ABSTRACT
Visual search experiments, in which subjects search an array of background objects for a target, are difficult to interpret because eye movements occur that are not under experimental control. The present experiments eliminated eye movements by presenting arrays of alphanumeric characters in brief flashes on a cathode ray oscilloscope. In previous experiments of this type we found that subjects searching for a numeral target in a background of letters could search simultaneously for an unknown one-of-ten numeral almost as well as for a single known numeral, and they could scan in parallel 15 to 25 characters in an array. The efficiency of this search was shown to be limited by the local density of characters.
In the attention experiments reported here, two different targets appeared in each sequence of character arrays. When the subject’s task was to search simultaneously for an unknown large-size and an unknown small-size numeral, there was considerable interference between the two tasks. Instructions to attend primarily to the large or primarily to the small numeral were highly effective. Two other pairs of visual search tasks also were studied, and data from the three experiments were used to trace out Attention Operating Characteristic (AOC) curves. An AOC is defined as the locus of points on a graph plotting performance on search task 1 against performance on search task 2; it is analogous to the Receiver Operating Characteristic (ROC) of signal detection theory. Using an AOC, it is possible to measure the compatibility of two tasks between which attention is divided. In order to do this one cannot use just one condition of attention for each pair of tasks, for this would be comparing one point from each of two curves instead of comparing two curves. (A similar problem occurs in signal detection with ROC curves).
Attention-switching and attention-sharing models of two-task performance are defined. Our data enabled us to show that, in these experiments, movement along the AOC was primarily due to switching between two attention states, although some sharing of attention also occurred.
