Semantic Priming 459associative and learning priming would occur, whereas at
longer lags, only learning priming would occur.
In summary, if priming at long lags holds up under addi-
tional experimental scrutiny, and if the paradoxical results
obtained by Joordens, Becker, and their colleagues can be
explained, long-term priming will provide compelling evi-
dence in support of distributed-network learning models and
virtually insurmountable evidence against other models of
semantic priming.
Forward Versus Backward Priming
Associations between primes and targets can be asymmetric.
Backward priming refers to the situation in which the associ-
ation from prime to target is weak, but the association from
target to prime is strong (e.g., baby-stork). Koriat (1981) was
the first to investigate backward priming, and he obtained
equal amounts of priming in the forward (e.g., stork-baby)
and the backward (e.g., baby-stork) directions. This result is
surprising, because if priming depends on strength of associ-
ation, it should be larger in the forward than in the backward
direction. A perusal of the backward priming literature re-
veals the following observations and findings.
One of the difficulties in comparing results across studies
is that different materials have been used. Several studies
have used asymmetrically associated, semantically related
primes and targets (e.g.,lamp-light,apple-fruit); other studies
have used semantically unrelated compound words (e.g.,
fruit-fly, sand-box); and still others have used a mixture of
these types of stimuli. Given the findings reviewed earlier on
pure semantic priming, one would expect to find some prim-
ing in the forward and in the backward directions for semanti-
cally related pairs, regardless of differences in associative
strength. In contrast, one wonders why priming would occur
at all for semantically unrelated compounds, unless it is strate-
gically mediated. Of the 20 compounds introduced into the lit-
erature by Seidenberg et al. (1984), and subsequently used by
Shelton and Martin (1992) and by Thompson-Schill, Kurtz,
and Gabrieli (1998), 18 prime words appear in the Nelson,
McEvoy, and Schreiber (1991) free-association norms. The
associative strength in the forward direction (e.g.,fruit-fly)
has a modal value of 0 and a mean of .02! These items are
therefore neither semantically related nor associated.
In fact, there is good evidence that forward and backward
priming for compounds is produced by strategic processes.
Priming does not occur in either direction for compounds
when conditions are consistent with automatic priming
(Shelton & Martin, 1992; Thompson-Schill et al., 1998). In
strategic conditions, processing of forward and backward as-
sociations is correlated with physiological indices of strategic
processing (Chwilla, Hagoort, & Brown, 1998). Those stud-
ies reporting reliable priming for compounds in the forward
or the backward directions (Kahan, Neely, & Forsythe, 1999;
Seidenberg et al., 1984; Shelton & Martin, 1992) employed
task parameters in the strategic regime (e.g., high RP, high
NR, or long SOA). We include in this mix experiments using
the naming task, as there is evidence that naming is not im-
mune to strategic processing (e.g., Keefe & Neely, 1990).
Asymmetrically associated, semantically related pairs
(e.g.,lamp-light) seem to prime each other in both directions,
and there is weak evidence that priming is greater in the for-
ward than in the backward direction. Only three published
studies have examined priming in both directions for seman-
tically related pairs (Chwilla et al., 1998; Koriat, 1981;
Thompson-Schill et al., 1998), and only one of them has used
procedures consistent with automatic priming (Thompson-
Schill et al.). Across all three studies, priming was 40%
larger, on the average, in the forward than in the backward
direction; in the experiments by Thompson-Schill et al., the
difference was approximately 30%.
Finally, there is evidence that backward priming in the
naming task may depend on the SOA (Kahan et al., 1999;
Peterson & Simpson, 1989). For example, Kahan et al. ob-
tained backward priming at an SOA of 150 ms but not at an
SOA of 500 ms. These findings are difficult to interpret, how-
ever, because the experiments have used RPs of at least .5. In
fact, only one experiment (Thompson-Schill et al., 1998) has
examined backward priming in naming under near automatic
conditions, and it only used one SOA (200 ms). Hence, the ap-
parent dependence of backward priming in naming on SOA
may be produced by strategic processing in this task.
In summary, semantic priming does not seem to occur for
compounds unless the conditions are ripe for strategic pro-
cessing, whereas priming occurs in both directions for asym-
metrically associated, semantically related pairs, and there is
some evidence that the magnitude of priming tracks associa-
tive strength. Because only the latter results seem to be
caused by automatic processes, only they are crucial for test-
ing models of priming.
Spreading activation models can account for these results
as long as appropriate semantic and associative relations
exist in memory (e.g., symmetric semantic relations but
asymmetric associative strengths). The predictions of com-
pound-cue models depend on which model of memory serves
as the base. The two models that have figured most promi-
nently in investigations of priming are SAM (Gillund &
Shiffrin, 1984) and TODAM (Murdock, 1982). SAM cannot
predict asymmetric priming unless the primes and the targets
differ in the strength of the association between the words
as cues and their representations in memory. TODAM also