Journal of Experimental Psychology: Human Perception and Performance 1977, Vol. 3, No. 3, 379-388

Attention and Reaction Times to Signals of Uncertain Modality Lawrence R. Boulter York University, Downsview, Ontario, Canada

Visual, auditory, and tactile reaction time (RT) signals were used in an a-reaction task. The main independent variable was the predictability of signal modality, which was varied by cuing the relevant modality or modalities before each trial. The response requirement was nondiscriminative with respect to modality. Three experiments showed that (a) RTs were longer when signal modality was uncertain, the more so with three possible modalities than with two; (b) this effect of uncertainty was approximately the same whether varied within subjects or between subjects; and (c) the effect of uncertainty was somewhat smaller on tactile RTs than on visual or auditory RTs. Experiment 4 examined change in this uncertainty effect with practice. The uncertainty effect declined over 11 daily sessions to the point of virtual absence from auditory and tactile RTs but was restored or increased with respect to all three signals following one session of discrimination RTs ("respond if visual, refrain if auditory or tactile"). The results are interpreted as showing that attention can be allocated to sensory modalities and that the implied selective process is concerned with modality ''identification," though not in a way consistent with a channel-switching model thereof. Considering the abundance of recently published experiments about human attention, it is surprising how few have expressly examined the effects of stimulus variation between or among modalities (Broadbent & Gregory, 1964; Eijkman & Vendrik, 1965; Kristofferson, 1965; LaBerge, 1973; Lindsay, Taylor, & Forbes, 1968; Massaro & Kahn, 1973; Shiffrin & Grantham, 1974; Treisman & Davies, 1973; Tulving & Lindsay, 1967). Furthermore, in only a few of

The research was partly supported by Defence Research Board Grant 9401-39. Experiments 1 and 4 were briefly reported at the annual meeting of the Canadian Psychological Association in Toronto, Ontario, June 9, 1976. I gladly acknowledge the assistance of Ron Bell, Frank Musten, and Derrnot Stewart in data collection; Jud Burtis contributed extensively to the data analysis for Experiment 4; Jenny Ono prepared the figures. I also wish to thank Len Theodor for some helpful discussions about the theory of signal detection. Requests for reprints should be sent to Lawrence R. Boulter, Department of Psychology, York University, 4700 Keele Street, Downsview, Ontario M3J 1P3, Canada. 379

these studies is such an approach regarded as providing any more than the practical advantage of enabling the simultaneous presentation of signals in more than one "channel," the familiar attraction of dichotic listening. This relative lack of multimodal research on attention is surprising for at least two reasons. The first stems from considering the broad functional role usually assigned to attention: (a) that of a selective process relating the class of "potential" sensory events, through a limited processing system, to perceiving and responding (e.g., Broadbent, 1958; Treisman, 1969); (b) that of a selective process to account for the relation between the relatively very large group of "nominal" stimuli in any situation and the relatively small group of "functional" stimuli in that situation, the stimuli of which the person's behavior can be shown to be a function (e.g., Trabasso & Bower, 1968) ; or, (c) as Swets and Kristofferson (1970) put it: "The emphasis is on the process of the organism's choosing to notice a particular part of his environment" (p. 339). It is

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LAWRENCE R. BOULTER

not clear why such amodal working definitions should have given rise predominantly to research involving stimulus variation within modalities. Even though flow charts have typically represented selection as a central process at which all sensory events arrive, their use in suggesting research has frequently conveyed the presumption that selection is invariably an intramodal process (Moray, 1969; Treisman, 1969), seemingly overlooking the fact that intermodal selection is inevitably a plausible factor in all such research. Second, this predominance of intramodal research is surprising because, at least from one point of view, it appears less promising. Not only does research involving stimuli in different modalities appear intuitively to pertain more directly to ideas about a central selective process but also it might be expected a priori to yield more robust evidence about which to theorize. Among the properties of a selective process must inevitably be included the capacity for dealing with simultaneous stimulation of all sensory systems; it is not unreasonable to expect that intermodal differentiation would occupy more of the limited capacity than would intramodal differentiation (Moore & Massaro, 1973). A further surprise is that a majority of the research on human attention investigates supposed selection among multiple contemporary stimuli, such as messages presented simultaneously to both ears, even though it is known that this presentation guarantees sensory and memory complications. Less used is the less ambiguous technique of examining responding to a single stimulus, both when an assumed selective process could be employed to advantage and when it could not. One might ask, for example, whether a person would be aided by knowing in which ear a message was to arrive if the task was to detect the occurrence of speech in either ear. If observers were equally proficient whether or not they knew in which ear speech would commence, one would conclude that the supposed selective process could not, or at least did not, enable selective discrimination between ears.

Furthermore, it seems clear that only from a test of this sort, wherein observers would be aided by knowing the ear in advance, could one distinguish the consequence of a selective process from, for example, the possible additional consequences of sensory or memory processes, as might be the case in many dichotic listening tasks. This is not at all to say that the literature on dichotic listening is irrelevant to attention, but only that as a starting point for theorizing about a selective process much of the dichotic listening research may be misleading. It is with these considerations in mind that the following experiments were conceived. They employ a simple reaction time (RT) task (Kristofferson, 1965; Mowrer, Rayman, & Bliss, 1940; Woodworth, 1938) in which visual, auditory, and tactile RT signals are presented and a nondiscriminative (a-reaction) response is required. The first experiment includes asking whether RTs are longer when subjects are not told which one signal will occur on each trial. The answer is clearly "yes." Experiment 1 Method Subjects. Twenty right-handed male undergraduate students at York University served as subjects, having volunteered from second- and third-year psychology classes. None had prior knowledge of the purpose of the research. RT signals. The three RT signals were easily discriminable events that were judged by the experimenters to be of moderate and approximately equal intensities. The visual (V) signal was the onset of light from a neon bulb displayed through a .019-m diameter milk-glass disk mounted at the subject's approximate seated eye level, 1.2 m away, at the center of a .36-m high by .48-m wide flat-black vertical panel. The auditory (A) signal was an increase of approximately 7 dB (re 20 /j, N/ma) in the intensity of white noise presented binaurally through earphones; the increase was in relation to a low level (approximately 70 dB) of white noise continuously presented through the earphones to mask apparatus sounds from the adjacent control room. The tactile (T) signal was 60-Hz vibration of a ,019-m diameter nylon disk lightly applied against the inner forearm, approximately .076 m distal to the elbow. The room was illuminated by one 60-W shaded lamp mounted above the display panel.

ATTENTION TO SIGNALS OF UNCERTAIN MODALITY Task. The right-hand armrest of the subject's chair contained a .025-m diameter, lightly springloaded button microswitch. The left-hand armrest contained a hand grip to control arm position and contact with the tactile signal, which was independently mounted in a gap in the armrest. Three green cue lights were aligned vertically on a gray panel (.18 m X .48 m) immediately above the visual signal panel. The top cue light was labeled ARM, the middle EARS, and the bottom EYES. Each RT trial commenced with the onset of one or more of the green cue lights, indicating for that trial the RT signal that would occur (one cue light) or could occur (two or three cue lights). The cue light (s) remained on until the subject pressed the RT button, at which time he also fixated the visual RT signal source. Depressing the RT button initiated the variable foreperiod (in random order, equally likely intervals of 1.7, 2.0, 2.3, 2.7, or 4.3 sec) at the end of which a single RT signal consistent with the cue light (s) occurred, which itself was response terminated. The RT response was button release. It was followed either by a 2-sec rest period or by a 2-sec exposure, through a shuttered aperture, of a digital counter displaying the RT in msec. This knowledge-of-results (KR) aperture was located immediately below the visual RT signal panel. The 2-sec KR period was terminated with the onset of the cue light (s) appropriate for the next trial. Procedure. Tape-recorded instructions were played to subjects individually, accompanied by a demonstration of the trial sequence and the three forms of RT signal. Speed was emphasized, as was care regarding the variable foreperiod. Thirty practice trials were administered of which, in random order, 10 were V, 10 A, and 10 T. All foreperiods occurred equally often with each RT signal. During these trials KR was provided to all subjects; in the subsequent criterion trials KR was omitted for half of the subjects, selected randomly according to their order of arrival for the experiment. Immediately after the practice trials, tape-recorded instructions informed subjects that on subsequent trials their task would remain the same except that frequently more than one of the cue lights would be on to begin a trial; if two cue lights appeared, one of the two corresponding RT signals would occur on that trial, but there was no way of knowing which of the two. Similarly, if three cue lights appeared, there was no way of knowing which one of the three would occur. Half the subjects were also told that KR would no longer be provided. All subjects then were given 80 trials in two randomized blocks of eight trial types combined with the five foreperiods. The trial types were as follows: V (EYES cue light and visual RT signals), A (EARS cue light and auditory RT signal), T (ARM cue light and tactile RT signal), Va (both EYES and EARS cue lights, visual RT signal),

381

Av, Vat, Avt, and Tva; the two-cue-light trial types involving T were omitted. Trials were self-paced, subjects having been instructed to maintain a steady pace but not to initiate a trial until they understood the cue light (s). Intertrial pauses exceeding 2-3 sec were rare. Intratrial events were controlled by electronic timers and were sequenced by a 21-channel tape reader located in an adjacent sound-insulated room. Reaction times were recorded to the nearest msec. Whereas the one longer foreperiod (4.3 sec) had been included for catch trials to reduce anticipatory responding, such responses were rare (eight in all, four by one person) ; despite this, the RTs associated with these foreperiods were not included in the calculations of subject and group mean and median RTs.

Results and Discussion Subjects' mean RTs and median RTs were calculated for the eight trial types. As expected, medians almost invariably were somewhat smaller than means. However, the pattern of results was essentially identical for the two measures, so only means are reported and used in further analyses. Figure 1 shows group mean RTs as a function of trial types (in order, left to right, of decreasing certainty as to modality), plotted separately for the three modalities. Panel a, which summarizes only responses to the visual RT signal, shows a substantial increase in RT over trial types; this effect of modality uncertainty appears to be unaltered by the presence or absence of KR. Panels b and c show similar results for responses to the auditory signal and the tactile signal, respectively. The relevant F ratios from three analyses of variance based on the data represented in Figure 1 indicate that in all three signal modalities trial types was a highly significant effect: visual, F(2, 36) = 35.95, /> < .001; auditory, F(2, 36) = 75.11, p < .001; and tactile, F(l, 18) = 83.10, p < .001. The KR groups differed significantly only in tactile RTs: visual, F(l, 18) = 3.57, .05 < p < .10; auditory, F(l, 18) = 1.27, p > .10; and tactile, F(l, 18) = 11.20, p < .005. None of the Groups X Trial Types interactions approached significance, Finally, it should be noted that relations

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Table 1 Visual, Auditory, and Tactile Reaction Times (msec) to the Different Trial Types of Experiments 2 and 3 Trial types V

Va

273.2 290.7

Vt

Vat

—

328.9

246.2 280.4 299.3 317.8

Av

At

Avt

T

Experiment 2 238.7 283.4 — 292.6 269.9

233.3

Ta

TV

Tav

314.6

Experiment 3 247.3 258.5 292.7 257.0 284.2 284.3 290.1

Note. Table entries are group mean RTs—in Experiment 2 (independent groups), based on 10 of 30 trials in Experiment 3, based on 10 of 120 trials. V = visual; A = auditory; T = tactile.

among RTs in the three modalities are approximately as expected (e.g., see Woodworth & Schlosberg, 1954, pp. 16-17), visual RTs being longer than either auditory or tactile RTs. Experiments 2 and 3 Experiments 2 and 3 might be regarded as "picture cleanup" experiments. First, it seemed possible that the within-subjects methodology could be responsible for the effect of trial types on RT in Experiment 1. A person given simple RT pretraining might regard the subsequently encountered two- and three-cue trial types as more complex, more demanding forms of the initial task, virtually justifying longer response times or at least evoking contrasting strategies. Since some of the people who served in Experiment 1 had made comments consistent with this interpretation, Experiment 2 was designed to evaluate the possibility. Accordingly, Experiment 2 could properly be considered simply a replication of Experiment 1 that used, instead, a betweensubjects design; five independent groups were tested to yield appropriate data on the eight trial types. Second, practical consideration had led to the inclusion of only eight trial types (Vt, TV, At, and Ta having been omitted) in Experiments 1 and 2. However, the apparently lesser impact of uncertainty on tactile RTs necessitated further examination. Thus, Experiment 3 can be considered simply a

replication of Experiment 1 except that all 12 trial types were included. As in the first experiment, trial types was a within-subjects variable. Because the results of Experiment 1 revealed no interaction between KR and trial types, Experiments 2 and 3 provided KR to all subjects. Method Subjects. Sixty-five right-handed male military personnel, ranging in age from 17 to 23 years, served as subjects as part of their military service. Nine of the young men were air cadets; the remainder, enlisted personnel. Seven of the people were rejected during pretraining, either because of equipment failure (four) or failure to follow task instructions (three). The RT signals, task, and procedures were the same as in Experiment 1, with the following exceptions : Experiment 2. Forty subjects were randomly assigned to one of five groups, 8 per group: V, A, T, VA, and VAT. The subjects were all given 10 pretest trials on each relevant signal (e.g., Group A subjects got 10 trials of A; Group VA got 20 trials, 10 of Va and 10 of Av) followed by 30 criterion trials (e.g., subjects in Group A got 30 A trials; Group VA got 15 Va and 15 Av trials, randomly intermixed; Group VAT got 10 Vat, 10 Avt, and 10 Tva, randomly intermixed). Thus, data from the subjects in Group VAT appear in the results for all RT signals; similarly, Group VA provides data on both RT signals. Experiment 3. Eighteen subjects were given 30 pretraining trials (10 V, 10 A, 10 T) followed after appropriate instructions by 120 criterion trials in two randomized blocks of 60, 5 of each trial type V, A, T, Va, Av, At, Ta, Vt, TV, Vat, Avt, and Tva.

ATTENTION TO SIGNALS OF UNCERTAIN MODALITY

383

Both Experiments 2 and 3 were carried out in a York University mobile testing laboratory situated at Camp Borden, Ontario, a Canadian Armed Forces training facility.

Results and Discussion Experiment 2. Group mean RTs for the eight trial types are presented in Table 1. It is obvious that, despite differences in amount of practice on the task, the effect of modality uncertainty (trial types) was approximately the 'same as in Experiment 1 (Figure 1). Analyses of variance based on •subjects' mean RTs for the different trial types indicated a significant effect of trial type for visual responses—Groups V, AV, and AVT: F(2, 21) = 6.77, p< .01; for auditory responses—Groups A, AV, and AVT: P(2, 21) = 3.14, .05 < p< .10; and for tactile responses—Groups T and AVT: F(l, 14) = 7.43, p < .05. Experiment 3. Group mean RTs associated with the 12 trial types of Experiment 3 are also shown in Table 1. They include three pairs that required separate initial examination. There was some tendency for At responses to be slower than Av and for Vt to be slower than Va. However, comparison by t tests of mean RTs on Av and At trials, and on Va and Vt trials, revealed no significant differences: £At-Av(17) = 1.39; *vt-va(17) = 1.99; and *Tv-T»(17) = .01. Consequently, in the remaining analyses a mean RT for each pair of these trial types constituted the datum from each subject. Again, the effect of modality uncertainty was approximately the same as in Experiment 1. Within-subjects analyses of variance based on subjects' mean RTs for the different trial types were calculated separately for the three RT signals. They indicated a significant effect of uncertainty for V responses, F(2, 32) = 44.32, p < .001; for A responses, F(2, 32) = 32.27, p < .001; and for T responses, F(2, 32) = 31.13, p < .001. As in Experiments 1 and 2, the effect of uncertainty was somewhat less on tactile responding than on visual or auditory. Over the three experiments the mean RT difference between Vat and V was 63.7 msec;

V

Va Vat

A Av'Avt" T RT Trial Types

Tav

Figure 1. Mean reaction time (RT) as a function of trial type (V = visual; A = auditory; T = tactile) and knowledge-of-results (KR) condition (see text; »=10 per group) plotted separately for the three RT signals of Experiment 1.

between Avt and A, 62.3 msec; and between Tva and T, 40.3 msec. Experiment 4 In sum, then, Experiments 1, 2, and 3 provide the clear "yes" answer: Reaction times to a signal are longer if the modality of the signal is uncertain. Because the sensory event is the same, for example, on A, Av, and Avt trials, and the response is the same regardless of the trial type, it seems appropriate to conclude that modality uncertainty has its effect on how the sensory event gets analyzed. Furthermore, it is evident that this implies the operation of a nonperipheral selective process having to do with intermodal differentiation. For several reasons, performance on complex trial types (e.g., Avt, Vat, and Tva) might be expected to improve with practice beyond the relatively small improvement common to simple reaction times (see Woodworth & Schlosberg, 1954). Mawbray and Rhoades (1959) and Mowbray (1960) showed substantial practice effects for choice reaction times. While in terms of information transmitted the responses to all our trial types are identical, it is conceivable

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LAWRENCE R. BOULTER

that a person could be responding, at least at the outset, in the manner of a discrimination reaction or, more accurately, Wundt's d-reaction (Woodworth, 1938). (In the dreaction, one common response was called for regardless of which of two or more signals occurred, but the response was to be made only after the signal had been identified.) Second, though they are neither understood nor studied, practice effects are common knowledge among researchers whose investigations involve reaction times (Kristofrerson, 1965; Moray, 1969). They probably are the reason why, in an experiment involving visual and auditory signals and highly experienced subjects, Kristofferson (1965) found no effect of signal uncertainty using a procedure substantially the same as in Experiments 1-3 above. Finally, even within the 80 and 120 trial sessions of Experiments 1 and 3, respectively, many individual subjects' complex trial-types data show some relative improvement from early to late trials. Accordingly, Experiment 4 constituted an evaluation of the proposition that if the difference between RTs to complex (Vat, Avt, and Tva) and simple (V, A, and T) trial types is attributable to a selective process, and if practice reduces that difference, then practice would have had its effect through change in the selective process. Two groups of undergraduate students were given extensive training on either simple or complex trial types. When performance had become relatively stable, one block of discrimination-RT trials (c-reactions) was administered prior to a final sequence of practice sessions, as a test for one possible consequence of the extensive practice.

To enable automated data collection, RTs were recorded by an electronic counter, coupler, and teleprinter system (Hewlett-Packard H 17-S321B 5-digit electronic counter; Hewlett-Packard 2S47A-K36 coupler; ASR 33 Teleprinter) to the nearest microsecond. All statistical analyses were based on RTs in microseconds, but for comparison with other experiments, graphical and tabular representations were rounded to milliseconds. Subjects. Ten right-handed male students at York University served as subjects. They were paid $2 each per session. None had prior knowledge of the purpose of the research. Procedure. All subjects were given four 60trial pretraining sessions comprised of 20 V, 20 A, and 20 T trials randomly intermixed. On the basis of mean auditory RT in the fourth pretraining session, subjects were rank ordered and randomly assigned in pairs, one member to the practice condition of modality certainty and the other to the practice condition of modality uncertainty. For all subjects, pretraining was followed by 11 daily practice sessions, 60 trials per session. The people in the certainty condition received 20 V, 20 A, and 20 T trials randomly intermixed in each practice session. Similarly, those in the uncertainty condition received 20 Vat, 20 Avt, and 20 Tva trials. Immediately prior to the 12th daily practice session, all subjects underwent a block of 60 creaction trials in which the format was that of an uncertainty session (a random mix of 20 Vat, 20 Avt, and 20 Tva trials), except that the people in both groups were instructed to make an RT response only when the signal was visual and to withhold the response (continue to press the button) for approximately 5 sec when the signal was either auditory or tactile. Following a 3-tnin rest interval the standard practice session (certainty or uncertainty) was then given (Session 12), and standard conditions were continued through Sessions 13 and 14. Finally, on Session 15 the people in the certainty condition, after appropriate instructions, underwent one 60-trial block of uncertainty trials. Similarly, uncertainty subjects served in a final block of certainty trials. Throughout all sessions, KR in milliseconds was provided to all subjects in both groups.

Results Method The RT signals, task, and procedures were the same as in Experiment 1, with the following exceptions. A few subjects in Experiment 3 had reported occasional apparent variations in the intensity of the tactile RT signal. Therefore, in Experiment 4 a Model V-47 Goodman vibrator (Sherrick, 1975) was used. It was isolated from possible fluctuations in line voltage and adjusted to provide a comparably discriminable tactile signal. In other respects it was unchanged.

Figure 2 shows group mean RTs as a function of sessions, plotted separately for the three RT signals. Because the corresponding analyses of variance included foreperiod as a factor, data from all foreperiods were included. Three separate analyses of variance were computed on the data from sessions 1-11. Visual RTs revealed a significant overall group effect, F(\, 8)= 6.25, / > < . 0 5 ; session effect, F(10, 80) = 3.30,

ATTENTION TO SIGNALS OF UNCERTAIN MODALITY

385

p < .005; and foreperiod effect, F(4, 32) = Table 2 7.12, p < .001. None of the interactions Visual, Auditory, and Tactile Reaction Times based on visual RTs was significant. The (msec) Following Different Foreperiods for the auditory RTs showed a significant session Certainty (C) and Uncertainty (U) Groups effect, F(10, 80) = 3.13, /> < .005, and fore- in Experiment 4 period effect, F(4, 32) = 21.97, p < .001, Foreperiods (sec) as well as the interaction of Group X Session, F(10, 80) =3.34, p< .005, and Group 1.7 2.0 2.3 2.7 4.3 Group X Foreperiod, F(4, 32) = 6.69, p < Visual .001. Tactile RTs showed only a significant U 288.1 273.9 276.9 273.3 277.4 effect of foreperiod, F(4, 32) = 22.78, p < c 2S7.4 249.4 2S2.2 247.9 257.2 .001, along with the interaction of Group X Session, F(10, 80) = 2.38, p < .05, and Auditory Group X Foreperiod, F(4, 32) = 3.42, p < 249.0 230.4 231.8 221.6 224.1 U .05. c 224.6 211.8 213.7 212.4 220.4 Thus, while the certainty group showed little systematic change over sessions 1-11, Tactile U

Visual 1

c-*u

'

260 260

240 "o"

1

I

1

l

l

I

'Auditory

S{ 260

^240 a. C220

^•0-0.

^200. 280

•I

l

i

~1 r (Jroup C Qroup U

Tactile

260 240

b.-0-o.o 220

'i i

i

238.2 229.4

227.5 224.0 221.3 212.7

220.4 217.5

227.8 230.1

Note. Table entries are group means over Sessions 1-11. In each session, subjects (n = 5 per group) received 4 trials per foreperiod per modality.

300

260

c

i i i

I I

2 3 4 5 6 7 6 9 10 I U I 2 13 14 15

Session Number T Figure 2. Mean reaction time (RT) as a function of session for Groups C (Certainty; n = S) and U (Uncertainty; » = S) plotted separately for the three RT signals. (The vertical arrow marks the interpolated discrimination-RT trials. The filled circles show mean RTs achieved by subjects when transferred for one session from C to U or U to C.)

the uncertainty RTs declined; by Session 11 the groups were not distinguishable on the basis of auditory or tactile RTs. Though the visual RTs of the two groups appeared to be converging over sessions, that interaction did not approach significance. The form of the significant effects of foreperiods is revealed in Table 2 and appears largely to be associated with the difference between the extreme (1.7 and 4.3 sec) and the intermediate foreperiods. The significant Groups X Foreperiods interactions found with both auditory and tactile RTs apparently reflect the fact that over sessions, the group difference is less at the longer foreperiods. Visual inspection of group means plotted separately for each foreperiod for sessions 1-11 suggests that practice reduces the uncertainty effect more quickly at the longer foreperiods. The effect of the block of c-reaction trials on subsequent RT performance was assessed by t tests comparing group mean RTs on Session 11 and Session 12 (see Figure 2) separately for the three RT signals. The uncertainty response times significantly increased from Session 11 to 12, t(4) = 2.96, p < .05; f (4) = 3.72, p < .05;

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LAWRENCE R. BOULTER

and *(4) = 5.71, p < .01, for Vat, Avt, and Tva -responses, respectively. Certainty responding was not significantly affected (t = .16, .32, and 1.64 for V, A, and T responses, respectively). The 'block of c-reaction trials apparently interfered substantially only with uncertainty responding; that is, the c-reaction task produced negative transfer for uncertainty responding but not for certainty responding. Finally, as shown in Figure 2, when certainty subjects performed a block of Vat, Avt, and Tva trials (Session 15), their RTs were approximately at the level of unpracticed subjects. At the same time, the people who had practiced the uncertainty task showed no influence of their adaptation to it in their session of T, A, and V responding. That is, Session 15 sh6ws no transfer effects either way. Discussion Experiment 4 shows that the effect of modality uncertainty on RT can be reduced by practice. While it is possible that practice might have enabled subjects somewhat to reduce signal uncertainty by learning sequential dependencies (e.g., there were always 10 of each signal in successive 30-trial sets), this would not easily explain differences between modalities. Nor would it explain the transfer from c-reaction trials on Session 12. Instead, in keeping with my assumption that a selective process is responsible for the uncertainty effect, I regard the practice-produced reduction in the uncertainty effect as one consequence of an adaptive change in the selective process. Furthermore, if we likewise regard the differential transfer on Session 12, then, taken together these consequences imply properties of the selective process. First, they imply that, at least in part, the selective process is occupied in this task with "identifying" the modality of an RT signal, even though identification is not part of the response requirements.1 The basis for this implication is that the unique characteristic of the c-reaction, which presumably produced the differential transfer, is

that it explicitly requires modality identification. Its differential influence on the certainty and uncertainty groups (Sessions 1214) implies that the improvement (Sessions 1—11) in uncertainty responding reflected reduction in a modality-identifying component of the RTs. This is, of course, consistent with an interpretation of attention as the system that specifies the level of analysis to be applied to stimuli (Craik & Lockart, 1972). Another implication, though less clear, stems from the observed rate of improvement; that it was relatively gradual over sessions 1-11 for all subjects bespeaks a basic learning mechanism more than simply a change in strategy (Swets, 1963) by subjects, and the data of sessions 12-14 (Figure 2) are consistent with this view. It may be pertinent to recognize the strong similarity between a reaction time task and classical conditioning procedures (Grant, 1964). Whatever their specific implications, generally these results support attention models. Therefore, it is appropriate to comment on recent, somewhat similar experiments (Shiffrin & Grantham, 1974) that also use visual, auditory, and tactile signals, though in a near-threshold detection task, and that have, in contrast, led the authors to conclude against attention models. Their experiments (see also Shiffrin, 1975) involve presenting three observation intervals per trial (one per modality) either simultaneously or successively, but at most one signal per trial. They reason that if the successive intervals are always in the same, known order, and if the observer can selec-

1

To some degree, even simple RTs (a-reaction) are "discriminative" reactions in this way; a person must refrain from responding to nonsignal events such as coughs, itches, and the like. The superiority of "motor" over "sensory" reaction times (Teichner, 1954; Woodworth & Schlosberg, 1954) may reflect a difference in this discriminative aspect, the motor set serving further to reduce its role. If so, it would help to account for the lack of continued improvement in Group C responding over sessions: The random schedule of RT signals within a session may interfere with establishment of a stable motor set.

ATTENTION TO SIGNALS OF UNCERTAIN MODALITY tively attend to a modality, then performance will be better under the successive condition than when the three observation intervals occur simultaneously. However, while this logic seems apt, their task does not, and the conclusion against attention models, based on finding no difference between their simultaneous and successive conditions, appears unwarranted. The confounding factor is that, in effect, their observers' task is not to detect, for example, a visual signal but to conclude that a visual and not an auditory or tactile signal has occurred. This memory component does suggest why performance in the simultaneous condition has sometimes been superior (Shiffrin, 1975). The effect of uncertainty in my experiments has been discussed as if modality uncertainty is the only conceivable difference between simple and complex trial types, but one can, of course, describe it otherwise. In particular, perhaps the most important difference is simply in the number of possible imperative events. From this view it could follow, for example, that similar results would occur if additional distinguishable visual events were used in place of the auditory and tactile signals. An experiment by Bernstein, Schurman, and Forester (1967, Experiment 2) provides pertinent data. They measured simple RTs to eight spatially arrayed lights presented within the fovea. When the RT signal could be, unpredictably, any one of the eight lights, RTs were longer than when instructions specified the light location in advance. However, the effect was relatively small, the mean RT difference being only 8 msec. I know of no published research involving auditory or tactile reaction times that directly addresses this question. Thus, evidence as to the magnitude of an intramodal effect on RT is available only for visual location, and that provides little empirical basis for discounting the significance of modality uncertainty in the present experiments. In any case, though the existing research is sparse, even in the event that studies of intramodal signal uncertainty were to show effects entirely parallel to those produced

387

by modality uncertainty, it would scarcely follow that modality uncertainty was a misnomer. What it might reflect, though, is that both forms of signal uncertainty engage a common selective process, which would be neither implausible nor surprising. Finally, although the complexity of differences among modalities cannot be essayed here, the data of Figure 2 invite one related comment. They suggest that change in the selective process is slower with respect to the visual than to either the auditory or tactile signals. Whereas such a difference among modalities would not follow easily from a channel-switching model, the importance of modality identification as part of the selective process suggests another possibility. The uniqueness of visual RTs does appear to be consistent with dominance relations among modalities (e.g., Kelso, Cook, Olson, & Epstein, 1975). Furthermore, the greater persistence of identification in response to visual signals may specifically be abetted by vision's more distinctive peripheral components, such as head and eye movements, and accommodation. A somewhat related interpretation comes from the theory of visual dominance outlined by Posner, Nissen, and Klein (1976). They propose, on the basis of several lines of evidence, that visual dominance may signify an attentional 'bias favoring vision over other modalities that balances, or is balanced by, vision's less efficient automatic alerting system. What this interpretation may suggest is that the auditory and tactile responses in Experiment 4 come to be 'based on their more effective automatic alerting systems, while visual stimuli continue to require specific attention. This point of view also draws attention to the fact that the changes in dealing with auditory and tactile signals (Figure 2) do not directly benefit visual RTs. References Bernstein, I. H., Schurman, D. L,, & Forester, G. Choice reaction time as a function of stimulus uncertainty, response uncertainty, and behavioral hypotheses. Journal of Experimental Psychology, 1967, 74, 517-524.

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Received November 23, 1976 •