Figure 3: (A) The left column shows the significant cluster in cortical source space of the comparison between the “attend to both rings” and “attend to the central fixation cross” pre-cue conditions. The upper panel of the right columns depicts the mean relative power across the sources pertaining to the significant cluster. The lower panel shows the corresponding paired observation plot for mean normalized values. Red lines represent participants with a difference exceeding the lower bound of the 95% confidence interval of paired differences. Grey lines represent differences that did not meet this criterion. (B) The upper panel shows the mean power across the significant source cluster of (A) for the right and homologous left hemisphere regions. The lower panel depicts the corresponding paired observation plot for mean normalized values, so that individual differences can be observed at the same scale. Red lines for the right hemisphere represent participants with a difference exceeding the lower bound of the 95% confidence interval of differences. Grey lines represent data that do not meet this criterion. Red lines for the left hemisphere represent participants with a difference within the 95% confidence interval including zero (indicating the absence of an effect). Grey lines represent differences outside these boundaries. Error bars in all subplots indicate the s. e. m. ri = rings, cr = cross.
However, the main objective of this study was to characterize how stimulus driven ssVEF responses are modulated by shifts of spatial attention when spatial attention is already directed towards the peripheral or central visual field (rings vs. cross). Therefore, we assessed the interaction of relative ssVEF power changes across experimental phases (pre-cue baseline, left or right visual hemifield not attended or attended) and pre-cue baseline condition (rings vs. cross). The nonparametric cluster-based permutation test based on previously formed F clusters (see methods) for the left visual hemifield (lvf) stimulation indicated an interaction between the pre-cue baseline condition (attend rings vs. central fixation cross) and experimental phase (pre-cue baseline, lvf not attended, lvf attended). The significant source cluster encompassed V1, V2, hMT+, the occipito-parietal and inferior-temporal visual cortex of the right hemisphere (summed F value = 1044, p < 0.001, maximum parametric F(2, 38) = 9.71, p < 0.001, ε = 0.84, η2 = 0.34; Figure 4A). However, right visual hemifield stimulation (rvf) resulted in an interaction between pre-cue baseline condition and experimental phase only at trend level localized in left V1, V2, and a small part of the occipital-parietal region (summed F = 137, p = 0.084; maximum parametric F (2, 38) = 8.09, p = 0.004, ε = 0.73, η2 = 0.30, Figure 4A).
Given the height of the left hemispheric cluster (F(2, 38) = 8.09) the trend level cluster based significance was probably due to the smaller extension of the source cluster. Therefore, in order not to rule out a significant interaction for the rvf stimulation in the left hemisphere, the mean ssVEF power values across the significant source clusters of the left and right hemispheres were entered in a repeated measurements ANOVA with the within-subject factors pre-cue baseline, experimental phase and hemisphere. An absent interaction between the pre-cue baseline condition and experimental phase in the left hemisphere should result in a pre-cue baseline by experimental phase by hemisphere interaction. However, no such interaction was observed (F(2, 38) = 1.66, p = 0.20, ε = 0.93, η2 = 0.08). Across hemispheres, the interaction pre-cue baseline by experimental phase indicated that ssVEF power was differently modulated depending on the pre-cue baseline condition (F(2, 38) = 8.57, p < 0.001, ε = 0.71, η2 = 0.31). Figure 4B depicts the mean relative ssVEF power changes across all significant cortical sources pertaining to the previously identified source clusters in the left and right cortical hemisphere for all experimental phases (pre-cue baseline, post-cue not attended, and post-cue attended) and the two different pre-cue baseline conditions (attend rings vs. fixation cross). Whereas the participants attended the peripheral rings during the pre-cue baseline task, the ssVEF responses across both cortical source clusters were enhanced for the pre-cue baseline and post-cue attend experimental phases compared to the unattended visual hemifield (pre-cue vs. unattended: t(19) = 3.80, p = 0.001, Cohen’s d = 0.85; attended vs. unattended: t(19) = 4.24, p < 0.001, Cohen’s d = 0.95). However, when the participants attended to small variations of fixation cross sizes during the pre-cue experimental phase, relative ssVEF power was similar to the ssVEF response elicited by the unattended hemifield stimulation during the post-cue phase (t(19) = 1.68, p = 0.110, Cohen’s d = 0.37). In contrast, attending the cued hemifield during the post-cue phase provoked enhanced ssVEF responses in comparison when the hemifield was not attended (t(19) = 4.43, p < 0.001, Cohen’s d = 0.99). Furthermore, when we tested the activation level for the attended side between the conditions cross vs. rings we found a trend for higher activation when subjects attended to central fixation first (t(19) = 1.93, p = 0.070, Cohen’s d = 0.43).