Perceptual grouping determines haptic contextual modulation

Since the early phenomenological demonstrations of Gestalt principles, one of the major challenges of Gestalt psychology has been to quantify these principles. Here, we show that contextual modulation, i.e. the influence of context on target perception, can be used as a tool to quantify perceptual grouping in the haptic domain, similar to the visual domain. We investigated the influence of target–flanker group- ing on performance in haptic vernier offset discrimination. We hypothesized that when, despite the apparent differences between vision and haptics, similar grouping principles are operational, a similar pattern of flanker interference would be observed in the haptic as in the visual domain. Participants dis- criminated the offset of a haptic vernier. The vernier was flanked by different flanker configurations: no flankers, single flanking lines, 10 flanking lines, rectangles and single perpendicular lines, varying the degree to which the vernier grouped with the flankers. Additionally, we used two different flanker widths (same width as and narrower than the target), again to vary target–flanker grouping. Our results show a clear effect of flankers: performance was much better when the vernier was presented alone compared to when it was presented with flankers. In the majority of flanker configurations, grouping between the tar- get and the flankers determined the strength of interference, similar to the visual domain. However, in the same width rectangular flanker condition we found aberrant results. We discuss the results of our study in light of similarities and differences between vision and haptics and the interaction between dif- ferent grouping principles. We conclude that in haptics, similar organization principles apply as in visual perception and argue that grouping and Gestalt are key organization principles not only of vision, but of the perceptual system in general