THE DEVELOPMENT OF EXECUTIVE FUNCTION IN EARLY ...

esponses were hypothesized to be verbal or attentional (e.g., Zeaman & House, 1963), it was argued on the
basis of research on discrimination learning that children under about 6 years of age fail to mediate their
responses to environmental stimuli. Discrimination-learning research was directed mainly at the question of
when children conceptualize specific stimuli (or cues such as "red") to be values of a general dimension (e.g.,
color). Dimensional responding was taken to reflect mediation and was inferred from the relative ease of
intradimensional (and reversal) versus extradimensional (and nonreversal) shifts in rule learning (as opposed to
rule use) tasks, and from the results of optional shift studies (T. S. Kendler & H. H. Kendler, 1966).
The results of this research are far from straightforward (see Esposito, 1975, for a review), and mediation theory
(T. S. Kendler, 1979) is not directed toward explaining changes in rule use in the preschool period because it
maintains that children in this age range cannot represent and use rules (which is a form of mediation). It is now
well established that 3year-olds can use rules (Zelazo & Reznick, 1991). However, results from the DCCS
suggest that in the absence of a higher-order rule for selecting between dimensions, 3- to 4-year-olds cannot be
said to understand a dimension qua dimension. Although 3- to 4-year-olds do use rules for sorting by a
dimension, such as color, it is not until children can reflect on these rules and contrast them with rules for
sorting according to a different dimension, such as shape, that the dimension itself becomes an object of
consideration for children. Being able to reflect on color rules as color rules that contrast with shape rules (or
rules from any other dimension) would seem to be required to comprehend the way in which different colors
form a coherent category of variation (i.e., a dimension).
Activation and Inhibition Processes
The results reported in this Monograph clearly indicate that both activation and inhibition play roles in
performance on the DCCS. This outcome suggests the following account of performance on the standard
version of the DCCS. During the preswitch phase, the preswitch rules are selected against competing alternative
rules, which are ignored. For example, based on the experimenter's instructions, children may select two
specific shape rules against two specific color rules. Then, in terms of this example, the activation level of the
shape rules is increased whereas the color rules are inhibited. Performance during the postswitch phase requires
that children overcome the inhibition of the values of the formerly irrelevant dimension and, at the same time,
deactivate the values of the formerly reinforced dimension.
The Negative Priming version is slightly (but not significantly) more difficult than the Partial Change version,
and to the extent that these versions provide pure measures of inhibition and activation, respectively, this may
indicate that the ignored rules are inhibited to a greater extent than the selected rules are activated. However,
there are good reasons to believe that these are not pure measures. New values are introduced in the postswitch
phase and these are likely to attract attention because attention to novelty appears to be a basic design feature of
the human attentional system (e.g., Desimone & Duncan, 1995). Attention to novel values would undermine
switching in the Negative Priming version because the new values introduced in the postswitch phase need to be
ignored. In contrast, attention to novel values would facilitate switching in the Partial Change version because
the new values are precisely those to which children need to attend. This difference may explain why the
Negative Priming version is more difficult than the Partial Change version. However, further empirical research
will be required to assess the relative influences of inhibition, activation, and novelty, and whether these
influences change with age. Further research will also be required to map out whether these influences interact,
and, if so, how.
Experiments 8 and 9 also raise a number of important questions about the processes underlying negative
priming. The findings from Experiment 9 suggest that negative priming in the DCCS is only operative when
children must select a pair of rules against a competing alternative. Negative priming thus appears to reflect
mechanisms that play an instrumental, inhibitory role in the selection of relevant rules in the presence of
competing distractors (see also Levy & Anderson, 2002). It is possible that in other tasks and/or at different
ages negative priming may occur in the absence of conflicting information (see Milliken et al., 1998), but this