Abstract: Abstract Object recognition is achieved through neural mechanisms reliant on the activity of distributed neural assemblies that are thought to be coordinated by synchronous firing in the gamma-band range (>20 Hz). An outstanding question focuses on the extent to which the role of gamma oscillations in object recognition is dependent on attention. Attentional mechanisms determine the allocation of perceptual resources to objects in complex scenes biasing the outcome of their mutual competitive interactions. Would object-related enhancements in gamma activity also occur for unattended objects when perceptual resources are traded off to the processing of concurrent visual material? The present electroencephalogram study investigated event-related potentials and evoked (time- and phase-locked) and induced (non-time- and phase-locked to stimulus onset) gamma-band activity (GBA) using a visual discrimination task of low or high perceptual load at fixation. The task was performed while task-irrelevant familiar or unfamiliar objects coappeared in the surrounding central area. Attentional focus was kept at fixation by varying perceptual load between trials; in such conditions, only holistic object processing or low-level perceptual processing, requiring little or no attention, are thought to occur. Although evoked GBA remained unmodulated, induced GBA enhancements, specific to familiar object presentations, were observed, thus providing evidence for cortical visual representation of unattended objects. In addition, the effect was mostly driven by object-specific activity under low load, implying that, in cluttered or complex scenes, attentional selection likely plays a more significant role in object representation.
Abstract: The present study investigated neuronal correlates of stimulus processing leading to conscious perception of a task irrelevant global structure in a visual display. To study the underlying neuronal processes, participants were presented different types of dot patterns (Glass patterns) either forming a global structure or forming no global structure while EEG was recorded. Participants were naive about the pattern types and performed a demanding colour discrimination task. Following the experiment, the degree to which participants acquired awareness of the global visual structure was assessed. Early gamma-frequency band responses (gamma, 25-100 Hz) over occipital, parietal, and central areas were enhanced to circular Glass patterns as compared to random dot patterns at 90 ms post-stimulus. This effect was observed exclusively in participants who were subjectively aware of the global pattern structure. In this group of observers, the pattern effect built up gradually during the course of the experiment. The significance of enhanced early gamma responses to global patterns for the production of awareness of the pattern might lie in the increased impact of information conveyed by well synchronised neuronal assemblies to upstream cortical areas.
Abstract: Glass patterns are randomized dot arrays that generate the perception of a global structure. They consist of correlated dot pairs which are generated by geometric transformations. The present study employed behavioral and event-related brain potential (ERP) measures to characterize the underlying neuronal processing when such patterns are perceived. Stimuli were circular, parallel, and randomized Glass patterns presented in two isoluminant colors using a choice reaction paradigm. Sixteen subjects were instructed to differentiate between colors with a button-press response. The N170 component increased in amplitude for circular patterns, and this effect was most pronounced bilaterally over occipito-temporal areas. The results suggest that the global perception of form generated by Glass patterns occurs at a stage of visual processing past area V1.
