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(1969). and Dale & Folarin (personal nearly of equiv:tlcl1l length (Fig. 3). Y cL A de,cription of the NASA·TCL Space Vi,ion communication). Posner found little or no these two rangcs produced the two Simulator. National Aeronautics and Space decay over a 20-sec interval; Blick found Administration CR 73305. 1%9. decay continuing beyond 5 sec; and Dale extreme group exponents. EK\IA;-". G .. HOS\IAN. B.. LIND\IAN. R .. range is a major can tributor to LJl"NGBERG, L.. & .t..KESSO'\. c. A. and Folarin found decay continuing group differences and to individual changes lnterindividual difference, in scaling beyond 10 sec. Closer examination of the in performance on a psychophysical scaling performance. Perceptual & \Iolor Skills. 1968. rationale behind the Posner result, however, task. The present data indicated that Ss 26, 815-823. suggests that it may be based upon a logical KUNNAPAS, T. Scales for subjective distance. adjust their response ranges with changes in Scandinavian Journal of . 1960. 1, error. It assumes that the discrepancy be­ stimulus range. However. the critical 187·192. tween the RT to physically identical terms measure of stimulus range affecting KONNAPAS, T. Distance as a and terms with the same name but response mnge appears to be the number of function of available visual cues. Journal of different case simply reflects the strength discriminable steps contained in the , 1968,77,523-529. of the physical trace, with the two RTs ~IARKLEY, R. P., BROWN, B. R., & stimulus range rather than the absolute ARNOl'L T. ~1. D. Fractionation of distance in becoming equal when the trace becomes physical range. These results point to simulated space. National Aeronautics and indistinguishable from the visual boundary conditions for Poulton's (1968) Space Administration CR 73 306, 1969. background noise. It is clear from Posner's Model I. There are situations in which Ss POCL TON, E. C. The new : Six own experiments, however, that at the will decrease their range of numerical models for magnitude estimation. Psychological Bulletin, 1968.69.1·19. same time as the physical trace is fading, responses in the face of a fivefold increase RULE, S. J. Subject difference in exponents the item is being translated into a name in physical range. Variation in subjective from circle size, numerousness, and line length. code. The point at which the difference range may not show up as a variation in the Psychonomic Science. 1969, 15,284-285. between the physical and name match RT power law's exponent when VINCENT, R. J., BROWN, B. R .. \IARKLEY, R. disappears, therefore, represents the P .. & ARNOlJLT, '.1. D. Magnitude estimation discriminability is variable over different of perceived distance over various distance combined effect of a fading visual trace portions of the physical continuum. ranges. Psychonomic Science, 1968, 13, and a developing name code. Once the 303·304. name code has developed to a point at REFERENCES WORLEY, J. K., & \IARKLEY. R. P. Distance which it allows faster RTs than the visual discrimination in a reduced cue setting. ARNOULT.~.D .. VINCENT.R.J.,BRO~N.R Psychonomic Science, 1969, 17,237·238. code, S will presumably use it in preference R .. \IARKLEY. R. P .. & HENSLEIGH. R. C. to the visual trace, even though visual trace continues to be available. Since S need no longer use the visual trace, his RTs will no longer reflect its strength. In short, Posner's technique confounds the fading of the visual trace with the developmen t of the name code, and as such cannot give a Reaction time and short-term visual * valid indication of the time course of visual STM. This suggests that the method of Posner & Keele (1967) may give an underestimate W. A. PHILLIPS and A. D. BADDELEY of the duration of visual STM, and that the Experimental Psychology Laboratory, University of Sussex longer times suggested by the other Brighton BN 1 9QY, Sussex, England experiments are more accurate. However, the discrepancy may be due to differences Posner's method of using differences in RT for physical and name matches to estimate in procedure. All three experiments the time constant of visual STM is criticized as confounding the decay of the visual trace suggesting longer times measured accuracy with the development of a name code. When this confounding is avoided by using stimuli rather than reaction time, and studied that are hard to name (a 5 by 5 matrix of randomly filled squares), the time constant memory for position or length rather than shown by both RT and errors is consistently longer than that reported by Posner. memory for form. Furthermore, Posner (I967) and Dale and Folarin used Posner and his co-workers have recently faster if the two are physically identical rather than recognition. The following devised an ingenious technique by which (i.e .. same letter, same case) than if the experiment. therefore, studies visual STM reaction-time (RT) measures may be used letters have the same name but are in a using a technique analogous to that to study visual , and from which different case. As the interval between the employed by Posner and Keele, but they have drawn interesting conclusions two increases, this difference decreases, till material which cannot easily be named in about short-term visual memory. The it disappears at delays of 1.5 sec or longer order to avoid the confounding of visual technique involves presenting S with a (Posner & Keele, 1967; Posner, Boies, trace decay with name-code development. letter, followed after a brief interval by a Eichelman, & Taylor, 1969). They METHOD second letter which may be either the same interpret this as evidence for a short-term Stimuli comprised a 5 by 5 matrix of or a different letter and may be either visual trace with a decay time of about squares in which each square had a 0.5 upper- or lowercase. When the second 1.5 sec. probability of being filled. A new pattern letter immediately follows the first, RT is The existence of such a trace is of was used each trial. After a randomly considerable interest. since its time selected delay of 0.3,1.0,3.0, or 9.0 sec, a constant is clearly longer than that of second matrix was presented. This was iconic (Neisser, 1967). On the *The authors are grateful for financial support either identical to the first or differed by to the Medical Research COLlllCil. The computer other hand, this time constant is having one square more or one square less facilities \\'ere made available throu~h a grant considerably shorter than that for visual filled. Ss were instructed to decide "as from the Science Research Council. • STM suggested by Posner (1967). Blick quickly as possible without error." whether

Psychon. Sci., 1971, Vol. 22 (2) 73 80 v~ (T =O. ,, =7 . P < . ()~ Ull Ihc Wikov'll w lest). \kall RT wa s ph) \ted ,;cPJrJtcJy fur a: • ,uneel alld illcorrect Rs and is shuwn in a: 75 0 Fi~ . ~ Al l Ss shu\\ JIl cm: rall Icndcncy for V • RT III in creaSe' wilh ddJ\ (T = O. ,,= 7 . 70 P < .O~). The' increase in RT bcl\\ccn 3 ~z and 9 sec is also sil!llificallt (T = I. " = 7. w P < .05). The trend is closely parallel for V 65 both correct Rs and errors. which are a: • w associated with slower RTs in all Ss (T = O. "=7.p < . O~). ~ 60 DISn:SSIO" z These re sults arc dearly at lariance with ct the time constant for visual ST\\ suggested w 55 by Posner ct aJ and. as such . Ihcy reinfor,e ~ • the daim that the physical vs name-match technique docs not provide a valid 50 indicator of sho rt-rerm visual memory. o 3 3 9 They suggest a rate of forget ting which is consistent with the experiments suggesting DELAY (s e es) a longer duration. despitc considerable differences ill procedure between this and Fig. \. Percentage correct recognition responses as a function of delay (chance level = the previous studies . Both accuracy and 509<). RTs give mUlllally consistent results. which suggests that RT may indeed provide a the second pattern was the same or comprising 15 trials at each of the four useful indicator of short-term . different. and press one of two delays in random order. The actual test Posner. Boies. Eichelman. & Taylor display-console keys accordingly . The involved a further six such blocks of 60 (1969) suggest that the decay of visual second pattern remained in view until the trials. Seven Ss comprising undergraduates STM may be reduced by appropriate key was pressed. Stimuli were exposed for and research workers were tested rehearsal strategies. They proVide evidence 0.5 sec on the visual display of an individually. for this by studying a pure-list condition in Elliot 4130 computer. The 5 by 5 matrix RESULTS which. as in the present experiment. a measured 0.6 x 0.6 in., and was viewed Figure I shows the overall mean physical match is always required. RTs from a distance of about I:: in. In order to percentage of correct Rs as a function of increased little over the I-sec interval minimize iconic storage effects, a masking delay. In contrast to the resul ts of Posner studied. They interpreted this as due to field comprising a IO by 10 matrix of e tal. our data clearly suggest that increased rehearsal of the visual aspect in randomJy filled squares was interpolated forgetting continues after the first 3 sec. the pure-list condition. In terms of this between the two 5 by 5 stimuli. All Ss show overall forgetting and all show view, the results of the present experiment Ss were given 60 practice trials a drop in retention between 3 and 9 sec would suggest that rehearsal can maintain a visual STM for the kinds of patterns used x beyond 3 sec, but not beyond 9 sec. e r As Posner et al note. their experiments do not show whether the decay observed is 1 5 a matter of reducing ease of accessibility, or a matter of complete loss of availability. By showing a large increase in errors. as .. well as in RT, the present experiment u .. " _____ x suggests that the decay observed involves a .. complete loss of availability . \ · 3 I

~ REFERENCES Z \ 2 RESPO SES BLICK , K. I. Decision and decay processes in the Q short·term memory for length. Journal of u~ / / ' Experimental Psychology. 1969. 82.224-230. • NEISSER, U. . New York • w" Appleton.(:entury.(:rofts, 1967. ~ POSNER, \1. I. Characteristics _of visual and z kinesthetic codes. Journal of Experimental w 0 Psychology. 1967,75 , 103-107. " / POSNER. \1. I., BOIES, S. J., EICHELMAN, W. I • H., & TAYLOR, R. L. Retention of visual and 0111 name codes of single letters. Journal of Experimental Psychology "Ionograph. 1969. 7 0 2 3 " 79, 1\0. I. Part 2. DE LA Y ( sec s ) POSNER. \1. I., & KEELE. S. W. Decay of visual information from a single letter. Science. Fig. 2. Mean reaction time for correct and incorrect responses as a function of delay. 1967. 158. 137·139.

74 Psychon. Sci., 1971, VoL 22 (2)