Leilei Mei

Spaced learning enhances subsequent recognition memory by reducing neural repetition suppression

Spaced learning usually leads to better recognition memory as compared with massed learning, yet the underlying neural mechanisms remain elusive. One open question is whether the spacing effect is achieved by reducing neural repetition suppression. In this fMRI study, participants were scanned while intentionally memorizing 120 novel faces, half under the massed learning condition (i.e., four consecutive repetitions with jittered interstimulus interval) and the other half under the spaced learning condition (i.e., the four repetitions were interleaved). Recognition memory tests afterward revealed a significant spacing effect: Participants recognized more items learned under the spaced learning condition than under the massed learning condition. Successful face memory encoding was associated with stronger activation in the bilateral fusiform gyrus, which showed a significant repetition suppression effect modulated by subsequent memory status and spaced learning. Specifically, remembered faces showed smaller repetition suppression than forgotten faces under both learning conditions, and spaced learning significantly reduced repetition suppression. These results suggest that spaced learning enhances recognition memory by reducing neural repetition suppression.

The "visual word form area" is involved in successful memory encoding of both words and faces

Previous studies have identified the critical role of the left fusiform cortex in visual word form processing, learning, and memory. However, this so-called visual word form areas (VWFA) other functions are not clear. In this study, we used fMRI and the subsequent memory paradigm to examine whether the putative VWFA was involved in the processing and successful memory encoding of faces as well as words. Twenty-two native Chinese speakers were recruited to memorize the visual forms of faces and Chinese words. Episodic memory for the studied material was tested 3h after the scan with a recognition test. The fusiform face area (FFA) and the VWFA were functionally defined using separate localizer tasks. We found that, both within and across subjects, stronger activity in the VWFA was associated with better recognition memory of both words and faces. Furthermore, activation in the VWFA did not differ significantly during the encoding of faces and words. Our results revealed the important role of the so-called VWFA in face processing and memory and supported the view that the left mid-fusiform cortex plays a general role in the successful processing and memory of different types of visual objects (i.e., not limited to visual word forms).

Orthographic transparency modulates the functional asymmetry in the fusiform cortex: An artificial language training study

The laterality difference in the occipitotemporal region between Chinese (bilaterality) and alphabetic languages (left laterality) has been attributed to their difference in visual appearance. However, these languages also differ in orthographic transparency. To disentangle the effect of orthographic transparency from visual appearance, we trained subjects to read the same artificial script either as an alphabetic (i.e., transparent orthography) or a logographic (i.e., nontransparent orthography) language. Consistent with our previous results, both types of phonological training enhanced activations in the left fusiform gyrus. More interestingly, the laterality in the fusiform gyrus (especially the posterior region) was modulated by the orthographic transparency of the artificial script (more left-lateralized activation after alphabetic training than after logographic training). These results provide an alternative account (i.e., orthographic transparency) for the laterality difference between Chinese and alphabetic languages, and may have important implications for the role of the fusiform in reading.

Artificial language training reveals the neural substrates underlying addressed and assembled phonologies

Although behavioral and neuropsychological studies have suggested two distinct routes of phonological access, their neural substrates have not been clearly elucidated. Here, we designed an artificial language (based on Korean Hangul) that can be read either through addressed (i.e., whole word mapping) or assembled (i.e., grapheme-to-phoneme mapping) phonology. Two matched groups of native English-speaking participants were trained in one of the two conditions, one hour per day for eight days. Behavioral results showed that both groups correctly named more than 90% of the trained words after training. At the neural level, we found a clear dissociation of the neural pathways for addressed and assembled phonologies: There was greater involvement of the anterior cingulate cortex, posterior cingulate cortex, right orbital frontal cortex, angular gyrus and middle temporal gyrus for addressed phonology, but stronger activation in the left precentral gyrus/inferior frontal gyrus and supramarginal gyrus for assembled phonology. Furthermore, we found evidence supporting the strategy-shift hypothesis, which postulates that, with practice, reading strategy shifts from assembled to addressed phonology. Specifically, compared to untrained words, trained words in the assembled phonology group showed stronger activation in the addressed phonology network and less activation in the assembled phonology network. Our results provide clear brain-imaging evidence for the dual-route models of reading.

It's a word: Early electrophysiological response to the character likeness of pictographs

Using unfamiliar and meaningless pictographs that varied in their degree of similarity to Chinese characters, the current study tested whether the early electrophysiological response was modulated by character likeness. We measured P100 and N170 while 20 native Chinese speakers were viewing Chinese characters, drawings of objects, and pictographs. Comparisons across the three categories of stimuli showed that pictographs elicited a smaller N170 amplitude than did Chinese characters and a stronger N170 amplitude than did objects, but did not differ in the P100 amplitude from the other two categories. Within the category of pictographs, stimuli with a higher degree of character likeness elicited larger N170 amplitudes and shorter N170 peak latencies, and this effect was again not observed in P100. These results suggest that N170 is sensitive to visual stimulis character likeness even though they are unfamiliar pictographs with no meanings or sounds.

Neural predictors of auditory word learning

The present fMRI study aimed to identify neurofunctional predictors of auditory word learning. Twenty-four native Chinese speakers were trained to learn a logographic artificial language (LAL) for 2 weeks and their behavioral performance was recorded. Participants were also scanned before and after the training while performing a passive listening task. Results showed that, compared to ‘poor’ learners (those whose performance was below average during the training), ‘good’ (i.e. above-average) learners showed more activation in the left MTG/STS and less activation in the right IFG during the pretraining scan. These results confirmed the hypothesis that preexisting individual differences in neural activities can predict the efficiency in learning words in a new language.

Long-term experience with Chinese language shapes the fusiform asymmetry of English reading

Previous studies have suggested differential engagement of the bilateral fusiform gyrus in the processing of Chinese and English. The present study tested the possibility that long-term experience with Chinese language affects the fusiform laterality of English reading by comparing three samples: Chinese speakers, English speakers with Chinese experience, and English speakers without Chinese experience. We found that, when reading words in their respective native language, Chinese and English speakers without Chinese experience differed in functional laterality of the posterior fusiform region (right laterality for Chinese speakers, but left laterality for English speakers). More importantly, compared with English speakers without Chinese experience, English speakers with Chinese experience showed more recruitment of the right posterior fusiform cortex for English words and pseudowords, which is similar to how Chinese speakers processed Chinese. These results suggest that long-term experience with Chinese shapes the fusiform laterality of English reading and have important implications for our understanding of the cross-language influences in terms of neural organization and of the functions of different fusiform subregions in reading.

Language-general and -specific white matter microstructural bases for reading

In the past decade, several studies have investigated language-general and -specific brain regions for reading. However, very limited research has examined the white matter that connects these cortical regions. By using diffusion tensor imaging (DTI), the current study investigated the common and divergent relationship between white matter integrity indexed by fractional anisotropy (FA) and native language reading abilities in 89 Chinese and 93 English speakers. Conjunction analysis revealed that for both groups, reading ability was associated with the FA of seven white matter fiber bundles in two main anatomical locations in the left hemisphere: the dorsal corona radiate/corpus callosum/superior longitudinal fasciculus which might be for phonological access, and the ventral uncinate fasciculus/external capsule/inferior fronto-occipital fasciculus which might be for semantic processing. Contrast analysis showed that the FA of the left temporal part of superior longitudinal fasciculus contributed more to reading in English than in Chinese, which is consistent with the notion that this tract is involved in grapheme-to-phoneme conversion for alphabetic language reading. These results are the first evidence of language-general and -specific white matter microstructural bases for reading.

Learning to read words in a new language shapes the neural organization of the prior languages

Learning a new language entails interactions with one׳s prior language(s). Much research has shown how native language affects the cognitive and neural mechanisms of a new language, but little is known about whether and how learning a new language shapes the neural mechanisms of prior language(s). In two experiments in the current study, we used an artificial language training paradigm in combination with an fMRI to examine (1) the effects of different linguistic components (phonology and semantics) of a new language on the neural process of prior languages (i.e., native and second languages), and (2) whether such effects were modulated by the proficiency level in the new language. Results of Experiment 1 showed that when the training in a new language involved semantics (as opposed to only visual forms and phonology), neural activity during word reading in the native language (Chinese) was reduced in several reading-related regions, including the left pars opercularis, pars triangularis, bilateral inferior temporal gyrus, fusiform gyrus, and inferior occipital gyrus. Results of Experiment 2 replicated the results of Experiment 1 and further found that semantic training also affected neural activity during word reading in the subjects׳ second language (English). Furthermore, we found that the effects of the new language were modulated by the subjects׳ proficiency level in the new language. These results provide critical imaging evidence for the influence of learning to read words in a new language on word reading in native and second languages.

Native language experience shapes neural basis of addressed and assembled phonologies

Previous studies have suggested differential engagement of addressed and assembled phonologies in reading Chinese and alphabetic languages (e.g., English) and the modulatory role of native language in learning to read a second language. However, it is not clear whether native language experience shapes the neural mechanisms of addressed and assembled phonologies. To address this question, we trained native Chinese and native English speakers to read the same artificial language (based on Korean Hangul) either through addressed (i.e., whole-word mapping) or assembled (i.e., grapheme-to-phoneme mapping) phonology. We found that, for both native Chinese and native English speakers, addressed phonology relied on the regions in the ventral pathway, whereas assembled phonology depended on the regions in the dorsal pathway. More importantly, we found that the neural mechanisms of addressed and assembled phonologies were shaped by native language experience. Specifically, one key region for addressed phonology (i.e., the left middle temporal gyrus) showed greater activation for addressed phonology in native Chinese speakers, while one key region for assembled phonology (i.e., the left supramarginal gyrus) showed more activation for assembled phonology in native English speakers. These results provide direct neuroimaging evidence for the effect of native language experience on the neural mechanisms of phonological access in a new language and support the assimilation-accommodation hypothesis.