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Memory Scanning Essay



The key purpose of this study was to measure reaction time and accuracy of responses to a certain stimulus (digit) during a memory-scanning task. This research originates from a series of experiments, conducted by Saul Sternberg, who examined the relation between the reaction time and the size of the set. This research aims to test a hypothesis which postulates the reaction time is directly proportionate to the size of the set.


Mental chronometry has long been of great interests to psychologists and neuroscientists; in particular, they study those factors that determine the response time (RT). Such studies are usually based on the so-called stage theory according to which perception and reaction to a stimulus or irritant consists of multiple-processes or mental operations, and RT depends on the number of these operations (Donders, as cited in Sternberg, 1969, p 61).

Overall, RT may also be determined by the type of stimulus, its intensity, duration, or the type of reaction, needed (Rosenbaum, 2009). Furthermore, one should not forget about individual characteristics of a person such as his age and the state of his health. In this paper, I would like to describe an experiment that has recently neen conducted. Its key objective was to measure the reaction time, needed for a memory-scanning task.

This experiment is similar to that one conducted by Saul Sternberg in 1968. He hypothesized that the reaction time, required for a memory scanning exercise is influenced by the type and number of mental operations, performed by the respondent (Sternberg, 1969, p 454). The essence of this experiment lies in the following: respondents are required to memorize a set of digits (the number of items in the set ranges from two to five); afterward the subjects are provided with a stimulus also in the form of a digit, from 0 to 9.

The responds need to determine whether the probe was present in the previous set of digits or not (Sternberg, 1968). By conducting such experiments, Saul Sternberg ascertained that reaction time was directly-proportionate to the number of items within the set of digits; in other words, if the experimenter increases the digit set, the response time will also increase, and vice versa.

He also postulated that the subject usually conducted exhaustive serial search, rather than self-terminating search, which means that he/she checked all items in the digit set, even despite the fact that the stimulus had already been identified (Sternberg, 1968). This is the key hypothesis, which needs to be tested in the course of this research.

On the whole, his experiments support the stage theory, which relies on the idea that reaction time is a sum of mental processes and that it is possible to decompose the reaction time into several parts (Sternberg, 1969, p 421). Sternberg relies on the idea that the reaction time is determined by the total amount of mental operations, such as recognition of the stimulus and organization of the response (Sternberg, 1969). In his study, he excludes such factors as the type of stimulus or its intensity.


The subjects for this experiment were seven students from an experimental psychology class. They were briefed on the purpose of the study and the experiment. Afterward, each of them was directed into an individual cubicle so that their attention was not distracted to any other stimuli such as light or noise. In the course of this research, the following tools were used: Windows XP desktop computers, placed in each room, and such program as SuperLab Pro which is quite suitable for such experiments.

The participants were asked to follow instructions that flashed on the screen. At first, they needed to memorize a number, (the number of digits ranged from one to six). Afterword, they were digit a digit.

They were asked whether this digit was present in the previous number or not. If they answer was positive, the participants needed to press slash (/) located at the right side of the keyboard, and if the answer was negative they needed to push Z, located at the left side. The task of the subjects was to respond as quickly and as accurately as possible. Finally, the participants were completely debriefed about the experiment. These are the key steps, taken in the course of this study.

It should be noted that in this experiment, the participants were allowed a limited amount of time in order to memorize the digit set; namely, they had only sixty seconds. The thing is that this mental scanning exercise is designed specifically for a short-term memory, which lasts for several seconds.

Furthermore, short-term memory can only hold 7±2 symbols, as it was ascertained by George Miller (1956, p 344). Although this article is not directly related to Sternberg’s experiment, it is crucial for our understanding of short-term memory and its functioning. It shows that the individual capacity of a short-term memory varies, and subsequently this individual characteristic impacts the reaction time.

In this research, it is possible to single out two independent variables: 1) the size of the initial set and 2) presence or absence of the stimulus (digit) in the initial set. In turn, the dependent variables are the reaction time and accuracy of responses. This study aims to measure the relations between these variables.

It should also be noted that the focus of this study is on digit recognition, not letters or any other symbol. The thing is that digit recognition and letter recognition are separate processes, and different parts of human brain are responsible for them (Polk & Farah, 1998). This is one of the reasons why the findings of this experiment cannot be applicable in all cases.


This experiment has demonstrated that the response time is longer when the stimulus (digit) is present in the set. In this case, mean (M) equaled 3371.83 milliseconds, while standard deviation was 3447.54.

In turn, when the stimulus is not present in the set, the average response time was 2135.68 milliseconds, while SD equaled 1176.20. Sternberg explains this phenomenon by the fact that the subject has to cope with an item recognition task, which increases the response time (1968, p 424). Yet, I would like to say that in my study the effect of present of absence was not statistically significant[1].

In this case, F equaled 1,6, or 3.246 milliseconds. In turn, p-value was 12. Under the circumstances, it is possible to speak about the so-called null hypothesis, which means that there is no relationship between the presence or absence of the stimulus, on the one hand, and response time, on the other. Interaction effect of presence and size was only marginally significant.

On the whole, these findings are in line with the hypothesis, proposed by Saul Sternberg who believed that while doing a memory-scanning task, a person relies on the exhaustive search rather than self-terminating search (1968, p 454). In other words, he/she checks all items (digits) of the set, even despite the fact that the probe has already been identified.

This experiment has also indicated that there is some marginal interdependence between the size of the initial set and response time. The findings of this research show that the main effect of size was also not very significant (5, 30) = 1.673, p=.17. It could be observed that the reaction time had been shorter if the initial set consisted of a smaller number of digits.

Nevertheless, there is another factor, which needs to be considered; it is the sequence of the digits within the set. We do not know what kind numbers the participants were required to memorize, we only know that they could consist of one to six digits. Let us suppose that that the number to be remembered is 612389, while the stimulus is 6; in such scenario, it is quite possible that the subject will not do a self-terminating search rather than exhaustive serial search.

As soon as he detects the stimulus, he will press the necessary button, and his reaction time will be much shorter. In part, this idea is supported by the findings of this study: in one case, the subject had to identify a stimulus within five-digit set, and it took them 7243.93 milliseconds; in the other case, they needed to do the same task but the set consisted of six digits and the reaction time was actually shorter 2553.9 milliseconds. This inconsistency can be accounted by the fact that the digit was at the very beginning of the set.

As far as accuracy of responses is concerned, I would to say that the effect of present was not statistically significant. In this case, F equals 1, 6 or 337, while p-value is 58[2]. This data indicates that participants were equally accurate in their responses to the stimuli irrespective of their presence of absence within the initial set. In addition to that, the findings suggest that the interaction between the presence and size did not affect the accuracy of responses.

Therefore, the key findings can be summarized in this way: 1) the presence effect is only marginally significant; it affects the reaction time and accuracy only to a small extent; 2) subsequently, the set size is also of little statistical importance. Nevertheless, these results do not refute the hypothesis, formulated by Saul Sternberg.

The reaction time is directly-proportionate to the number of digits in the set. Still, one has to bear in mind that some these results can be partially explained by the limitations of this study, which will be discussed in the next section of the paper.


On the whole, the results of this experiment cannot be regarded as conclusive due to several reasons. First, while selecting the subjects for this research, I did not take into account the individual differences of short-term memory. Again, as George Miller (1956) points out it can hold 7±2 symbols items; this means that some people can easily memorize a set of six or five digits, whereas others cannot cope with this task. Thus, it is quite probable that memory-scanning exercise would more or less difficult for some of the participants.

This is why a researcher has to pay more attention to the choice of subjects, as these people must have the same capacity of short-term memory. For this purpose, one has to carry out a set of preliminary tests that evaluate memorization skills of the person. The second limitation of this research is insufficient sampling. This research included only seven subjects; while as a rule, such studies encompass at least fifty participants; otherwise, it is hardly permissible to make any generalizations.

Again, this discussion leads us to the debate as which type of search the subjects usually undertake while doing memory-scanning exercises. It can be either a serial exhaustive search, which means that the search does not cease as soon as the digit is identified, more likely, the search goes on until all items in the set have been checked or it can be a self-terminating search that ends as soon as the target has been located (Townsend, 2001, p 1102).

Even at this point, scholars cannot state for sure which type of search, a person does while doing memory scanning exercises. The study by James Townsend (2001) has indicated that the type of search may depend upon the position of the stimulus within the search set.

Another issue, which should be discussed, is that reaction time strongly depends upon the type of stimulus. For example, Thad Polk and Martha Farah believe that the reaction time for letter and digit recognition is not the same, and that a person requires less time to recognize a letter rather than a digit (1998, p 852). Furthermore, the authors argue that the neural substrates, which are responsible for letter recognition, are separated from those ones, underlying the recognition of letters (Polk & Farah, 1998, p 852).

This research does not cast doubt on the validity of our study; however, it does suggest that there are some other factors, affecting the reaction time, namely, the type of stimulus. To a large extent, this study by Polk and Farah extends the scope of Sternberg’s experiments as it shows that there is another factor that influences reaction time and it is the type of the stimulus.

Moreover, one should not forget that such in itself, such research method as a controlled experiment has several disadvantages; one of them is the so-called observer effect, which means that the behavior of a subject changes when he/she is aware of being observed.

One should remember that the participants were informed about the tasks that they will need to perform prior to the start of the experiment, which means that to some extent, they were ready for this memory-scanning exercise. There is great likelihood that this circumstance could have affected the results of the experiment.

These are the major drawbacks of the study, and one can argue that aspects of the research design should have been changed. First, the number of participants should be much larger because the initial sample size is sufficient for such studies.

Secondly, the short-term memory of each subject should be tested beforehand because the participants of such studies must have similar memorization skills. Finally, it might be prudent to test not only digit recognition but letter recognition as well, because the reaction time for these processes may vary.


Mental chronometry still remains one of the most thought-provoking issues in psychology and neuroscience. There are several questions, which have yet to be answered, for instance, scholars have not ascertained which type of mental search a person undertakes, while performing a memory-scanning task. The choice is between two options: self-terminating search or serial exhaustive search.

The results of Sternberg’s experiments support exhaustive search hypothesis, however, this assumption, even now but this assumption has not been proved completely. Another area of research that is of great interest to psychologists, linguists, and neuroscientists is the difference between digit recognition and letter recognition. Apart from that, it is necessary to examine the impact of short-term memory capacity on recognition process and reaction time.

The experiment which has been conducted for this study substantiates the hypothesis, which we have advanced at the very beginning: namely, the size of the digit set increases the reaction time. Furthermore, the present or absence of the stimuli within the set also influences accuracy of responses and reaction. Nonetheless, one should take it into consideration that the statistical data in support of these assumptions is only marginally significant.


Figure 1

Reaction Time on Memory Recall Graph.

Figure 2

Accuracy Rate on Recall Graph.

Reference List

Miller G. (1956) . Psychological Review, (101), 2, pp. 343-352. Web.

Polk T. & Farah M. (1998). The neural development and organization of letter recognition: Evidence from functional neuroimaging, computational modeling, and behavioral studies. The National Academy of Sciences, pp. 847-852. Web.

Rosenbaum D. 2009 Human Motor Control. NY: Academic Press.

Sternberg S. (1969). Memory-scanning: mental processes revealed by reaction-time experiments. American Scientist, (57), 4, pp. 421- 457.

Townsend. J. (2001). A clarification of self-terminating versus exhaustive variances in serial and parallel models. Perception & Psychophysics. (63), 6, pp. 1101-1106. Web.


  1. Refer to Figure 1
  2. Refer to Figure 2
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