Segmentation by single and combined features involves different contextual influences

Orientation discrimination of a texture line having orientation different from that of static background lines is facilitated when the lines are aligned along their orientation axis and when their separation is small (Experiment la). The facilitation by alignment remains when motion is added to the target (Experiment 1b). However, when the motion rather than the orientation has to be judged, alignment reduces sensitivity (d') regardless of whether the target has orientation the same as (iso-oriented) or different from background elements (Experiment 2). The inhibitory effect of alignment is confirmed when subjects have to discriminate the motion direction of an iso-oriented target (Experiment 3). Such inhibition by alignment is stronger when elements are close and may reflect a property of lateral interactions of motion detectors, since it is only present when observers have to judge the target motion direction. Overall, our results indicate an opposite role of the lateral interactions that facilitate grouping of iso-oriented and collinear elements, in segmentation by orientation contrast and motion contrast. In other words, global grouping (i) facilitates discrimination of orientation contrast, indicating a global process, and (ii) inhibits both detection and discrimination of motion contrast, suggesting the presence of a local process.