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Interference and short term memory

Interference And Short Term Memory


The Effects of a Distracting Activity on Short-Term Memory


The present study examined the effects of short-term memory forgetting under the delay of a distracting task. Eleven subjects were asked to remember three consonants while counting backwards by 3’s for varying amounts of time (3-, 9-, 18-sec). An Analysis of Variance (ANOVA) was used to analyze the data and post hocs were done to measure the differences between the levels. The results showed that the retention of the consonants depended on the retention interval, with good performance over short intervals and poor performance after intervals that just 18-sec. in duration. The implications of these findings are discussed.

The Effects of a Distracting Activity on Short-Term Memory

We encounter a great deal of new information in our daily lives. The particular way in which we think about new information affect the ease with which we learn it and the likelihood we can remember it later on. Memory is said to be the primary aspect of cognitive processes. Generally cognitive psychologists divide memory into three stores: sensory store, short-term store, and long-term store. The sensory store is the component of memory that holds the information that has been received in its original unencoded form. Everything that the body is capable of seeing, hearing, or otherwise sensing is stored in the sensory memory. The sensory store has a large capacity but can only hold the information for a short period of time with visual information lasting less than a second, and auditory information lasting two to three seconds. Short-term memory, sometimes known as working memory is the component of memory where new information is held while it is mentally processed. S!

hort-term memory is also the component of memory where much of our thinking, or information processing occurs; it is a temporary holding bin for new information. The capacity of short-term memory is limited and can only hold a small amount of information. Miller (1956) presented the idea that short-term memory could only hold 5-9 chunks of information (seven plus or minus two) where a chunk is any meaningful unit. An item usually remains in short-term memory, unless it was rehearsed, for only an average of 20- to 30-sec. Long term memory is the component of memory that holds knowledge and skills for a relatively long period of time, from a few minutes to potentially a lifetime, and has no known limit on how much it can hold.

There are three main theories as to why we forget. Among the three, Keppel & Underwood’s (1962) theory on interference, when one set of information impairs the acquisition or retention of another set of information has been the most studied. Pellegrino (1976) examined the effect of different types of distraction tasks and the length of the delays had on recall of a list. Subjects in the study were presented with either a word or picture, and were then distracted by the acoustic (counting form a particular number) or acoustic + visual distraction (counting backwards while trying to locate the shape of a nonlabelable structure from a matrix of confusing lines). The results showed that recall of pictures were greater for the acoustic, whereas the acoustic + visual distraction task together, word recall was significantly different. Pictures were superior to words at all delay intervals under single acoustic distraction, whereas dual distraction consistently reduced picture retention while simultaneously facilitating word retention.

Peterson and Peterson (1959) reported a study on short-term retention of individual verbal items. In this study the results of two identical experiments with the exception of a few delay were reported. Both experiments examined the progress of retention after the brief presentation of an item. The subjects were presented with a consonant syllable followed by a three-digit number from which they had to count backwards by 3’s or 4’s for varying amounts of time, until prompted by the experimenter to stop and recall the consonant syllables. Subjects were given practice trials of which they were coached on the ways they should respond to the stimulus in the study. The only difference between the two experiments was that the latter was a between-subject study in which the subjects were either given no time, 1-sec. or 3-sec. to rehearse the consonant syllable before starting to count backwards. The results reported by Peterson & Peterson (1959) was that the retention of the consonants depended on the retention interval, with good performance over short intervals and poor performance after intervals that were just 18-seconds in duration. The probability of recall decreased with the duration of the distracting activity. Peterson & Peterson (1959) attributed this forgetting effect to decay rather than interference.

Murdoch (1961) reported a series of experiments in which a number of different factors that could affect one’s memory, was examined. Among the three experiments that were reported was the replication of the Peterson and Peterson (1959) experiment with the exception of a few delays. It was conducted in a series of four sessions, which varied from a single consonant syllable to single, monosyllable non-homophonic words from a list of very common words. The results showed that session 1 and 3 were not significantly different from one another. However, session 2 was significantly different from sessions 1and 3 combined. Recall was greater if the items formed a word that can be associated with a particular thing as opposed to the single consonant items.

The present study is a replication of the Peterson and Peterson study. The independent variable in this study was the length of time of the delay before recall. The dependent variable was the mean percentage correct of recall. It is hypothesized that the results from the present study will be similar to those of the Peterson and Peterson (1959) study. It is hypothesized that as the length of delay with some a distraction task got longer, the lower the percentage of recall would be.



Eleven undergraduate students in an advanced experimental psychology class took part in this study. All the participants were Queens College students of the City University of New York. The sample consisted of nine females and two of ages ranging from 23 to 32 (mean= 23, SD= 3.4). The level of education ranged from lower juniors to upper seniors. All the participants were right handed and had vision that was normal or corrected-to-normal. Each participant gave informed consent prior to the beginning of the experiment.


Researchers used an experiment, Short-term Memory, a computer based program designed for the disk of the MEL Lab Manual: Experiments, in Perception, Cognition, Social Psychology and Human Factors. The experiment took place in a small classroom where the participants were placed in cubicles where the computers were positioned.


To determine if there was a significant difference in the percentage of recall a one-way Analysis of Variance (ANOVA) was done. Post hoc tests were done between the different values of the lengths of delay in order to determine where the effects were.

Design and Procedure

The experiment was a single factor within-subject counterbalanced blocked design with 3 different levels of delay of 45 different trials. There were 15 trials at each delay. Partial counterbalancing was used to ensure that sequence effects were properly controlled. The independent variable in this study was the varying length of delay of the distraction task (3-, 9-, 18-sec.). The dependent variable was the percent of letters recalled after the distraction task. Subjects were seated in a small lab room in front of a computer. Then, the subjects were presented with a set of three consonants and a three-digit numbers. The experimenter read those aloud to the subject (letter and the numbers), pressing the spacebar as the lasts digit was read. After that, the subject was to count backwards by threes (out loud) from the number the experimenter read to them, counting in time to the tones that the computer presented once each second. The experimenter was to hit the spacebar !

each time the subject reported a number. For example, if the number was 765, the subject would say 762, 759, 756, 753, 750, and 747, etc., until told to stop.

The counting backwards by threes continued until a different tone was heard. At that point, the subjects were asked to report the letters they could recall out loud to the experimenter, who entered them into the computer. The computer then reported whether the recall was accurate, and the experimenter reported this information to the subject. The letters had to be recalled in the same order in which they were presented. Only a completed recall in order was counted as correct. There were 45 trials and no practice trials were given. The experiment took each participant eighteen minutes to complete.


The results from the data indicated that as the length of delay increased, the percentage of recall simultaneously decreased also. An alpha level of .05 was used for all statistical tests.


The percentage of recall decreased significantly as the length of delay increased [F (1, 10)= 7.41, p*. 05]. The percentage of recall at the 3-sec. delay was significantly higher than that at the 18-sec. delay. Post Hoc on recall as a function of delay revealed an overall significance [F (1, 10)= 5.027, p* .05] within the three levels of delay. The percentage of recall in the 3-sec. delay was almost twice as much as the percentage of recall in the 18-sec. delay. There was a significant difference [F (1, 10)= 6.586, p* .05] on recall between the 3-sec. delay and the 9-sec. delay. There was also a significant difference [F (1, 10)= 6.076, p* .05] on recall between the 3-sec. delay and the 18-sec. delay. However there was no significant difference in recall between the 9-sec. delay and the 18-sec. delay, p= 0.182.

Results in figure 1, the percentage of recall as a function of the length of delay was illustrated. There was a 16% decrease in recall between the 3-sec. delay and the 9-sec. delay conditions, whereas, the percentage of recall between the 3-sec. delay and the 18-sec. delay decreased by 30%. The data revealed that the variability at the 9-sec. delay and the 18-sec. delay did not differ as much as the variability between the 3-sec. delay and the 18-sec. delay.

Distraction Task

Researchers were only able to obtain data for the measures of central tendencies, as opposed to the inferential statistics needed to analyze the data. The data revealed that as the length of the delay increased, the number of entries per second increased notably as well. The mean number of entries between the 3-sec delay and the 9-sec. delay increased nearly by 50%, and between the 3- sec. delay and the 18-sec. delay the number of entries increased by 150%.



The results indicated that as the delay period was prolonged, the proportion of recall decreased drastically. This supported the hypothesis that as the length of delay with a distraction task increased, the percentage of recall would decrease. The results of the present study are similar to those obtained by Peterson and Peterson (1959) under the delay of the different rates of distractions. Peterson and Peterson (1959) argued that their forgetting effects were not due to retroactive interference (new information causing loss of old information) because the numbers from the counting backwards task were sufficiently different from the items being retained that no interference should occur. They also argued that the forgetting effect was not due to proactive interference (old information interfering with the learning of new information) because they found that overall performance was the same across blocks in the experiment. Peterson and Peterson concluded that the forgetti!

ng must be due to trace decay (the gradual weakening of a memory record with the passage of time). If correct, then this would mean that the forgetting from short-term memory occurs by a different mechanism than forgetting from short-term memory.

The scores in the 3-sec. delay and the 18-sec. delay were so diverse because short-term memory can only hold a small amount of information, up to seven plus or minus two items at a time. Short-term memory can hold items fairly well for the first few seconds. After about ten seconds, however, recall is poor, and after twenty seconds the information has disappeared completely. As long as the items are rehearsed they can be held indefinitely. When once rehearsal has stopped, memory for the information is often lost. Another possible reason might be that many of the subject used different memory techniques and word association to recall the consonant syllables, (e.g., BMW might have been associated with the automobile).

In both the Murdock and the Peterson and Peterson experiments the subjects were given practice trials and were consequently "trained" on how to perform in the different elements of the experiment. However, the subjects in the present study were not given any practice trials. This concurrently might have produced a practice effect for the subjects in the earlier studies that the subjects in this present study did not benefit from. Future research can be done to further study this assumption. Experimenters can possibly replicate the Peterson and Peterson study and extend it to where group A would get the practice trials and group B would not get the practice trials.

Distraction Task

The results for the distraction task had some kind of effect on recall and memory. Since the statistical test were unattainable it was difficult to imply the cause or reason for those results. Shown in figure 2, is the percentage of the number of entries that were to be made per second as a function of the length of the delay. Rehearsal is needed to retain in formation in the short-term memory for as long as twenty seconds. Counting interrupts rehearsal and causes loss of items. This contrasts to those items in memory that aren’t forgotten. The expected number of entries per second is also presented. Subjects did not follow the instructions given prior to beginning the experiment and for this reason the levels were not as expected. As shown in figure 2, if the subjects were following instructions, there would have been an equal number of entries as the length of delay increased. For example, three entries at the 3-sec. delay and 9 entries at the 9-sec, etc. However, the subjects were probably rehearsing the numbers causing a slower subtraction rate and a lower entry rate. As seen in figure 2, the number of entries nearly doubled between the 3-sec delay and the 9-sec. delay, and nearly triples between the 3-sec. delay and the 18-sec. delay. Future research can examine to see if three letter words are treated the same in short-term memory as nine letters or three words. Researchers can extend this study by having only three or four trials per subject, with no practice trials, and had a different group of subjects for each delay.

Figure Captions

Figure 1 Percentage of recall as a function of the length of deal.

Figure2Percentage of the number of entries per second that were to be entered as a function of the length of delay. The expected number of entries is also illustrated


James, J.S., Schneider, W., & Rodgers, K.A. (1994). Short-Term Memory. MEL LAB: Experiments in Perception, Cognition, Social Psychology and Human Factors, 113-116.

Keppel, G. & Underwood, B.J. (1962). Proactive inhibition in short-term retention of single items. Journal of Verbal Learning and Verbal Behavior, 1, 153-161

Miller, G.A. (1956). The magical number seven, plus or minus two: Some limits on our capacity for processing information. Psychological Review, 63, 81-97.

Murdock Jr., Bennet B. (1961). The retention of individual items. Journal of Experimental Psychology, 62(6), 618-625.

Pelegrino, James W., Siegel, Alexander W., Dhawan, Meena (1976). Short-term retention of pictures and words as a function of type distraction and length of delay interval. Memory & Cognition, 4(1), 11-15.

Peterson, Lloyd R. & Peterson, Margaret J. (1959). Short-term retention of individual verbal items. Journal of Experimental Psychology, 58(3), 193-198.

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