Induced Gamma-Band Activity and Fixational Eye Movements are Differentially Influenced by Low-and High-Level Factors in a Visual Object Classification Task

Until recently induced high frequency oscillatory activity (gamma-band activity; >30 Hz) was considered a neural marker of cortical object representation. However, Yuval-Greenberg et al (2008; Neuron) demonstrated that induced gamma-band activity (GBA) in the elecetroencephalogram (EEG) is susceptible to artifacts caused by miniature eye movements, which account for the major part of the signal in the crucial time window of 200-400 ms after stimulus onset. Is there an underlying cortical-induced gamma-band response that is obscured by ocular artifacts but can still be recorded with EEG? Furthermore, if object-specific modulations of induced GBA in previous studies were caused by ocular artifacts, should we instead study fixational eye movements as a response that can reflect higher-level representational processes in vision? In order to investigate this, we conducted an eye tracking experiment and an EEG experiment using the same design. Participants were asked to classify line drawings of objects or non-objects. To introduce low-level differences, their contours were defined along different directions in cardinal colour space: 1) S-cone-isolating (S), or 2) intermediate isoluminant (S and L-M), or 3) a full-colour stimulus, containing an additional achromatic component (S; L-M; L+M+S). In both experiments, behavioural performance was optimal for full-colour stimuli. In the eye tracking experiment, fixational eye movement rates 200-400 ms after stimulus onset depended on low-level factors, with no difference between objects and non-objects. In the EEG experiment, miniature eye movements were identified and removed using the saccadic filter approach. The artifact-free induced GBA exhibited a lateralised distribution, with enhancements at left and right posterior sites. Activity was higher for full-colour objects on the left, with the opposite effect observed on the right. We conclude that induced GBA can be observed in the EEG. While it showed high-level object-specific modulations, miniature eye movements were driven solely by low-level information