Saved by a log: how do humans perform hybrid visual and memory search?

Abstract

Could you find 1 of your 1,000 Facebook friends in a crowd of 100? Even at a rate of 25 ms per comparison, determining that no friends were in the crowd would take more than 40 min if memory and visual search interacted linearly. In the experiment reported here, observers memorized pictures of 1 to 100 targets and then searched for any of these targets in visual displays of 1 to 16 objects. Response times varied linearly with visual set size but logarithmically with memory set size. Data from memory set sizes of 1 through 16 accurately predicted response times for different observers holding 100 objects in memory. The results would be consistent with a binary coding of visual objects in memory and are relevant to applied searches in which experts look for any of many items of interest (e.g., a radiologist running through a mental checklist of what might be wrong in a car-crash victim or an airport screener looking for any of a list of prohibited items in a carry-on bag).

Figure 1

(a) Observers memorized a set of 1–16 (Exp 1) or 100 (Exp 2) objects and were tested to confirm that these were retained (which they were with >90% accuracy). (b) Os then performed visual search for any memorized target among 1–16 other objects. (c) RTs increased linearly as a function of the visual set size. The slope of the RT × visual set size function is steeper when larger numbers of targets are held in memory. (d) In contrast, RTs increased as a logarithmic function of memory set size. (e–f) Exp 2 extends the result to memory set size of 100. RT rises linearly with the log of the memory set size (note log x-axis). Solid lines are best-fit regression lines for RT × log2(memory set size) for the Exp 1 data. Dashed lines are the extrapolation of those lines to a memory set of 100. Solid data points at set size 100 show the results for Exp 2 based on data from the entirely different set of observers in Exp 1. Clearly, the extrapolation of Exp 1 results predicts Exp 2 results. In all graphs, solid symbols are average data for 10 observers +/− 1 s.e.m.

Figure 2

Average RT data for Experiment 3 in which targets were present on each trial and observers localized the target with a mouseclick. (a) RT rises linearly with visual set size. (b) RT rises with the logarithm of memory set size. Open circles show the predicted RT for set size 16 based on a logarithmic relationship. Asterisk symbols show the unsuccessful linear predictions.