Object-based saccadic selection during scene perception: Evidence from viewing position effects

Abstract

The goal of the present study was to further test the hypothesis that objects are important units of saccade targeting and, by inference, attentional selection in real-world scene perception. To this end, we investigated where people fixate within objects embedded in natural scenes. Previously, we reported a preferred viewing location (PVL) close to the center of objects (Nuthmann & Henderson, 2010). Here, we qualify this basic finding by showing that the PVL is affected by object size and the distance between the object and the previous fixation (i.e., launch site distance). Moreover, we examined how within-object fixation position affected subsequent eye-movement behavior on the object. Unexpectedly, there was no refixation optimal viewing position (OVP) effect for objects in scenes. Where viewers initially placed their eyes on an object did not affect the likelihood of refixating that object, suggesting that some refixations on objects in scenes are made for reasons other than insufficient visual information. A fixation-duration inverted-optimal viewing (IOVP) effect was found for large objects: Fixations located at object center were longer than those falling near the edges of an object. Collectively, these findings lend further support to the notion of object-based saccade targeting in scenes.

On the optimal viewing position for object processing

“…During these refixations, the eyes showed no systematic bias towards either side of the object. This is consistent with Pajak and Nuthmann's (2013) findings for objects in natural scenes. They observed that refixations launched from object boundaries bring the eyes closed to the object's center, whereas refixations that are launched from the center keep the eyes close to the center.…”

“…Fixations were longer towards the center of isolated words and objects, which is in line with previous reports on word identification (Vitu et al, 2007), sentence reading (Hyönä & Bertram, 2011;Nuthmann et al, 2005Nuthmann et al, , 2007Vitu et al, 2001), object recognition (Henderson, 1993) and scene viewing (Pajak & Nuthmann, 2013). This effect, referred to as the inverted OVP (I-OVP) effect, might appear counterintuitive when assuming that fixation duration is an index of processing difficulty.…”

“…To what extent do these effects generalize to objects embedded in natural scenes? Two previous studies investigated this question Pajak & Nuthmann, 2013). In line with isolated-object studies, Pajak and Nuthmann observed a fixation-duration I-OVP effect (for large objects).…”

“…Pajak and Nuthmann (2013) found an I-OVP for large objects (3.15-6°), but not for medium (.85-3.15°) and small (1-1.85°). Foulsham and Kingstone (2013, Experiment 2) did not observe an I-OVP effect.…”

“…Fixating on this position facilitates word identification (O'Regan & Jacobs, 1992) and lowers the chance of within-word refixations (McConkie et al, 1989;O'Regan & Lévy-Schoen, 1987;Rayner et al, 1996;Vitu et al, 1990), but lengthens initial fixation durations (I-OVP effect, Vitu et al, 2001). Several studies have shown that initial viewing position also influences object processing Henderson, 1993;Pajak & Nuthmann, 2013). However, these studies did not directly compare the resulting (I-)OVP curves between words and objects.…”