Gamma frequency transcranial alternating current stimulation over the visual cortex modulates contour integration.

INTRODUCTION: The visual system must integrate spatially fragmented information to perceive coherent object boundaries. Contour integration represents a fundamental process underlying this perceptual organization by linking discrete edge elements into continuous contour. Gamma oscillations have been associated with perceptual grouping processes, from contour integration to object perception; however, their causal contribution to contour integration remains unclear.

METHODS: Participants performed a contour detection task in which they identified the orientation of a target contour composed of four collinearly aligned segments embedded among randomly oriented and positioned distractors. To investigate the role of gamma oscillations in contour integration, participants were randomly assigned to one of three groups: gamma-tACS (40 Hz), theta-tACS (7 Hz; active control), or sham stimulation (control condition). Participants completed both pre-stimulation and stimulation sessions. During the stimulation session, the assigned tACS was applied over the early visual cortex.

RESULTS: A mixed-effects linear model analysis revealed a significant session × group interaction, such that contour detection performance improved from the pre-stimulation session to the stimulation session specifically in the gamma-tACS group, whereas no comparable improvement was observed in the sham and theta-tACS groups.

DISCUSSION: These findings provide causal evidence that gamma-frequency stimulation over occipital cortex can facilitate contour integration, suggesting that gamma-band oscillations contribute to the integration of spatially distributed visual elements into coherent contours.