Neural mechanism underlying preview effects and masked priming effects in visual word processing

Abstract

Two classic experimental paradigms - masked repetition priming and the boundary paradigm - have played a pivotal role in understanding the process of visual word recognition. Traditionally, these paradigms have been employed by different communities of researchers, with their own long-standing research traditions. Nevertheless, a review of the literature suggests that the brain-electric correlates of word processing established with both paradigms may show interesting similarities, in particular with regard to the location, timing, and direction of N1 and N250 effects. However, as of yet, no direct comparison has been undertaken between the two paradigms. In the current study, we used combined eye-tracking/EEG to perform such a within-subject comparison using the same materials (single Chinese characters) as stimuli. To facilitate direct comparisons, we used a simplified version of the boundary paradigm - the single word boundary paradigm. Our results show the typical early repetition effects of N1 and N250 for both paradigms. However, repetition effects in N250 (i.e., a reduced negativity following identical-word primes/previews as compared to different-word primes/previews) were larger with the single word boundary paradigm than with masked priming. For N1 effects, repetition effects were similar across the two paradigms, showing a larger N1 after repetitions as compared to alternations. Therefore, the results indicate that at the neural level, a briefly presented and masked foveal prime produces qualitatively similar facilitatory effects on visual word recognition as a parafoveal preview before a single saccade, although such effects appear to be stronger in the latter case.

Keywords: Masked priming; N1; N250; Preview effect; Single word boundary paradigm.

Fig. 1

Schematic representation of a typical trial in the single word boundary paradigm (A) and the masked priming (B). The dashed line in the single word boundary paradigm indicates the invisible vertical boundary. Once the eyes cross this boundary, during the saccade, the preview is exchanged to the target. The figure shows an example of a trial in which the prime/preview was unrelated (left) and repeated/valid (right) to the target character, respectively. In this example, “術” means technique, “灣” means bay. All stimuli were presented in black on a white background

Fig. 2

(A) Waveforms at left occipital-temporal (LOT), right occipital-temporal (ROT) and central regions for each paradigm. Shading indicates the time windows used for the N1, N250 and N400 components. (B) Effect topographies (repeated minus unrelated) of the N1, N250 and N400 repetition effects for each paradigm. Black dots highlight the electrodes used to define the regions of interest (LOT, ROT and central regions), the white dots are the adjusted ROI for the N400 repetition effect for the single word boundary paradigm

Fig. 3

Sample-by-sample TANOVA for the single word boundary paradigm (left) and the masked priming paradigm (right) with global duration statistics. When using global duration statistics, the duration threshold is used to identify whether an effect is significant. The "duration threshold" denotes the minimum duration that sub-threshold p-values must surpass for an effect to be deemed significant in the overall analysis. This duration is derived from comparing results from various randomization runs. An effect is considered significant when its duration exceeds 95% of all durations obtained under the null hypothesis from random runs. This specific threshold is then utilized in the TANOVA analysis for the corresponding effect. For the masked priming paradigm, the duration threshold was identified to be 51 ms. For the single word boundary paradigm, the duration threshold was identified as 56 ms. The thresholds were then applied to the TANOVA plot, where significant periods longer than this estimated duration threshold are marked in green. The gray areas mark non-significant time points while the white areas mark periods of significant differences between ERP maps of different factor levels

Fig. 4

Scalp distribution of effects on the target character (repeated minus unrelated) averaged across consecutive 20-ms windows from 100 to 600 ms. Shown are results for the single word boundary paradigm (left panel) and the masked repetition priming paradigm (right panel)