Temporal sequence of hemispheric network activation during semantic processing: a functional network connectivity analysis

Abstract

To explore the temporal sequence of, and the relationship between, the left and right hemispheres (LH and RH) during semantic memory (SM) processing we identified the neural networks involved in the performance of functional MRI semantic object retrieval task (SORT) using group independent component analysis (ICA) in 47 healthy individuals. SORT requires participants to determine whether word pairs describing object features combine to retrieve an object. Functional network connectivity (FNC) was used to assess the correlations between components' time courses. Results showed that semantic left and right hemisphere networks comprise two independent ICA components. The components' time courses were highly correlated and RH activation preceded that of the LH. Moreover, this correlation was significantly stronger in better vs. poorer performers of the SORT. These results indicate an early activation of the RH that is closely followed by activation of the LH, to facilitate performance during word retrieval from SM.

Figure 1. Schematic representation of the semantic object retrieval task (SORT)

The task includes 46 retrieval pairs (R, orange square) and 46 non-retrieval pairs (NR, green square). The words pairs are presented for 2.7 seconds with a fixation cross of 5.5 seconds in between them. Colors are presented for demonstration purposes only. During the actual run the word pairs are presented in the center of a screen without a frame.

Figure 2. Functional network connectivity map

The map includes the seven components (A–G) selected for the correlation and lag analysis. Each component was entered into a one-sample t-test in SPM2 and is thresholded at p<0.05 (corrected for multiple comparisons using the family wise error approach in SPM2). All lines show significant correlations within the entire sample (q_FDR<0.05) and the arrows represent the direction of the delay between two components. Black lines represent correlations equal or greater than r=0.2 while gray lines represent weaker correlations. The dashed line depicts a correlation where there was a significant difference in the correlation values of good and poor performers, such that good performers show greater correlation than poor performers (p=0.02). In all maps the left (L, in map G) and right (R, in map C) panels represent the left and right hemispheres, respectively.