The effects of target template speci fi city on visual search in real-world scenes : Evidence from eye movements University of Edinburgh , UK

cue takes longer to set up before becoming fully useful, whereas a precise target template created from a picture cue can be established more quickly but decays over time. Each of the previous studies, and the current one, used a similar type of categorical cueing in the word cue condition (e.g., the word cue apple as opposed to fruit), suggesting that our failure to replicate the previously found interaction was unlikely to be due to different text labels. An obvious difference between the studies of Vickery et al. (2005) and Wolfe et al. (2004) and the current one is that both previous studies used objects arraysVsets of objects with no established spatial relations between each otherVwhereas the present study used semantically rich real-world scenes. In object arrays the only top–down information available to guide search is knowledge of the target’s appearance, so RTs might be sensitive to any changes to the specificity of the target template. In real-world scenes other information is available to guide search (e.g., scene context: Castelhano & Henderson, 2007; Eckstein et al., 2006; Torralba et al., 2006). A potential reason for not finding an interaction between cue type and SOA in the present study is that when a participant knows that other information will be available in the scene to guide search, they may devote fewer cognitive resources to creating a detailed target template. Participants may simply store a coarser version of the templateVeither extracting and storing fewer features from a picture cue or forming a less specific template from a word cueVleaving the template less susceptible to changes in SOA. The present results also appear to contrast with those of Foulsham and Underwood (2007), who found no difference in search time or fixation count between categoricaland instance-cued search. The two studies differed, however, in that Foulsham and Underwood (2007) used a single word cue prior to a block of trials, either indicating a categorical target type (in their case fruit, meaning that several different types of fruit could be the target over the block of trials) or an instance cue (for example, apple, meaning that the target for every trial in the block would be an apple). Participants were never given an exact picture of a cue, and since the angle, size, luminance, color, and other features of the target naturally changed from trial to trial, it would be impossible for their participants to generate a specific target template. Even if participants could store a few features from trial to trial in the instance cue block (e.g., it was a red apple), our results (Experiment 3) and those of previous experiments (Wolfe et al., 2004) indicate that seeing a picture cue of the target is more effective than recalling it. Toward an integrated real-world search model Recent research has identified several sources of information that help guide search in novel, real-world scenes. However, most research to date has focused on how the visual system processes these forms of information Journal of Vision (2009) 9(11):8, 1–13 Malcolm & Henderson 10 Downloaded from jov.arvojournals. org on 04/09/2019 individually. For instance, the processing of image properties has tended to be studied in isolation in the saliency model (Itti & Koch, 2000; Parkhurst et al., 2002). Similarly, the effect of scene context has tended to be studied in isolation (Castelhano & Henderson, 2007; Eckstein et al., 2006; Neider & Zelinsky, 2006; Zelinsky & Schmidt, 2009). And in the present study, we studied the effect of target template specificity in isolation. In the real world, however, when all these forms of information are available, a more efficient method of guiding visual attention would be to integrate two or more information sources during a search task (see Ehinger, Hidalgo-Sotelo, Torralba, & Oliva, 2009). The benefit of integrating different processes in realworld search has been demonstrated in the contextual guidance model (Torralba et al., 2006). In this model saliency at a local spatial scale is constrained by scene context at a global spatial scale. Areas of high salience within a selected global region are given higher weights on an activation map than those that fall outside the selected global region. The model accurately predicted participant’s first few eye movements in a counting search task. This suggests that the visual system benefits from integrating multiple sources of information. Recent empirical evidence, however, indicates that the visual system relies less on low-level saliency than the contextual guidance model suggests (Einhäuser et al., 2008; Foulsham & Underwood, 2007; Henderson et al., 2007; Zelinsky et al., 2006). A future integrated real-world search model may benefit from substituting a form of top– down information, such as a target template, in place of saliency at the local spatial scale. Instead of selecting where within a selected global scene region to fixate based on saliency, a model could compare the target template stored in VWM with the incoming scene percept to weight the activation map accordingly. Local regions with high correlations to the target template are given higher weights on the activation map, particularly if they fall within the selected global region. This way the visual system is looking for target-similar features within a target-probable region of the scene rather than highly salient features which may not have any correlation with the target object. In addition, a complete model of real-world search must consider fixation durations. When sites of fixations are also weighted by their durations, the distribution of attention changes dramatically (Henderson, 2007). The current data demonstrate that when there is a more specific target template available, scanning fixations are shorter as the visual system will have a more definite representation of the target to compare with a fixated region. Since fixation durations vary with the internal representation of the target while the display image and task stay constant, the current data support the hypothesis that fixation durations are, at least in part, under direct control during real-world tasks (Henderson & Pierce, 2008; Henderson & Smith, 2009). Conclusion The present study indicates that a target template can guide attention during real-world visual search. A more specific target template facilitates scanning for and verification of the target. Search initiation time appears to be an automatic process affected only by SOA, with longer SOAs producing shorter initial saccade latencies. Familiarizing participants with the target prior to testing did not affect the target template guidance process. The current study also demonstrates that eye-tracking allows us insight into the processes underlying real-world visual search. By separating traditional unitary RT measures into three behaviorally defined epochs that probe the time taken to initiate search, the time to locate the target, and the time to accept an object as the target, we were able to examine the underlying processes that are affected by target template specificity and so increase our knowledge of search processes.