The fixation-related lambda response: Effects of saccade magnitude, spatial frequency, and ocular artifact removal.

Fixation-related potentials (FRPs) enable examination of electrophysiological signatures of visual perception under naturalistic conditions, providing a neural snapshot of the fixated scene. The most prominent FRP component, commonly referred to as the lambda response, is a large deflection over occipital electrodes (O1, Oz, O2) peaking 80-100 ms post fixation, reflecting afferent input to visual cortex. The lambda response is affected by bottom-up stimulus features and the size of the preceding saccade; however, prior research has not adequately controlled for these influences in free viewing paradigms. The current experiment (N = 16, 1 female) addresses these concerns by systematically manipulating spatial frequency in a free-viewing task requiring a range of saccade sizes. Given the close temporal proximity between saccade related activity and the onset of the lambda response, we evaluate how removing independent components (IC) associated with ocular motion artifacts affects lambda response amplitude. Our results indicate that removing ocular artifact ICs based on the covariance with gaze position did not significantly affect the amplitude of this occipital potential. Moreover, the results showed that spatial frequency and saccade magnitude each produce significant effects on lambda amplitude, where amplitude decreased with increasing spatial frequency and increased as a function of saccade size for small and medium-sized saccades. The amplitude differences between spatial frequencies were maintained across all saccade magnitudes suggesting these effects are produced from distinctly different and uncorrelated mechanisms. The current results will inform future analyses of the lambda potential in natural scenes where saccade magnitudes and spatial frequencies ultimately vary.