It has previously been believed that short term memory demanded constant neurological maintenance. As such it was believed that short term memory was a product of constant maintenance of representations, placing short term memory as a product of focus of attention.
Recent research conducted by Lewis-Peacock et. al. (2011) shows that attention and short term memory is neurologically distinct. While the constant neurological activity that maintained focus of attention shifted into representing some other information, short term memory would be retained and was correctly recalled a short period of time afterwards. The study conducted utilized fMRI trace neurological activity during the time which the test subjects conducted one of two tests.
In the first test conducted, the test subjects were to pair different pictures. There were pictures from three categories (faces, places and objects) with six pictures from each category. The pictures were paired into nine pairs, pairing each picture with a picture from another category. During a training the test subjects learned these pairings. Then the actual test began. In the test the test subjects first saw a picture that was to be paired with another picture based on what pairings they had already learned. At the end of the test they saw another picture and their task were to answer yes or no if the two pictures were matching, according to the pairings they had learned. In half of the tests there was a distraction between the two pictures. The distraction was always chosen to not match any of the other categories in the test.
The test subjects were told to use one of two strategies to accomplish their task. There was a prospective and a retrospective strategy. A prospective strategy meant that the test subjected watched at the first picture and retrieved from their long term memory what picture it was paired with then they tried to remember this picture. A retrospective strategy meant that the subjects were meant to keep the first picture they saw in their memory, then when they were prompted to see if the last picture in the test matched the first they used their long term memory finish the task.
In the top line, where the distraction were absent, we can see that the target picture created activity in the brain. However for the test subjects that used the prospective strategy the target associated brain areas quickly diminished while areas associated to the second picture (associated) increased slightly. When using the prospective strategy the the test subjects where already thinking about the picture when it showed up on the screen and their task were simply to see if it matched. For the retrospective strategy the test subjects task were to remember if it was the correct pair when the second picture showed up.
When the distraction was added to the test is when things starts to get interesting. As can be seen on the graph, brain activity in areas associated to the irrelevant (irrel) category greatly increased after it has been presented to the test subject. While the target and associated categories both diminished to a baseline activity. Even though the test subjects were able to keep it in their short term memory and accomplish their task. This proves that a persistent brain activity in the brain region and therefore any persistent focus on the category, is not essential to complete a short term memory task.
In the second test of the study the researchers wanted to be more precise. The experiment used again three different categories; English words, pronounceable pseudo-words, and line segments. The test subjects task was to respond whether the probe presented to them, yes or no, was synonymous in meaning (in the English words category) if it was rhyming (for pseudo-words category) or of the lines matched (for line segments category). At the beginning of the test they were presented by two stimuli, from two different categories, on a computer screen. Then after a short period of a blank screen, they received a clue to which of the two categories that were relevant for their response. Shortly afterwards a new stimuli were presented to them and the test subjects were to response if the word or line segment were correct, according to the rules explained earlier. Then the clue and and response session were repeated. In 50% of the tests the second clue and response was of the same category as the first (this was called a repeat), while in the other 50% it was of the second category that was presented earlier (this was called a switch).
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The red and blue colors represent the two relevant categories presented at
the beginning of each test. The red triangle represents the clue received for
the red category. The blue triangle represents the clue for the blue category.
The squares represents the where the test subjects were probed and asked
to answer. Grey represents the category that was not present at each test.
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This study proves to us that short term memory is not something that is created through a constant neurological activity. It also proves to us that there is a discrete distinction between attention and short term memory. This is because focus on a specific category or stimuli demands neurological activity, while short term memory works regardless of this. Although we know that short term memory is entangled with attention and as such attention and short term memory are two phenomenons that affects each one another. This study also creates a understanding of the nature of short term memory and how we should understand it.
As Lewis-Peacock et. al. describes short term memory can describe as an emergent property of the interaction between long term memory and attention. This study gives support for that theory commonly described as "embedded component" theory.
Reference:
Lewis-Peacock J. A., Drysdale A. T., Oberauer K., Postle B. R., (2011). Neural Evidence for a Distinction between Short-term Memory and the Focus of Attention. Journal of Cognitive Neuroscience 24:1, pp. 61–79.
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