short-term memory and working memory

Short term memory

Conscious representation of ‘the present moment’

Temporary store in which we integrate current sensory experience with long-term memory to achieve current goals

Sensory memory

STM’s duration is rather brief although significantly longer than sensory memory - being a matter of seconds, rather than milliseconds.

Atkinson and Shiffrin’s model of STM

Proposed a mechanism called the maintanence rehearsal (or sometimes just rehearsal) to keep information active in STM. This was conceived of as an ‘inner voice’ (a language-based auditory code) that can be used to mentally rehearse information until it is transferred to LTM.

Capacity of STM

Limited - estimates from 3 or 4 items to between 5 to 9 items. However the definition of what constitutes an ‘item’ in STM can be tricky.

Measuring verbal STM capacity

Digit span task - immediately serial recall of verbally presented digits (ie number names) in the order they were presented. Length of sequence is increased by one item after each successful attempt to determine the upper limit or “span”.

Participant’s span is reached when they fail on two trials at a given series length.

So if you were unable to recall both trials for a series of 8 items, then your digit span would be 7 items.

Duration of STM

15-30 seconds

Measuring duration of STM

The Brown-Peterson task

Recall the names of 3 consonants eg D-P-R

Memory probed (tested) at 3 second retention intervals

To prevent rehearsal, participants were required to count backwards from a given number in 3’s until told to stop ie “filled retention interval”

For example” participants hear “D-P-R - 306” count backwards aloud from 306 until asked to recall the sequence of letter names

Forgetting due to “trace decay” or interference from pervious trials?

**results

After a 3 second interval only 50% of trials were likely to be recalled correctly with all three letters named in the correct order

By 9 seconds this has dropped to 20%

After 12-18 seconds 0% could recall

Maintenance rehearsal in transfer to LTM

Information in STM comes from sensory registers and retrieval from LTM.

Verbal rehearsal keeps information active in STM and strengthens the trace to increase the chance it will be stored in LTM.

Encoding in STM relates information from sensory stores to long-term memory.

Serial position effects and transfer to LTM

Immediately free recall of lists of numbers or words is affected by the position of items in the studied list.

Primacy effect and recency effect

Primacy effect - beginning position of items in a list

Provides evidence for transfer to long term memory for items that receive more rehearsal

We can easily recall the first few items in a list because the opportunity to rehearse them increases the likelihood that they are transferred into long-term memory

Primary effects are eliminated if rehearsal is prevented by introducing a concurrent task (repetition of a word)

Recency effect - end position of items in a list

Reflects availability of information still in short term memory

We can easily recall items near the end of a list because they are still contained in short term memory

Recency effect is reduced by introducing a filled retention interval before recall

Middle position of items in a list

Hardest to recall because they were presented too long ago to still be in short term memory and so many items came before and after them that there was little opportunity for rehearsal, limiting transfer into LTM.

Levels of processing - towards working memory

Purpose of STM is to encode information meaningfully.

Meaningfully or “deep” processing of information during encoding will produce long term memory traces

‘Shallow’ processing is less effective for long-term retention

Eg. Upper or lower case, rhyme, sentence

Questions that preceded each of the words served to control the “level of processing: used to encode each of the words

Participants were given another task to do for 5 minutes. Then, they were given a surprise recognition memory test. The recognition memory task consisted of 180 words presented in a different random order to each participant. 60 of these words were those that had been in the question task. 120 are random words.

Baddeley’s working memory model

Rather than focusing on maintaining information for immediate recall, the focus shifts to thinking about STM as providing a mental work-space that helps us to achieve our current goals and update our understanding of the world.

Three-component model of working memory consists of an attentional controller, the central executive, aided by two subsidiary systems, the phonological loop, capable of holding speech-based information, and the visuospatial sketchpad, which performs a similar function for visual information.

Episodic buffer later added to provide a storage system that binds together the inputs from the visual/spatial and auditory systems and integrates them in multimodal (ie multi-sensory) representation of the current contents of awareness.

Central executive

Executive processes are used in planning and coordinating complex behaviour:

• goal orientation

• Focus attention

• Control of social behaviour

• Switching between tasks, updating memory, inhibition of distracting information

• Planning and problem solving

Executive processes are governed by circuitry in the pre-frontal cortex, especially dorsa-lateral prefrontal cortex and the anterior cingulate cortex (ACC)

Phonological loop

A mental workspace for manipulating auditory and verbal information.

Digit span backwards is considered a test of phonological/verbal working memory

Must actively manipulate the information in memory, rather than just maintaining the sequence.

Visio-spatial sketchpad

A temporary store for representations of visual and spatial information such as faces, objects written words and cognitive maps.

Enables the mental manipulation of visually and spatially represented information:

• mental rotation of objects

• Visual and spatial mnemonics

• Mental arithmetic

• “Cognitive maps” for navigation

Neural bases of working memory

• executive processes are based within networks in the pre-frontal cortex

• The phonological loop (PL) is a left-hemisphere frontal-temporal lobe network

• The Visio-spatial sketchpad (VSS) is a right-occipital-parietal network

• The episodic buffer integrates multi-modal information in an integrated ‘episodic trace’ within the parietal cortex (association cortex)

Summary

Early accounts of STM focused on verbal maintanence of information for immediate recall and transfer to LRM.

• verbal STM capacity approximately 7 ± 2 items (digit span)

• Duration is very brief, with rapid decay if rehearsal is prevented (Brown-Peterson task)

Rehearsal in STM leads to LTM transfer

• primary reflects LTM, recency reflects STM

Levels of processing affects transfer to LTM and suggests a working-memory rather than a system for shallow maintenance of information - Craik and Tulving (1975)

Baddeley’s model of working memory expands the concept of STM to a multi-component, multi-modal system governed by executive processes

• active workspace for reasoning and problem-solving, not mere maintanence of information

The WM system is located across an integrated cortical network

• left and right hemisphere systems for PL and VSS, respectively, governed by pre-frontal executive system (DLPF and ACC), producing an integrated episodic trace in parietal cortex