Repetition enhancement and perceptual processing of visual word form

Abstract

The current study investigated the cerebral basis of word perceptual repetition priming with fMRI during a letter detection task that manipulated the familiarity of perceptual word form and the number of repetitions. Some neuroimaging studies have reported increases, instead of decreases, in brain activations (called "repetition enhancement") associated with repetition priming of unfamiliar stimuli which have been interpreted as the creation of new perceptual representations for unfamiliar items. According to this interpretation, several repetitions of unfamiliar items would then be necessary for the repetition priming to occur, a hypothesis not explicitly tested in prior studies. In the present study, using a letter detection task on briefly flashed words, we explored the effect of familiarity on brain response for word visual perceptual priming using both words with usual (i.e., familiar) and unusual (i.e., unfamiliar) font, presented up to four times for stimuli with unusual font. This allows potential changes in the brain responses for unfamiliar items to be assessed over several repetitions, i.e., repetition enhancement to suppression. Our results reveal significant increases of activity in the bilateral occipital areas related to repetition of words in both familiar and unfamiliar conditions. Our findings support the sharpening hypothesis, showing a lack of cerebral economy with repetition when the task requires the processing of all word features, whatever the familiarity of the material, and emphasize the influence of the nature of stimuli processing on its neuronal manifestation.

Keywords: fMRI; priming; repetition enhancement; repetition suppression; sharpening; visual processing.

Figure 1

Design of the study. Subjects were instructed to detect whether the letter “A” was into the word. Unprimed items, corresponding to the 1st presentation of the word (U1 and UN1), and target items, corresponding to following presentation(s) of repeated words (U2, UN2, UN3, and UN4) were mixed together into each run. The familiarity of word perceptual form was manipulated using an usual (U) font (Abadi MT Condensed Light) and an unusual (UN) font (Matisse ITC). Presentations of words were preceded by a fixation cross (variable duration) and followed by a mask. The words in usual font were repeated twice (U1 and U2) while the words in unusual font were repeated four times (UN1, UN2, UN3, and UN4).

Figure 2

Brain render with Anatomist (www.brainvisa.info) for each priming condition with the analysis mask (for words with usual font) in yellow and corresponding graphs of the haemodynamic time course data for each priming condition. Green clusters represent the significant differences between the 2nd presentation of words in usual font (U2) and the 1st presentation of words in usual font (U1). The red cluster represents the significant differences between the 4th presentation of words in unusual font (UN4) and the 1st presentation of words in unusual font (UN1). The blue cluster shows the regions significantly correlated with the number of presentations of words in unusual font (UN). All clusters coordinates are given in the MNI space. Note that fMRI statistical analyses were performed on beta estimators of the BOLD response and not the peak of the hemodynamic response function shown here for the purpose of illustration.