Abstract
Visual processing is classically modeled as a hierarchy of feedforward stages of increasing complexity (Hubel & Wiesel, 1962; Riesenhuber & Poggio, 1999). However, processing of complex stimuli may be performed initially at a coarse level in high-level visual areas, with reentrant processing to low-level areas to refine object representations (Mumford, 1992; Hochstein & Ahissar, 2002). Whether this is also true for a simple element, such as a single orientated bar, is not clear: it may be that it is processed before the detection of complex configurations, without any re-entrant processing. We tested a strong form of the re-entrant hypothesis (Gorea & Julesz, 1991), that even the detection of a line-element stimulus in a typical visual search task would be influenced by its insertion into a complex configuration. We measured the detection speed for vertical target elements embedded within arrays of 22.5° oriented noise elements, presented for a maximum of 3000ms. The vertical target element was clustered with three horizontal lines to form one of four patterns: an upright or inverted schematic face, or a symmetric or asymmetric non-face pattern. In other conditions the vertical line appeared outside the cluster. Finally, the vertical line could appear alone or with three randomly distributed horizontal lines (Fig1a). The target was detected faster when clustered with horizontal lines than when presented alone (p<0.01), outside the cluster (p<0.0001), or amongst random horizontal lines (p<0.0001). Most importantly, it was detected faster in the face-like pattern than in the asymmetric (p<0.0001), symmetric (p<0.0001), and inverted face-like pattern (p<0.05) (Fig1b). This novel face-superiority context effect points to a simultaneous activation of lower and higher visual processes. The results provide evidence for a non-hierarchical organisation in the visual system, suggesting that even simple perceptual decisions are reached through a recursive exchange of information between low and high processing levels.
Meeting abstract presented at VSS 2013