Orientation repulsion and attraction in alignment perception

Orientation masking induces changes of discrimination thresholds and perceived orientation. Studies on alignment discrimination of Vernier stimuli concentrated on masking induced changes of discrimination thresholds, without considering possible changes of perceived orientation and/or alignment of the two-line segments. Measuring both parameters in an orientation discrimination task, we confirmed a standard repulsion effect between a single line target and a mask grating that co-varied with elevated orientation discrimination thresholds. Masking a Vernier stimulus in an alignment discrimination task, we observed a strong misperception of alignment that was accompanied with elevated alignment discrimination thresholds. Orientation masking on perceived orientation and alignment of a Vernier stimulus revealed orientation repulsion and attraction that depended on the spatio-orientation configuration of the superimposed stimuli. Control of task-dependent effects confirmed that our observed pattern of results was independent of attentional or cognitive demands.