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Sequential effects in an eight choice serial reaction time task
Acta Psychologica 0 North-Holland
39 (1975) 205-216. Publishing Company.
SEQUENTIAL EFFECTS IN AN EIGHT CHOICE SERIAL REACTION TIME TASK N. H. KIRBY Dept. of Psychology, Received
December
University of Adelaide, Adelaide, Australia 1974
Data from an eight choice serial reaction time experiment using two different response-stimulus intervals were examined for sequential effects by separating out the mean reaction times for each stimulus following every other stimulus. This analysis revealed differences in performance under the two response-stimulus interval conditions not shown by the conventional partitioning of the reaction times into those for repetitions and for non-repetrttons.
Following the discovery of the ‘repetition effect’ in two choice tasks by Bertelson (196 l), experiments concerned with sequential effects in four, six or eight choice tasks have usually divided the data into two mean reaction times (RTs), one for repetitions, that is, for all stimuli following themselves, and the other for alternations or non-repetitions, that is, for all stimuli following all other stimuli than themselves (e.g. Rabbitt 1968; Schvaneveldt and Chase 1969; Keele 1969). This partitioning ignores a good deal of potential information. It has already been pointed out (Alegria and Bertelson 1970; Welford 1971) that in eight choice tasks the RTs for individual fingers can be significantly different. Hence the question arises as to whether sequential effects might also differ amongst different sequential combinations of stimuli. In order to investigate this question, sequences were constructed which could be analyzed into an 8 X 8 matrix allowing a comparison of the mean RTs for each stimulus following every other stimulus in an eight choice task. This analysis enables several questions to be answered. For example, can an overall repetition effect be further broken down into hand and finger repetition effects; what is the relative contribution of each; and do they differ depending on the position of the particular stimulus in the display? Are there also proximity effects, in the sense that a
206
N. H. Kirby / Sequential effects ill X-choice RT task
stimulus close to an immediately preceding stimulus is responded to faster than stimuli further removed? Answers to these questions might be expected to throw some light on the cause of the repetition effect in eight choice tasks. Two explanations that have been suggested are, first, in terms of (i) a facilitative after-effect of a response, (ii) such that RT to a repeated stimulus is decreased due to the effects of having just responded to that stimulus. This explanation llas taken two forms, (a) in terms of a physiological activation effect which continues, and (1~) due to some portion of a complex coding or decoding operation which does not have to be done again. The second one is in terms of the application of particular processing strategies where the S is assumed to prepare for the occurrence of a particular stimulus (e.g. a repetition of the immediately preceding stimulus) on the basis of some expectation, or to check the stimuli in the display in a particular order for the correct stimulus. Recent evidence (Welford 197 1, 1973) has favoured an explanation of multiple choice RTs in terms of a serial dichotomization or trichotomization process in which the S is assumed to arrive at the correct signal and choice on each occasion by a series of inspections. He is assumed to divide the display into two or three groups of stimuli and inspect each group in turn for the correct signal. If it occurs in the group he inspects first, he divides that group into two or three smaller groups and again inspects each of them in turn. If the correct signal is not in the group first inspected, the S is assumed to check the remaining groups until he positively identifies the group in which it does occur, before proceeding to divide that group into two or three smaller groups. In an eight choice task with compatible S-R arrangement, Welford found that the inner fingers of each hand, that is the middle and ring fingers, responded more slowly than the outer fingers, that is the index and little fingers. This difference was apparently due to perceptual factors because when the outer stimuli were paired with the inner instead of the outer fingers, and vice versa, RTs to the outer stimuli were still shorter than those to the inner. Welford accounted for this finding by assuming that whenever the S has to decide between an outer and an inner stimulus, he tends to inspect the former before the latter. However, the model allows for some flexibility in the types of strategy that may be adopted by the S, and certainly does not preclude the possiblity that other more subtle sequential effects may also influence performance.
N. H. Kirby /Sequential
effects in 8-choice RT task
207
Since the response-stimulus interval (RSI) has been shown in previous research with two choice RT tasks to affect sequential effects (Bertelson 196 1; Hale 1967), two RSIs were used in this experiment.
Method Subjects The Ss were three male and three female volunteers enrolled for a first year psychology course at the University of Adelaide. Their ages ranged from 18 to 25 years and they were naive as to the aims of the experiment.
Apparatus The display consisted of a 20 cm square flat black board in which were set eight 16 mm diameter neon bulbs. These were arranged in a horizontal row at 22 mm intervals. A verticle white line separated the lights into two groups of four. Each S was tested individually, sitting at a table 2.8 m in front of the display and operating 8 morse keys with flat tops, arranged in two arcs of four placed so as to be convenient for the two hands. The lights and keys were not labelled but will be referred to as if numbered 1 to 8 in order from left to right. Each light was responded to by pressing its corresponding key. The allocation of fingers to keys was in order from 1 to 8, left little finger, left ring, left middle, left index, right index, right middle, right ring and right little fingers respectively. A pressure of approximately 140 gms was required to operate the keys. RT was recorded in milliseconds from the onset of the stimulus to the depression of any of the keys. The RSIs used were, as measured from the release of any key to the onset of the next stimulus, 1 msec and 2000 msec.
Stimulus sequences Three runs were prepared each beginning with scored, followed by 192 stimuli arranged in three random order one of every possible combination stimulus. In addition, 16 practice trials, not included
four stimuli, reactions to which were not blocks of 64, each block containing in a of each stimulus following every other in the results, preceded each run.
Procedure Three of the Ss completed each of the three stimulus sequences for the 2000 msec condition on one day, followed by the same three sequences for the 1 msec condition on the next. The other three Ss completed the two conditions in the reverse order. The Ss were told that they were participating in an eight choice RT experiment and that their task was to respond as quickly as possible to whichever stimulus light came on by pressing the appropriate key, keeping the number of errors down to less than 5%. They were informed of the RSI before each session and were told that the stimuli would be presented in a random order. The three
208
N. H. Kirby / Sequential effects in 8-choice R T task
different sequences were presented in a random order, and there was a rest period of approximately two minutes between each sequence. The stimuli were presented and the data recorded by means of computer programmed paper tapes. Errors amounted to approximately 6%. These and all RTs less than 100 msec were omitted from the analyses.
Results and discussion Table 1 shows the mean RTs for repetitions and alternations as they have usually been measured in previous experiments in the literature. It can be seen that the repetition effect for the 2000 msec condition appears to be slightly less than that for the 1 msec condition. However, this difference was not significant by a related samples t test. Table 1 Mean RTs for repetition
and alternation
responses.
Response different from previous response Response same as previous response Difference
2000 msec
1 msec
599 531 68
687 613 74
Table 2 Mean RTs for the 2000 msec condition. LAST 1 509
RESPONSE
2
3
4
5
6
540
522
507
544
549
576
557
606
648
657
65816351566!575!1648!671
Means
540
499
526
685
678
629
j7191651 1541
538’522~537i477~~514 606
8
I
_
627
7
570
621
589
73616991695/691
497
597
16401
~555~572~532\ 590
579
5.31
594
590
/6191548/602i688(6601
t :_~~~~~, 604
594
589
582
574
584
600
N. H. Kirby / Sequential effects in 8-choice RT task
209
Table 3 Mean RTs for 1 msec condition. LAST 1234567
RESPONSE 8
Means
Tables 2 and 3 show the mean RT for each stimulus when preceded by each other stimulus for both the 2000 msec and 1 msec conditions. One way of condensing the 8 X 8 matrices is by combining the mean RTs for each response for both hands and for each of the four fingers, whether little, ring, middle or index, ( 1) when it is repetition, that is, follows itself (REP); (2) when it follows a response by a finger of the same hand but is not a repetition (SHNR); (3) when it follows a response by a finger of the other hand (OH). These mean RTs are joined by the lines shown in fig. 1 for both the 2000 msec and 1 msec conditions. Also included in the graphs are the RTs which constitute each of these mean RTs. These are indicated by letters each of which represents the preceding response whether little (L), ring (R), middle (M), or index (I) of the same hand or of the other hand. Duncan multiple comparison analyses were used to compare the little, ring, middle and index REP, SHNR and OH RTs. Any difference referred to as significant in the text will be at least with p < 0.05.
N. Ii. Kirby / Sequential effects
210
in &choice RT task
2000 mwc RSI condition It is clear that,
finger Outer
as was found by Welford (I 971), the RTs for Inner responses, that is, middle and ring, are slower than those for finger responses, that is, little and index. Comparing the four STIMULUS
INTER 2,000
INTERVAL 1 m set
m set
900
500
-
REP
o--a
SHNR OH
400
t
l-
I
I
I
I
I
I
I
I
INDEX
MIDDLE
RING
LITTLE
INDEX
MIDDLE
RING
LITTLE
THIS
FINGER
Fig. 1. Mean RTs in msec for the two response-stimulus intervals for the index, middle, little finger responses when each, (1) is a repetition of the last response (REP), (2) response by a finger of the same hand but is not a repetition (SHNR) and, (3) response by a finger of the other hand (OH). Also shown are the RTs which constitute these mean RTs. These are indicated by letters, each of which represents the preceding whether little (L), ring (R), middle (M), or index (I) finger of the same hand or of hand.
ring and follows a follows a each of response the other
N. H. Kirby / Sequential effects in S-choice RT task
211
fingers, for both REP and OH responses the only significant differences in each case were those between either of the Inner when compared with either of the Outer responses. For the SHNR responses the middle finger RT was significantly different from both the little and index finger RTs, but the ring finger RT was significantly different only from the little finger RT. It can also be seen that the repetition effect tends to be greater for the Inner fingers than for the Outer, although only when REP RTs are compared with OH RTs. Considering the differences between REP, SHNR and OH responses, for the little finger it is clear that there is little difference in RT between these conditions, none of the differences being significant. For the index finger the pattern is similar except that the REP RT is significantly faster than the rest. For both the ring and middle fingers the REP RTs are fastest but there are no significant differences between these and the SHNR RTs. However, there are much larger differences between SHNR and OH RTs, both of these differences and the differences between REP and OH being significant. The partitioning of the overall mean RT into REP, SHNR and OH RTs would seem to be justified in so far as there seems to be little difference between the RTs, whether L, R, M or I, which constitute each of the OH Outer, each of the OH Inner and each of the SHNR Outer mean RTs. The one exception would seem to be the SHNR Inner mean RTs, particularly where the middle following L RT seems to be substantially longer than the remaining SHNR middle RTs. The significant differences between the REP, SHNR and OH conditions indicate separate effects due to a repetition of the same hand as well as of the same finger. This finding could be explained by assuming either a spread of a facilitative after-effect to the same hand as that involved in initiating the last response, or a saving in coding the required hand for the next response. However, the fact that the repetition effects exist mostly for the Inner fingers is incompatible with an explanation in terms of a facilitative after-effect, since this predicts that all fingers would benefit equally. An explanation in terms of a saving in coding time could perhaps be made by assuming that the Inner fingers require extra coding and it is this which is saved.
N. H. Kirby / Sequential effects in 8-choice RT task
212
1 msec RSI cnrzdition The same analysis as that used for the 2000 msec condition was also used for the 1 msec condition. It can be seen from the graph that the overall pattern of RTs is similar to that of the 2000 msec condition in so far as once again the repetition effects seem to be greatest for the Inner fingers. There are no significant differences between REP, SHNR and OH responses for either of the two Outer fingers and only between SHNR and OH, and REP and OH responses for the two Inner fingers. Once again this would seem to be incompatible with an explanation in terms of a facilitative after-effect. However, three differences from the 2000 msec condition may be noted: (1) the differences between the Inner and Outer OH responses seem to be slightly larger. The differences are again significant between either of the Outer fingers and either of the Inner fingers; (2) all the REP responses are very similar with no significant differences between them, whereas in the 2000 msec condition the Inner REP RTs are longer than the Outer. Although the pattern of SHNR RTs appears to be similar to that in the 2000 msec condition, no differences between any of the four fingers are significant; (3) generally, stimuli acljacent to the last stimulus are responded to as quickly as, and sometimes more quickly than, a repetition of the last stimulus. In the 2000 msec condition, these RTs all tend to be longer than the REP RT. It may also be noted that both middle following L and ring following 1 responses tend to be longer than the remaining SHNR RTs. In order to investigate this apparent proximity effect the mean RT for all REP responses was compared with that for all responses to stimuli immediately adjacent to the last stimulus. These means and their differences for both the 2000 msec and 1 msec conditions are shown in table 4. Each difference was tested by a related samples t test Table 4 Proximity
effects.
RT to response adjacent to repetition RT to repetition response Difference
response
2000 msec
1 msec
519 531 48
618 614 4
N. H. Kirby / Sequential effects in d-choice RT task
213
and was found to be significant (p < 0.02) for the 2000 msec condition but not for the 1 msec condition. The proximity effect could be explained in terms of a facilitative after-effect by assuming that the effect may spread to adjacent fingers. Its absence in the 2000 msec condition could be explained with the additional assumption that the effect decays with time. A saving in coding time, however, would require some additional explanation. Models In previous experiments concerned with sequential effects in two choice tasks, the results have been explained in terms of either (I) a facilitative after-effect of the last response (e.g. Bertelson 196 l), or (2) subjective expectancy and preparation on the part of the S (e.g. Schvaneveldt and Chase 1969; Kirby 1972). While a facilitative after-effect can explain both the proximity effect and the fact that there are differences overall between REP, SHNR and OH responses, it cannot explain the differences between the repetition effects for Inner and Outer responses. As far as preparation and expectancy are concerned, it might seem less likely that the S in an eight choice task would prepare for a repetition since its probability of occurrence is only 0.125. Nevertheless there could be some preparation for the stimuli of a particular hand although, if any such preparation occurred, one would expect some advantage of the Outer as well as the Inner SHNR and REP responses over their OH equivalents. An explanation of the above results in terms of the model put forward by Welford (197 1) would require certain modifications to its assumptions. For example, the fact that there is no difference between the RTs for Inner and Outer REP responses in the 1 msec condition is incompatible with the assumption that Outer stimuli are always inspected before Inner. This difficulty might be overcome by assuming that the REP stimulus is inspected first, then SH stimuli and finally OH stimuli, with the S still checking Outer stimuli before Inner. According to this strategy, the S in the 1 msec condition might be expected to be still attending to the last stimulus when the next stimulus arrives. He would be expected to do so to a lesser extent in the 2000 msec condition as there would be time for attention to be changed to OH stimuli on some occasions. However, while such simple biasing of
214
N. H. Kirby
/ Sequential
effects
in &choice
RT task
inspections would also explain the differences between Inner finger RTs in both conditions, it would not explain the lack of difference between the Outer REP, SHNR and OH responses, since all REP RTs should then be faster than all OH and SHNR RTs. Such an assumption would also fail to explain the proximity effect in the 1 msec condition, since REP RTs should be faster than all SHNR RTs, including those to the stimulus adjacent to the last stimulus. It is clear from the results that the fastest RTs occur to stimuli that are : (1) at the ends of the display: (2) adjacent to the midline; or (3) repetitions of the last stimulus, if the RSI is short enough. An alternative explanation could then be made by assuming that the ends of the display, the midline, and the position of the last stimulus act as reference points, and that the S first detects the stimulus. and then locates it in the display by seeking its nearest reference point. Thus, on this view, Ss would locate little and index stimuli quickly, regardless of the position of the previous stimulus, as being next to either an end of the display or the middle of it, Middle and ring stimuli would need to be located as next to a little or index stimulus except when they are repetitions of the previous stimulus and hence reference points themselves. The proximity effect in the 1 msec condition might be accounted for by assuming that the S is able to use the position of the last stimulus as a guide to the position of the adjacent stimulus if the RSI is short enough. If the position of the last stimulus is dependent on a memory trace which fades with time, similar to that postulated by Bertelson (1963), it might be expected not to operate to the same extent as a reference point after 2000 msec. Since Inner SH stimuli have no other alternative reference point, RTs to these stimuli would tend, with increasing RSI, to increase relative to the RTs to Outer SH stimuli which are adjacent to reference points. These Inner RTs would include REP responses and responses to Inner stimuli which are adjacent to the last stimulus. The use of the last stimulus as a reference point only for repetition responses, and for responses to stimuli immediately adjacent to the last stimulus, would also explain why middle following L responses and, to some extent, ring following I responses tend to be closer to the OH RTs, since in both these cases the last stimulus would not be an adjacent stimulus and hence they would have to be located in the same way as an OH inner stimulus. The fact
N. H. Kirby / Sequential effects in &choice RT task
21s
that the REP RT for the index finger is faster than its SHNR and OH equivalents could be due to the midline not being as clear a reference point (since it has a stimulus light on each side of it) as the ends of the display. Hence a repetition of the index stimulus might be expected to be a better reference point for an index response than the midline, thus accounting for the REP index response being faster than the SHNR and OH index responses. Such a difference in the relative efficiency of the midline and ends of the display as reference points might also account for the fact that the middle following L response appears to be slower than the ring following I response. In the latter case the end of the display is the more likely reference point to be used; in the former, the midline. The above explanation assumes that basically the repetition effect in an 8-choice situation is due to a saving in identifying the stimulus, the greater effect for the Inner stimuli being due to the fact that, for non-repetitions, these stimuli take longer to identify than their Outer equivalents. Assuming a decaying memory trace of the position of the last stimulus would seem to imply an overall decrease in the repetition effect as the RSI increases. As previously pointed out, there is a decrease, but it was not found to be significant. However, the usual method of measuring the repetition effect might now be seen to obscure the real repetition effect since (1) only some stimulus positions benefit from a repetition of the previous stimulus and (2) responses to repeated stimuli are not the only ones which may benefit from a repetition of the previous stimulus, those immediately adjacent also benefitting. In fact, there would seem to be a substantial decrease in the repetition effect with the increase in RSI if the Inner repeated responses are compared with the Inner OH responses, although this effect was only found to be significant by a one tailed related samples t test with p < 0.05. Overall, the mean RT for the 1 msec condition is longer than that for the 2000 msec condition and, since all RTs are in fact longer in the 1 msec condition, it is presumably due to some constant factor which affects all RTs. In personal communication, Welford has suggested three possible constants that might be considered: (1) the time needed for optimum preparation; (2) the time needed to recover from responding to the last stimulus; (3) the time needed to monitor the last response.
216
N. H. Kirby /Sequential
efjects in &choice R T task
No decision in favour of any one of these can be made on the basis of the present results. In conclusion, the above results do not support an explanation of sequential effects in eight choice RT tasks in terms of some facilitative after-effect. Nor do they support an explanation in terms of a simple sequential processing strategy. They do, however, offer some support for an explanation in terms of a saving in stimulus identification time, due to the last stimulus acting temporarily as a reference point.
References Alegria, J. and P. Bertelson, 1970. Time uncertainty, number of alternatives and particular signal-response pair as determinants of choice reaction time. Acta Psychol. 33, 36-44. Bertelson, P., 1961. Sequential redundancy and speed in a serial two-choice responding task. Quart. J. Psychol. 13,90-102. Bertelson, P., 1963. S-R relationships and reaction times to a new versus repeated signals in a serial task. J. Exp. Psychol. 65,478-484. Hale, D. J., 1967. Sequential effects in a two-choice serial reaction task. Quart. J. Exp. Psychol. 19,133-141. Keele, S. W., 1969. Repetition effect: a memory-dependant process. J. Exp. Psychol. 80 (11, 243-248. Kirby, N. H., 1972. Sequential effects in serial reaction time. J. Exp. Psychol. 96, 32-36. Rabbitt, P. M. A., 1968. Repetition effects and signal classification strategies in serial choiceresponse tasks. Quart. J. Exp. Psychol. 20, 232-239. Schvaneveldt, R. W. and W. G. Chase, 1969. Sequential effects in choice reaction time. J. Exp. Psychol. 80 (l), l-8. Welford, A. T., 1971. What is the basis of choice reaction-time? Ergonomics 14, 6, 679-693. Welford, A. T., 1973. Attention, strategy, and reaction time: a tentative metric. In: S. Komblum (ed.), Attention and performance IV. New York, London; Academic Press, 37-53.
39 (1975) 205-216. Publishing Company.
SEQUENTIAL EFFECTS IN AN EIGHT CHOICE SERIAL REACTION TIME TASK N. H. KIRBY Dept. of Psychology, Received
December
University of Adelaide, Adelaide, Australia 1974
Data from an eight choice serial reaction time experiment using two different response-stimulus intervals were examined for sequential effects by separating out the mean reaction times for each stimulus following every other stimulus. This analysis revealed differences in performance under the two response-stimulus interval conditions not shown by the conventional partitioning of the reaction times into those for repetitions and for non-repetrttons.
Following the discovery of the ‘repetition effect’ in two choice tasks by Bertelson (196 l), experiments concerned with sequential effects in four, six or eight choice tasks have usually divided the data into two mean reaction times (RTs), one for repetitions, that is, for all stimuli following themselves, and the other for alternations or non-repetitions, that is, for all stimuli following all other stimuli than themselves (e.g. Rabbitt 1968; Schvaneveldt and Chase 1969; Keele 1969). This partitioning ignores a good deal of potential information. It has already been pointed out (Alegria and Bertelson 1970; Welford 1971) that in eight choice tasks the RTs for individual fingers can be significantly different. Hence the question arises as to whether sequential effects might also differ amongst different sequential combinations of stimuli. In order to investigate this question, sequences were constructed which could be analyzed into an 8 X 8 matrix allowing a comparison of the mean RTs for each stimulus following every other stimulus in an eight choice task. This analysis enables several questions to be answered. For example, can an overall repetition effect be further broken down into hand and finger repetition effects; what is the relative contribution of each; and do they differ depending on the position of the particular stimulus in the display? Are there also proximity effects, in the sense that a
206
N. H. Kirby / Sequential effects ill X-choice RT task
stimulus close to an immediately preceding stimulus is responded to faster than stimuli further removed? Answers to these questions might be expected to throw some light on the cause of the repetition effect in eight choice tasks. Two explanations that have been suggested are, first, in terms of (i) a facilitative after-effect of a response, (ii) such that RT to a repeated stimulus is decreased due to the effects of having just responded to that stimulus. This explanation llas taken two forms, (a) in terms of a physiological activation effect which continues, and (1~) due to some portion of a complex coding or decoding operation which does not have to be done again. The second one is in terms of the application of particular processing strategies where the S is assumed to prepare for the occurrence of a particular stimulus (e.g. a repetition of the immediately preceding stimulus) on the basis of some expectation, or to check the stimuli in the display in a particular order for the correct stimulus. Recent evidence (Welford 197 1, 1973) has favoured an explanation of multiple choice RTs in terms of a serial dichotomization or trichotomization process in which the S is assumed to arrive at the correct signal and choice on each occasion by a series of inspections. He is assumed to divide the display into two or three groups of stimuli and inspect each group in turn for the correct signal. If it occurs in the group he inspects first, he divides that group into two or three smaller groups and again inspects each of them in turn. If the correct signal is not in the group first inspected, the S is assumed to check the remaining groups until he positively identifies the group in which it does occur, before proceeding to divide that group into two or three smaller groups. In an eight choice task with compatible S-R arrangement, Welford found that the inner fingers of each hand, that is the middle and ring fingers, responded more slowly than the outer fingers, that is the index and little fingers. This difference was apparently due to perceptual factors because when the outer stimuli were paired with the inner instead of the outer fingers, and vice versa, RTs to the outer stimuli were still shorter than those to the inner. Welford accounted for this finding by assuming that whenever the S has to decide between an outer and an inner stimulus, he tends to inspect the former before the latter. However, the model allows for some flexibility in the types of strategy that may be adopted by the S, and certainly does not preclude the possiblity that other more subtle sequential effects may also influence performance.
N. H. Kirby /Sequential
effects in 8-choice RT task
207
Since the response-stimulus interval (RSI) has been shown in previous research with two choice RT tasks to affect sequential effects (Bertelson 196 1; Hale 1967), two RSIs were used in this experiment.
Method Subjects The Ss were three male and three female volunteers enrolled for a first year psychology course at the University of Adelaide. Their ages ranged from 18 to 25 years and they were naive as to the aims of the experiment.
Apparatus The display consisted of a 20 cm square flat black board in which were set eight 16 mm diameter neon bulbs. These were arranged in a horizontal row at 22 mm intervals. A verticle white line separated the lights into two groups of four. Each S was tested individually, sitting at a table 2.8 m in front of the display and operating 8 morse keys with flat tops, arranged in two arcs of four placed so as to be convenient for the two hands. The lights and keys were not labelled but will be referred to as if numbered 1 to 8 in order from left to right. Each light was responded to by pressing its corresponding key. The allocation of fingers to keys was in order from 1 to 8, left little finger, left ring, left middle, left index, right index, right middle, right ring and right little fingers respectively. A pressure of approximately 140 gms was required to operate the keys. RT was recorded in milliseconds from the onset of the stimulus to the depression of any of the keys. The RSIs used were, as measured from the release of any key to the onset of the next stimulus, 1 msec and 2000 msec.
Stimulus sequences Three runs were prepared each beginning with scored, followed by 192 stimuli arranged in three random order one of every possible combination stimulus. In addition, 16 practice trials, not included
four stimuli, reactions to which were not blocks of 64, each block containing in a of each stimulus following every other in the results, preceded each run.
Procedure Three of the Ss completed each of the three stimulus sequences for the 2000 msec condition on one day, followed by the same three sequences for the 1 msec condition on the next. The other three Ss completed the two conditions in the reverse order. The Ss were told that they were participating in an eight choice RT experiment and that their task was to respond as quickly as possible to whichever stimulus light came on by pressing the appropriate key, keeping the number of errors down to less than 5%. They were informed of the RSI before each session and were told that the stimuli would be presented in a random order. The three
208
N. H. Kirby / Sequential effects in 8-choice R T task
different sequences were presented in a random order, and there was a rest period of approximately two minutes between each sequence. The stimuli were presented and the data recorded by means of computer programmed paper tapes. Errors amounted to approximately 6%. These and all RTs less than 100 msec were omitted from the analyses.
Results and discussion Table 1 shows the mean RTs for repetitions and alternations as they have usually been measured in previous experiments in the literature. It can be seen that the repetition effect for the 2000 msec condition appears to be slightly less than that for the 1 msec condition. However, this difference was not significant by a related samples t test. Table 1 Mean RTs for repetition
and alternation
responses.
Response different from previous response Response same as previous response Difference
2000 msec
1 msec
599 531 68
687 613 74
Table 2 Mean RTs for the 2000 msec condition. LAST 1 509
RESPONSE
2
3
4
5
6
540
522
507
544
549
576
557
606
648
657
65816351566!575!1648!671
Means
540
499
526
685
678
629
j7191651 1541
538’522~537i477~~514 606
8
I
_
627
7
570
621
589
73616991695/691
497
597
16401
~555~572~532\ 590
579
5.31
594
590
/6191548/602i688(6601
t :_~~~~~, 604
594
589
582
574
584
600
N. H. Kirby / Sequential effects in 8-choice RT task
209
Table 3 Mean RTs for 1 msec condition. LAST 1234567
RESPONSE 8
Means
Tables 2 and 3 show the mean RT for each stimulus when preceded by each other stimulus for both the 2000 msec and 1 msec conditions. One way of condensing the 8 X 8 matrices is by combining the mean RTs for each response for both hands and for each of the four fingers, whether little, ring, middle or index, ( 1) when it is repetition, that is, follows itself (REP); (2) when it follows a response by a finger of the same hand but is not a repetition (SHNR); (3) when it follows a response by a finger of the other hand (OH). These mean RTs are joined by the lines shown in fig. 1 for both the 2000 msec and 1 msec conditions. Also included in the graphs are the RTs which constitute each of these mean RTs. These are indicated by letters each of which represents the preceding response whether little (L), ring (R), middle (M), or index (I) of the same hand or of the other hand. Duncan multiple comparison analyses were used to compare the little, ring, middle and index REP, SHNR and OH RTs. Any difference referred to as significant in the text will be at least with p < 0.05.
N. Ii. Kirby / Sequential effects
210
in &choice RT task
2000 mwc RSI condition It is clear that,
finger Outer
as was found by Welford (I 971), the RTs for Inner responses, that is, middle and ring, are slower than those for finger responses, that is, little and index. Comparing the four STIMULUS
INTER 2,000
INTERVAL 1 m set
m set
900
500
-
REP
o--a
SHNR OH
400
t
l-
I
I
I
I
I
I
I
I
INDEX
MIDDLE
RING
LITTLE
INDEX
MIDDLE
RING
LITTLE
THIS
FINGER
Fig. 1. Mean RTs in msec for the two response-stimulus intervals for the index, middle, little finger responses when each, (1) is a repetition of the last response (REP), (2) response by a finger of the same hand but is not a repetition (SHNR) and, (3) response by a finger of the other hand (OH). Also shown are the RTs which constitute these mean RTs. These are indicated by letters, each of which represents the preceding whether little (L), ring (R), middle (M), or index (I) finger of the same hand or of hand.
ring and follows a follows a each of response the other
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fingers, for both REP and OH responses the only significant differences in each case were those between either of the Inner when compared with either of the Outer responses. For the SHNR responses the middle finger RT was significantly different from both the little and index finger RTs, but the ring finger RT was significantly different only from the little finger RT. It can also be seen that the repetition effect tends to be greater for the Inner fingers than for the Outer, although only when REP RTs are compared with OH RTs. Considering the differences between REP, SHNR and OH responses, for the little finger it is clear that there is little difference in RT between these conditions, none of the differences being significant. For the index finger the pattern is similar except that the REP RT is significantly faster than the rest. For both the ring and middle fingers the REP RTs are fastest but there are no significant differences between these and the SHNR RTs. However, there are much larger differences between SHNR and OH RTs, both of these differences and the differences between REP and OH being significant. The partitioning of the overall mean RT into REP, SHNR and OH RTs would seem to be justified in so far as there seems to be little difference between the RTs, whether L, R, M or I, which constitute each of the OH Outer, each of the OH Inner and each of the SHNR Outer mean RTs. The one exception would seem to be the SHNR Inner mean RTs, particularly where the middle following L RT seems to be substantially longer than the remaining SHNR middle RTs. The significant differences between the REP, SHNR and OH conditions indicate separate effects due to a repetition of the same hand as well as of the same finger. This finding could be explained by assuming either a spread of a facilitative after-effect to the same hand as that involved in initiating the last response, or a saving in coding the required hand for the next response. However, the fact that the repetition effects exist mostly for the Inner fingers is incompatible with an explanation in terms of a facilitative after-effect, since this predicts that all fingers would benefit equally. An explanation in terms of a saving in coding time could perhaps be made by assuming that the Inner fingers require extra coding and it is this which is saved.
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1 msec RSI cnrzdition The same analysis as that used for the 2000 msec condition was also used for the 1 msec condition. It can be seen from the graph that the overall pattern of RTs is similar to that of the 2000 msec condition in so far as once again the repetition effects seem to be greatest for the Inner fingers. There are no significant differences between REP, SHNR and OH responses for either of the two Outer fingers and only between SHNR and OH, and REP and OH responses for the two Inner fingers. Once again this would seem to be incompatible with an explanation in terms of a facilitative after-effect. However, three differences from the 2000 msec condition may be noted: (1) the differences between the Inner and Outer OH responses seem to be slightly larger. The differences are again significant between either of the Outer fingers and either of the Inner fingers; (2) all the REP responses are very similar with no significant differences between them, whereas in the 2000 msec condition the Inner REP RTs are longer than the Outer. Although the pattern of SHNR RTs appears to be similar to that in the 2000 msec condition, no differences between any of the four fingers are significant; (3) generally, stimuli acljacent to the last stimulus are responded to as quickly as, and sometimes more quickly than, a repetition of the last stimulus. In the 2000 msec condition, these RTs all tend to be longer than the REP RT. It may also be noted that both middle following L and ring following 1 responses tend to be longer than the remaining SHNR RTs. In order to investigate this apparent proximity effect the mean RT for all REP responses was compared with that for all responses to stimuli immediately adjacent to the last stimulus. These means and their differences for both the 2000 msec and 1 msec conditions are shown in table 4. Each difference was tested by a related samples t test Table 4 Proximity
effects.
RT to response adjacent to repetition RT to repetition response Difference
response
2000 msec
1 msec
519 531 48
618 614 4
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and was found to be significant (p < 0.02) for the 2000 msec condition but not for the 1 msec condition. The proximity effect could be explained in terms of a facilitative after-effect by assuming that the effect may spread to adjacent fingers. Its absence in the 2000 msec condition could be explained with the additional assumption that the effect decays with time. A saving in coding time, however, would require some additional explanation. Models In previous experiments concerned with sequential effects in two choice tasks, the results have been explained in terms of either (I) a facilitative after-effect of the last response (e.g. Bertelson 196 l), or (2) subjective expectancy and preparation on the part of the S (e.g. Schvaneveldt and Chase 1969; Kirby 1972). While a facilitative after-effect can explain both the proximity effect and the fact that there are differences overall between REP, SHNR and OH responses, it cannot explain the differences between the repetition effects for Inner and Outer responses. As far as preparation and expectancy are concerned, it might seem less likely that the S in an eight choice task would prepare for a repetition since its probability of occurrence is only 0.125. Nevertheless there could be some preparation for the stimuli of a particular hand although, if any such preparation occurred, one would expect some advantage of the Outer as well as the Inner SHNR and REP responses over their OH equivalents. An explanation of the above results in terms of the model put forward by Welford (197 1) would require certain modifications to its assumptions. For example, the fact that there is no difference between the RTs for Inner and Outer REP responses in the 1 msec condition is incompatible with the assumption that Outer stimuli are always inspected before Inner. This difficulty might be overcome by assuming that the REP stimulus is inspected first, then SH stimuli and finally OH stimuli, with the S still checking Outer stimuli before Inner. According to this strategy, the S in the 1 msec condition might be expected to be still attending to the last stimulus when the next stimulus arrives. He would be expected to do so to a lesser extent in the 2000 msec condition as there would be time for attention to be changed to OH stimuli on some occasions. However, while such simple biasing of
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inspections would also explain the differences between Inner finger RTs in both conditions, it would not explain the lack of difference between the Outer REP, SHNR and OH responses, since all REP RTs should then be faster than all OH and SHNR RTs. Such an assumption would also fail to explain the proximity effect in the 1 msec condition, since REP RTs should be faster than all SHNR RTs, including those to the stimulus adjacent to the last stimulus. It is clear from the results that the fastest RTs occur to stimuli that are : (1) at the ends of the display: (2) adjacent to the midline; or (3) repetitions of the last stimulus, if the RSI is short enough. An alternative explanation could then be made by assuming that the ends of the display, the midline, and the position of the last stimulus act as reference points, and that the S first detects the stimulus. and then locates it in the display by seeking its nearest reference point. Thus, on this view, Ss would locate little and index stimuli quickly, regardless of the position of the previous stimulus, as being next to either an end of the display or the middle of it, Middle and ring stimuli would need to be located as next to a little or index stimulus except when they are repetitions of the previous stimulus and hence reference points themselves. The proximity effect in the 1 msec condition might be accounted for by assuming that the S is able to use the position of the last stimulus as a guide to the position of the adjacent stimulus if the RSI is short enough. If the position of the last stimulus is dependent on a memory trace which fades with time, similar to that postulated by Bertelson (1963), it might be expected not to operate to the same extent as a reference point after 2000 msec. Since Inner SH stimuli have no other alternative reference point, RTs to these stimuli would tend, with increasing RSI, to increase relative to the RTs to Outer SH stimuli which are adjacent to reference points. These Inner RTs would include REP responses and responses to Inner stimuli which are adjacent to the last stimulus. The use of the last stimulus as a reference point only for repetition responses, and for responses to stimuli immediately adjacent to the last stimulus, would also explain why middle following L responses and, to some extent, ring following I responses tend to be closer to the OH RTs, since in both these cases the last stimulus would not be an adjacent stimulus and hence they would have to be located in the same way as an OH inner stimulus. The fact
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that the REP RT for the index finger is faster than its SHNR and OH equivalents could be due to the midline not being as clear a reference point (since it has a stimulus light on each side of it) as the ends of the display. Hence a repetition of the index stimulus might be expected to be a better reference point for an index response than the midline, thus accounting for the REP index response being faster than the SHNR and OH index responses. Such a difference in the relative efficiency of the midline and ends of the display as reference points might also account for the fact that the middle following L response appears to be slower than the ring following I response. In the latter case the end of the display is the more likely reference point to be used; in the former, the midline. The above explanation assumes that basically the repetition effect in an 8-choice situation is due to a saving in identifying the stimulus, the greater effect for the Inner stimuli being due to the fact that, for non-repetitions, these stimuli take longer to identify than their Outer equivalents. Assuming a decaying memory trace of the position of the last stimulus would seem to imply an overall decrease in the repetition effect as the RSI increases. As previously pointed out, there is a decrease, but it was not found to be significant. However, the usual method of measuring the repetition effect might now be seen to obscure the real repetition effect since (1) only some stimulus positions benefit from a repetition of the previous stimulus and (2) responses to repeated stimuli are not the only ones which may benefit from a repetition of the previous stimulus, those immediately adjacent also benefitting. In fact, there would seem to be a substantial decrease in the repetition effect with the increase in RSI if the Inner repeated responses are compared with the Inner OH responses, although this effect was only found to be significant by a one tailed related samples t test with p < 0.05. Overall, the mean RT for the 1 msec condition is longer than that for the 2000 msec condition and, since all RTs are in fact longer in the 1 msec condition, it is presumably due to some constant factor which affects all RTs. In personal communication, Welford has suggested three possible constants that might be considered: (1) the time needed for optimum preparation; (2) the time needed to recover from responding to the last stimulus; (3) the time needed to monitor the last response.
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No decision in favour of any one of these can be made on the basis of the present results. In conclusion, the above results do not support an explanation of sequential effects in eight choice RT tasks in terms of some facilitative after-effect. Nor do they support an explanation in terms of a simple sequential processing strategy. They do, however, offer some support for an explanation in terms of a saving in stimulus identification time, due to the last stimulus acting temporarily as a reference point.
References Alegria, J. and P. Bertelson, 1970. Time uncertainty, number of alternatives and particular signal-response pair as determinants of choice reaction time. Acta Psychol. 33, 36-44. Bertelson, P., 1961. Sequential redundancy and speed in a serial two-choice responding task. Quart. J. Psychol. 13,90-102. Bertelson, P., 1963. S-R relationships and reaction times to a new versus repeated signals in a serial task. J. Exp. Psychol. 65,478-484. Hale, D. J., 1967. Sequential effects in a two-choice serial reaction task. Quart. J. Exp. Psychol. 19,133-141. Keele, S. W., 1969. Repetition effect: a memory-dependant process. J. Exp. Psychol. 80 (11, 243-248. Kirby, N. H., 1972. Sequential effects in serial reaction time. J. Exp. Psychol. 96, 32-36. Rabbitt, P. M. A., 1968. Repetition effects and signal classification strategies in serial choiceresponse tasks. Quart. J. Exp. Psychol. 20, 232-239. Schvaneveldt, R. W. and W. G. Chase, 1969. Sequential effects in choice reaction time. J. Exp. Psychol. 80 (l), l-8. Welford, A. T., 1971. What is the basis of choice reaction-time? Ergonomics 14, 6, 679-693. Welford, A. T., 1973. Attention, strategy, and reaction time: a tentative metric. In: S. Komblum (ed.), Attention and performance IV. New York, London; Academic Press, 37-53.