“…Second, the number of covariates that can be simultaneously matched under practical conditions is limited and likely smaller than the number of variables that are already known to modulate the FRP. In particular, variables shown to affect the waveform include the durations of the preceding and current fixation, the temporal overlap with the stimulus-onset ERP (Coco et al, 2020), the amplitude (e.g., Gaarder et al, 1964;Thickbroom et al, 1991) and direction (Cornelissen et al, 2019;Meyberg et al, 2015) of the incoming saccade, the fixated region of the screen (Dimigen et al, 2013), and local image properties at the foveated spot, such as the local luminance (Armington, Gaarder, & Schick, 1967;Gaarder et al, 1964;Ossandon et al, 2010) and spatial frequency spectrum (Armington & Bloom, 1974;Armington et al, 1967;Ries et al, 2018b;Yagi, Ishida, & Katayama, 1992). In our experience, in a task like sentence reading, it is rarely possible to match more than two to three covariates, at least if these covariates show sizeable condition differences in the first place.…”