Guidance and selection history in hybrid foraging visual search

Abstract

In Hybrid Foraging tasks, observers search for multiple instances of several types of target. Collecting all the dirty laundry and kitchenware out of a child's room would be a real-world example. How are such foraging episodes structured? A series of four experiments shows that selection of one item from the display makes it more likely that the next item will be of the same type. This pattern holds if the targets are defined by basic features like color and shape but not if they are defined by their identity (e.g., the letters p & d). Additionally, switching between target types during search is expensive in time, with longer response times between successive selections if the target type changes than if they are the same. Finally, the decision to leave a screen/patch for the next screen in these foraging tasks is imperfectly consistent with the predictions of optimal foraging theory. The results of these hybrid foraging studies cast new light on the ways in which prior selection history guides subsequent visual search in general.

Keywords: Priming; Visual search; Working memory.

Fig 1:

Stimuli for Exp. 1. After starting in this rough grid, items moved about the screen to thwart a strategy of systematically selecting items from, say, top left to bottom right.

Figure 2:

Distributions of runs of different lengths. Green circles show the observed distribution. Red squares show the results of simulating random selection. Circles with black outlines show points where the functions are statistically different by 1-sample t-tests (p<0.01). Significance was not calculated for runs > 10 because the chance probability is extremely low.

Figure 3:

RT as a function of Effective Set Size for run and switch trials for each condition. Effective set size is the number of remaining items divided by the number of remaining targets. For graphing, effective set sizes are rounded to the nearest integer and averaged.

FIGURE 4:

Distances in pixels between successive target collections in each condition. Different lines represent runs of different lengths. Data are plotted in terms of reverse clicks, with 0 being the last click in a run before a switch, 1 being the penultimate click in the run, etc.

Figure 5:

Percentage of targets left on the screen when observers chose to move to the next screen. Each data point shows the percentage for one type of target (e.g. p or d) left on screen for each screen for each observer (many data points overlap). Dark bars show mean +/− 1 s.d. for each observer. Observers are sorted by the percentage left behind in the letter task.

Figure 6:

Rate of selection as a function of Reverse Click. Reverse Click #1 is the final click in the patch. #2 is the penultimate click and so forth. Horizontal lines are the average rate for the task as a whole. Shaded areas are +/1 s.e.m. around that average. Error bars are +/− 1 s.e.m.

Figure 7:

Rate (1/RT) as a function of reverse click position in a run within a patch. The first trial in a run is a “switch trial” and those lower rates are plotted as larger, outlined symbols. Dashed lines represent the average rates for the entire task.

Figure 8:

(A) Target types for Experiment 2. Targets can share 0, 1, 2, or 4 basic features with each other. (B) Percent deviation from baseline for selection of one type of target on the current click, given selection of one type of target with the previous click. Thus, for example, selection of the horizontal orange target twice in a row occurs 21% more often than would be expected given the prevalence of those targets in the display. Colors are simply a coarse coding of the deviation percentages.

Figure 9:

RT × Effective Set Size results for Normal and Amnesic conditions. Run and Switch clicks are plotted separately. The amnesic manipulation produces a substantial, additive increase in RT.

Figure 10:

Transition probabilities between each of the four types of targets in Normal (A) and Amnesic (B) conditions of Experiment 4. Results show the difference in the percentage of actual transitions and the percentage of possible transitions. Thus, if there were 25% blue crescents in the display but they were selected only 10% of the time following a green rectangle, the table would show a −15% difference. Results do not differ significantly between conditions. Colors coarsely code the percentages.

Figure 11:

A theoretical snapshot of hybrid foraging. The memory set resides in Activated Long-Term Memory (ALTM). The current object of attention is selected into Working Memory (WM) from the Stimulus. The WM representation is matched to the memory set in ALTM in order to determine if it is a target. If the item, represented in WM, is a target (as it is, in this case), its features prime subsequent selection of similar items in the stimulus.