Short-Term Memory in Signed Languages: Not Just a Disadvantage for Serial Recall

The higher short-term memory (STM) capacity for spoken language compared to signed language is well-documented: speakers have a digit span of 7 ± 2, signers only 5 ± 1 (see Hall and Bavelier, 2010, for a review). A consensus has been developing that speech is “special” in supporting the temporal sequencing of linguistic information, giving spoken-language users a serial recall advantage (e.g., Bavelier et al., 2008; Conway et al., 2009). The “speech supports temporal sequencing” hypothesis predicts that the difference between signed and spoken languages should disappear in tasks that do not require recall of a sequence of signs. However, recent data from a non-sign repetition task do not support this prediction. We created a sign language equivalent of the non-word repetition task, a task widely used to investigate phonological STM in speakers. Test items were phonotactically plausible but meaningless signs, manipulated for complexity of handshape and movement (Mann et al., 2010). Importantly, serial recall was not involved in this task. At most, a non-sign contained a change from one handshape to another or from one location to another (or both). We tested 91 deaf children aged 3–11 years, divided into three age-bands. All had early and continued regular exposure to BSL, comprehension skills within the normal range (as measured by the BSL Receptive Skills Test; Herman et al., 1999), normal nonverbal cognitive development and no identified special educational need additional to deafness. The task proved to be surprisingly difficult, with low scores across the age groups in comparison to results from non-word repetition studies of hearing, English-speaking children of equivalent ages, including those for whom English is an additional language (Figure 1). Generally, signs are of longer average duration than words, so we measured the duration of a selection of 3 and 4 syllable non-words used in a new non-word repetition study (Marshall et al., 2011). Duration for non-words ranged from 0.97 to 1.36 s and non-signs were slightly longer, with a mean duration of 1.31 s for phonologically simple and 1.35 s for phonologically complex non-signs (overall range 1–1.84 s). Is it possible that non-signs are more difficult to repeat because of these differences in temporal duration? After all, hearing children are less successful at repeating longer non-words (for a review, see Gathercole, 2006). For hearing children, STM capacity increases rapidly throughout childhood, and at the age of 3 years it is already equal to about 3 digits (Chi, 1977). Yet, in non-sign and non-word repetition tasks, stimuli are presented singly and not in a span, so there is no new material to block rehearsal, and our stimuli do not exceed the estimated 2 s of available time capacity in STM (Baddeley et al., 1975). Rather than duration, we argue that the way in which phonological material is structured is likely to be a more important limiting factor in repetition. The “speech supports temporal processing” view has recently been modified by Hall and Bavelier (2010), who argue that the advantage for speech arises from speakers being more likely to rely on the temporal chunking of units and on articulatory rehearsal. Under this more nuanced view, they might predict that the repetition of non-signs would be disadvantaged relative to the repetition of non-words because the latter benefit from the chunking of temporally adjacent units. However, in a direct comparison between serial recall and nonword repetition Archibald and Gathercole