The neural basis for perceptual grouping operations in the human visual system, including the processes which generate illusory contours, is fundamental to understanding human vision. We have employed functional magnetic resonance imaging to investigate these processes noninvasively. Images were acquired on a GE Signa 1.5T scanner equipped for echo planar imaging with an in-plane resolution of 1.5 x 1.5 mm and slice thicknesses of 3.0 or 5.0 mm. Visual stimuli included nonaligned inducers (pacmen) that created no perceptual contours, similar inducers at the corners of a Kanizsa square that created illusory contours, and a real square formed by continuous contours. Multiple contiguous axial slices were acquired during baseline, visual stimulation, and poststimulation periods. Activated regions were identified by a multistage statistical analysis of the activation. for each volume element sampled and were compared across conditions. Specific brain regions were activated in extrastriate cortex when the illusory contours were perceived but not during conditions when the illusory contours were absent. These unique regions were found primarily in the right hemisphere for all four subjects and demonstrate that specific brain regions are activated during the kind of perceptual grouping operations involved in illusory contour perception.Our goal is to identify areas of human cortex involved in the operation of perceptual grouping of local features into a global percept. We used functional magnetic resonance imaging (fMRI), a noninvasive neuroimaging technique which relies on local variations in blood supply and 02 concentration during neural activity (1-3), to investigate these processes. Neural activation within the cerebral cortex is believed to be associated with an increase in blood flow that is out of proportion to the 02 consumption, thus decreasing the capillary 02 extraction fraction and delivering more oxyhemoglobin to the local venous circulation (4-7). The resulting decrease in the local capillary and venous deoxyhemoglobin concentration results in an increase in the T2* weighted magnetic resonance signal due to the decreased paramagnetic effects of deoxyhemoglobin (8, 9). These local changes in blood chemistry can be observed without the use of exogenous contrast enhancing agents on clinical magnetic resonance scanners (3,(10)(11)(12), which has enabled fMRI localization of visual (10-16), motor (3,(17)(18)(19), auditory (20), speech (21), taste (22), and olfactory (23) processing in the human brain. Since fMRI is now established as a method for exploring the functional organization of the human brain (24), we applied this technique to investigate the global processes of perceptual grouping in vision.Borders between visual objects and their background are usually defined by changes in luminance or color. However, perceptual borders can be created by inducing elements distant from the perceived border, as in Kanizsa's triangle (25). The phenomena of perceived visual borders not associated with net lumi...

Rapid sensory adaptation is observed across all sensory systems, and strongly shapes sensory percepts in complex sensory environments. Yet despite its ubiquity and likely necessity for survival, the mechanistic basis is poorly understood. A wide range of primarily in vitro and anesthetized studies have demonstrated the emergence of adaptation at the level of primary sensory cortex, with only modest signatures in earlier stages of processing. The nature of rapid adaptation and how it shapes sensory representations during wakefulness, and thus the potential role in perceptual adaptation, is underexplored, as are the mechanisms that underlie this phenomenon. To address these knowledge gaps, we recorded spiking activity in primary somatosensory cortex (S1) and the upstream ventral posteromedial (VPm) thalamic nucleus in the vibrissa pathway of awake male and female mice, and quantified responses to whisker stimuli delivered in isolation and embedded in an adapting sensory background. We found that cortical sensory responses were indeed adapted by persistent sensory stimulation; putative excitatory neurons were profoundly adapted, and inhibitory neurons only modestly so. Further optogenetic manipulation experiments and network modeling suggest this largely reflects adaptive changes in synchronous thalamic firing combined with robust engagement of feedforward inhibition, with little contribution from synaptic depression. Taken together, these results suggest that cortical adaptation in the regime explored here results from changes in the timing of thalamic input, and the way in which this differentially impacts cortical excitation and feedforward inhibition, pointing to a prominent role of thalamic gating in rapid adaptation of primary sensory cortex.

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