Parietotemporal Stimulation Affects Acquisition of Novel Grapheme-Phoneme Mappings in Adult Readers

Abstract

Neuroimaging work from developmental and reading intervention research has suggested a cause of reading failure may be lack of engagement of parietotemporal cortex during initial acquisition of grapheme-phoneme (letter-sound) mappings. Parietotemporal activation increases following grapheme-phoneme learning and successful reading intervention. Further, stimulation of parietotemporal cortex improves reading skill in lower ability adults. However, it is unclear whether these improvements following stimulation are due to enhanced grapheme-phoneme mapping abilities. To test this hypothesis, we used transcranial direct current stimulation (tDCS) to manipulate parietotemporal function in adult readers as they learned a novel artificial orthography with new grapheme-phoneme mappings. Participants received real or sham stimulation to the left inferior parietal lobe (L IPL) for 20 min before training. They received explicit training over the course of 3 days on 10 novel words each day. Learning of the artificial orthography was assessed at a pre-training baseline session, the end of each of the three training sessions, an immediate post-training session and a delayed post-training session about 4 weeks after training. Stimulation interacted with baseline reading skill to affect learning of trained words and transfer to untrained words. Lower skill readers showed better acquisition, whereas higher skill readers showed worse acquisition, when training was paired with real stimulation, as compared to readers who received sham stimulation. However, readers of all skill levels showed better maintenance of trained material following parietotemporal stimulation, indicating a differential effect of stimulation on initial learning and consolidation. Overall, these results indicate that parietotemporal stimulation can enhance learning of new grapheme-phoneme relationships in readers with lower reading skill. Yet, while parietotemporal function is critical to new learning, its role in continued reading improvement likely changes as readers progress in skill.

Beyond Reading Modulation: Temporo-Parietal tDCS Alters Visuo-Spatial Attention and Motion Perception in Dyslexia

“…The possibility to modulate reading in individuals with and without dyslexia using transcranial electrical stimulation, such as tDCS, has been documented in the literature [ 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 ]. Nevertheless, the underlying neurocognitive mechanisms mediating reading changes induced by tDCS are still not well understood.…”

“…A number of studies have demonstrated the positive effect of tDCS on reading [ 54 , 55 , 56 ] and, particularly, in dyslexia [ 57 , 58 , 59 , 60 , 61 , 62 ]. In typically reading adults, an overall improvement in word reading efficiency has been observed after the application of left anodal/right cathodal tDCS over the temporo-parietal areas [ 54 ] as well as following left anodal stimulation over the inferior parietal lobule [ 55 , 56 ]. In adults with dyslexia, an enhancement in text reading fluency was demonstrated after left anodal stimulation over the visual extrastriate area—a region commonly implicated in the magnocellular system [ 57 ].…”

Dynamics of brain activation during learning of syllable-symbol paired associations

“…Additionally, the left occipitotemporal area, together with primary auditory and visual areas, showed increased activity during audio-visual learning (Tanabe et al, 2005). The important role of the posterior brain regions was also shown in a transcranial direct current stimulation study in which the learning rate and outcome of audio-visual associations was modulated by stimulation injected to left inferior parietal lobe (Younger & Booth, 2018). Furthermore, the frontal cortex has been suggested to mediate cross-modal learning, particularly in the case of arbitrary associations and sub-optimal presentation of cross-modal information, and to show activity change as a function of consistent audio-visual pairings (Gonzalo et al, 2000;Calvert, 2001;van Atteveldt et al, 2009).…”

“…Audio-visual processing engages specific cross-modal sites and primary sensory areas (e.g., Raij et al, 2000;van Atteveldt et al, 2004;Molholm et al, 2006;Kayser et al, 2009;Murray et al, 2015). The superior temporal sulcus in the left hemisphere has been implicated particularly in processing of well-established letterspeech sound combinations, thus mostly reflecting long-term audio-visual memory representations (Raij et al, 2000;van Atteveldt et al, 2004;Hashimoto & Sakai, 2004, Blomert, 2011. The fusiform gyrus in the left hemisphere is particularly involved in learning letters (Cohen et al, 2000) but is sensitive to long-term learning occurring over several months (e.g., Maurer et al, 2008;Brem et al, 2010).…”

“…Furthermore, the frontal cortex has been suggested to mediate cross-modal learning, particularly in the case of arbitrary associations and sub-optimal presentation of cross-modal information, and to show activity change as a function of consistent audio-visual pairings (Gonzalo et al, 2000;Calvert, 2001;van Atteveldt et al, 2009). Previous studies have also shown that overt attention can mask subtle effects of audiovisual processing that are observable only during implicit or passive presentation of the audio-visual material (van Atteveldt et al, 2004;Blau et al, 2009). In order to make the neural processes during learning of audio-visual associations observable at the cortical level, non-optimal presentation of the stimuli might therefore be needed (cf.…”