To test these predictions, the following experiment was carried out. Participants had to press one key if a capital E was present in a display comprising Ls and Fs and another key if there was no E present. Each participant pressed the present key with their right hand and the absent key with their left hand.
The experiment was designed such that across the trials the number of non-targets varied. Four display sizes were used, containing respectively, 5,10 ,15, or 20 letters. On present trials the target took the place of one of the non-targets. Both kinds of non-targets were equally represented in the displays. For each of the four display sizes there were 20 present trials and 20 absent trials.
On each trial a central fixation point was presented for 100 msec and then was irnrnediately replaced by a display which remained on for 150 msec. Participants were told to respond as quickly as possible. The data from this experiment are given in Table 1.
Table 1. Mean reaction times in msecs and percentage errors for different display sizes. Present trials are where the target E was present in the display and absent trials where it was not.
| Display size | ||||
|---|---|---|---|---|
| 5 | 10 | 15 | 20 | |
| Present trials | ||||
| RTs | 429 | 435 | 454 | 470 |
| %Errors | 5.0 | 7.5 | 2.7 | 1.5 |
| Absent trials | ||||
| RTs | 550 | 587 | 604 | 675 |
| %Errors | 2.3 | 4.5 | 6.8 | 5.6 |
Having obtained this support for their neural network model the researchers went on to design a further experiment. This was to test the counter-intuitive prediction that carrying out a simultaneous cognitive task would have no effect on visual search. To test this, half the participants were given a concurrent short-term memory task on every trial. They were presented with a list of six random digits and had to hold them in mind whilst the visual display was presented. When the trial was complete, the participant then had to recall the digits in the correct serial order. This was the dual-task condition. In the single task condition, participants viewed the displays and made the appropriate responses without an additional cognitive load being imposed on them.
In Experiment 2, participants were presented with the same displays as before; however, now the task was simply to press a single key when the target was present and not make any response when it was absent. RT and errors were recorded as in Experiment 1 and means for these data are presented in Table 2.
Table 2. Reaction times in msecs and percentage errors for different display sizes.
| Display size | ||||
|---|---|---|---|---|
| 5 | 10 | 15 | 20 | |
| Single task | ||||
| RTs | 413 | 427 | 460 | 468 |
| %Errors | 2.3 | 4.5 | 6.8 | 5.6 |
| Dual task | ||||
| RTs | 347 | 367 | 370 | 675 |
| %Errors | 22.5 | 37.5 | 23.7 | 24.5 |
From this pattern of results the researchers conclude that their predictions basically hold up but that there is some additional effect of the secondary short-term memory task. Specifically, a short-term memory task prevents conscious processing of the primary search task and thus reduces interference from the non-target items. This additional effect was duly programmed into the neural network model.