Further evidence for the spread of attention during contour grouping: a reply to Crundall, Dewhurst, and Underwood (2008)

In a contour-grouping task, subjects decide whether contour elements belong to the same or different curves. Houtkamp, Spekreijse, and Roelfsema (2003) demonstrated that object-based attention spreads gradually over contour elements that have to be grouped in perception. Crundall, Dewhurst, and Underwood (2008) challenged this spreading-attention model and suggested that attention in the contour-grouping task is not object based but rather has the shape of a zoom lens that moves along the relevant curve. To distinguish between object-based and spatial attention, they changed the stimulus and measured the impact on performance. Subjects were not able to correct for changes at the start of the relevant curve toward the end of the trial. They suggested that attention did not stay at the beginning of the curve, in accordance with a moving zoom lens model. Here, we examine the task of Crundall et al. and find that subjects perceive the changes but fail to correct their response. By measuring change detection directly, we find that performance is much better for the start of the relevant curve than for an irrelevant curve, at all times. Our findings do not support the zoom lens model but provide further support for the spreading attention model