The Phonological Loop

An Overview of the Phonological Loop

The phonological loop has a specific function and is limited in the type of information it stores.

Also the phonological loop transforms perceptual stimuli into phonological codes that include the acoustic, temporal, and sequential properties of the verbal stimulus (Gilliam & van Kleeck, 1996).

Phonological codes are then matched with existing codes (i.e., phonemes and words) stored in long-term memory and also linked with meaning representations.

Higher level processing of the verbal information, such as putting the words together to form an idea, involves complex working memory functions that are conducted by the central executive Opens in new window.

The bulk of the empirical effort on working memory Opens in new window has been spent on the phonological loop, and it is said to consists in two components (Figure X-1):

1. a passive phonological store directly concerned with speech perception;
2. an articulatory rehearsal/recording process linked to speech production that gives access to the phonological store.

Figure X-1. Baddeley Model of Working memory (Focus on Phonological Loop). Adapted from Psychology Wizard Opens in new window

Orally presented verbal information gains immediate, direct, and automatic access to the phonological loop, where it is briefly stored in phonological form (Hitch, 1990; Logie, 1996).

The phonological loop is analogous to an “audio tape recorder loop” of specific length.

Words or other auditory units are recorded in the order they are perceived, and they will quickly decay or be recorded over by new auditory units unless rehearsal Opens in new window re-records them onto the tape.

According to Baddeley, words presented auditorily are processed differently from those presented visually.

Auditory presentation of words produces direct access to the phonological store regardless of whether the articulatory control process is used. In contrast, visual presentation of words only permits indirect access to the phonological store through subvocal articulation.

Baddeley et al. (1975) found that subject’s ability to reproduce a sequence of words was better with short words than with long words. The subjects recalled as many words as they could read out in 2 seconds.

This finding suggested that the capacity of the phonological loop is determined by temporal duration like a tape loop, and that memory span is determined by the rate of rehearsal.

Verbal Short-Term Memory Span and Articulatory Rehearsal

Unless action is taken to preserve the phonologically coded information, the phonological loop will hold information for only 2 seconds or less (Baddeley, 1986; Hulme & Mackenzie, 1992).

The number of verbal items that can be fitted onto the phonological “tape” loop depends on the time taken to articulate them. This tendency explains why recall of short, one-syllable words is better than that for longer words; longer words take longer to articulate and therefore take up more space on the phonological tape loop.

Adult recall of a five-word sequence of monosyllabic words is about 90%, whereas it drops to about 50% when the equivalent number of words consists of five syllables each (Baddeley, 2003a). Thus, the capacity of the phonological loop can be expressed as:

Research has found the length of the normal phonological tape loop to be about 2 seconds, regardless of the individual’s age.

Subjects can recall as many words as they can articulate in that amount of time (Baddeley, 1986; Hulme & Mackenzie).

For example, if an individual’s speech rate is two words per second, his or her memory span will be about four words. The number of words recalled is not a function of how many items are presented within 2 seconds but rather the number of words the individual can articulate within 2 seconds.

The implications are that any retention of verbal information in short-term memory beyond 2 seconds depends on rehearsal (repetition) and that the amount of information that can be rehearsed is also constrained by the 2-second loop.

Subvocal rehearsal rate is thought to be equivalent to overt speech rate. This relationship accounts for the findings that verbal short-term memory span varies according to the length of the items and the span has a strong positive correlation with speech rate.

Individuals with faster articulation rates can usually maintain more items than individuals who are slow articulators. For adults, normal phonological memory span has long been assumed to be approximately seven units (Miller, 1956).

The span is typically measured with tasks such as digit or word span and is often referred to as verbal short-term memory span or verbal working memory span. The finding that memory span is highly related to the time it takes to articulate the stimulus words implies that working memory is not necessarily limited to seven, plus or minus two, units of information as is usually believed.

With a few short words, individuals are able to subvocally rehearse the complete sequence in less time than it takes for the memory trace to decay, thereby extending maintenance of the sequence indefinitely.

The immediate serial recall of word sequences decreases as the constituent words become longer (Baddeley, 1990). This phenomenon, known as the word length effect, has been attributed to the greater time it takes to subvocally rehearse items of longer articulatory duration (Gathercole & Martin, 1996). The crucial feature is the spoken duration of the word and not the number of syllables. When subvocal articulation of the sequence exceeds the decay time, errors begin to occur.

Therefore, verbal memory span should be expressed as the number of words that can be articulated in approximately 2 seconds rather than thinking of it as a specific number of spoken words (Baddeley, 1990).

Phonological Similarity Effects

Some of the most convincing evidence that we use the phonological loop on short-term memory tasks comes from the phonological similarity effect, a phenomenon in which serial recall of a short list of visually presented words is worse when the words are phonologically similar than when they are phonologically dissimilar.

Individuals with normal phonological processing ability find it more difficult to remember lists of words that sound similar, such as man, map, and mat. Phonological similarity effect most likely results from confusions that occur in the passive phonological input store and misidentifications during rehearsal and later retrieval (Hulme & Mackenzie, 1992).

Any loss of information due to decay leads to confusion between acoustically similar items. Phonological similarity can have profound effects on recall. For instance, in a study by Baddeley (1986), dissimilar words were recalled correctly 82.1% of the time while similar-sounding words were correctly recalled only 9.6% of the time.

The effect of phonological similarity supports Baddeley’s claim that short-term memory Opens in new window encoding for verbal information is phonetically based (Logie, 1996), whereas long-term encoding is based more on meaning. For example, phonological similarity has no effect on long-term retrieval, indicating that, while it is the basis for immediate encoding, it is not the basis for long-term encoding (McElree, 1998).

More evidence for the phonological similarity effect comes from the study of how unattended background speech and noise impact short-term verbal span.

Concurrent but irrelevant speech in the background can have a deleterious effect on word retention, especially when the words to be recalled are phonologically similar to the irrelevant material (Gathercole & Baddeley, 1993).

Additional evidence that short-term verbal memory is based on phonological coding comes from the fact that orthographic (the visual representation of words) similarity has very little influence on word retention.

Phonological similarity effects may be only one aspect of a broader interference effect that arises whenever there is similarity between content being stored and content being operated on.

For example, recall of digits is substantially lower when subjects are required to engage in arithmetic calculation while trying to maintain a string of digits, whereas processing the meaning of sentences during digit span causes less interference (Conlin & Gathercole, 2006).

When exactly the interference is most disruptive is unclear. There are indications that the detrimental interference occurs mainly during retrieval when it is difficult to discriminate between phonologically similar items (Conlin & Gathercole).

Recency and Primacy Effects

The recency effect is often cited as further evidence for the existence of a temporary phonological store (Baddeley, 1990).

The recency-based phenomenon seems to result from the displacement or overwriting of earlier cues; recent items are remembered because they are still retained in the phonological store at the time of recall. As such, they are automatically recalled without rehearsal Opens in new window being necessary or without there having been time for rehearsal.

The fact that little or no rehearsal occurred is borne out by the finding that subsequent long-term retrieval of items at the end of the list is poorer than for items at the beginning or middle (Cowan, 2005), indicating that earlier items were rehearsed and encoded into long-term storage.

Apparently, the lack of rehearsal for the final items limits the encoding of the items into long-term memory—an effect that has implications for academic instruction. The recency effect also indicates that the last item or chunk heard still remains active in awareness or is still the focus of attention.

Proactive interference Opens in new window, which is interference from previously learned similar information, has no impact on immediate recall, indicating that no retrieval processes are needed for items that are still maintained in active awareness (McElree, 1998), unless they have been lost and retrieval from long-term memory is necessary.

Primacy, the superior recall of items from the beginning of a list compared to the middle items, is another memory constant. The effect is particularly strong when there is subvocal rehearsal, most likely because there is an opportunity to repeat these items more than subsequent items.