Human Memory Human Memory Lesson 7
STM & Capacity
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The Nature & Structure of Memory • Several theories of memories are based on the assumption that there are three kinds of memory: sensory memory short-term memory long-term memory
What I need to know about STM & LTM What is the difference between STM & LTM
Capacity Duration Encoding
What evidence supports The distinction between STM & LTM
Case Studies Primacy & Recency Encoding Differences
The Capacity of STM
Activity How much information can you hold in STM? work out the following sums mentally (a) 5 x 7
(b) 56 x 7
(c) 56 x 89
• You probably found problem (a) extremely easy and problem (b) was easy. Problem (c) is much more of a challenge: It would have stretched the limits of your STM by requiring you to carry too much information at once. It can be frustrating to hold onto relevant information whilst manipulating others. • The above exercise indicates that the STM has a limited capacity, that is it can only hold a small number of items at any one given moment and time.
• One way of assessing how much your STM can hold (this is the STM’s capacity) is by finding out how many digits you can repeat in correct order immediately after hearing them. This is known as your immediate digit span. • This technique usually involves reading a list of random digits and requiring the participant to repeat them back in the correct order. The sequence usually begins with three digits and steadily increases in length until it becomes impossible to recall them in serial order.
• Over a number of trials, the sequence length at which the participant is correct 50% of the time is defined as their digit span. Most people have a digit span of ‘seven plus or minus two’ according to Miller (1956). This has been called Miller’s magic number 7. • According to Miller ‘chunking’ occurs when we combine individual letters or numbers into a larger meaningful unit. For example, the digits 7, 1, 8, 7 would represent four separate items to most people, but would form a single chunk for you, if the numbers happened to be your bank PIN number.
Key Study Capacity and STM
Study of immediate memory span by Baddeley et al. (1975) Aim : • To see whether people could remember more short words than long words in a serial recall test, and in so doing demonstrate that pronunciation time , rather than number of items to be recalled , determines the capacity of short-term memory.
Procedure: • The reading speed of the participants was measured. • The participants were then presented with sets of five words on the screen. • The words were taken from one of two sets : a set of one syllable words (e.g. harm, pot, sat) or a set of polysyllabic words (e.g. opportunity, aluminium, university). • Participants were asked to write down the five words in serial order immediately after presentation. They recalled several lists of both short and long words.
Findings: • Participants could recall considerably more short words than long words. • They were able to recall as many words as they were able to articulate in about 2 seconds. • There was a strong positive correlation between reading speed and memory span. Conclusion: • Immediate memory span represents the number of items of whatever length that can be articulated in 2 seconds.
Criticisms: • It might just be that short words are easier to recall than long words because they are more familiar to us. • This was a laboratory experiment using lists of unconnected words and did not reflect everyday use of STM. • Miller was not able to account for the findings of research that showed that immediate span depends on the nature of the stimulus, i.e. the kind of words and the language in which they are spoken. This study can explain such findings.
Supporting Evidence for Pronunciation Time Naveh –Benjamin & Aryes (1986) • They tested immediate memory span for speakers of various world languages. They found for example that the digit span for native English speakers is considerably greater than that of Arabic speakers. • The only explanation for this finding is that Arabic numbers take longer to pronounce than English numbers. These results confirmed earlier findings in a classic study by Baddeley at al. (1975).