Method:
Design.
All participants were tested on all conditions, which enabled analysis as a repeated measures design. As there was thought to be a possibility of gender difference in reaction times, the gender of each participant was recorded to enable the results of this experiment to be analysed as independent samples. The different conditions were: identical letters, e.g. A A, same name, e.g. A a, & different, e.g. A B or A b. The inter stimulus interval was also varied; for one block the stimuli were presented together, for another there was a one second interval, & for the other there was a two second interval. Therefore the independent variables were the three conditions & the inter stimulus interval. The dependant variable in all cases was the reaction time. Regarding the independent samples aspect of the experiment the two groups were simply male & female.
Participants
There were 32 participants in this experiment, 16 male & 16 female, aged between 15 & 60. One subject’s results were discarded because their (handwritten) results were unreadable. English was not necessarily the first language of each participant although they all had an adequate familiarity with the English alphabet to take part in this investigation.
Materials.
The experiment was presented on a PC, & used ‘Mel lab’ Micro Experimental Laboratory, v1.61. © Psychological Software Tools Inc. The experiment used was from ‘Experiments in Perception’ part 1.4; Perceptual Matching. The results were recorded on pen & paper until each subject’s results had been collected.
Procedure.
Participants were informed that the experiment would take about 24 minutes & that they had to make a choice as to whether the 2nd letter presented was the same as, or different from, the 1st letter. The letter(s) were preceded by a fixation point (+) & due to having to press the space bar between each trial, the experiment could be carried out at the participants own pace. There were full instructions included on screen & prior to the actual experiment there were 3 blocks each of 10 practice trials, in which the participant was required to attain an 80% correct response rate. The experiment consisted of 3 blocks of 64 trials each, all incorrect responses were discarded. The full ‘on screen’ instructions for this experiment are included in Appendix A.
Results:
All results were analysed using SPSS v.10. The mean reaction times for all subjects’, as shown in fig.1, demonstrate that RT’s for identical stimuli were generally faster across the various inter stimulus intervals.
Raw data for all participants mean RT’s can be found in Appendix B.
Hypothesis 1 predicted that group 1; (males) would have significantly different reaction times to group 2; (females) in the same name condition. The corresponding null hypothesis was that there would be no significant difference in RT’s between the two groups. As this was between subjects a one way ANOVA was used for analysis. Table 1 shows the results obtained from this analysis. As p>0.05 across all inter stimulus intervals, the null hypothesis, that there would be no significant difference in reaction times between the two groups, is retained & H1 is rejected.
Table 1: Between subjects’ comparisons on same name condition at 0, 1 & 2 second inter stimulus intervals.
Hypothesis 2 predicted that there would be a significant difference in reaction times between the identical & same name conditions, when stimuli were presented simultaneously or with a 1 second interval. The corresponding null hypothesis was that there would be no significant difference in RT’s between the two conditions. As this was within subjects a paired sample t-test was used for analysis. Table 2 shows the results obtained from this analysis. As the (2 tailed) ‘p’ value for both conditions is .000 the null hypothesis is rejected & H2 is retained.
Table 2: Within subjects’ comparison between identical/same name conditions at 0 & 1 second inter stimulus intervals.
Hypothesis 3 predicted that when the inter stimulus interval was of 2 seconds duration there would be a significant difference in reaction times, on any condition. The corresponding null hypothesis was that there would be no significant difference between the RT’s on any condition when the inter stimulus interval was 2 seconds. As this was within subjects a paired samples t-test was used for analysis. Table 3 shows the results obtained for this analysis. As the (2 tailed) ‘p’ value for each pair is <0.05 the null hypothesis is rejected & H3 is retained.
Table 3: Within subjects comparisons in all conditions at 2 second inter stimulus interval.
Discussion.
Although some of the results for different aspects of the experiment show similar results to past studies, such as Posner & Keele, (1967) & Posner, et al, (1969) there are marked differences to these prior studies in a number of them, particularly those that have the longer, (2 sec) time interval. Also, there were no significant differences between the two groups, on the same name condition. According to Gross, (1994) a difference would not have been unexpected, due to males generally having superior abilities at visual-spatial tasks.
This study had the greatest similarity to Posner & Keele, (1967) although their presentation of the stimuli differed in that they always used an uppercase letter first. Results for the identical/same name condition replicated their findings, showing a significant difference between the two conditions, as well as a similar difference between identical/different conditions, which was also found in Posner & Mitchell, (1967) but whereas they found no significant difference in RT’S when the interval was increased to 2 seconds, this study found a significant difference across all inter stimulus time intervals. The differences in the presentation of the stimuli may account for this. In the previous study the first letter was always presented as uppercase, the change to a random presentation of both upper & lower case as the initial stimuli may have affected the response times. Part of the cause of this difference may be that their participants could react faster when able to anticipate the case of the 2nd letter & therefore reduce the RT to a level similar to that of the identical condition.
An interesting aspect reported by Posner, et al, (1969) regarded error rates; they found that the error rate tended to be relatively small, excepting the same name condition, when presented simultaneously, the higher error rate correlating to the faster RT In this study, the error rates were not recorded, a future study might include error rate, as Posner, et al, (1969) found this increase was related to speed of RT, it may be of interest to look at those subjects correct RT’s, in order to ascertain if they were significantly slower than their incorrect response times.
The theory that an object or shape, in this case, a letter, can be reduced to certain key features that bring about a response seems to be appropriate when matching identical letters, but does not work in the case of same name letters, which leaves the question of what process is in action when judging two letters of the same name, but of different case to be the same. The theories of object recognition, such as feature detection, template matching & the pandemonium system are not sufficient to explain how we recognise an ‘A’ as another format of ‘a’. According to Bruce, et al, (1996) all of these theories reject the information that differentiates different occurrences of the same patterns. Structural descriptions are possibly closer to describing human pattern recognition, as they provide a more flexible framework, & this seems to agree with Posner & Raichle, (1994) that what may be happening is a (learned) ability to visually identify a letter of one case with a letter of the other.
Posner, et al, (1969) varied their experiment by introducing mixed visual/aural presentation of the stimuli, which provided interesting differences to the results, in that same name RT’s generally became slower that identical RT’s, but this tendency could be reduced. They mention that attention to different aspects of the stimuli possible affects the results, future studies could include aural presentation with a divided attention aspect.
In this study there was a very wide age range, (15-60) & the participants were selected randomly. A future study with more selectively chosen participants may yield different, possibly more reliable results.
References.
Bruce, V., Green, P. & Georgeson, M. (1996) Visual Perception: Physiology, Psychology, & Ecology. Psychology Press. East Sussex.
Gross, R.D. (1992) Psychology: The Science of Mind & Behaviour. Hodder & Stoughton. Kent.
Posner, M.I., Boies, S.J., Eichelman, W.H., & Taylor, R.L. (1969). Retention of visual & name codes of single letters. Journal of Experimental Psychology Monograph, 79, No 1, Part 2.
Posner, M.I., & Keele, S.W. (1967) Decay of visual information from a single letter. Science, 158, 137-139
Posner, M.I., & Mitchell, R.F. (1967) Chronometric analysis of classification. Psychological Review, 74, 392-409.
Posner, M.I., & Raichle, M.E., (1994) Images of Mind. Scientific American Library. New York.
Williams, S. (2003) Coursework notes.
Wolfe, J, Oliva, A, Horowitz, T, Butcher, S. & Bompas, A. (2002) Segmentation of objects in visual search tasks. Vision Research, 42, 2985-3004.
Appendix A.
On screen instructions for perceptual matching task:
Respond as quickly as possible while keeping errors to a minimum.
In this experiment you are asked to compare two letters & determine if they are the same letter or not. The letters may be uppercase, lowercase, or both. If they are NOT the same, press the ‘1’ key; otherwise, press the ‘2’ key. For example you might see:
Z
z
Since these two letters are the same, you would press the ‘2’ key.
1 - MISMATCH & 2 – MATCH
We will start the experiment with 10 practice trials. You must get 8 of the practice trials correct to begin the experiment. The experiment has three blocks of 64 trials. For a total of 192 trials. In one block both letters appear simultaneously, as in the practice block, in the other two blocks, the 2nd letter will appear after some delay.
Please respond as quickly & accurately as possible.
Remember-on half of the trials the letters match & on half the trials they are different. The order in which they occur is random.
Appendix B
Raw data-all participants mean reaction times.
ID_0 S/N_0 D_0 ID_1 S/N_1 D_1 ID_2 S/N_2 D_2 GROUP
498 621 707 370 466 551 471 557 532 1
489 574 605 381 472 530 395 444 502 1
673 809 841 541 628 664 636 726 694 1
590 705 722 734 884 828 826 898 860 1
355 593 615 460 493 580 436 572 563 1
549 620 718 626 665 671 769 795 888 1
659 880 773 506 577 697 623 697 631 1
654 686 691 489 512 615 532 613 716 1
727 824 870 601 687 811 886 1236 1108 1
720 838 840 670 680 758 858 710 743 1
553 828 838 535 670 742 583 690 743 1
519 593 671 460 527 591 403 454 491 1
672 713 782 497 544 518 456 492 501 1
497 585 631 339 476 439 392 442 491 1
545 592 670 397 395 385 393 420 481 1
745 850 880 680 768 744 651 820 767 2
587 602 644 489 624 586 514 498 552 2
589 805 802 606 695 731 684 670 720 2
515 651 641 388 427 468 491 429 514 2
670 702 692 621 646 687 643 707 643 2
582 200 654 486 620 549 516 502 562 2
601 764 800 590 742 723 645 799 731 2
758 629 663 457 498 579 606 629 645 2
676 785 845 535 627 598 603 611 662 2
588 705 682 477 529 536 537 560 553 2
542 620 686 558 623 613 547 567 657 2
644 858 868 620 740 780 600 658 790 2
634 972 984 618 834 926 602 871 916 2
652 681 845 582 687 637 734 805 842 2
530 532 632 442 375 437 429 400 438 2
543 756 643 502 477 532 527 496 538 2
