is preparation of responses precluded by choice reaction time testing?
TRANSCRIPT
Human Movement Science 1 (1982) 273-288 North-Holland Pubhshmg Company
273
IS PREPARATION OF RESPONSES PRECLUDED BY CHOICE REACTION TIME TESTING?
Ann HARRISON * and Andrew BISHOP Sheffield Unmerslty, UK
Harrison, A. and A. Bishop, 1982. Is preparation of responses pre- cluded by choice reaction time testing? Human Movement Science 1, 273-288.
Choice reactlon times for short (Dlt) and long (Dah) Morse key presses were measured using a number of mfferent left hand/right hand Dlt/Dah 2-response combmatlons When Dlt and Dah responses were made ulth the same hand, Dots were nutlated slgmflcantly faster, whch confums the fmdmg of Klapp et al (1974) The absolute sizes of Dlt and Dah CRT’s, however, vaned slgrnflcantly from one test situation to another, and the Dlt Dah difference was shown to be mtranslhve Such context effects cast doubt on the extrapolative power of CRT findings, and on the usefulness of CRT as a measure of movement complexity Latencles were reduced when subJects knew m advance whch hand would be used, but the usefulness of response duration preknowledge was not unambiguously demonstrated Preparation of responses IS, therefore, not precluded by CRT testing, and possible explanations for these savmgs m terms of response selection and motor preprogrammmg were discussed On balance, the hypothesis that the longer reaction times of Dah responses are due to their greater movement complexity (Klapp et al 1974) was rejected, m favour of the explanation that subjects adopt a strategy of preferentially preparmg Dlt responses
Introduction
An implicit assumption in very many studies, and one made explicit in Klapp et al. (1974), is that a choice reaction time paradigm prevents any preparation of actions taking place until the required response is
* The work reported m thus paper forms part of the research programme supported by a grant from the Natlonal Fund for Research mto Cnpphng Diseases (grant number 421-700) Dr Bishop was supported by a research student&p from the Social Science Research Councd Both are gratefully acknowledged
Requests for reprmts should be addressed to Dr A Harnson, Dept of Commumty Medicine and Behamoral Sciences, Faculty of Medlcme, Kuwrut University, P 0 Box 24923, Kuwat
0167-9457/82/0000-0000/$02.75 0 1982 North-Holland
274 A Harmon, A Bishop / Preparatron of responses
testing is programming of may be advanced by time the to move given; and such as (1940) (cited Welford 1968) succeeded m pre- paratory m relevant groups. SRT, does not
a proper of the time taken select a and complete programmmg for to begm; if the
outlined above correct, choice time (CRT) In a study (Bishop Harrison 1982), compared CRT’s
single and Jointed actions two main conditions. In ‘matched’ condition, responses were single or Jointed actions; one single one dual were paired the ‘mixed’ Latency differences not statistically but there a strong for ‘matched’
CRT’s to shorter. This mto question prepro- gramming really prevented CRT testing; one possible
for these is that were programming advance elements to the action programs. effects are unknown in time studies, these have been interpreted terms of selection rather response pro-
The term Compatibihty was by Fitts to describe mutual influence responses m choice set; effects of spatial compatibihty, common performance have been
Poor discriminabihty to prejudice speed, it when alternative are easily or are performed together. (1973) showed when movements the index
middle finger the same are paired, are longer when the and middle of different are used. this is
due to discrmunabihty, created fingers of same hand often used conJunction, then would predict it would quicker to the simultaneous of fingers the same than of hands, which exactly what and Ritchie found. Blyth 1964) measured when a and a action are and found latencies for pairs were than for pan-s. Substitution were also
under ipsilateral which could confusion at response selection or mistakes response programmmg;
former appears more likely.
A Harmon, A Bzshop / Preparation of responses 215
In contrast to discrimmability effects, when responses share certain common performance components, CRT’s are shorter. What forms of response similarity lead to poor discriminabihty and which to perfor- mance compatibility have yet to be elucidated; but common component effects have been demonstrated, and have been attributed to response preprogrammmg. Gottsdanker (1969) studied step-tracking responses and found that when paired actions differed only m extent, CRT’s were not very much greater than SRT’s; whereas, if responses differed only in direction, latencies were very much longer. These fmdmgs were confirmed by Megaw (1972), who developed a merarchrcal model of motor programmmg to account for such effects (Megaw 1974). He proposed that a series of decisions underlies the construction of any motor program, and that these proceed in a standard sequence. The first decision involves selecting which muscle groups are to be used; next, the broad sequence of muscle activity is specified, and then inter-muscle timing is detailed; fmally, the duration of activity for each muscle group is designated. When a pair of responses differ only m extent, the person can presumably preprogramme very extensively, leaving himself only intra-muscle timmg to add when the exact re- sponse is known. He may even be able to mitiate activity immediately, fuushing the programmmg after movement has commenced (Bishop and Harrison 1982). Far less preparatory programmmg is possible when actions differ m direction; for at best these will involve the same muscle groups, and decisions concerning sequencmg and timing have to wait until the required response is known. Support for Megaw’s model comes from studies in wluch subjects were asked to modify prepared motor programs, and the times taken to do so measured. Delays were longest when primary decisions (such as choice of muscle groups) had to be revised, and minimal when all that needed changmg was the duration of activity m exlstmg muscle groups. A third type of R-R compatibility concerns the spatial similarity of responses. Berlyne (1957) defined forward and backward movements of the same hand as being spatially incompatible, and isodirectional movements of both hands as being compatible; and he offers some evidence that children are quick- est when performmg the latter. Such a phenomenon is probably best viewed as a form of cognitive compatibility; for m Megaw’s terms, movements of different hands share less primary motor programmmg decisions.
A well-established phenomenon in CRT studies is the shortening of
276 A Harmon, A &shop / Preparation of responses
latencies when a response is repeated in successive trials (Bertelson 196 1; Landauer 1964). Whether this is due mainly to response or to stimulus famiharity is not clear. Rabbitt (1965) studied serial CRT’s, and concluded that improvements were due to the re-use of existing motor programs; however, given the spatial arrangement of lights and response keys that he used, stimulus factors cannot be ruled out.
The literature then, contains a variety of converging evidence sug- gesting that the context in which a response is made (i.e. the nature of the other responses whtch rmght be demanded) can influence its latency. Unfortunately, much of this evidence has been collected unsystemati- cally. The present study sought to remedy this by systematically varying the choice sets in which responses are placed, and studying what effects these have on CRT. As a starting point, we chose a well-established CRT phenomenon reported by Klapp et al. (1974), which is that when short (Dit) and long (Dah) switch closures are paired, latencies for Dah responses are reliably longer. The present study investigated four differ- ent contexts (table 1). Condition 4 replicates that used by Klapp et al. (1974): both responses are performed with the same hand, and differ only in duration. In Conditions 1 and 2, the paired responses are of the same duration, but different hands are used to execute them. In Condition 3, responses differ both in duration and hand used. By studying a range of response pairings, it was hoped to establish whether Dit and Dah latencies are fixed or vary, and whether the Dit : Dah difference is transitive. The study also sought to investigate whether initiation of a response is facilitated when the performer knows m advance which hand will be used or which type of switch-closure will be demanded.
Table I Response pamngs tested
Condltlon
Condltlon 1
Condltlon 2
Condition 3 Condltlon 4
Response pair
Dlt, Dlt,
Dah, Dah,
Dlt, Dah,
Dlt, Dah,
Parameter(s) vaned
Hand
Hand
Hand and response type
Response type
Note Subscnpt letter denotes the hand to be used 1 e r = r&t, 1 = left
A Harmon, A Bishop / Preparatron of responses 217
Method
Subjects
Four independent groups of fourteen subjects participated m the study. Subjects were randomly assigned, with the constraint that groups should be as balanced as possible with respect to sex ratio, age and handedness. Each group had 3 or 4 males, a median age of 20 yrs, and 0 or 1 left handers.
Apparatus
Presentation of stimuli, assessment of responses, and feedback about errors were managed on-line, usmg a specially-written program runnmg on a Nova 840 computer. The computer’s m&second timer was used throughout. The stirnull for Dit and Dah responses were red and green 1 cm squares, back-projected to precisely the same location on a frosted plastic screen; these were illummated by 1.2 watt incandescent bulbs, and viewed m semidarkness. The fixation stimulus was a white square at the same spot. Stimuli were positroned m the visual midline, 60 ems from the sublect. Standard Morse keys were used; m order to register a response, a travel of 0.5 mm and a force of about 250 gms wt were needed. Keys were placed 25 ems apart, with the display tube midway between. A light-display box provided feedback about response errors and changes m CRT (detail below).
Response speclficatlons and error feedback
Klapp and Wyatt (1976) found that the switch closure for a Dit response characteristically lasts 100 msecs, and for a Dah response 360 msecs. In the present study, Dit responses were operationally defined as switch closures lasting less than 170 msecs, and Dah responses as ones lasting longer than 300 msecs. Failure to meet these specificattons was signalled on the light display box provided. A second light indicated the followmg errors (1) wrong hand, (ii) wrong hand and duration, (iii) double tap, denoting poor control or an attempt to correct an initial Dit response, (iv) RT less than 70 msecs, taken as sigmfymg a false start or guessing, since the response was presumed too fast to have been a reaction to the signal, and (v) RT greater than 900 msecs, indicating
278 A Harmon, A Buhop / Preparatron of responses
that the sublect had for some reason failed to respond. Although only one light was used, data for each type of error were classified sep- arately.
Procedure
Seven subjects operated with the stimulus-response mapping red = Dit, green = Dah, and seven with the converse. The procedure for all subjects was essentially the same, except for the precise pair of re- sponses tested, and it began with a practice session. This consisted of a block of 20 trials, ten of each response type presented m randomised order. Subjects concentrated on learning to interpret error feedback and differentiate appropriate responses. Testing involved 20 blocks of 20 trials, completed in a single session of about 50 mms If errors were recorded, trials were repeated at the end of a block. The experiment was machine-paced within a block (fig. 1); 20 set rests were allowed be- tween blocks, with a break of 2 mms at the end of the 5th, lOth, and 15th. Subjects were given the customary mstructions: ‘respond as quickly as possible, making a minimum of errors’. In an attempt to maintain motivation, a subject was gtven feedback at the end of each block indicating whether average RT was better or worse than his previous best performance. It was suggested that subjects should aim to produce no more than two errors m any one block, and sometimes none
at all. Fig. 1 summanses the procedure for a trial. It differs from that used
by Klapp et al. (1974) in that a fixed rather than a variable foreperiod
CHECK FOR ERRORS, OUTF'UT FEEDBACK
f1xatron J 1
lqht 'on', “ar*lIlg warnrng tone ‘cm tone 'off' st1mu1us MORSE KEY
light 'on' 'off'
Fig 1 Tnal sequence
I350 I 1200 msecs 1 LATENCY 1 1 1400 msecs I msecs T
response duratron
A Harmon, A Brshop / Preparatton of responses 219
was used, this was done so that subjects could gauge exactly when a response would be demanded and organise to culminate preparations then. A warning tone signalled the start of a trial, and the white square fixation stimulus was switched on. 1200 msecs later, this was replaced by either a red or a green square, and reaction timing began. The stimulus remamed on until a response was made. Switch closure stopped the clock, and CRT was computed. If any errors were detected, suitable feedback was given, and the trial was aborted.
Results
Errors
Error rates (table 2) were slightly higher than those reported by Klapp et al. (1974), probably because of improved error detection. A very clean response was needed to avoid double tap errors, as the high percentage of these (32%) shows. Very few early or late responses were recorded. Total error counts were subjected to a Kruskal-Walhs one-way analysis of variance; the four conditions were not found to differ significantly (p > 0.05). Error rates for mixed duration pairs (Condi- tions 3 and 4) were higher than for matched (Conditions 1 and 2), but a U-test proved non-significant (p > 0.05). More errors were made in Condition 1 than Condition 2, suggesting that Dit responses are more prone to error, but a U-test revealed no sigmficant difference. When Dit and Dah performances m Condition 3 were compared, however, Dit responses were found to be more errorful (t = 16.0, N = 14, p <
0.02). This is unlikely to have been due to the different hands used, for no such effect was found in Conditions 1 and 2. In Condition 4, again the trend was for a lugher rate of Dit errors, but a U-test proved non-significant.
There is, therefore, some evidence that context affects error rate. When responses of the same duration were paired no difference in error rate was observed; but when responses differed in duration, there was a clear trend (sometimes significant) for a lngher rate of Dit errors.
Reachon tmes
The twenty blocks of test trials were partitioned mto 4 consecutive groupings of 5 blocks so that practice effects could be analysed. For
Tab
le
2
A b
reak
dow
n of
th
e er
rors
re
cord
ed
Subj
ect
Con
dltlo
n 1
Dlt,
D
lt,
Tot
al
Con
dltlo
n 2
Dah
, D
ah
r T
otal
Con
dltlo
n 3
Dlt,
D
ah,
Tot
al
Con
dltlo
n 4
Dlt,
D
ah
I T
otal
1 2 3 4 5 6 I 8 9 10
11
12
13
14
Tot
al
17
3 30
21
21
42
30
26
56
33
36
7 9
16
6 7
13
18
10
28
24
11
15
21
36
14
11
25
4 39
43
15
14
4
36
40
16
26
42
22
13
35
14
4
13
14
27
11
7 18
12
11
23
22
31
8
15
23
19
12
31
17
11
28
13
11
0 5
5 5
9 14
48
30
78
13
10
11
5 16
10
6
16
36
2 68
18
9
18
11
29
10
9 19
5
4 9
18
14
II
21
32
5 15
20
16
8
24
18
19
30
16
46
7 6
13
48
21
69
7 6
5 9
14
5 12
17
11
5
16
17
14
16
36
52
19
34
53
7 10
17
16
17
16
7
23
18
11
29
30
17
47
9 9
171
218
389
186
166
352
334
207
541
237
205
Num
ber
subs
tltut
lon
erro
rs
Han
d 42
60
43
11
44
12
_
Dur
atio
n 46
59
67
17
17
0 11
7 13
4 13
7
Bot
h 4
0 13
12
14
50
2
3
Ove
rall
erro
r ra
te
65%
59
%
88%
73
%
69
35
29
18
53
24
23
27
32
37
13
31
33
18
442
- -
..r.
.I,_
.
.I -
_,,
_.-2
-
-- _
.-
- _
._
__ ^
.
-
A Harmon, A Bzshop / Preparatron of responses 281
each subject, the median RT for each group was computed and figs. 2 and 3).
(see table 3,
The Dzt : Dah dzfference Condition 4 rephcated that used by Klapp et al. (1974), and the
results are m good agreement with then-s* Dit responses were on average 35 msecs faster, compared with then 22 msecs. A two-way analysis of variance was apphed, and both response duration (F( 1, 13) = 40.69, p < 0.001) and extent of practice (F(3, 39) = 11.62, p < 0.001) emerged as htghly sigmficant. Similar results were seen m Condition 3, when Dit and Dah responses were performed with different hands. Dtt responses were, on average, 39 msecs faster; and an analysis compara- ble to the above showed this to be significant (F( 1, 13) = 24.22, p c 0.001). Practice effects were reliable (F(3, 39) = 10.92, p < 0.001); and so was the response duration X practice level mteractton (F(3, 39) = 4.02, p -C 0.05), mdicatmg that Dit performance improved stgmficantly faster. It seems reasonable to rule out the possibility that the Dit : Dah difference is caused by using different hands, because no significant hand differences were found m Conditions 1 and 2. When perfor- mances in Conditions 1 and 2 were compared (fig. 2), there was no evidence of a Dit : Dah difference (p > 0.05); and so although the findings of Klapp et al. (1974) were replicated, the Dit advantage did not generahse to situations where Dit and Dab responses were per- formed in separate CRT tasks.
Table 3 Group reactlon times (mean of medians, msecs)
Blocks l-5 Blocks 6- 10 Blocks 11-15 Blocks 16-20
Means Means
Dlt Dah
difference =
Condalon 1 Condltlon 2 Condltlon 3 Condlhon 4 Means
Dlt , Dlt, Dah, Dab, Dlt, Dab, Dlt r Dah,
434 444 450 452 441 463 403 442 441 410 414 417 421 403 442 384 418 412 410 403 408 411 389 434 366 398 402 394 390 381 390 378 427 351 386 387
412 413 414 419 403 442 376 411 412 416 422 394
4 msecs 39 msecs 35 msexx
282 A Harmon, A Bishop / Preparatm of responses
44c
430
420
410
400
390
380
‘h,
h t, Dahl
Dltr
1 1 -ix
Condltlan 1 Condltun 2 Candltmn 3 Condltmn 4
Dah r
Fig 2 Group choice reactlon times (mean of medians)
Pre-knowledge of response duration Dit, performance m Condition 1 was contrasted with that m Condr-
tion 3 using an analysis of variance; but preknowledge concerning duration was not shown to confer any sigmfrcant advantage. Dah, responses were markedly faster in Condition 2 (where response dura- tion was known) than in Condition 3 (where it was not), but again tms difference was not verified statistically, even when only the last 10 blocks of trials were considered.
Pre-knowledge of hand Knowing in advance which hand was to be used facilitated respond-
ing. Dit r responses were mrtrated more qmckly m Condition 4 than m Condrtron 1 (by an average of 37 msecs); and thrs was shown to be
A Harmon. A Buhop / Preparation of responses 283
460
420
460
420
380
Condlemn 3
I I I ,
1 2 3 4
460
420
420
380
360
r\ Dab,
DItl
Fig 3 Changes m group choice reactlon time performance (mean of medians) across test sessions
(Ordmate = CRT (msecs), abscissa = practxe level (1 = blocks 1-5, 2 = blocks 6-10, 3 = blocks
11-15, 4 = blocks 16-20))
significant (F( 1, 26) = 5.51, p -c 0.05). Dah, responses were performed, on average, 31 msecs faster m Condition 4 than 3; and thts very nearly reached srgmficance (F( 1, 26) = 4.14, p = 0.052). This difference is srg- mfrcant when only the second half of testing IS considered (F( 1, 26) = 5.50, p < 0.05).
Although there is no firm evidence that prior knowledge about the
284 A Harmon, A Bishop / Preparatron of responses
duration of switch closure required confers any advantage, pre-knowl- edge as to hand significantly speeds up respondmg.
Discussion
It is clear from the results that CRT’s for Dit and Dah responses are not fixed, but vary significantly from response set to response set. The relative sizes of Dit and Dah CRT’s are not even rehable. When ‘mixed’ Dit : Dah pairings were studied, Dit responses were initiated sigmfi- cantly faster than Dahs, just as Klapp et al, (1974) reported. But when Dlt and Dah responses were performed under ‘matched’ conditions there was no evidence of a latency difference. At the very least, thts means that the researcher must be careful always to measure latencies in precisely those contexts he 1s interested in; for mtransltivity means that he cannot depend on extrapolatmg from putatively comparable circumstances.
What sorts of explanations could account for the pattern of CRT’s seen? If Klapp et al. (1974) are wrong m supposing that a choice reaction time paradigm prevents preprogrammmg, then differing amounts of preparation could underlie variations m CRT when the same response is tested in different contexts. The superior performance of Dit and Dah responses when subjects knew m advance which hand would be used is evidence that effective preparation is possible. The data relating to response duration preprogrammmg are more difficult to mterpret. If Condition 3 is accepted as a proper control (the subject knowmg neither which hand nor which response duration was required), then the data suggest that prior knowledge of duration (Conditions 1 and 2) leads to faster Dah responses but slower Dots. Thts implies some problem with Condition 3 as a control, for why should initiation of Dits be retarded by removmg uncertainty? A more likely explanation is that subjects in Condition 3 did not wait for the stimulus light to come on before preparing a response, but adopted some other performance strategy.
Let us accept for a moment that the reason for the difference between Dit and Dah latencies observed m Condition 4 is the greater movement complexity of Dah responses, as Klapp et al. (1974) propose. Error rates for the two responses are much the same, and so a differential speed : accuracy trade-off is not rmphcated. Why then is
A Hanwon, A Bishop / Preparatron of responses 285
there no evidence of this differential when Conditrons 1 and 2 are compared? Could it be that duration is being preprogrammed as happens m SRT testing, with the effect that complexity differences are being masked? Firstly, it is not certain that effective duration prepro- gramming is possible without knowing which muscle groups will be used; and secondly a comparison of Condition 1, 2 and 3 reaction times would indicate that such preprogramming does not sigmficantly shor- ten latencies. A simple movement complexity model is, therefore, not sufficient to account for the variations in CRT found.
What happens if instead credance 1s given to the error scores and reaction times recorded m Conditions 1 and 2, and the conclusion drawn that Dit and Dah responses do not differ in complexity, but can be imtiated with equal facility? The problem now becomes one of accountmg for the differentially faster performance of Dit responses m Conditions 3 and 4. A possible explanation is that subjects opted to prepare one of the responses m advance, and Dit was the one most often chosen. The practice curves for Condition 3 mdmate a growing disparity between Dit and Dah speed, which is compatible with sub- jects perfecting such a strategy. If the person prepares a Dit, response m advance, then he must change both hand and duration parameters if Dah, is demanded. Using the average speed difference m the last quarter of Condition 3 testing as the best estimate available of the time needed for thrs reprogramming, the figure is 51 msecs. Throughout Condition 4, there was a relatively constant Drt : Dah latency difference of about 35 msecs; which, following the reasoning above, would reflect the time taken to convert from a Dit, to a Dah, response. There is no proper basis for deciding whether such figures are reasonable; modifi- cations were completed more quickly m the tasks studied by Gotts- danker (1969) and Megaw (1972, 1974), but the level of strmulus-re- sponse compatibility in these was higher. It is perfectly reasonable that if subjects were preferentially preparing one response, then the Dit rather than the Dah would be chosen; for as Megaw (1974) and Vince and Welford (1967) have shown it is easier to increase existing muscle activity than to decrease it. It is also encouragmg that the time taken for putatively changing hand and duration parameters is longer than that for altering Just duration. Another important point to remember is that subjects were encouraged to reduce average RT in consecutive blocks, and it could be that a preferential preparation strategy offers the most effective way of doing so urlth ‘mtxed’ response pairs. Support
286 A Harnson, A Btshop / Preparatron of responses
for such a model is not unambiguous, however, when error scores are consrdered. Substrtution errors were more common for Drts than for Dahs in Condition 3; whereas if subjects were modifymg Drts mto Dahs, one would expect the observe. No error breakdown 1s given in the original Klapp paper; but in a more recent study (Klapp 1977), Dah responses were found to be more errorful than Dits. He took this as evidence that shorter Drt latencres stem not from a speed : accuracy trade-off, but from their greater srmphcrty. The pattern of errors they report, however, is also consistent with preferential Dit preparation. Another difficulty is that Drt responses were quicker m Condition 4 than in Condition 3. If subjects were preparing a Drt, m Condition 3 and a Dit r in 4, why should latencres differ when they drd not do so m Condrtron l? It may simply be that identifying the required response was easier in Condition 3, for only one hand was mvolved. Certain strategies could overcome thrs, such as simply deciding to output the prepared response rf the appropriate coloured square appears; and maybe rf testing had continued long enough, latency differences be- tween Condition 3 and 4 would have drsappeared.
The speculative nature of the drscussion so far reflects the difficulties encountered in interpreting the data. Extending the study to explore other response pairings probably would not help, because context effects limit the usefulness of such cross-comparisons. There seems to be no adequate way of accountmg for the CRT differences seen if one adopts the premisses that movement complexity is greater for Dah responses, and choice reaction time is an mdex of movement complex- ity. A more consistent picture emerges when a preferential preparation model is adopted; but such an explanation must be considered highly tentative, and a number of possible drffrcultres have already been mentioned.
Setting aside for a moment the question of response duration pre- paration, the study clearly indicates that pre-knowledge about hand srgmfrcantly accelerates response mitration. This can be interpreted m two ways: either as a response selection effect, where the given mforma- tion acts to reduce uncertainty, or in terms of anticipatory motor programmmg. The model of motor programmmg provided by Megaw (1974) predicts that hand information wrll be more useful to the subject than response duration data because hand selection is a primary decision m program constructron. For the data to be accounted for in terms of response selection, it would need to be demonstrated that hand
A Harmon, A Brshop / Preparation of responses 287
pre-knowledge has a higher mformation content than response dura- tion. In Megaw’s model, the decisions mvolved in constructing a motor program are tackled m an mvariant order, with inter-muscle timing decisions preceding mtra-muscle time specification. In terms of transfer of training effects, however, it is known that when people tackle tasks they have previously mastered usmg different muscles groups, then some type of non-muscle based prescription of sequencing and timmg helps them do so. So why shouldn’t performers be flexible m the order of programming decisions they use, so that they can take maximum advantage of whatever information is available? Perhaps physiological constraints sometimes prevent this happening, because motor neurons must be identifed and primed m a given sequence. If so, this represents an essential difference between response selection and preprogramming effects; for any information which effectively reduces the number of alternative responses should speed-up selection and reduce latency, but only information which is relevant to current programming decisions can hasten effector preparation. If it is indeed true that people cannot preprogramme conflicting hand responses, than this may indicate Just such a physiologxal constraint; for parallel-processmg is well-estab- lished in other spheres of human activity (Neisser 1963; Egeth et al. 1972). The present paper clearly highlights the need for further work on the nature and temporal characteristics of stages in response prepara- tion.
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