Age Differences in Visual Working Memory Capacity: Not Based on Encoding Limitations

It is well-accepted that younger children perform more poorly than older children and adults on tests of working memory, the current-task information kept in an active state for short-term recall. It is clear that the development of working memory ability is an important component of cognitive development across many tasks (e.g., Andrews & Halford, 2002; Cowan et al., 2005, 2006; Gathercole, Pickering, Ambridge, & Wearing, 2004; Hitch, Towse, & Hutton, 2001; Johnson, Im-Bolter, & Pascual-Leone, 2003). What has been more controversial for many years is the reason behind the age differences in working memory performance. One simple hypothesis, the one advocated here, is that some brain system operates by retaining a limited number of items in an active form, and that this brain system holds fewer items in young children than in older participants (e.g., Burtis, 1982; Case, 1995; Cowan, 2001; Pascual-Leone & Smith, 1969). The notion that there is a working memory faculty limited to no more than a few items is supported by considerable recent research in adults (Awh, Barton, & Vogel, 2007; Cowan & Rouder, 2009; Rouder et al., 2008; Zhang & Luck, 2008; for an opposing view see Bays & Husain, 2008, 2009). The challenge for advocates of a capacity-growth hypothesis, however, is that it is not logically necessary; other possibilities exist (e.g., Barrouillet, Gavens, Vergauwe, Gaillard, & Camos, 2009; Case, Kurland, & Goldberg, 1982; Dempster, 1991; Hulme & Tordoff, 1989). The older participants may excel at focusing on more task-relevant information, in which case the holding system in the brain may be more cluttered by information irrelevant to the task at hand in younger children. Also, older participants may be better able to encode the stimuli in a manner that allows the information to be retrieved. In particular, they may form verbal labels for the stimuli that allow these stimuli to be retained using multiple brain systems, adding redundancy to the representation and making recall more reliable. In fact, verbal rehearsal is one of the main ways in which mnemonic processing improves in childhood (e.g., Cowan, Cartwright, Winterowd, & Sherk, 1987; Cowan, Saults, & Morey, 2006; Flavell, Beach, & Chinsky, 1966; Ornstein, Naus, & Liberty, 1975; Tam, Jarrold, Baddeley, & Sabatos-DeVito, 2010). The present research was designed to determine whether an age difference in working memory performance could be obtained across manipulations in factors that could affect the encoding of the stimuli. It was based on the procedure used by Cowan, Morey, AuBuchon, Zwilling, and Gilchrist (2010), illustrated in Figure 1. On each trial in that study, an array was presented for 500 ms and the task was to retain items from the array for a probe item recognition test shortly afterward. The probe either was identical to the array item in the same location or differed from that array item in color, in one of three ways shown in the figure. In the most critical condition, the one to be replicated here, the instructions said to pay attention to items in one shape and not another (e.g., circles and not triangles) throughout the session. The probe tested was in the attended shape 80% of the time, but in the other shape the remaining 20% of the time. After Gold et al. (2006), it was possible to obtain two meaningful measures from the results. First, the sum of items recalled from the attended and unattended (or less-attended) shape provided an estimate of the number of items held in working memory. Second, the difference between the number of items recalled in the attended versus unattended shape provided an estimate of the efficiency of the allocation of attention preferentially to the shape that was usually tested, and therefore more relevant than the other shape for maximum performance. Figure 1 After Cowan, Morey, AuBuchon, Zwilling, and Gilchrist (2010, Figure 2). An illustration of the array memory procedure. The present study differed from Cowan et al. in that the array items were presented one at a time, and in that a verbal response was ... Results of the previous study were clearest for small arrays, which included two circles and two triangles. Cowan et al. (2010) found that children in Grades 1-2 recalled far fewer items in total, but that they favored the relevant shape to the same degree as older children (Grades 6-7) or adults (college students). This strongly suggests that visual working memory capacity increases with age in childhood. The reason for this finding, however, is not yet clear. It could be that the effect occurs because children are not as good at entering the items from a brief, concurrent visual display into working memory. To examine whether that encoding difference can explain the age difference in capacity, the present experiment mirrored that of Cowan et al. but with the items presented one at a time, at a slow rate of 1 s per item (Figure 2). Figure 2 Comparison of the simultaneous array presentation method of Cowan, Morey, AuBuchon, Zwilling, and Gilchrist (2010) to the sequential array presentation method of the present study. If age differences in the number of items retained in working memory survive this change in procedure, though, there is still an explanation aside from basic capacity growth. It could be that this slower procedure allows time for older participants to engage in verbal encoding and rehearsal of the visual items, enhancing recall. To examine this factor, each participant carried out trials in each of three articulation conditions. In the first condition, the participant remained silent and therefore was free to adopt whatever strategy he or she wished. In a second condition, the color of each item was to be named as it was presented. This should enhance recall generally for individuals who did not spontaneously name the colors. Finally, in a third condition, the participant was to say the word “wait” after each item presented. This is essentially an articulatory suppression condition (Baddeley, 1986) in which the need to say a word irrelevant to the memory task interrupts more task-relevant verbal processing, such as labeling and covert rehearsal. If the working memory advantage of older participants in this procedure accrues only because they use verbal processing better than younger children, then the age difference might be substantially diminished when naming of the color is required (because item colors are then verbally labeled in all groups, instead of just the older participants) and it should be diminished also when an irrelevant word is required (because it disrupts verbal processing that may otherwise be an advantage for older participants). This last condition, presented after the participants are well-practiced in the task, provides the best indication of each age group's basic visual working memory capacity without the assistance of verbal strategies.