Processing directional information in stimuli inhibits the spatial association of luminance levels

Previous studies have found that the spatial-numerical association of the response codes (SNARC) effect automatically occurred when processing both numbers and non-symbolic magnitudes. However, this conclusion was challenged by several recent studies that found no SNARC effect when processing non-symbolic magnitudes with a directional cue. In the present study, we hypothesized that automatic spatial association of non-symbolic magnitudes would be inhibited by directional cues; thus, we utilized left and right arrow stimuli with different luminance levels to systematically investigate the spatial association of luminance. To ensure that participants could effectively discriminate the luminance stimuli, we first replicated the SNARC effect in Experiment 1, by presenting rectangles with different luminance levels. Then, arrows with the same luminance levels as the rectangles were randomly presented to participants on the centre of a screen; participants completed direction classification (Experiment 2), colour classification (Experiment 3), or luminance classification (Experiment 4) tasks with these arrow stimuli. We found that (1) the SNARC effect was present when processing rectangles with different luminance levels (Experiment 1); however, (2) the Simon-like effect rather than the spatial association of luminance was observed when processing arrows with different luminance levels in the luminance-irrelevant classification tasks (Experiments 2 and 3) and the luminance-relevant classification task (Experiment 4). These results indicate that processing of a directional cue inhibited the spatial association of luminance in both luminance-relevant and luminance-irrelevant classification tasks.