Simultaneous EEG and fMRI reveals stronger sensitivity to orthographic strings in the left occipito-temporal cortex of typical versus poor beginning readers

Abstract

The level of reading skills in children and adults is reflected in the strength of preferential neural activation to print. Such preferential activation appears in the N1 event-related potential (ERP) over the occipitotemporal scalp after around 150-250 ms and the corresponding blood oxygen level dependent (BOLD) signal in the ventral occipitotemporal (vOT) cortex. Here, orthography-sensitive (print vs. false font) processing was examined using simultaneous EEG-fMRI in 38 first grade children with poor and typical reading skills, and at varying familial risk for developmental dyslexia. Coarse orthographic sensitivity was observed as an increased activation to print in the N1 ERP and in the BOLD signal of individually varying vOT regions in 57% of beginning readers. Finer differentiation in processing orthographic strings (words vs. nonwords) further occurred in specific vOT clusters. Neither method alone showed robust differences in orthography-sensitive processing between typical and poor reading children. Importantly, using single-trial N1 ERP-informed fMRI analysis, we found differential modulation of the orthography-sensitive BOLD response in the left vOT for typical readers only. This result, thus, confirms subtle functional alterations in a brain structure known to be critical for fluent reading at the very beginning of reading instruction.

Keywords: Children; EEG-informed fMRI analyses; Reading; Simultaneous EEG-fMRI; Visual N1; Visual word form system.

Fig. 1

Visual one-back task with three conditions: blocks of words, nonwords and false font strings were presented pseudorandomly to the children. The children had to respond to immediate repetitions of items by a button press.

Fig. 2

Print sensitive locations: The masks (green, top left) to identify the individual print sensitive areas were defined as the cluster of functional activation over all three conditions (W, NW, FF) overlapping with the anatomically defined left fusiform gyrus (logical operation W > baseline OR NW > baseline OR FF > baseline AND FFG) and mirrored to the right hemisphere. Depicted are the individually identified clusters for the four contrasts words-false font strings, nonwords-false font strings, nonwords-words, and words-nonwords. Clusters of typical readers (yellow) and poor readers (light blue) are shown in the render view (left) and slice (right) view (z-coordinates -21, -18, -15). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

Fig. 3

Top row: ERP waveforms for all three conditions over LOT and ROT clusters for the whole group and the reading groups. Grey horizontal bars show the N1 interval (184–244 ms) defined by mean GFP peak (214 ms) ±30 ms. Bottom row: Mean N1 amplitudes per condition and group for LOT and ROT clusters shown in the topographical map. Error bars show 95% CI. ERP = event-related potential, LOT = left occipito-temporal, ROT = right occipito-temporal electrode clusters, CI = confidence interval.

Fig. 4

EEG-informed fMRI analysis in vOT ROIs. The single-trial N1 ERP mean values modulated the hemodynamic response in the left vOT stronger for words than for false font strings in typical readers. Error bars show 95% CI. The right pannel shows LOT and ROT electrode clusters (orange circles) and the a-priori-defined left and right vOT ROIs (light blue). vOT = ventral occipito-temporal, ERP = event-related potential, ROI = region of interest, LOT = left occipito-temporal, ROT = right occipito-temporal. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).