To manipulate perceived position, we exploited the
De Valois effect: Objects with stationary contrast envelopes and moving carriers show a pronounced displacement in their perceived position in the direction of the carrier motion (De Valois & De Valois,
1991). Investigations of the De Valois effect using functional MRI have shown that its magnitude does not correlate with activity in V1 (Whitney et al.,
2003) and that it is disrupted only by transcranial magnetic stimulation (TMS) over area MT/V5, with no discernable effects from TMS over V1 (McGraw, Walsh, & Barrett,
2004). Further, visual transients can restore the veridical position of moving elements, suggesting that these veridical signals are maintained within V1 throughout (Kanai & Verstraten,
2006). Positional shifts can nonetheless be induced by crowded motion signals (Whitney,
2005), suggesting that these effects are produced by mid-level mechanisms. Indeed, a dependence on perceived (rather than physical) position is a hallmark of cortical areas such as V3a and V4, as seen with both moving (Maus, Weigelt, Nijhawan, & Muckli,
2010; Sundberg, Fallah, & Reynolds,
2006) and static stimuli (Fischer, Spotswood, & Whitney,
2011). The cortical locus for these motion-induced shifts in position might therefore set a lower limit on the locus of crowding. Were crowding associated with processing as early as V1, we would expect its magnitude to follow the physical position of flankers. A later-stage process should instead follow the perceived position of these elements.