Abstract
Until recently induced gamma-band activity (GBA) was considered a neural marker of cortical object representation. However, induced GBA in the electroencephalogram (EEG) is susceptible to artifacts caused by miniature fixational saccades. Recent studies have demonstrated that fixational saccades also reflect high-level representational processes. Do high-level as opposed to low-level factors influence fixational saccades? What is the effect of these factors on artifact-free GBA? To investigate this, we conducted separate eye tracking and EEG experiments using identical designs. Participants classified line drawings as objects or non-objects. To introduce low-level differences, contours were defined along different directions in cardinal color space: S-cone-isolating, intermediate isoluminant, or a full-color stimulus, the latter containing an additional achromatic component. Prior to the classification task, object discrimination thresholds were measured and stimuli were scaled to matching suprathreshold levels for each participant. In both experiments, behavioral performance was best for full-color stimuli and worst for S-cone isolating stimuli. Saccade rates 200–700 ms after stimulus onset were modulated independently by low and high-level factors, being higher for full-color stimuli than for S-cone isolating stimuli and higher for objects. Low-amplitude evoked GBA and total GBA were observed in very few conditions, showing that paradigms with isoluminant stimuli may not be ideal for eliciting such responses. We conclude that cortical loops involved in the processing of objects are preferentially excited by stimuli that contain achromatic information. Their activation can lead to relatively early exploratory eye movements even for foveally-presented stimuli.
Highlights
In order to acquire sufficient information from the complex and dynamically changing environment, the visual system implements various strategies
PSYCHOPHYSICS: THRESHOLD MEASUREMENTS Figure 3 presents scaled, suprathreshold contrasts as well as contrasts at threshold for the eye movement experiment, while Figure 4 presents these contrasts for the EEG experiment
The scale factors were significantly larger in the EEG experiment [t(16.29) = 3.08, p = 0.007]
Summary
In order to acquire sufficient information from the complex and dynamically changing environment, the visual system implements various strategies One such strategy is to perform eye movements in order to scan the visual scene, while intermittently maintaining gaze at objects of interest. Engbert and Kliegl (2004) refined Cornsweet’s (1956) suggestions Their analysis revealed that microsaccades operated on two time scales of different characteristics. On a short time scale (up to 20 ms), microsaccades increased fixation errors, increasing retinal image shifts. This most likely contributes to the prevention of perceptual fading (see Hubel and Wiesel, 1968). The purpose of these fixational saccades is likely to be selection or re-selection of scene attributes that are relatively close to fixation
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