What Determines the Capacity of Short-Term Memory?

ScienceDaily (Dec. 15, 2011) — Short-term memory plays a crucial role in how our consciousness operates. Several years ago a hypothesis has been formulated, according to which capacity of short-term memory depends in a special way on two cycles of brain electric activity. Scientists from the Nencki Institute of Experimental Biology of the Polish Academy of Sciences in Warsaw have now demonstrated this experimentally for the first time.


A human being can consciously process from five to nine pieces of information simultaneously. During processing these pieces of information remain in the short-term memory. In 1995 researchers from Brandeis University in Waltham suggested that the capacity of short-term memory could depend on two bands of brain's electric activity: theta and gamma waves. However, only now, through carefully designed experiments conducted at the Nencki Experimental Biology Institute of the Polish Academy of Sciences (Nencki Institute) in Warsaw, it was possible to unambiguously prove that such a relationship really exists.


For an electroencephalography exam (EEG) several electrodes are placed on patient's head. The recorded brain electric signals contain waves of different frequencies, among other theta waves with the frequency of 4-7 Hz and gamma waves with the frequency of 25-50 Hz. It has been known for some time that these waves are used for retaining information in the brain. It was observed for example that the amplitudes of theta and gamma waves increased when people were forced to store more information in short-term memory.

"The hypothesis formulated by Lisman and Idiart in 1995 assumes that we are able to memorise as many 'bites' of information, as there are gamma cycles for one theta cycle. Research to date provided only indirect support for this hypothesis," say psychologist Jan Kamiński, PhD student from the Nencki Institute and main author of experiments conducted by the team of Prof. Andrzej Wróbel in cooperation with Dr. Aneta Brzezicka from the Warsaw School of Social Sciences and Humanities.


A 'bite' of information refers to its portion in memory. A 'bite' may be a number, letter, idea, situation, picture or smell. "Designing experiments on the capacity of memory one needs to be very careful not to make it too easy for the subject to group many 'bites' into one," stresses Kamiński and as an example gives the following sequence of letters: 2, 0, 1, 1. "Such four 'bites' of information are easy to group into the number corresponding to current year. Instead of four bites of information we are left with just one."

Interpreting the length of theta and gamma waves from EEG recording is not easy either. These waves are not directly visible in the EEG signal. Kamiński proposed a new method of determining them. Researchers recorded brain's electric activity in seventeen volunteers resting with closed eyes for five minutes. Next they filtered the signals and analysed not the cycles themselves but their correlations. Only based on discovered correlations the ratio of the length of theta wave to gamma wave was determined and the likely capacity of verbal short-term memory was determined.


Following the EEG recording, the volunteers, were subjected to classic short-term memory capacity test. It consisted of repeated display of longer and longer sequences of numbers. Each number was presented for one second. Then volunteers had to reconstruct the sequence from memory. At first the sequence consisted of three numbers but at the end of the exam of as many as nine. "We have observed that the longer the theta cycles, the more information 'bites' the subject was able to remember; the longer the gamma cycle, the less the subject remembered. Next we determined the correlation between the results of the tests and estimates from the EEG measurements. Just as expected the correlation turned out to be very high and it confirmed the hypothesis of Lisman and Idiart," says Kamiński.


Capacity of short-term memory impacts the effects of reasoning -- the greater the capacity, the better the effects. Currently researchers conduct studies on developing the most effective ways of training short-term memory.