The Magical Number One, Plus Or Minus Zero
Ronald A. Rensink, Cambridge Basic Research, Cambridge, Massachusetts, USA.

Investigative Ophthalmology & Visual Science, 40:52. 1999. [ARVO 1999; Ft. Lauderdale, FL.]

Purpose. To determine how many objects we can attend to at the same time.

Methods. Visual search was carried out using arrays of horizontal and vertical rectangles under flicker conditions, where displays continually alternated between "on" (each exposure 80-800 ms) and "off" (blank fields of 120 ms). In variant 1, the target had a changing orientation and the distractors a constant orientation. In variant 2, the target had constant orientation, and the distractors changed.

Results. In variant 1, search rate was constant (indicating a process limit) for on-times up to 600 ms. Beyond this, rate was proportional to on-time (indicating a memory limit), with an average memory capacity of roughly 5 items. In variant 2, search was more difficult, but rate was still constant (process limited) for on-times up to about 300 ms. Beyond this, rate again became proportional to on-time (memory limited). Average capacity here was 1.4 items, with individual observers rarely able to hold on to more than 1.6 items.

Responses of high-capacity observers in variant 1 (capacity of 6 items) were further analyzed in terms of the distribution of reaction times (RTs). For these observers, memory at set sizes 2 and 6 was loaded to capacity, and so only comparison operations were needed to test the items of the subsequent display. Although mean RTs for these conditions showed a significant increase with set size, the spread of the RT distributions (RT variances) did not, indicating that the comparison process could not be terminated until all items had been processed.

Conclusions. Variant 1 shows that attention can access up to 5 stimulus items at a time; variant 2 indicates that these are collected into a single nexus that mediates the perception of change. Finally, the performance of the high-capacity observers shows that this nexus is unitary, i.e., that all the items it contains are considered to belong together. Thus, although we can attend to several stimulus items at the same time, these are treated as parts of just one unitary object.

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