Masked repetition priming is often greater when a larger proportion of trials involve repetition primes, suggesting that a context-sensitive unconscious process may be operating. Two recent studies have failed to obtain an effect of prime proportion in the perceptual identification (PI) task, suggesting that the effect may not occur in nonspeeded tasks. Contrary to this possibility, we report proportion effects with masked repetition primes in two nonspeeded tasks: PI and fragment completion (FC). A proportion effect occurred in the accuracy measure only in the FC task, but it occurred in the reaction time measure in both tasks. Prior failures to find proportion effects in the PI task thus may have been due, in part, to the dependent measure used. We interpret our findings in light of several recent accounts of prime proportion effects with brief primes.
The masked priming paradigm is commonly used to evaluate how unconscious stimuli affect responses to consciously perceived targets. A short-duration prime (e.g., horse for 45 msec) is preceded by a premask (e.g., XXXXX for 495 msec) and is followed by a target (e.g., horse) to which the subject responds. Using this arrangement, Forster and Davis (1984) found that masked repetition primes (e.g., horse-horse) facilitated lexical decisions to word targets, relative to an unrelated-prime control condition (e.g., stair-horse). They suggested that this paradigm is useful for isolating the effects of "low-level" lexical processes from those of context-sensitive "high-level" processes (see also Forster, Mohan, & Hector, 2003).
Recent masked priming studies in which the proportion of repetition (vs. unrelated) primes in the stimulus list (i.e., RP) was manipulated appear to challenge this conclusion. If masking primes eliminates context-sensitive higher level processes, responses should not be sensitive to the list-wide RP. Contrary to this possibility, several lexical decision studies have reported that facilitation of reaction times (RTs) from repetition primes is greater when RP is high rather than low (e.g., Bodner & Masson, 2001; Bodner, Masson, & Richard, 2006; Bodner & Stalinski, 2008), and analogous proportion effects have now been obtained with a variety of stimuli in a variety of tasks (Bodner & Dypvik, 2005; Bodner & Masson, 2004; Jaskowski, Skalska, & Verleger, 2003; Kinoshita, Forster, & Mozer, 2008; Klapp, 2007).
These influences of prime proportion on masked priming indicate that either the process that produces masked priming or some aspect of the decision process is context sensitive. In Forster and Davis's (1984) account, masked priming reflects the automatic process of opening of a lexical entry; hence, proportion effects on masked priming must be attributed to the decision stage. Kinoshita and her colleagues (e.g., Kinoshita et al., 2008) have recently posited an adaptation to the statistics of the environment (ASE) account that builds on Lupker, Brown, and Colombo's (1997) suggestion that when performing a speeded task, subjects attempt to set a response deadline that will allow them to do so while minimizing their errors.
In the ASE account, the deadline established on a given trial is based on the system's attempt to minimize the combination of its estimated error rate and the cost it associates with taking longer to respond. The estimated error rate is based on a combination of the difficulty of target processing on the current trial and on recent trials. This error estimate decreases over time as decision evidence accumulates, and it decreases more rapidly for "easy" repetition-primed trials than for "hard" unrelated-primed trials. In contexts in which there are more easy trials than difficult trials, such as in a high-RP context, the estimated error rate for the easy trials decreases more rapidly than in a low-RP context, but it does not change much for the hard trials. This difference produces elevated priming in the high-RP group (see Kinoshita et al. …