Abstract
In computer vision, the detection of ‘salient regions’ (SRs) — a set of contiguous image pixels that likely corresponds to a major surface in the scene — has been shown to enhance performance of scene segmentation algorithms. The brain may use a similar strategy to accomplish scene segmentation. We have shown (Stanley & Rubin 2003) that the lateral occipital complex (LOC), known to respond to Kanizsa-type illusory squares, maintains its response to SRs. SR stimuli were created by rounding the corners of the Kanizsa ‘pac-men’ inducers and misaligning the edges, eliminating the illusory contours (ICs) but maintaining the enclosed SR. Here we combine psychophysics and fMRI to examine the concept of SRs in the brain. We adapted an apparent motion stimulus (Ramachandran 1985) where illusory squares are seen hopping through an array of fixed, rotating ‘pac-men’ inducers. Observers performed a 2-alternative forced choice task on the direction of apparent motion. In addition to the ICs, observers viewed a SR condition, and a control condition of scrambled inducers with no enclosed regions. We found that observers performed significantly better in both IC (acc=%100, rt=0.912±0.041secs) and SR conditions (acc=%94.44±3.87, rt=1.288±0.091secs) than in control conditions (acc=%55.56±8.39, rt=2.839±0.149secs. Performance in the control conditions improved (acc=%83.78±6.14, rt=2.113±0.158secs) when the inducers defining the pattern were made brighter than the other inducers in the array. We also obtained fMRI data from three observers on the same paradigm. The LOC was active during illusory contour and salient region conditions, but not during control conditions, supporting our previous findings. Interestingly, Seghier et al (2000) reported fMRI activation in V1/V2 to apparent motion of IC stimuli but we found none in either IC or SR condition. Our findings indicate that SRs are automatically processed by the visual system and may play an important role in segmentation.