Research ReportThe temporal dynamics of masked repetition picture priming effects: Manipulations of stimulus-onset asynchrony (SOA) and prime duration
Introduction
As we move through our environment, we rapidly recognize objects without any apparent effort. How we achieve this feat of object recognition with such ease has eluded researchers for decades, as the processes underlying recognition are not well understood. One way the mechanisms underlying object recognition have been studied is through priming. Priming consists of a visible presentation of a prime (word, picture, etc.) that is followed by a target that is either related to or a repetition of the prime or unrelated to the prime. The basic assumption is that presentation of the prime activates certain representations in memory and that the subsequent processing of the related or identical target item, which occurs shortly thereafter, benefits from the overlap of representations between the two items. In the case of unrelated primes and targets, there is little or no overlap in representations and therefore processing of target items is not facilitated. The typical behavioral effect observed in unmasked repetition priming is facilitation in reaction time or accuracy to target objects/word preceded by the same or related prime in comparison to targets preceded by unrelated or unrepeated primes.
Forster and Davis (1984) modified the standard priming paradigm and reduced the visibility of the prime by using a forward mask. They successfully used masked priming with words, where the visibility of the prime is limited, to show the same behavioral repetition effects observed with unmasked primes. The idea behind masked repetition priming is that the briefly presented prime followed by a pattern masked interrupts the processing of the prime prior to conscious identification; thus by examining subtle differences in target stimulus processing that immediately follows the masked prime that is either the same or different stimulus, it should be possible to examine the residual processing influences of the prime on the system. Because participants are not aware of the prime, effects of supraliminal processes are reduced in these masked priming behavioral studies (Forster et al., 2003) and masking has been shown to reduce recurrent processing between V1 and higher level cortical areas (Lamme et al., 2002).
Masked priming has been successfully combined with event-related potential (ERP) recordings to provide precise temporal information about the mechanisms involved in recognition (see Eddy et al., 2006, Holcomb and Grainger, 2006, Eddy and Holcomb, 2009). ERPs are ideal for examining the processes involved in masked priming because of their precise temporal resolution. ERPs provide an advantage over behavioral measures, as they give a measure of brain activity within milliseconds (ms) of stimulus presentation, whereas behavioral responses occur several hundred milliseconds after the presentation of the stimulus. Previous ERP experiments aimed at elucidating these neural mechanisms have examined recognition using masked and unmasked repetition priming with pictures.
There are fewer studies combining these two methodologies with pictures (see Eddy et al., 2006, Eddy and Holcomb, 2009) than with words; however a similar pattern of results has emerged. ERP studies investigating both masked and unmasked picture priming reveal a series of components that occur between 100 and 500 ms (Eddy and Holcomb, 2009, Eddy et al., 2006, McPherson and Holcomb, 1999, Holcomb and McPherson, 1994). Even using brief masked prime presentation, robust effects have been observed (Eddy et al., 2006). Masked priming has been widely used in the study of visual word recognition (see Forster et al., 2003, Grainger and Jacobs, 1999). These studies using a short prime duration and a short stimulus-onset asynchrony (SOA) have found a series of effects (N150/P150, N250, and N400) (e.g. see Holcomb and Grainger, 2006) reflecting a cascade of processes involved in word recognition. The earliest of these ERP components, the N150/P150 is thought to reflect the activation of a perceptual feature representation (N150/P150). Once initial processing of features occurs, a more form specific representation is activated, reflected by the N250 effect. The representation then becomes more abstract, activating semantic, meaning based representation (N400). Some studies only report N400 effects and not earlier ERP components (Misra and Holcomb, 2003, Holcomb et al., 2005). The main difference between studies that describe earlier effects (N150/P150; N250) and those in which only later effects are noted (N400) is the SOA between the prime and target: the experiments yielding only N400 effects were designed with longer SOAs, between 500 and 1000 ms, while those showing earlier effects had SOAs shorter than 500 ms (e.g., 70 ms).
To further explore the relationship between SOA and the series of word ERP components reported in the masked and unmasked priming literature, Holcomb and Grainger (2007) systematically examined the effects of SOA and prime duration on masked repetition priming ERP components. Overall, they found no effect of repetition priming at shorter prime durations (10 and 20 ms), but the typical cascade of visual word priming effects were observed at longer prime durations (30 and 40 ms). They suggest that only a minimal amount of prime exposure is necessary to start the cascade of processing that leads to earlier and later ERP components. Additionally, by manipulating the SOA (60, 180, 300, and 420 ms), they were able to examine how masked priming effects with words are affected by increasing the time interval between the prime and target presentation. They found N400 effects for all SOA manipulations; while, the N250 effect was only observed for the 60 and 180 ms SOAs. These observations suggest that the effect of early perceptual processing of word features fades quickly after the presentation of the prime.
There exists a similar disparity in the picture priming literature. Unmasked repetition priming studies that use longer SOAs (e.g., 500–1000 ms) and longer prime durations (e.g., 400 ms) (McPherson and Holcomb, 1999, Holcomb and McPherson, 1994), have found N300 and N400 effects. However, with masked repetition priming, effects begin as early as 100 ms after target stimulus presentation (N/P190) (Eddy et al., 2006, Eddy and Holcomb, 2009) and are followed by the previously observed N300 and N400 effects with a short SOA (110 ms) and prime duration (50 ms). The early effect (N/P190), not previously observed in unmasked priming, presents itself as an early anterior negativity that inverses in polarity in posterior cortices, occurring between 100 and 250 ms (N/P190; Eddy et al., 2006). This effect has also been observed when using masked priming with faces (Henson, Mouchlianitis, Matthews, and Kouider, 2008). Specifically, Henson et al. (2008) found a repetition effect between 100 and 150 ms with greyscale faces. This effect likely parallels the process observed in the N190/P190 effect with objects. In addition, the observation of this early effect with faces and objects is similar to the effect reported with masked word priming studies where word were either partial or full repetitions (e.g., Holcomb and Grainger, 2006). The next component typically observed with objects is the N300 effect, a middle level, object representation specific component, observed with both masked and unmasked repetition priming and may parallel the form specific N250 found with words. With pictures, the N300 is a greater negativity for unrepeated/unrelated pictures compared to repeated/related pictures and is found less reliably with masked priming (Eddy et al., 2006, McPherson and Holcomb, 1999, Holcomb and McPherson, 1994). We are describing this component as a “mid level” effect because it seems to parallel mapping of perceptual information to a more form specific percept in memory that matches the primed object best, however, it is still reliant upon the physical form of the object and not higher level abstract conceptual and semantic features of the object. This idea is consistent with models of object recognition such as Schendan and Kutas' (2007) where this second level representation allows for higher level cognitive processes to occur. The third component observed in these studies, the N400 is thought to reflect a general process mediated by semantics. This component is larger for unrepeated/unrelated pictures compared to repeated/related pictures in masked and unmasked conditions and reportedly occurs with both longer and shorter SOAs and prime durations (Eddy and Holcomb, 2009, Eddy et al., 2006, McPherson and Holcomb, 1999, Holcomb and McPherson, 1994).
The N/P190 effects are not observed with unmasked picture priming paradigms, leaving a question about the potential role of prime duration or SOA unanswered. In addition, reports that middle (N300) and later (N400) effects are observed with both masked and unmasked priming leaves unresolved the issue of whether the longer prime exposure or longer SOA is responsible for the discrepancy between masked and unmasked picture priming. Typically the N300 and N400 effects observed with unmasked primes are typically larger in magnitude than those observed with masked primes. These findings leave open the possibility that the prime duration or SOA modulates ERP effects observed in masked and unmasked repetition priming. In this study, we aim to examine how the duration of the prime and the SOA modulates these middle and late effects, as well as the early effect (N/P190) observed in rapid masked repetition priming.
We predict if longer exposure to the prime in the unmasked studies leads to larger amplitude N300 and N400 effects, then increasing the prime duration in a masked priming paradigm would result in larger amplitude N300 and N400 effects. Alternatively, if larger effects are observed in the unmasked studies due to more in-depth prime processing because of the longer prime–target SOA, then increasing the SOA between the prime and target in a masked priming paradigm, while keeping the prime duration constant at a short duration, should produce larger N300 and N400 effects. Experiments 1 and 2 are aimed at addressing these issues by systematically manipulating the SOA (Experiment 1) and prime duration (Experiment 2) while holding the other parameter constant.
The first experiment examined how manipulating the interval (110, 230, 350, and 470 ms) between the prime and target (SOA) affects ERP components while holding the prime duration constant (50 ms). Participants were presented with brief color picture primes (50 ms) that were forward and backward pattern masked and followed by a visible target picture for 300 ms (see Fig. 1). The target picture was either a repetition of the prime or completely unrelated to the prime. The SOA was manipulated by varying the duration of the backward mask (60, 180, 300, and 420 ms). Participants performed a semantic categorization task where they were asked to press a button to non-critical, occasional food pictures. The critical conditions for this experiment contained non-probe pictures.
By manipulating the SOA between the prime and target we were able to determine whether object processing reaches a more in-depth level with increased time between prime and target presentation. In particular, we hypothesized that the N300 and N400 effects should be larger in amplitude for longer SOAs if more in-depth processing of the prime occurs with an increased duration between the prime and target. It is also predicted that the N/P190 effect may be limited to only shorter prime–target SOA durations based on the results Holcomb and Grainger (2007) reported where the N250 effect dissipated with longer SOAs (sometime between 180 and 300 ms).
The second experiment manipulated the duration of the prime (30, 50, 70, and 90 ms) while keeping the SOA constant (110 ms). Participants were presented with brief color picture primes for either 30, 50, 70 or 90 ms that were forward and backward pattern masked and followed by a visible target picture for 300 ms. The target picture was either a repetition of the prime or completely unrelated to the prime. The SOA was held constant by varying the backward masked duration (80, 60, 40, and 20 ms). Like in Experiment 1, participants performed a semantic categorization task where they were asked to press a button to non-critical, occasional food pictures. The critical conditions for this experiment contained non-probe pictures.
For this experiment, it was predicted that if increasing prime duration leads to more in-depth processing, then larger N300 and N400 effects would be expected. However, if more time is necessary for elaborate processing to occur before presentation of the target, then larger effects are not predicted with increased prime duration when the SOA (110 ms) remains constant across conditions. Additionally, the N/P190 effect should remain intact across all prime durations, but may be larger for longer prime durations since more perceptual information processing may be occurring with longer exposures. However, because the SOA is sufficiently short enough between the prime and target, the N/P190 cannot be disrupted at the shorter prime durations.
Section snippets
Experiment 1 results
Experiment 1 manipulated the SOA between the prime and target while holding the prime duration constant. Participants performed a go no-go semantic categorization task where they pressed a button anytime a food item appeared. Since the prime duration was held constant at 50 ms for the four SOA manipulations the d′ values for probe items were collapsed across SOA. The average d′ for probe items in the prime position was 1.86 and the average d′ for probe food items appearing in the target position
General discussion
The results of Experiment 1 and 2 suggest that longer prime exposure leads to more in-depth processing as indexed by larger N400 effects rather than a longer time window between the prime and target to allow for more extensive processing. Effects of prime duration on later processing components, observed with a relatively short SOA (110 ms), further support the idea that more time between the prime and target does not lead to increased processing of the prime. However, since we limited the prime
Participants
Twenty-four volunteers (10 female, mean age = 21, range 20–27, SD = 1.65), all undergraduate students at Tufts University, were paid $20 to participate in this experiment. All were right-handed with normal or corrected to normal visual acuity.
Stimuli and procedure
Color photographs of 620 common objects taken from conventional views were displayed on a white background (each 256 × 256 pixels) on a 19-in. display (visual angle 2°) time-locked to the vertical refresh signal of the video card (100 Hz resolution). Each subject
Acknowledgments
This research was supported by grant number HD25889. The authors would like to thank Alexandra Cheetham, Tarun Sridharan and the students in Psychology 49 at Tufts University for their help in collecting data.
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