in vertical or horizontal background elements than when these orientation are interchanged. If the underlying visual mechanisms also provide an input to the oculomotor system, then a similar asymmetry should be observed in eye-movement behaviour. To test this hypothesis, an experiment was undertaken in which eye movements were recorded while subjects searched for a line-element target in background of line elements; orientations were chosen from the range 0 degree, 30 degrees, 60 degrees, and 90 degrees to the vertical. Data from three subjects showed that (1) latencies for the initial saccade, (2) angular errors in initial-saccade direction, and (3) manual response times depended similarly on the combination of target- and background-element orientations, performance being better for 30 degrees or 60 degrees targets in 0 degree or 90 degrees backgrounds than vice-versa. The early orientation-selective mechanisms responsible for the rapid detection of oriented-line targets are probably the same as those providing signals for saccadic eye movements.