Probability effects in choice reaction time tasks

Transcription

1 Perceptin & Psychphysics '0/. 15. N. I. / Prbability effects in chice reactin time tasks ELENA KRINCHIK* Mscw State University, Lenin Hills, Msc... V-234, U.S.S.R. This paper cntains a shrt review f the main results that were btained by the authr in a series f experiments that cnstituted a study f the effects f signal prbability n chice reactin time. The effects f stimulus prbability are shwn t be influenced by the fllwing variables: (1) differences in the methd f varying stimulus prbability, (2) differences in task cmplexity, (3) differences in S-R cde, and (4) differences in Ss' mtivatin. The data that are cnsidered here are the verall mean RT fr particular signals and the mean RT fr sequential repetitins. Tw questins, related t the psychlgical "nature" f the prbability effects in chice RT are discussed: (1) The questin f the relatinship between the relative frequency and the number f alternatives as tw different ways f determining the prbability effect in chice RT; and (2) the questin f identifying the main determinants f the trial-t-trial variability f RT in such experiments. The first effrts t apply a prbabilistic apprach t the analysis f animal and human behavir were made several decades ag (Brunswick, 1928, 1939, 1943; Rmanv, 1935; Skinner, 1938). S much data and s many mdels using this apprach have accumulated since then that a branch f psychlgy has nw emerged that specializes in the study f stchastic aspects f behavir. Within this area, a special place is ccupied by the investigatin f man's behavir in prbabilistic situatins, such as the analysis f prbability learning, the effects f stimulus prbability in reactin time (RT), etc. This paper attempts t present a systematic analysis f sme aspects f the prbability effect in chice RT experiments and t frmulate a hypthesis cncerning its main determinants. METHOD Since the results f a large series f experiments will be cnsidered in this paper, the experimental methd will be described in a general way at first. The methdlgical details pertaining t a particular experiment will then be given at the time the results fr that experiment are reprted. The principal resul ts were btained by using three types f experiments: (1) experiments with tw-chice tasks, where the independent variable was the relative frequency f the stimuli; (2) experiments with multiple-chice tasks, where the independent variable was the number f equiprbable alternative signals; and (3) experiments with multiple-chice tasks where the tw independent variables were the number f alternatives and their relative frequencies. Each experiment began with a practice sessin, in which the simple RT and the chice RT were measured with equiprbable signals. Signals were presented randmly accrding t a special prgram, with the intertrial interval (ITI) set between 5 and 10 sec, depending n the experiment. In thse experiments where the ITI exceeded 5 sec, a warning signal was presented sec befre the test signal. Varius kinds f stimulus-respnse cdes were used: lights t keys (Figs. 2E and 2F), lights t verbal symbls (Figs. la and 1B, and Figs. 2G t "'I wuld like t express my deep gratitude t Prfessr A. N. Lentjev, n whse initiative this series f researches was started several years ag, and under whse direct guidance the first f the studies described here was perfrmed. I am als especially indebted t Sylvan Krnblum fr his detailed cmments n several previus drafts f this paper. 21), gemetric ftgures t keys (Fig. 10, sunds t keys (Figs. 2A and 2B), tw-digit numbers t their verbal names (Fig. 2C), tw-digit numbers t keys (Tables 3 and 4). When lights were used as stimuli, they were cmmutatr's lamps (24 V, A, 10 mm in diam). In the tw-chice experiments, they were placed hrizntally n a vertical panel, with a distance between their centers f 75 mm (Lentjev & Krinchik, 1962; Krinchik, Mednikarv, & Brisva, 1970). In the mul tiple-chice experiments, these lamps were placed n a vertical panel in a 3 x 3 matrix, with a distance between centers f the tw next lamps f 25 mm (Lentjev & Krinchik, 1964). Gemetrical figures were drawn in black n white cards. Their size was 20 x 20 mm. They had cnturs f a square, a circle, a triangle, a star, a crss, the sign f infinity, and a spiral. They were presented in the windw (95 x 50 mm) f a tachistscpe (Krinchik & Alexandrva, 1966). When numbers were used as stimuli, slides with a black number inscribed in white squares were presented n the screen with an epidiascpe (Krinchik & Akimva, 1970; Krinchik & Alexandrva, 1970). The prjected size f each number was equal t 30 x 15 mm, and f the white square, 100 x 100 mm. Data fr the sunds used as stimuli are presented in Sectin B. When keypress (r buttnpress) reactins were used, the reactive keys (r buttn) were placed either n the hrizntal panel arranged n the table befre S r n the armchair. Verbal reactins were registered thrugh the micrphne r laringphne and with special equipment which stpped the timer at the start f the verbal reactin. In mst experiments, RT was measured with an electrnic timer, MSK-l, in msec. Depending n the experiment, the length f a stimulus sequence varied between 80 and 240 trials. In the case f shrt sequences, Ss might be given up t tw r three sequences per day. In the main sessins, the sequences having the same stimulus prbability distributins were replicated frm tw t eight times, depending n the experiment. In all the experiments, the S was required t react as fast as pssible withut making errrs. Data f errr rate, as well as statistics f trials per data pint, training per S, and ther details f the experimental methds are presented in Tables 1 and 2. Errneus RTs were nt included in the data. During the experiment, the S was nt infrmed abut the prbabilistic structure f the sequence, nr was he given feedback n his speed r accuracy, except in thse experiments where differential reinfrcement was the variable (see Figs. 2F-21). In the latter case, special methds f reinfrcement were used when Ss exceeded the predetermined time deadline during the current curse f the sessin: "equipment breakdwn" reinfrcement in the tw-chice task [see explanatins in (0) belw], and three different types f reinfrcement (red light, shck, and "equipment breakdwn") in 131

2 132 KRINCHIK Fig. la Experiment Lentjev & Krinchik, 1964 Table I Trials Per Data Pint and Errr Rate fr Each Experiment' Cnditins Tw-chice task Lights t Verbal Symbls Intertrial Interval (ITI)-l 0 Sec Freperid (FP)-2 Sec Warning Signal (WS)-Bell Trials Per Days Sessin N = 240 Ss = 6 Fig. 1B (Upper Curve) Multiple-Chice Task With Equiprbable Signals Lentjev & Krinchik, 1964 Lights t Verbal Symbls ITI-IO Sec (Data fr All Alternative Signals) FP-2 Sec WS-Bell N = 240 Ss = 6 Fig. ic- Multiple-Chice Task With Nnequiprbable Krinchik & Alexandrva, 1966 Signals n = 2 Gemetrical Figures (Data fr the Critical Signal) t Keys n=4 ITI-7 Sec FP-2 Sec WS-Bell N = 120; n = 8 Ss = 6 Fig. 2A Krinchik & Rajevskij, 1970 (Data fr the Critical Signal) Fig.2B Krinchik & Rajevskij, 1970 (Data fr the Critical Signal) Dnders's A, C, and B Reactin Task (Withut Warning Signals) Sunds t Keys ITI-5 Sec N = 120 Ss = 5 Dnders's A, C, and B Reactin Tasks (With WarningSignal) Sunds t Keys ITI-5 Sec FP-1.5 Sec WS-Tne f 1000 Hz N = 120 Ss= 4 Fig. 2C' Multiple-Chice Task With Nnequiprbable Krinchik & Akirnva, 1970 Signals n = 2 Numbers t Keys (Data fr the Critical Signal) IT!-I0 Sec FP-2Sec n=4 WS-Bell N=120;n=8 Ss =6 Stimulus Prbability A Reactins C Reactins B Reactins A Reactins C Reactins B Reactins 0,07 Trials Per Data Pint O Errr Rate Per Data Pint (in Percent) Ttal Per S

3 Table 3 3 Experiment Krinchik & Akimva, 1970 (Data fr the Critical Signal) Fig.2D Krinchik & Alexandrva, 1970 Fig.1E 4 Krinchik, Mednikarv, & Brisva, 1970 Fig.2F Lentjev & Krinchik, 1962 (Data fr the Critical Signal) Fig. 2G-I s Lentjev & Krinchik, 1964 (Data fr the Critical Signal) PROBABILITY EFFECTS I~ CHOICE REACTION TIMETASKS 133 Table 1 Cntinued Errr Rate Per Data Stimulus Trials Per Pint (in Percent) Prba- Data Cnditins bility Pint Ttal Per S Multiple-Chice Tasks With Nnequiprbable Signals n= Numbers t Keys ITl-I0 Sec n= FP-2 Sec WS-Bell N= 120;n=8;Ss= Slving a Divisin Prblem Negative Respnses ITI-I0 Sec FP-2 Sec Psitive Respnses WS-Bell N = Ss = Tw-Chice Task Experiment 1 With Interleaved Distractr Task Experiment 11 Lights t Keys ITl-I0 Sec fr all three experiments Experiment 111 N = Ss = Tw-Chice Task Backgrund Experiment Experiments Withut Rein frcement and With Equip ment "Breakdwn" Reinfrcement Reinfrced Signal Lights t Keys ITl-IO Sec FP-2 Sec Nnreinfrced Signal WS-Bell N = Ss = Fig.2G: Backgrund Sessins Multiple-Chice Task With Equiprbable Signals and Different Types f Reinfrcement Reinfrced Signal Lights t Verbal Symbls ITl-lO Sec Fig.2H: Backgrund Sessins FP-2 Sec WS-Bell Reinfrced Signal N = Ss = Fig. 21: Backgrund Sessins Reinfrced Signal

4 134 KRI~CHII. Experiment Lentjev & Krinchik, 196* (Data fr Nnreinfrced Signals) Cnditins See Preceding Sectin Table 1 Cntinued Stimulus Prbability Errr Rate Per Data Trials Per Pint (in Percent) Data Pint Ttal Per S Nnreinfrced I Trials per data pint and errr rate d nt include the data f the training sessins. Errr rate data are given in tw variants: as a ttal errr rate per data pint and as an average errr rate per 5. 1 Data fr the critical signal are the average data fr tw signals that were presented symmetrically in all the three set size cnditins (S ; :. 4. S). Tw different figures were used as critical signals fr different Ss. 3 Here nly aile critical signal was used and it was different fr each f the three set size cnditins. But these three different critical signals remained the same fr all Ss in these experiments. Each prbability cnditin has 90 trials. Tw such cnditins were used ill 1 day's sessin. SAll three set size cnditins were used in each day's sessin in the backgrund sessins as well as in reinfrced sessins. Ttal trials per day's sessin were equal t 240 (80 trials per each set size cnditin). the multiple-chice task [see explanatins in (D) belw). At the start f the sessins with reinfrcement, the Ss were infrmed f the time deadline. It was different fr each S in the tw-chice task and was equal t the fastest mean RT shwn by this 5 in the backzrund sessins (i.e., the sessins withut reinfrcement) with -equiprbable signals. In the multiple-chice task, the time deadline was equal t 400 msec fr all Ss. Tw sessins with reinfrcement were cnducted in the tw-chice experiment: ne with the reinfrced rare signal, and the ther with the reinfrced frequent signal. In bth cases, the same signal was reinfrced. Three sessins with reinfrcement were cnducted in the multiple-chice experiment. All three set size cnditins (N = 2, 4, 8) were presented in 1 day's sessin, and three f eight signals were reinfrced with three different types f reinfrcement. Training reinfrcement sessins preceded the main sessins with reinfrcement in bth experiments. In the tw-chice task, there were tw such sessins with equiprb able signals in which the red light flashed when Ss exceeded the time deadline. In the mul tiple-chice task, there was ne training sessin in which three f eight signals (different frm the reinfrced signals in the main sessins) were reinfrced with the tne when Ss exceeded the time deadline. Main data frm the reinfrcement cnditin were cmpared in all cases with well-practiced data frm the backgrund cnditin, which always preceded the reinfrcement cnditin and which cnsisted f many sessins (see Tables 1 and 2). The Ss were students frm different departments at Mscw State University. They were naive abut the purpses f the experiments, but sme f them tk part in several f the experiments. Additinal changes in experimental cnditins cncerned the methd in which stimulus prbability was varied, the character f the experimental task, and the type freinfrcement that was used. RESULTS (A) The Effects f Different Methds f Varying Stimulus Prbability Figure 1A presents the results f the tw-chice experiment in which stimulus prbability was varied by changing the relative frequency f the signals (Lentjev & Krinchik, 1964). Stimulus prbabilities ranged frm t and were cunterbalanced fr bth signals. Under these cnditins, and with the range f signal Signal Table 2 The Average Amunt f Training Per Subject Figures Fig.1A Fig.1B (Upper Curve) Fig. ic Fig.2A Fig.2B Fig.2C Fig.2D Table 3: Numbers t Keys Fig.2E Slving a Divisin Prblem Backgrund Sessins Fig.2F Reinfrced Sessin Backgrund Sessins Fig.2G-l Reinfrced Sessins Stim- Average ulus Amunt f Prba- Training bility Trials Per S , , , , N = 2 N=4 N'" ,25 0,50 1 0,50 1 0,50 1 0,50 0,50 1 0,125 0,25 0,50 0, , prbabilities that was used, the effect f prbability n verall RT appears t be satisfactrily apprximated by a straight line. The similarity between these results and

5 PROBABILITY EFFECTS IN CHOICE REACTION TIME TASKS 135 thse btained in ther studies is striking (Lentjev & Krinchik, 1962; Bertelsn & Barzeele, 1965; Laming, 1969; Remingtn, 1969). Differences in the intercept are apparently due t differences in sme particular cnditins f the experiment. In Fig. 1B, the effect f stimulus prbability in a tw-chice task is cmpared with the effect f stimulus prbability when it is varied by changing the number f equiprbable signals frm tw, t fur, t eight (Lentjev & Krinchik, 1964). Bth experiments were cnducted with the same Ss and the same equipment. The data suggest that these tw different methds f varying stimulus prbability cmprise tw different cmpnents in the prbability structure f stimulus sequences. The first deals with the relative frequency f a fixed number f signals and will be dented as the "frequency" cmpnent; the secnd deals with differences in the number f alternatives and will be dented as the "alternative" cmpnent. In Fig. 1C are pltted the data f an experiment in which bth the number f alternatives and the relative frequency f the signals were changed simultaneusly (Krinchik & Alexandrva, 1966). In each f a tw-, fur-, and eight-chice task, the prbability f ne f the signals, the critical signal, was changed frm.07, t.5, t.93; the remaining signals were presented equiprbably. The data in Fig. 1C are fr the critical signal nly and indicate the presence f a clear interactin between prbability and the number f alternatives. Similar results were btained by Krinchik and Akimva (1970) (see Tables 3 and 4, the numbers t keys task). Krnblum (1967, 1969) als analyzed the relatinship between stimulus prbability and the number f alternatives in a situatin in which he varied the prbabilities f repetitins and nnrepetitins f the signal in a tw-, fur-, and eight-chice equiprbable task. His results were quite similar t urs, bth fr the repeated and fr the nnrepeated signals. Hwever, he fund that the repetitins and nnrepetitins were differentially affected by the number f alternatives and, in cmparisn t ur data, were less affected by the signal prbability. Sternberg (1969), wh als studied an interactin between stimulus prbability and the number f alternatives in a binary classificatin task, finds that the slpe is affected by the prbability, whereas the intercept is affected by the number f alternatives. Ij{KI Fig. 1. (A) Tw-chice data with nnequiprbable signals. The abscissa represents stimulus prbability, the rdinate represents verall mean RT. The stimuli used were lamps, and the respnses were verbal. a--a Lentjev & Krinchik, 1964 (RT = p): a a Bertelsn, 1965 (RT = p, FP = 5 sec): Bertelsn, 1965 (RT= p, FP =1/2 sec): 0--0 Laminlk_ 196. (RT = p);.--. Remingtn, 1969 (RT = 326-1Op). (8) Overall mean RT fr a tw-chice task with nnequiprbable signals a--a (RT = 363-9Op) and fr a rw-, fur-, and eight-chice task in which the signals were equiprbable.--. (RT = p) the stimuli cnsisted f lamps and the respnses were verbal (Lentjev & Krinchik, 1964). The data are fr ne particular signal that was used in bth experiments. (C) Overall mean RT fr the critical signals in a tw-chice a--a (RT = 343-8Op), fur-chice.--. (RT = p), and eight-ehice.--. (RT =512-24Op) tasks; the stimuli were gemetric figures and the respnses were keypresses (Krinchik & Alexandrva, 1966). Table 3 Mean RTs t the Critical Stimulus fr Different Numbers f Alternatives and fr Different Stimulus Prbabilities Stimulus Prbabilities Tw Alternatives Fur Alternatives Eight Alternatives Study Task Krinchik & Alexandrva (1966) Krinchik & Akimva (1970) Gemetric Figures t Keys Numbers t Keys Numbers t Names ~

6 136 KRINCHIK Table 4 Prbability Effect (Slpe and Intercept) fr Experiments With Different Degree f Chice (Data fr Critical Signal) Number Krinchik & Krinchik & Krinchik & f Alexandrva, 1966 Akimva, 1970 Akimva, 1970 Alternatives Krnblum, 1969 Gemetric Task Figures Numbers Numbers Repeated Nnrepeated Parameter t Keys t Keys t Names Signal Signal Intercept Slpe (B) Prbability Effects at Different Stages f Infrmatin Prcessing Krinchik and Rajevskij (1970) next studied the effects f stimulus prbability in tw experiments in which Dnders' A, B, and C tasks were used in rder t cmpare the prbability effect in a task representing different stages f infrmatin prcessing. Each experiment used a different grup f Ss. Except fr the Band C tasks f Experiment I, n warning signal was used in this study. The stimuli in Experiment I were tnes f 400 and 800 Hz. The stimuli in Experiment II were tnes f 250 and 600 Hz. The stimuli were presented with prbabilities f.07,.5, and.93 in each cnditin f each experiment, Where the task was a simple RT task, ne f the tnes was simply mitted frm the randm sequences which had been prepared fr ne f the ther tasks; in this way, the tempral distributin f the slitary signal was cmparable t its distributin with ne mre signal. The results, which are presented in Figs. 2A and 2B, indicate that the effect f prbability was quite similar in the A and B tasks. This finding suggests that the effect f prbability is clsely related t tempral structures ("time mrphlgy") f the stimulus sequences (Krinchik, 1969; Krinchik & Mednikarv, 1970). Secnd, the warning signal did nt have an effect n the slpe f the curve in the 'B task. This is in cntrast t Bertelsn and Barzeele (1965), whse findings imply that, with the warning signal acting as a cue, varying the length f the freperid wuld affect the slpe (Fig. la) f the line. Finally, prbability appears t have had a peculiar effect n the C reactin. In bth experiments, the data fr the C task depart frm linearity by what appears t be the prlnged RT in the equiprbable situatin. This indicates the imprtance f signal discriminatin prcesses in the C-reactin task, since these prcesses must be mre active in the equiprbable situatin because f the frequent alternatin f the signal.' Mrever, the curve fr the C task reveals a clear tendency t change its psitin in the rdinate accrding t the absence r presence f the warning signal. In the frmer case, the RT in general is clser t the B-reactin data; in the latter case, it is clser t the A-reactin data. (C) Prbability Effects with Differences in Task Cmplexity Figure 2C illustrates the data fr the critical signal f a study in which bth the number f alternatives and stimulus prbability were varied in a number-naming task (Krinchik & Akimva, 1970). In this study, tw-digit numbers were used as stimuli, with a different number being used as the critical stimulus at each level f chice ("23" fr tw-chice, "27" fr fur-chice, and "25" fr eight-chice tasks). Stimulus prbability exerts an effect at all degrees f chice; this effect als appears t be mre prnunced than in previus studies (Fig. l C and Table 4). Hwever, the number f alternatives des nt seem t have had an effect in the number-naming task, whereas it did have an effect in the keypressing task (Tables 3 and 4). Bth experiments were cnducted with the same grups f Ss, and the same stimuli were used. It is difficult t say nw whether the absence f an alternative effect is due t S-R cmpatibility factrs f this number-naming task (Mwbray, 1960) r t the unfamiliarity f tw-digit numbers which led t the elevatin f RT fr tw- and fur-chice tasks (Fitts & Switzer, 1962). In any case, these data cnfirm Mwbray's results (1960) and in a different situatin prve that the number-naming cde des nt ablish the effect f frequency cmpnent f stimulus sequence. Mwbray, t, has bserved a clearly expressed effect f a frequency cmpnent in an experiment with a number-naming task using selective reactin and a different number f equiprbable stimuli: fr N = 2, 4, 8, it had an intercept f 549 and a slpe f Cmparisn f the data btained in the number-naming task and in the number key pressing task supprts Sternberg's finding (1969) that there is different lcalizatin f prbability and set size effects n infrmatin prcessing stages. Mrever, this result appears t suggest that the prbability effect is cnnected nt nly with respnse bias prcesses and the respnse rganizatin stage, fr the investigated tasks requires very different respnse rganizatin prcesses, whereas the prbability effect was significantly great in bth tasks.

7 PROBABILITY EFFECTS IN CHOICE REACTION TIME TASKS 137 1",, Fig. 2. (A) RT as a functin f stimulus prbability. The stimuli were tnes (400 '"" and 800 Hz) and the respnses were " " keypresses. The results are shwn fr three "'", tasks: Danders' Type A e_ (RT = 22~ -79p), Dnders' Type B (RT = 361 -llop), and Dnders' Type C 110 c e~ - - -e. The data are fr ne particular 0001 s OJ 0'1S l signal which was used in each f the three p P tasks (Experiment I, Krinchik & Rajevskij, 1970)....,~--_ -.. (B) RT as a functin f stimulus prbability. Stimuli were tnes (250 and 600 Hz) and the respnses were keypresses.... The results are shwn fr three tasks: Dnders' Type A e--e (RT = -: ''''1""""'".. 21~ -70p), Dnders' Type B"'-' { J:,). (RT = 322-1l5p), and Dnders' Type C JCX., """" e e. The data are fr ne particular I ' signal which was used in each f the three J tasks. (Experiment II, Krinchik & Rajevskij, 1970). (C) Overall mean RT fr the critical stimuli in number-naming tasks where the ''''"I> <, ~~ n umber f alterna tives and stim ulus :» OV1 0 ~ C'J~ I, p prbability were varied simultaneusly. p Tw-chice e--e (R:T = p), fur-chice (R:T =550 -!lop), and eight-chice e (RT = p) (Krinchik & Akimva, 1970). (D) Respnse latency in slving a divisin prblem. See text fr explanatin. Latency fr psitive set --e (RT = p),-_Iatency fr negative set (RT = p) (Krinchik & Alexandrva, 1970). (E) Mean RT data fr nnequiprbable tw-chice tasks in which the stimuli were lamps and the respnses were keypresses; with a distractr task (RT = p, Experiment I),!:> c:, (R:T = p, Experiment III), and withut a distractr task e--e (R:T = c p, Experiment II). The RT fr the 0...,... equiprbable cnditin with the distractr p task is indicated by e (Krinchik, Mednikarv, & Brisva, 1970). (F) Overall mean RT as a functin f stimulus prbability in the tw-chice nnequiprbable task with the "equipment breakdwn::.jeinfrcement. RT fr reinfrced signal...~ (R:T = p), RT fr this signal in backgr~lid experiment e_(rt = 329-1I5p), RT fr the nnreinfrced signal in the experiment with reinfrcement: (RT = p). Stimuli were lights and respnses were keypresses (Lentjev & Krinchik, 1962). (G-l) Overall mean RT as a functin f stimulus prbability fr rw-, fur-, and eight-chice equiprbable tasks with differential reinfrcements given n selected signals fr respnse times exceeding 400 msec. The RT t reinfrced signals is indicated as...~, and t the same signal in the backgrund experiment, as e e. The stimuli were lamps, the respnses were verbal symbls (Len9.~ & Krinchik, 1964). (G) Reinfrcement cnsisted f a red light ging n fr excessively lng RTs. With reinfrcement, RT = p, withut reinfrcement, RT = p. (H) Reinfrcement cnsisted f a shck being given after excessively lng RTs. With reinfrcement, R:T = p, withut reinfrcement, RT = p. (I) Reinfrcement cnsisted f the equipment "breaking dwn" after excessively lng RTs. With reinfrcement, RT = p, withut reinfrcement, RT = p. Figure 2D shws the effect f stimulus prbability n verall mean RT fr the slutin f a simple arithmetic prblem (Krinchik & Alexandrva, 1970). The task cnsisted f determining whether a three-digit number was r was nt divisible by 3 (Bdrv, Genkin. & Zarakvski, 1966). The numbers were presented n a screen with an epidiascpe, and were turned ff by the S's verbal "yes" r "n" respnse, The prbability f

8 138 KRINCHIK TableS Average Percent Errrs and Average Percent Omissins in the Perfrmance f a Distractr Task (Krinchik, Mednikarv. & Brisva, 1970) Sessins With the Distractr Task Befre the Backgrund Series Sessins With the Distractr Task After the Backgrund Series Percent Percent Percent Percent P Errrs Omissins Errrs Omissins negative and psinve instances was varied in different sessins and cunterbalanced ver sessins (p =. 0.5.). The interactin f psitive and negative respnses with prbability is quite clear in these data. In additin, the negative RT is cnsiderably lnger than the psitive RT. which cnfirms previus findings (Sternberg, 1967, 1969: Nickersn, 1969). The interactin in ur wn data. hwever. disagrees with the results btained in ther binary classificatin tasks (Sternberg, 1969; Krueger. 1970). where negative and psitive tests prduced the same value f prbability effect. The cnsiderably larger value f the prbability effect in this task can be attributed t the fact that the task invlved prblem slving. In cntrl sessins f this experiment, simple RT fr psitive and negative numbers by themselves was measured. We used tw types f instructins: In the first. Ss were required t name the number that had been presented: in the secnd, Ss were required t answer "ves" r "n" accrding t the categry f the presented numbers. Neither the number identificatin time nr the verbal respnse time revealed any differences cnnected with these tw categries. The abve-mentined difference is bserved nly when S is faced with the prblem fdetermining the divisibility f numbers, and this difference increases when stimulus prbability decreases. One way in which the difference between the RT fr psitive and negative respnses in binary classificatin tasks has been accunted fr is in terms f a serial search mdel (Nickersn, 1969). That mdel, hwever, hardly accunts fr the results in Fig.2D. This questin, therefre, requires serius theretical and experimental study. Nevertheless, it is tempting t cnjecture that this phenmenn is due t the fact that befre negative decisins are made they are "checked" nce mre, while psitive decisins are made immediately. Pssibly, this is what causes the increase in the differences between psitive and negative RT when test number prbability decreases, i.e., when S's dubt abut the crrectness f negative decisins may increase. Figure 2E shws the results f a study in which Ss perfrmed a tw-chice task in which lights were used as stimuli and keypresses as respnses. Interleaved between successive trials n this task was a distractr task cnsisting f the divisin prblem described abve. Tw divisin prblems were presented during the lo-se.c III f the majr task (Krinchik, Mednicarv, & Bnsva, 1970). The study cnsisted f the fll~wing experiments: practice sessins, measurement f SImple and chice RT (tw-chice equiprbable task); Experiment I, tw-chice RT wi~h nnequiprba~le stimuli and distractr task; Expenment II, tw-chice RT with nnequiprbable stimuli and withut distractr task; Experiment III, the same as Experiment I. In all these experiments, the same set fstimulus prbabilities was used. An equiprbable cnditin with the distractr task was presented nly nce-immediately after the practice sessin. Analysis f the data indicated that there were cnsiderable differences in the effects f prbability in these three experiments. In Experiment I, the verall mean RT data shws n prbability effects at all (slpe = -12). In Experiment II, the prbability effect was naturally the same as in usual tw-chice tasks. In Experiment III, this effect is restred cmplete~y i~ spit.e f the distractr task, which was used again In this experiment.. The intrductin f a distractr task nt nly increased mean RT (see data fr Experiments I and III, Fig. 2E), but variance as well (see Fig. 4D, the upp~r curves). In Table 5, errrs and missins made by Ss ill slving the distratr prblem are shwn. They characterize the Ss' level f perfrmance n the distractr task in Experiments I and III. Slight differences in this level suggest that there was n remarkable adaptatin f the S t the distractr task in Experiment III as cmpared with Experiment I. Therefre, the cnsiderable differences in prbability effect bserved in these experiments cannt be ascribed t this adaptatin factr. Hypthetically, the absence f prbability effect in Experiment I culd be explained by the fllwing: The distractr task makes it impssible fr S t frm any adequate subjective mdel f prb ability structure f stimulus sequence and accrdingly t cnstruct apprpriate expectancy and respnse strategy. Hwever, wrking during the sessins withut the distractr task (Experiment II) des enable the S t d that, and the experience gained here was prbably transferred t Experiment III. (D) Prbability Effects with Different Levels f Mtivatin The influence f different levels f mtivatin n the magnitude f the prbability effect is illustrated in Figs. 2F t 21 (Lentjev & Krinchik, 1961,1962,1964). The data in Fig. 2F are frm an experiment using a tw-chice task with lights as stimuli, keypresses as the respnse, and "equipment breakdwn" as the reinfrcing event. That is, at the beginning f the reinfrcement sessins, the S was tld that if his RTs t

9 PROBABILITY EFFECTS IN CHOICE REACTION TIME TASKS 139 ne f the stimuli exceeded a predetermined limit, the equipment had brken dwn and that day's sessin was terminated. When that type f reinfrcement was used, unbeknwn t the S, the E switched ff the equipment that presented the stimulus sequence. That type f reinfrcement caused a cnsiderable decrease in the magnitude f the parameters that characterize the prbability effect. The fact that the effect f prbability n nnreinfrced signals was almst the same as in the experiment in which bth the signals were nnreinfrced is evidence in supprt f a hypthesis which accunts fr the decrease in RT t reinfrced signals in terms f the rerganizatin and intensificatin f infrmatin prcessing as well as in terms f special mbilizatin f sme "inner" resurces f reactivity which are develped in a reinfrced sensry-mtr system (Lentjev & Krinchik, 1962). These results suggested that the parameter values f the prbability effect might be used t scale the degree f subjective significance fr the signals. This suggestin was cnfirmed by Lentjev and Krinchik (1964) in an experiment with tw, fur, and eight equiprbable chices, in which three methds f reinfrcement were used: (a) infrmatin reinfrcement cnsisting f flashing a red light n the S's panel if his RT t a designated signal exceeded 400 msec (Fig.2G); (b) shck reinfrcement cnsisting f delivering an electric shck in accrdance with the same criterin as used in (a) abve (Fig.2H); and (c) breakdwn reinfrcement, which has already been described (Fig. 21). Mrever, many "backgrund" sessins in which all signals were nnreinfrced preceded the three sessins with reinfrcements. The results f these experiments indicated that the parameters f prbability effect are quite sensitive t differences in degree f subjective significance f the signals. Infrmatin reinfrcement prved t be the least effective. In these experiments, the slpe decreased nly by 7.9% as cmpared t the backgrund experiment. The ther tw kinds f reinfrcement prduced a sizable effect: as cmpared t the backgrund experiment, the slpes decreased by 28.7% fr shck reinfrcement and by 27.2% fr breakdwn reinfrcement. As befre, data frm these experiments indicated that the effects f prbability fr the nnreinfrced signals did nt change significantly as cmpared with data btained fr the same signal in the "backgrund" experiment. (E) Prbability Effect and Sequential Effects The analysis f sequential effects was intrduced int RT experiments by Bertelsn (1961) and Falmagne (1965) and became the basis f sme imprtant mdels fr reactin time in general and fr the effects f signal prbability in particular (Falmagne, 1965; Falmagne & Theis, 1969; Krnblum, 1967, 1969; Laming, 1969; Theis & Smith, 1970). The effects f stimulus repetitins fr sme f the 4-4O f 3"00,' 4.;() ; '-. '. acho:ce " "CHOICE ''-..., :2C~CICE,!, 5 " 7 8 NU~5ER OF RE?c.TITI~~S,......"..._ SCHOiCC 3?J. 4 QiO:CE 3'-'J~ 2CHO!CE Z30; C 1 I I ' ' t,, I : :~ achoice 440 ( I:' 2CHOICE ~. u O;Z~4-S678 Fig. 3. Mean RT fr successive stimulus repetitins t stimuli which ccurred with p =.5. The tasks were as fllws: Panel A, tw-digit numbers t keys (Krinchik & Akimva, 1970); Panel B, tw-digit numbers t verbal respnses (Knnchik & Akimva, 1970): Panel C, upper half, tw-chice task with distractrs (Krinchik, Mednikarv, & Brisva, 1970; als see text): Panel D, lwer half, tnes t keys with Dnders' Type A, B, and C tasks (Krinchik & Rajevskij, 1970; als see text). experiments that have been discussed thus far are illustrated in Figs. 3 and 4. The data in Fig. 3 are all fr stimuli whse prbability was.5; the data in Fig. 4 are fr stimuli with a prbability f.93. The data in the A panels fr bth figures are frm a study by Krinchik and Akimva (1970), in which the stimuli were tw-digit numbers and the respnses were buttn presses (see als Tables 3 and 4). The data in the B panels are frm the same study (Krinchik & Akimva, 1970), in which the stimuli were tw-digit numbers and the respnses were verbal (see als Fig.2C), The data in the C panels f

10 140 KRINCHIK 1 <20 ~S :!>O':l 8 c 5 10 '5 tt) :2 Z c I~ ~20 ~--~-_--l_--..l:---~=---~ :0 15 -, 280..,-_.*-... _ ~"~'... _-,... _-,_ '",E,IZl' sec "",, '~-''::--.::;::<::::'':-'':=':.--...e- -...,...: ~ 0 5 NUM8EROF REPETITIONS Fig. 4. Mean RT fr successive stimulus repetitins t stimuli which ccurred with p =.93. The tasks in each panel are frm the same studies as the crrespnding panels in Fig. 3. Figs. 3 and 4 are frm a study by Krinchik and Alexandrva (1966), in which the stimuli were gemetric figures and the respnses were keypresses (see als Fig. 1C). The upper prtin f the D panels illustrate data frm a study by Krinchik, Mednikarv, and Brisva (1970), in which the effects f a distractr task were examined in a tw-chice task (see als Fig. 2E). The lwer prtin f the D panels illustrate data frm the Krinchik and Rajevskij (1970) study, in which tnes were used as stimuli and keypresses as the respnses in a Dnders A, B, and C task. An inspectin f Fig. 3 makes the fllwing results quite apparent. (l ) The tw-chice equip rbable data d nt demnstrate the strng, mntnic decrease f RT with successive repetitins, even in the range f ne t fur repetitins. This is in agreement with previusly reprted results (Hyman, 1953: Hale, 1967a, b, 1969). (2) When the number f alternatives is increased beynd tw, the drp in RT with successive repetitins becmes greater fr the keypressing tasks (Figs. 3A and 3C), but nt fr the number-naming task (Fig. 3B). The frmer cnfirms similar findings by Hale (l967a). (3) N mntnic decrease f RT with successive repetitins appears t ccur beynd the furth repetitins. In cmparing the data fr the first fur repetitins with the data f ther investigatrs, similar results are fund (Bertelsn, 1961; Falmagne, 1965; Lenard, Newman, & Carpenter, 1966: Hale, 1967a, b, 1969; Laming, 1969; Remingtn, 1969; Krinchik, 1969; Krinchik & Mednicarv, 1970). In all these studies, either the data were nt reprted beynd fur repetitins r the curve apprached asymptte after fur repetitins f the signal. (4) The distractr task appears merely t have shifted the data alng the intercept withut affecting the slpe f the functin. The data in Fig. 4 are the mean RTs fr successive repetitins t high prbability signals (p =.93). (1) As was true f Fig. 3, all the data in Fig. 4 (except fr 4B) shw a clear tendency t becme asympttic after the first fur repetitins. (2) The steepness f the curves in the range f ne t fur repetitins increases with increasing number f alternatives, again with the exceptin f the experiment with the number-naming task (4B). (3) In the experiments with the distractr task, the RT variability is significantly greater than in the ne withut the distractr task (Fig. 4D). (4) In all the experiments, as the number f alternatives increases, RT variability decreases. In summary, an analysis f sequential effects fr highly prbable stimuli indicates that sequential effects are mre cmplex than had hithert been thught. DISCUSSION Since it is impssible t discuss all aspects f the data within the framewrk f this paper, discussin will be restricted t tw questins that deal directly with the psychlgical nature f the prbability effect in chice RT tasks. The first questin deals with the relatinship between the number f alternatives and the relative frequency f a signal as the tw variables that determine the prbability effect. The secnd questin deals with the identificatin f the main determinants f trial-by-trial variability f RT. in these experiments. The

11 PROBABILITY EFFECTS IN CHOICE REACTION TIME TASKS 141 data, presented in Sectin A (Figs. 1A and 1B) permitted us t separate the traditinal cncept f stimulus uncertainty (Hyman, 1953; Bricker, 1953; Welfrd, 1960) int tw cmpnents: (l) alternative uncertainty related t the number f alternatives, and (2) tempral uncertainty related t the relative frequency f the signals. The frmer was interpreted as a determinant f S's uncertainty regarding which f the alternative signals wuld ccur at a given mment; the latter was interpreted as a determinant f S's uncertainty as t when, i.e., at which pint in the stimulus sequence, the given signal wuld ccur 2 (Krinchik, 1963a, b; Lentjev & Krinchik, 1964; Krinchik & Alexandrva, 1966). The identificatin f these tw cmpnents.f stimulus uncertainty led t a new apprach tward the understanding f Hyman's (I953) results, and t a mdificati0 n f his cnclusins regarding the psychlgical equivalence f different ways f varying stimulus infrmatin (Krinchik & Alexandrva, 1966; Krnblum, 1969). The data presented in Figs. 1, 2A, 2B, and 2C prvide strng evidence in supprt f the ntin that prbability effect in chice RT tasks is the result f the jint actin f these tw cmpnents f stimulus uncertainty. Furthermre, adjustment f these cmpnents sets befre S's tw tasks f different cmplexity (see, fr instance, Fig. 1B). The cntributin f each f these cmpnents t the prbability effect appears t depend largely n the methd used t vary stimulus prbability. Thus, the relatively slight differences in the prbability effects between Dnders' A and B tasks (Figs. 2A and 2B) wuld imply that in tw-chice tasks with unequal stimulus prbabilities the effect f tempral uncertainty is dminant. In cntrast, very large differences in the prbability effect between the tw-chice task with nnequiprbable signals and multiple-chice tasks with equiprbable signals (Fig. 1B) implies that, in the latter, the effect f alternative uncertainty predminates, given that tempral uncertainty was the same in bth situatins. One f the factrs cntributing tward such a significant difference in the prbability effect between the tw-chice and the multiple-chice data is the difference in the degree f predictability fr a particular signal, hence the degree f efficiency with which Ss can develp anticipatry and preparatry prcesses in these tw tasks. Sme bservatins indicate that, in chice RT tasks with equiprbable signals, Ss either refuse t make guesses regarding the next signal r, if they d, it results in t many errrs (Krinchik, 1968; Sternberg, 1969; Swenssn & Edwards, 1971). The latter may be due t the degradatin f the tempral structure that accmpanies an increase in the number f alternatives. It therefre seems reasnable that mdels which describe prbability effect in RT wuld include the number f alternatives and the relative frequency f the signals as separate parameters. In additin, the way these wuld be related wuld depend n the particular methd in which stimulus prbability was varied. With regard t the secnd questin dealing with the trial-by-trial variability f the data, Fig. 4 appears t prvide sme new evidence regarding the behavir f sequential effects in chice RT tasks. The absence f mntnic decrease in RT with the increasing number f stimulus repetitins, even in the range f the first five repetitins, the rapid apprach f the curve t asymptte, and the strngly expressed variability f pints in the asympttic sectin demnstrate that this dependence is the result f rather a cmplex determinatin that cannt be reduced t the simple case f the influence f the preceding signal. Several cnjectures regarding the determinants f sequential effects in chice RT tasks are cnsidered belw. There appear t be at least three aspects f a stchastic stimulus sequence which culd determine RT n a particular trial: (I) the immediately preceding stimuli (extending frm 1 back t 4 r 5), (2) sme particular patterns f the stimulus sequence, and (3) the ttal stchastic structure f the stimulus sequence. The first gives rise t the well-knwn recency effects, and represents the mst elementary frm f a sequential determinant in human RT experiments. The secnd represents a mre cmplex type f sequential determinant, wherein the S perceives separate structural patterns within the stimulus sequence as segments and builds his expectatins in terms f such segments. The simplest example f such an effect is illustrated belw frm data in a nnequiprbable tw-chice task (Lentjev & Krinchik, 1962): Subject N.A., Recrd N.59/1 ( ; PI =, P2 = ) , 190, 180, 180, 290, 310-,360,240,340*,220,190,300*,200,190, 190,180, 180, RTs t the rare signal are indicated by being underlined.f the asterisk (*) dentes lengthened RTs t the mre frequent signal. This lengthening may be interpreted as being due t S's expectancy f the rare signal at this pint." Such patterns f stimulus sequence can be impressed clearly by the S and determine his further expectatins, because repetitins f rate signals were very surprising and untypical events in the sequences with given prbability distributin. This effect culd be amplified als with the S's errr respnse t the repeated rare signal. These first tw determinants may be thught f as giving rise t shrt-term sampling f the stimulus sequence and determining shrt-term selective preparatin (Bertelsn & Barzeele, 1965; Lenard et al, 1966). The third determinant cnsists f the ttal sequence structure and its subjective representatin in the frm f a subjective prbability mdel (Krinchik, 1963a. b, 1968). The data that wuld be needed fr a cmplete descriptin f such a mdel are nt available nw, but the mdel wuld undubtedly include: (I) a subjective estimate f the differences between the relative

12 142 KRINCHIK Signal Run Psitin Table 6 Mean RT t the Rare (+) and t the Frequent Signals" First Sessin Start Middle End Sessin With Reinfrcement f Frequent Signal Start ~liddle End Sessin With Reinfrcement f Rare Signal Start Middle End + :! "Frequeni signals are listed accrding t their rder f ccurrence in runs up t fire repetitins (0-5). Within each f the three sessins, mean R Ts fr trials that fllw three successive ccurrences f the rare signal are presented fr the start. middle. and end f. a se.ssln. In each Sessin the rare signal was presented 15 times and the frequent ne 225 times. Data f S SA. (Lentiev & Krinchik, 196J.). frequencies f the signals; (2) a subjective estimate f the Prprtin f stimulus repetitin and nnrepetitin; Kr.nblum's studies (1967, 1969) demnstrated cnvmcingly the imprtance f these variables in the prbability effect: (3) a subjective representatin f the tempral structure ("time mrphlgy") that is ass~clated with the ccurrence f a particular signal and which distinguishes sequences with different stimulus prbability distributins (Krinchik, 1969). This determinant influences sme general levels f expectancy and preparatry state t the rare and t the frequent signals, which characterize the sessin as a whle. These levels are established by S in accrdance with his subjective prbability mdel, his subjective criterin f perfrmance (including the relatinship between speed and accuracy), and a respnse strategy which the S selected t guide his perfrmance in prbabilistic situatins. The demnstratin f the effect f this third determinant is cntained in several studies. In these experiments, either false instructins abut the structure f the stimulus sequence were given t the Ss (Knpkin, 1965, 1966; Knpkin & Strjukv, 1971; Feigenberg & Tsiskaridze, 1967) r the surprise change f the stimulus sequence was prduced (Guldan & Osnitskij, 1968; Krinchik & Vilenskaja, 1970). In bth cases, the RT data did nt crrespnd t the actual structure f the stimulus sequence at the start f the test. Instead, they crrespnded t the subjective prbabilities, which, in the first case, were cnstructed by the S n the basis f the instructins, and, in the secnd case, n the basis f experience received frm the primary sequence. Hwever, as the tests prceeded, the RT data began t change and t crrespnd mre clsely t? the. actual stimulus sequence. This determinant als gives nse t the specific type f anticipatins that take place in separate segments f the stimulus sequence and ~hich are smewhat antagnistic with respect t the influence f the immediately preceding stimulus (Hale, 1967a; Krinchik, 1968, 1969; Svjadsh, Shendrik, & Jamplsky, 1968; Ivannikv, 1969; Thmas, 1970). In the light f these cnjectures, we may hypthetically interpret the data in Fig.4A in the fllwing manner. The asympttic segments f these curves represent the average general level f RT. t a frequent signal and, cnsequently, the level f reactivity f the apprpriate sensry-mtr system (Byk, 1964: Chuprikva, 1967), which is established and regulated by the S n the basis f infrmatin abut the ttal statistical structure f the stimulus sequence. The large trial-by-trial deviatins f the individual RTs frm this general level represent the fluctuatins f the reactivity f the given sensry-mtr system which are due t the first tw determinants mentined abve.. They reflect dynamics f the S's shrt-term preparatry state. The initial sectin f the Curves, i.e., the first five repetitins f the signal, can be regarded as a segment f a functin in which RTs are becming asympttic. This appears t be necessary, because at the start f this sectin, the sensry-mtr system, which is respnsible fr reactins t frequent signals, is inhibited by the S's reactin t a rare signal. It seems reasnable t suppse that this sectin represents a prcess in which this inhibitry state is vercme and the system reactivity is raised t an asympttic level. This prcess can prceed at different rates, depending n special factrs, such as the S's experience in these experiments r the degree f subjective significance f the signals, etc. Table 6 presents data which supprt such an interpretatin. As can be seen frm Table 6, the S's RT did nt apprach asympttic level during the first five repetitins f the frequent signal at the start f the first sessin; hwever, at the end f that sessin, it appears t have dne s just after the first repetitin. In the sessin where the frequent signal is reinfrced, the asympttic level appears t have been reached very rapidly with the first ccurrence f the frequent signal (see the middle and end f that sessin). In the sessin where the rare signal is reinfrced, the RTs seem t apprach the asympttic level mre gradually. The cnjectures that are presented here in cnnectin with the nature f the sequential effects in chice reactin time tasks must be regarded as speculative and requiring new experimental cnfirmatin. Nevertheless, it seems reasnable t utline in summary that the prbability effect as well as the sequential effects in chice RT tasks are the result f the cmplex interactin

13 PROBABILITY EFFECTS IN CHOICE REACTION TIME TASKS 143 f several determinants and that the ttal stchastic structure f the stimulus' sequence in its subjective representatin takes the principal place between them as a determinant which must be mst inherent in the behavir f human Ss in such experiments. REFERENCES Bertelsn, P. Sequential redundancy and speed in a serial tw-chice respnding task. Quarterly Jurnal f Experimental Psychlgy, 1961, 12, Bertelsn, P., & Barzeele, 1. Interactin f time uncertainty and relative signal frequency in determining chice reactin time. Jurnal f Experimental Psychlgy, 1965,70, Bdrv, V. A., Genkin, A. A., & Zarakvskij, G. M. Znachenije statisticheskj struktury pri dnznachn-determinirvannm reagirvanii na signaly dvukh vidv, Vprsy Psykhlgii, 1966, 6, (The significant f a statistical structure in case f the simply determined respnding t signals f tw types. Questins f Psychlgy.) Bricker, P. D. Infrmatin measurement and reactin time. In H. 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P. 0 determinatsii prvedenija verjatnstnj strukturj situatsii. Vprsy Psychlgii, 1968,3, (On determinatin f behavir by the prbabilistic structure f the situatin. Questins f Psychlgy.) (Als published in English in Sviet Psychlgy, VL 7, N.4, Summer 1969.) Krinchik, E. P. The prbability f a signal as a determinant f reactin time. In W. G. Kster (Ed.), Attentin and perfrmance, II. Acta Psychclgica, 1969, 30, Krinchik, E. P., & Akirnva, M. K. Issledvanije effekta verjatnsti signala na vremja reaktsii vybra v uslvijakh raznj stepeni slznjsti vybra i razlichnj stepeni svrnestimsti stimula i reaktsii. Printsypy Verjatnstnj Organizatsii Pvedenija. (Tezysy dkladv Vsesjuznj Knferentsii). Leningrad, 1970, (The study f the stimulus prbability effect in RT experiments with the different degrees f chice and S-R cmpatibility. Paper given at the All-Unin Cnference n the Prbabilistic Principles f Behavir Organizatin, Recgnitin, and Medical Diagnstics. (Thesis, Leningrad.) Krinchik, E. P., & Alexandrva, L. N. 0 stnshenii vremennj i alternativnj nepredelennsti v uslvijakh peredachi infrmatsii chelvekm. Vprsy Psykhlgii, 1966, 2, (The relatinship between tempral and alternative uncertainties. Questins f Psychlgy.) Krinchik, E. P., & Alexandrva, L. N. Effekt verjatnsti i vremja reshenija elementamykh aritmeticheskikh zadach v situatsii binarnj klassifikatsii bjectv. Printsipy Verjatnstnj Organizatsii Pvedenija. (Tezisy Dkladv Vsesjuznj Knferentsii), Leningrad, 1970, (prbability effect and time f the decisin f the elementary arithmetical prblems in binary classificatin tasks. Paper given at the All-Unin Cnference n the Prbabilistic Principles f Behavir Organizatin, Recgnitin and Medical Diagnstics. (Thesis) Leningrad.) Krinchik. E. P., & Mednikarv, P. D. 0 mekhanismakh vlijanija

14 144 KRINCHIK verjatnsti signala na vremja reaktsii chelveka. Vprsy Psykhlgii, 1970, 6, (On the mechanisms f signal prbability effect in chice reactin time experiments. Questins f Psychlgy.) Krinchik, E. P., Mednikarv, P. D., & Brisva, E. M. Determinatsija pvedenija chelveka verjatnstnj strukturj situatsii v uslvijakh dejstvija "pmekh." Prinstipy Verjatnstnj Organizatsii Pvedenija (Tezisy Dkladv Vsesjuznj Knferentsii), Leningrad, 1970, (On the determinatin f human behavir by the prbabilistic structure f the situatin in the task with the distractr. Paper given at the All-Unin Cnference n the Prbabilistic Principles f Behavir Organizatin, Recgnitin and Medical Diagnstics. (Thesis) Leningrad.) Krinchik, E. P., & Rajevskij, A. M. Zavisimst vremeni reaktsii t verjatnsti predjavlenija signala v uslvijakh "bnaruzjenija,' "raslichenija" i "vybra" signala. Printsipy Verjatnstnj Organizatsii Pvedenija (Tezisy Dkladv Vsesjusnj Knferentsii). Leningrad, 1970, (Prbability effect in Dnders' A, Band C reactin tasks. Paper given at the All-Unin Cnference n the Prbabilistic Principles f Behavir Organizatin, Recgnitin and Medical Diagnstics. (Thesis) Leningrad.) Krinchik, E. P., & Vilenskaja, E. G. Reaktsija "zjidanija" i eje elektrmigraficheskje vyrazjenije v uslvijakh verjatnstnj rganizatsii sistemy signalv. Ibid, 1970, (Expectancy and its electrmygraphical representatin in the RT experiments with the prbabilistic structure f the stimulus sequence.) Krueger, 1. Effect f stimulus prbability n tw-chice reactin time. Jurnal f Experimental Psychlgy, 1970,84, Laming, D. R. J. Subjective prbability in chice reactin time experiments. Jurnal f Mathematical Psychlgy, 1969,51, Lenard, J. A., Newman, R. C., & Carpenter, A. On the handling f heavy bias in a self-paced task. Quarterly Jurnal f Experimental Psychlgy, 1966,18, Lentjev, A. N., & Krinchik, E. P. 0 primenenii terii infrmatsii v knkretnpsychlgicheskikh issledvanijakh (svremennye issledvanija reaktsii vybra). Vprsy Psychlgii, 1961, 5, (On the applicatin f infrmatin thery t cncrete psychlgical investigatins. Questins fpsychlgy.) Lentjev, A. N., & Krinchik, E. P. 0 nektrykh sbennstjakh prtsessa pererabtki infrmatsii chelvekm, Vprsy Psikhlgii, 1962, 6, (On sme aspects f human infrmatin prcessing. Questins f Psychlgy.) (Als published in English in Sviet Psychlgy & Psychiatry, 1963, 1,24-30.) Lentjev, A. N., & Krinchik, E. P. Nektryje sbennsti prtsessa pererabtki infrmatsii chelvekm, In A. I. Berg (Ed.), Kibemetica, myshleniie, zjizn. Mscw: Mysl, pp (Sme aspects f human infrmatin prcessing. In Cybernetics. thinking life.) Mwbray, G. H. Chice reactin times fr skilled respnses. <)larterly Jurnal f Experimental Psychlgy, 1960, 12, Nickersn, R. S. Binary-classificatin respnse time, memry search and questin f serial versus parallel prcessing. Paper presented at 19th Internatinal Cngress f Psychlgy, Lndn, Remingtn, R. J. Analysis f sequential effects in chice reactin time. Jurnal f Experimental Psychlgy, 1969,82, Rmanv, N. A. 0 vzmzjnsti kntakta mezjdu terijej verjatnsti i uchenijem akad, I. P. Pavlva f uslvnych refleksakh. Dklady A.N.SSSR, 1935, I, (On the pssibility f cntact between the prbability thery and the thery f acad I.P. Pavlv n cnditining. Reprts f the Academy f Sciences f the USSR.) Skinner, B. F. The behavir f rganisms. New Yrk: Appletn-Century-Crfts, Sklv, E. N. Vsprijatije i uslvnyj refleks. Mscw, MGU, (perceptin and cnditining. Mscw State University, Mscw.) Sternberg, S. Tw peratins in character recgnitin: Sme evidence frm reactin-time measurements. Perceptin & Psychphysics, 1967,2, Sternberg, S. The discvery f prcessing stages: Extensins f Dnders' methd. In W. G. Kster (Ed.), Attentin and perfrmance, II. Acta Psychlgica, 1969, 30, Svadsh, A. M., Shendrick, E., & Jamplsky, 1. T. Analiz dnj mdeli adaptivng pvednija. In A. N. Kchergin and P. P. V lkv (Eds.), Prblemy mdelirvaniia psikhicheskl dejatelnsti. Vl. 2. Nvsibirsk: NGU, Pp (The analysis f sme mdel f adaptive behavir. In Prblemsf the mdeling fpsychic activity-) Swenssn, R. G., & Edwards, W. Respnse strategies in a tw-chice reactin task with a cntinuus cst fr time. Jurnal f Experimental Psychlgy, 197 1,88, Theis, J., & Smith, P. G. Sequential dependencies in chice reactin times. Reprt N. 70-8, Wiscnsin Mathematical Psychlgy Prgram, University f Wiscnsin, Thmas, E. A. On expectancy and average reactin time. British Jurnal f Psychlgy, 1970,61, Welfrd, A. T. The measurement f sensry-mtr perfrmance: Survey and reappraisal f twelve years' prgress. Ergnmics, 1960,3, NOTES 1. The absence f this effect in the equiprbab1e sessins f the B-reactin task can be accunted fr by the well-knwn phenmenn f an increase in the sensitivity f the analyzers ccurring when an indifferent stimulus (requiring n active respnse, as it was with ne f tw signals in the Cereactintask) becmes a signal stimulus (requiring an active respnse, as it was with this stimulus in the B-reactin task) (Sklv, 1958). 2. It seems useful t distinguish this type f tempral uncertainty frm the traditinal cncept f time uncertainty which dentes the variability f the interstirnulus interval and freperid. 3. The secnd ccurrence f the rare signal resulted in an errr (-). (RTs were measured t the nearest.01 sec). 4. The quantitative analysis f these lengthened RTs t the frequent signals indicated (Krinchik, 1963b) that they were nt the result f randm fluctuatins. Furthermre, the EMGs recrded in the curse f a tw-chice task with nnequiprbab1e signals (Krinchik, 1968; Krinchik & Vilenskaja, 1970) indicated that these lengthened RTs were ften accmpanied by increased tnic tensin in the muscles respnsible fr the reactin t the rare signal. These EMGs develped during the interstimulus interval which preceded the trial in which the frequent signal ccurred. (Received fr publicatin January 22, 1973; revisin received August 27,1973.)