Magnetoencephalographic responses to illusory figures: early evoked gamma is affected by processing of stimulus features

We examined evoked and induced responses in event-related fields and gamma activity in the magnetoencephalogram (MEG) during a visual classification t ask. The objective was to investigate the effects of target classification and the different levels of discrimination between certain stimulus feature s. We performed two experiments, which differed only in the subjects' task while the stimuli were identical. In Experiment 1, subjects responded by a button-press to rare Kanizsa squares (targets) among Kanizsa triangles and non-Kanizsa figures (standards). This task requires the processing of both stimulus features (collinearity and number of inducer disks). In Experiment 2, the four stimuli of Experiment 1 were used as standards and the occurre nce of an additional stimulus without any feature overlap with the Kanizsa stimuli (a rare and highly salient red fixation cross) had to be detected. Discrimination of collinearity and number of inducer disks was not necessar ily required for task performance. We applied a wavelet-based time-frequenc y analysis to the data and calculated topographical maps of the 40 Hz activ ity. The early evoked gamma activity (100-200 ms) in Experiment 1 was highe r for targets as compared to standards. In Experiment 2, no significant dif ferences were found in the gamma responses to the Kanizsa figures and non-K anizsa figures. This pattern of results suggests that early evoked gamma ac tivity in response to visual stimuli is affected by the targetness of a sti mulus and the need to discriminate between the features of a stimulus. (C) 2000 Elsevier Science B.V. All rights reserved.