460 Semantic Memory and Priming
has difficulty explaining asymmetric priming because associ-
ations are modeled by a commutative operation, convolution.
Hence, a demonstration of reliable asymmetric priming with
primes and targets of equal word frequency would be prob-
lematic for these models. All of the distributed network mod-
els predict priming for semantically related primes and
targets, but only two (Moss et al., 1994; Plaut & Booth, 2000)
have an associative mechanism that would allow them to pre-
dict greater priming in the forward than in the backward
associative direction.
Subliminal Priming
Several researchers have reported evidence that semantic
priming occurs even when the prime is presented under con-
ditions in which it cannot be identified or its presence cannot
be detected (e.g., Marcel, 1983). After conducting a compre-
hensive review of this literature, Holender (1986) concluded
that the effects were unreliable and that the stimuli had
probably been consciously identified (also see Cheesman &
Merikle, 1984). More recent studies addressed many of these
problems, but effects were still small and inconsistent (e.g.,
Greenwald, Klinger, & Schuh, 1995).
Greenwald and his colleagues (e.g., Draine & Greenwald,
1998; Greenwald, Draine, & Abrams, 1996) have recently
claimed that robust unconscious priming effects can be ob-
tained under the proper experimental conditions. An impor-
tant feature of these experiments is that they used evaluative
or gender judgments as the priming task rather than standard
semantic priming tasks (e.g., lexical decision or naming). For
example, in the evaluative judgment task, participants judged
whether words had positive or negative meanings (e.g., happy
vs.vomit). Priming was assessed by examining the effect of
the prime’s category membership (e.g., positive vs. negative)
on responses to targets. Greenwald and his colleagues found
that under appropriate conditions primes increased the proba-
bility of responding in a manner consistent with their category
membership even when direct perception of the primes ap-
proached zero sensitivity. They attributed this result to the un-
conscious activation of the meaning of the prime.
There are at least two reasons to question this conclusion,
however. First, Klinger, Burton, and Pitts (2000) replicated
the priming effects obtained by Greenwald and his colleagues
(e.g., Draine & Greenwald, 1998; Greenwald et al., 1996) but
also showed that semantic priming of the lion-tigervariety
did not occur in the same paradigm. Second, Abrams and
Greenwald (2000) have shown that the priming obtained in
the basic paradigm does not occur unless primes previously
occur as targets; the effect may be due to procedural learning
in the task (see the chapter in this volume by A. Johnson).
Our (admittedly conservative) conclusion is that there is little
or no convincing evidence that the meaning of a word can be
activated unconsciously.Prime Task EffectsGiven that priming occurs when participants read the prime
but make no response to it, one might predict that semantic
priming would occur regardless of the task performed on the
prime. In fact, this is not true. M. C. Smith, Theodor, and
Franklin (1983) showed that semantic priming was elimi-
nated if participants searched the prime for a letter or re-
sponded whether or not an asterisk was next to the prime
(also see Friedrich, Henik, & Tzelgov, 1991; Henik,
Friedrich, & Kellogg, 1983; Henik, Friedrich, Tzelgov, &
Tramer, 1994; M. C. Smith, 1979). A general conclusion
from these studies is that if attention is directed away from
the semantic level early in the processing of the prime, se-
mantic priming is eliminated or attenuated (e.g., Stolz &
Besner, 1996). In a related line of research, Besner and Stolz
(1999) demonstrated that Stroop interference was reduced in
magnitude if attention was directed to individual letters of a
word (rather than to the whole word).
These and related findings led Stolz and Besner (1999) to
conclude that attentional control is needed to activate the
meanings of words. They argue that attention determines
how activation is distributed across levels of representation
(e.g., letter, word, semantic) during word recognition. The
attentional mechanisms implied by this explanation are
qualitatively different from those implied by the traditional
distinction between automatic and strategic priming (as dis-
cussed in a previous section); in particular, they must be fast
acting and need not be conscious. Prime-task effects create
difficulties for all of the models of priming. The fundamental
problems are that the models do not cast the proper roles for
attention, or they do not distinguish between levels of repre-
sentation in a manner that would allow, for instance, attention
to be directed to one level (e.g., letter) but not to another (e.g.,
semantic), or both. These problems are not insurmountable,
but they are not trivial to solve either.
Neely and Kahan (2001) have recently argued that prime-
task effects may be caused, at least in part, by effects of spatial
attention on visual feature integration. The hypothesis is that
when attention is directed to individual components of prime
words, such as letters, the visual features of unattended letters
may not be properly integrated, and hence the primes may not
be perceptually encoded as words. Semantic activation of the
primes would not be expected under such circumstances. If