In two experiments, the effect of the duration (40, 80 and 120 ms) of pattern masked prime words on subsequent target word processing was measured using event-related potentials. In Experiment 1, target words were either repetitions of the prior masked prime (car-CAR) or were another unrelated word (job-CAR). In Experiment 2, primes and targets were either semantically related (cap-HAT) or were unrelated (car-HAT). Unrelated target words produced larger N400s than did repeated (Exp 1) or semantically related (Exp 2) words across the different prime durations and these N400 priming effects tended to be smaller overall for semantic than repetition priming. Moreover, there was only a modest decline in the size of N400 repetition priming at the shortest prime durations, and there was no relationship between this N400 effect and a measure of prime categorization performance. However, the size of semantic priming at the shortest durations was relatively smaller than at longer durations and was correlated with prime categorization performance. The findings are discussed in the context of the functional significance of the N400 as well as a model that argues for different mechanisms during masked repetition and semantic priming.

Keywords: ERPs, N400, Masked priming, Semantic priming, Repetition priming

If I understand correctly, you are not interested in the prime processing as such but in the influence the prime has during target processing. What the answer above hints at is that you might want to code the type of prime (condition) that was presented before the target as a fixed factor in your repeated measures ANOVA. If you then find a main effect of condition or interactions with the factor condition, this should give you an indication of how different primes influence target processing. Hope this helps.

Thanks for your response. However, I focus more on a technique factor.

If the epoch is locked to the onset of the prime, the ERP caused by the target is confonded more or less. Is it right?

If the epoch is locked to the onset of the target, the baseline is not optional.

Then, my question is what is the better choice if I am not interested in the prime processing but in the influence the prime has during target processing.

As I understand it, the issue is as follows: you are interested in the effect of the prime on the target, not the response to the prime itself. Therefore the better choice is to lock the epoch to the target onset. However, the prime may influence the baseline from which the target ERP arises, therefore a measured amplitude difference may be due to the baseline shift rather than the change in processing of the target caused by the prime. It might be a good idea to assess the effect of the prime alone at time t=0 for the target onset, but not present the target. If the baseline returns to zero, then there is no worry. If there is some off response to the prime that extends into the time region of the target ERP then the difference wave could be computed by subtraction. If your design is comparing the difference in response to valid and invalid cueing, again it should be possible to subtract out the ERP to the prime itself.

Dear Michael Wright,

My understanding of your idea is that the epoch should be locked to the prime onset, then subtracting the ERP of prime display only ("A" or "B") from the ERP of both prime and target display ("A-B" or "B-B"). The final substracted ERP results ("A-B"-"A" or "B-B"-"B") would reflect the pure processing of the target and the effect of the prime on the targe. Because of the same target processing in two conditions, the difference ERP, if existed, indicates the pure effect of the prime on the targe. Have I understood correctly?

Dear Shen Tu,

I didn't get from your question wheter the time interval between prime and target is constant or not.

If it is constant it actually doesn't matter what time point you are locked to. The only difference in this case could be in the absolute values of peaks' amplitudes - if the baseline is also locked to the same time. However, more common way to deal with amplitudes is to calculate them peak-to-peak, and if you add the same number of microvolts to both, the difference is unchanged.

And if the interval between prime and target varies, you certainly have to lock your ERP to the target (as you study reaction to target), otherwise your target-ralated ERPs will be "smeared".

Dear Evgeny Levin,

Whether the peak-to-peak analysis is to caculate the difference between the maximum amplitude of the peak that I focus on (For exaple, posive at 300ms) and the maximum amplitude of the last peak (For exaple, negative at 200ms). Have I understood correctly? Thank you.

supplement:

Maybe the difference between the maximum amplitude of the peak that I focus on (For exaple, posive at 300ms) and the maximum amplitude of the "next" peak (For exaple, negative at 400ms) is also right?

To my experience, peak-to-peak amplitude is usually measured in comparison with previous peak of opposite sign, i.e. as in your first variant.

But actually choosing any of time points to lock to, as well as use peak-to-peak amplitudes don't help in solving the problem of superimposing of prime-related activity on target-related ERP. This problem does especially matter if the primes differ significantly between each other. One variant to solve it is what Michael Wright suggested, but it requires changing the experiment design and could lengthen the test time.

Alternatively, you could just use proper statistical analysis design. Say, you have two types of primes (a, b) and two types of targets (A, B). In this case you have 4 conditions: aA, aB, bA and bB. And if you perform 2 (primes) by 2 (targets) two-way ANOVA, you will catch as the main effects of the targets and primes (if differences between the targets or between the primes are strong enough), as interactions between primes and targets. If you do so, it's not a problem using such measures as absoute values of peak amplitudes, or area under the curve etc. The only drawback is that in this case you have to have enough trials in each condition to calculate robust ERPs, and it could be a problem if there are many types of primes and/or targets. In this case Michael's variant could be preferrable.

Could I use the time interval before prime as the baseline, however at the same time, the epoch is locked to the onset of the target? I think this might get better results.

Yes, surely you can. Moreover, if you analyze not only ERPs but event-related oscillatory reactions, such as ERD/ERS or ERSP it would be the only correct variant .

I agree with the above responses suggesting time-locking to the target. In fact that is what we do and have done in the past (see e.g. Timmer & Schiller, 2012; Timmer et al., 2012; Timmer et al., in press a,b) and what is done in behavioural work using masked priming (see the mass of literature on visual word recognition and reading aloud). The baseline issue could be resolved if you use a backward mask, i.e. between the offset of the prime and the onset of the target, because you may use that period as your baseline. However, if you do that, you should be aware of the fact that the backward mask may in fact add to the processing time of your masked prime, so prime processing may be prolonged by the backward mask.

I strongly recommend doing both while varying the prime-target SOA. That way you can see which parts of the signal are invariant over time, or are time-locked to the prime or target. For example, in response priming we find that the earliest part of priming effects in pointing responses, keypress force, and LRPs is invariant when time-locked to the prime. This is a strong prediction of feedforward sweep accounts of priming (including diffusion and Poisson models), because it implies that there is a period of time where the prime alone controls the response (see our "rapid-chase theory of response priming", Schmidt et al., 2006, Vath & Schmidt, 2007).

.The current study further investigates masked repetition and semantic priming. Two modifications were made to the Holcomb et al. (2005) paradigm. First, semantic/associated and repetition trials were included within a single session. Holcomb et al (2005) used two separate experiments with two different groups of subjects for their semantic and repetition priming effects. It could be argued that different strategies due to the mix of repeated versus semantic pairs altered the pattern of priming across experiments. Second, the priming paradigm used by Holcomb et al (2005) had a long prime-target SOA of 500 ms. In the current study the interval between primes and targets was a short 70 ms. Using a short prime-target SOA, Holcomb and Grainger (2006; 2007) also found that the N400 component was larger to target words following unrelated primes than primes that were identical to (table-TABLE) or nearly identical to (teble-TABLE) the target. However, they also found repetition effects on earlier ERP components as well. The most interesting for the current study was a widely distributed ERP negativity that started around 170ms and peaked near 250 ms. Holcomb and Grainger referred to this as the N250 component, and argued that it was sensitive to the degree of orthographic overlap between the prime and target stimuli. They found that the amplitude of the N250 was proportional to the number of letters that the prime and target words had in common, being largest when primes and targets shared no letters, intermediate when prime and target shared several letters (partial repetition) and smallest when they shared all letters (full repetition).

The current experiment sought to determine if there are different target ERP effects produced by masked repetition and associative priming when the prime-target SOA was short. Participants were presented with pairs of items consisting of a brief pattern masked prime word and a subsequent clearly visible target word. Masked primes could be followed by a clearly visible target word that was either a repetition of the prime word (repeated targets), a semantic associate of the target (associated targets) or another unrelated word (unrelated targets). Primes were sandwiched between two pattern masks (forward and backward) and had a duration of 50 ms. Participants performed a semantic categorization task in which they were instructed to monitor all stimuli for occasional exemplars (probes) from the category of animal names, and to press a button when such items were detected. No response was required to the critical prime or target stimuli. One feature of this task is that it provides a measure of the effectiveness of the masking procedure by allowing subjects to respond on trials with animal names in the prime position.

The following predictions were made. First, following Holcomb and Grainger (2006) it was predicted that there would be an attenuation of both the N250 and N400 components to target words immediately preceded by identity masked primes as compared to target words preceded by an unrelated masked prime words. This is because the N250 is thought to be sensitive to processing at the interface between sub-lexical orthographic and lexical representations and the N400 either because of processing at the lexical form and semantic interface (Grainger & Holcomb, in press) or within the semantic system itself. Second, following Grossi (2006) it was predicted that there would be an attenuation of the N400 (but not the N250) to target words immediately preceded by semantically associated primes as compared to target words preceded by an unrelated masked prime words. N400 effects are predicted because of the semantic overlap between prime and target words for associated but not unrelated pairs. No N250 effects were predicted for the associatively related pairs because these items have no more overlap at the orthographic level of representation than unrelated items.