Capacity Studies

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Sperling (1960)

Several experiments were conducted to measure the amount of information that was available to a person after brief exposures to visual stimuli; a.k.a. recall of the sensory register.

Method

Participants were shown grids of letters: three rows of four letters.
Sperling chose letters as they have a large amount of information and they have been used by previous investigators.
Control tests were done first to measure the individual’s ability to recall the letters as this will be unique.
Essentially, these were meant to establish a baseline for the individual.

Method cont.

The experiment showed the participants a grid of letters (three rows of four letters) for 0.05 seconds (50 milliseconds).
There were two different scenarios:
- Recall the whole grid.
- Recall a single row.
The row to be recalled was identified by a high/medium/low tone that was played directly after the grid was flashed.
The participants did not know which row was to be selected.

Results

First scenario - recall the whole grid:
- Participants only managed to recall four or five letters out of a total of 12 letters.
Second scenario - recall a single row:
- They could only recall three letters (out of four letters in one row).
These results were consistent no matter which row was selected.

Conclusions

First of all, the sensory register is finite. Humans can only process and recall a small amount of sensory information.
Secondly, as the participants did not know which row was to be selected, they viewed the whole grid. This means the entire grid was in their sensory register.
But they could not recall all of the letters as the trace faded and the information decayed.

Evaluation

This was a highly controlled laboratory experiment, so it was scientific and it is easy to replicate.
- This means that the results are more reliable.
But laboratory experiments such as this do not reflect real-life scenarios (you are hardly ever expected to recall a grid of letters).
- So the study had low ecological validity.

Jacobs (1887) - Capacity of Short Term Memory

Capacity is the measure of how much information can be stored in memory. Jacobs studied the capacity of short-term memory (STM) and found that STM had a limited capacity.

Method

This was a laboratory experiment.
Participants were asked to recall a sequence of either letters or digits.
They had to recall them in the order in which the sequence was presented.
They repeated this procedure until they could not recall the sequence accurately.

Letter and digit sequence example

Letter sequence example:
- A, AO, AOP, AOPH, AOPHQ, AOPHQR, … etc.
Digit sequence example:
- 9, 98, 984, 9845, 98452, 984523, 9845230, 98452309… etc.

Results

Generally, participants could recall a longer string of digits than of letters.
On average, they could recall nine digits and seven letters.
The capacity of short-term memory increased with age.

Jacobs (1887) - Conclusions

Capacity is the measure of how much information can be stored in memory. Jacobs studied the capacity of short-term memory (STM) and found that STM had a limited capacity.

Capacity

Jacobs concluded that the capacity of short term memory is finite.
There is a limit on how much information can be stored, and so recalled, accurately within short-term memory.
Jacobs established the capacity to be between five to nine pieces of information.

Influence of age

The capacity may increase with age because of learning or memory techniques.
- E.g. chunking the information:
The digit sequence “98452309” could be chunked into two sets of numbers “9845” and “2309”.

Digits vs letters

Finally, Jacobs concluded that the digits may have been easier to recall because there are only 10 possible digits compared to 26 possible letters.

Extraneous variables

A strength of this study is the control of extraneous variables because it was a laboratory experiment.

Ecological validity

But because it was a laboratory experiment, it was artificial and lacked ecological validity.
Strings of random digits and letters are not very meaningful, so are less likely to be recalled.
A test on meaningful information, such as actual words or phone numbers, may show that STM capacity is actually larger than concluded in this study.

Methodological issues

A final weakness could be the confusion that could result from similar sequences between current and previous trials.
If presented with similar sequences, participants could muddle them up, confounding the results.