Encoding- and decision-related brain activity during a motion judgment task

Abstract

We have previously used steady-state VEPs to show that responses in visual cortex are strongly suppressed when motion stimuli produce inter-ocular velocity differences (Kohler et al., 2018), regardless of whether these differences give rise to movement in depth (MID). Here we used an event-related VEP design to explore how activity evoked by lateral movement and MID unfolds over time. Participants (n=20) viewed stereoscopically presented random-dot patterns in which a central region moved in one of four directions. Dots underwent single-shot apparent motion on every trial: in-phase motion between the two eyes produced lateral movement (left/right), while anti-phase motion gave rise to MID (towards/away). Participants indicated movement direction as quickly and accurately as possible using the keyboard arrow keys. For lateral movement, accuracy was at ceiling and reaction time (RT) was ~800ms, while MID produced lower accuracies (~80%) and longer RTs (~1000ms). We used reliable components analysis to extract maximally correlated signal components from the EEG data (Dmochowski et al., 2012). A medio-frontal component appeared to capture decision-related activity, exhibiting a ramp-like shape that was shallower for MID, consistent with a longer integration period leading to slower RTs. An additional component, centered over occipital cortex, appeared to primarily capture encoding-related activity. Lateral movement and MID produced indistinguishable positive peaks at ~70ms, but diverged at later negative (~180ms) and positive (~350ms) peaks. MID responses were enhanced (more negative) around the negative peak (100–200ms) and suppressed (less positive) leading up to the second positive peak (250–350ms). This late-onset suppression is consistent with the finding that suppression depends on second-order processes, including extraction of relative motion and relative disparity (Kohler et al., 2018). The current results provide a time-resolved electrophysiological analogue to psychophysical data showing that temporal integration underlying decision making is near-perfect for lateral motion, but sub-optimal for MID (Katz et al., 2015).