THE DEVELOPMENT OF EXECUTIVE FUNCTION IN EARLY ...

specialized for the processing and storage of limited amounts of information within specific domains: whereas
the phonological loop operates on sound-based information, the visuo-spatial scratchpad operates on spatial
and visual information. In contrast to these slave systems, the central executive is capable of coordinating and
integrating information of either type.
In terms of this account, the capacity of the phonological loop can be assessed by memory span tests such as
digit span and other serial recall tests. These tests show that memory span (a measure of the maximum number
of unrelated verbal items that can be remembered in correct sequence) increases from two to three items at 4
years of age to about six items at 12 years (e.g., Cowan, 2001; see Gathercole, 1998, for a review). The span of
the visuospatial scratchpad is measured in an analogous fashion (e.g., using a pattern span task; Gathercole,
1998) and shows similar age-related improvements in childhood.
In contrast to these relatively simple span tasks, which mainly measure storage, the capacity of the central
executive is assessed by relatively complex working memory tasks such as reading and listening span. In these
tasks, children need to process incoming information (either reading or listening to sentences) while
remembering old information (e.g., the final words of the preceding sentences). Another complex measure of
the central executive is backward digit span, in which children are asked to remember a list of digits and
repeat them in the reverse order. Performance on these tasks also improves with age (Gathercole, 1998).
Several theoretical accounts have suggested that the development of executive function and behavior related to
executive function is made possible by the growth of short-term or working memory (e.g., Case, 1992, 1995;
Demetriou, Christou, Spanoudis, & Plastidou, 2002; Gordon & Olson, 1998; Olson, 1993; Pascual-Leone,
1970). It is possible that the growth of memory, perhaps particularly of the phonological loop
(which would be required for rehearsing verbal rules), constrains the number of rules that children can use
in the DCCS. For example, perhaps 3- to 4-year-olds can only hold two rules (i.e., a preswitch pair of rules)
in mind (e.g., Gordon & Olson, 1998), and consequently fail the postswitch phase.
A memory account of the development of executive function has also been suggested by Morton and Munakata
(2002), who presented a connectionist model of children's performance on the DCCS. Morton and Munakata
distinguished between active memory representations and latent memory traces. Active memory representations
take the form of sustained activity in the prefrontal cortex, whereas latent memory traces are formed in more
posterior cortex when processing a stimulus brings about a change in the subsequent processing of that
stimulus. Because latent traces are instantiated implicitly as changes in the strength of connections between
units, they are not accessible to other brain regions, although they influence the processing of these other
regions via changes in the activation of subsequent stimuli. In contrast, active representations are accessible to
other brain regions, and they can influence processing in these areas even if the stimulus is no longer
perceptually available.
The architecture of Morton and Munakata's (2002) network model of the DCCS consists of three input layers
("Visual Features, Verbal Features, and Rule") for representing test cards, instructions, and the relevant
dimension; an internal representation layer (i.e., a layer of hidden units); a prefrontal cortex layer that
corresponds to active memory of the relevant dimension (shape or color); and an output layer corresponding to
sorting trays. The units interact through connections of various types. For example, feedforward connections are
established between input and hidden units and between hidden and output units, and the strength of these
connections changes with experience according to a Hebbian learning rule (i.e., connections between units that
are simultaneously active increase in strength). Consequently, when the network sorts by color, for example, the
feedforward connections for color become stronger and the network develops a latent memory for sorting by
color. There are also feedfoward connections to the prefrontal cortex units that change with experience, but
these change at a much slower rate, such that the prefrontal cortex part of the model (which represents the
relevant dimension) is less susceptible to bias than the rest of the system.