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Lora T. Likova, Christopher W. Tyler; Structure-from-transients: HMT/MST mediates figure/ground segmentation. Journal of Vision 2005;5(8):1059. doi: 10.1167/5.8.1059.
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Transient changes may be considered as the ‘elemental events’ structuring dynamic visual scenes. Previously we established that synchrony/asynchrony in a spatiotemporal pattern of transients was the critical factor in novel illusions of contextual motion (Likova & Tyler, JOV 2003; SPIE 2003). Now we report strong image segmentation from transient asynchronies in fields of featureless visual noise. We term this phenomenon ‘Structure-from-Transients’ (SFT). BOLD fMRI (GE Signa 3T, spiral acquisition, 23 coronal slices 3 mm3) was used to reveal cortical mechanisms involved in SFT. The stimuli were random dot fields (RDF) of 30×40 deg, replaced by uncorrelated dots every 500ms. Asynchronous updates in subregions of the RDF results in SFT. Figure/ground organization was generated in the test stimuli by transient-asynchrony between an elongated horizontal region (figure) and its surrounding area. The transient changes between these areas in the null stimuli however were synchronized, generating no SFT. Thus the global percepts switched from figure/ground (test) to a homogenous RDF (null) every 9 sec, in 36 blocks per scan. Exp 2: Figure/ground organization was eliminated by SFT-segmentation of the RDF into equal horizontal stripes. In Exp 3, the SFT stripes were defined by unidirectional but asynchronous apparent motion of the noise. Our data show dramatic reorganization of the cortical activation pattern with manipulation of the perceptual SFT organization. Consistent figure/ground-specific activation was seen only in hMT/MST, while surround-specific suppression occurred in V1 (Exp 1). Both were abolished by eliminating the figure/ground organization (Exps 2&3) - the activation shifted to higher retinotopic areas. The results support a view of a recurrent architecture with inhibitory feedback from hMT/MST to the V1 surround projection. We suggest that this cortical network mediating the perceptual reorganization may be activated for figure/ground in dynamic structures in general.
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