Top‐down and bottom‐up influences on the left ventral occipito‐temporal cortex during visual word recognition: An analysis of effective connectivity

Abstract

The functional role of the left ventral occipito-temporal cortex (vOT) in visual word processing has been studied extensively. A prominent observation is higher activation for unfamiliar but pronounceable letter strings compared to regular words in this region. Some functional accounts have interpreted this finding as driven by top-down influences (e.g., Dehaene and Cohen [2011]: Trends Cogn Sci 15:254-262; Price and Devlin [2011]: Trends Cogn Sci 15:246-253), while others have suggested a difference in bottom-up processing (e.g., Glezer et al. [2009]: Neuron 62:199-204; Kronbichler et al. [2007]: J Cogn Neurosci 19:1584-1594). We used dynamic causal modeling for fMRI data to test bottom-up and top-down influences on the left vOT during visual processing of regular words and unfamiliar letter strings. Regular words (e.g., taxi) and unfamiliar letter strings of pseudohomophones (e.g., taksi) were presented in the context of a phonological lexical decision task (i.e., "Does the item sound like a word?"). We found no differences in top-down signaling, but a strong increase in bottom-up signaling from the occipital cortex to the left vOT for pseudohomophones compared to words. This finding can be linked to functional accounts which assume that the left vOT contains neurons tuned to complex orthographic features such as morphemes or words [e.g., Dehaene and Cohen [2011]: Trends Cogn Sci 15:254-262; Kronbichler et al. [2007]: J Cogn Neurosci 19:1584-1594]: For words, bottom-up signals converge onto a matching orthographic representation in the left vOT. For pseudohomophones, the propagated signals do not converge, but (partially) activate multiple orthographic word representations, reflected in increased effective connectivity.

Interactive activation and competition models and semantic context: From behavioral to brain data

“…During word recognition, in contrast, Jacobs and Grainger (1992) found that lowering the top-down excitation from the orthographic word layer does equally well account for human performance. When testing this hypothesis by connectivity analyses of brain regions, several dynamical causal modeling (DCM; Friston et al, 2003) studies show that the bottom-up excitation of the fusiform gyrus is stronger than its top-down excitation during word recognition (Richardson et al, 2011;Schurz et al, 2014). Therefore, they support the initial proposal of Jacobs and Grainger (1992), which was based on IAM simulations of behavioral data.…”

Uncovering phonological and orthographic selectivity across the reading network using fMRI-RA

“…This particular study also showed a positive correlation in brain activity between these two areas (the IFC and VWFA). Indeed, there is evidence for anatomical and functional connectivity between the VWFA and the IFG (Bitan et al, 2006; Koyama et al, 2010, 2011; Mechelli et al, 2005; Schurz et al, 2013), suggesting that tuning in these areas may be due to the neuronal connection between these two areas.…”

Neural correlates of the lexicality effect in children

“…Furthermore, their ROI analysis revealed the same pattern (pseudoword > word activation) along the entire left vOT. Studies with German speaking adolescents and adults, which incorporated the same phonological lexical decision task, also found greater activation for pseudohomophones as compared to words in the left anterior FG (MNI: x,y,z = -45,-48,-15/-18) (Kronbichler et al, 2007; Schurz et al, 2014). This effect was interpreted as indicating an increased demand on lexical search in this region for pseudohomophones.…”