The VWFA: it's not just for words anymore

Abstract

Reading is an important but phylogenetically new skill. While neuroimaging studies have identified brain regions used in reading, it is unclear to what extent these regions become specialized for use predominantly in reading vs. other tasks. Over the past several years, our group has published three studies addressing this question, particularly focusing on whether the putative visual word form area (VWFA) is used predominantly in reading, or whether it is used more generally in a number of tasks. Our three studies utilize a range of neuroimaging techniques, including task based fMRI experiments, a seed based resting state functional connectivity (RSFC) experiment, and a network based RSFC experiment. Overall, our studies indicate that the VWFA is not used specifically or even predominantly for reading. Rather the VWFA is a general use region that has processing properties making it particularly useful for reading, though it continues to be used in any task that requires its general processing properties. Our network based RSFC analysis extends this finding to other regions typically thought to be used predominantly for reading. Here, we review these findings and describe how the three studies complement each other. Then, we argue that conceptualizing the VWFA as a brain region with specific processing characteristics rather than a brain region devoted to a specific stimulus class, allows us to better explain the activity seen in this region during a variety of tasks. Having this type of conceptualization not only provides a better understanding of the VWFA but also provides a framework for understanding other brain regions, as it affords an explanation of function that is in keeping with the long history of studying the brain in terms of the type of information processing performed (Posner, 1978).

Keywords: fMRI; occipito-temporal cortex; orthography; reading; resting-state fMRI; resting-state functional connectivity; resting-state networks; visual word form area.

Figure 1

There is more activity for Amharic character strings than letter strings in the left OT cortex. (A) Activity profile and location of region of the left OT cortex defined in a whole brain analysis of stimulus type. The location of the region closest to the VWFA is denoted with an arrow. The timecourses of BOLD activity for each stimulus type is shown for this region. (B) Timecourses of BOLD activity for each stimulus type in an region applied to the classic VWFA coordinates (coordinates in MNI, original Talaraich coordinates taken from Cohen and Dehaene, 2004). Figure adapted from Vogel et al. (2012b).

Figure 2

The left OT processes unfamiliar stimulus strings as individual characters and familiar strings as groups of characters. (A) Location of the left OT region defined in a whole brain pair type by timecourse analysis (−44, −67, −4 in MNI coordinates). (B) Reaction times and timecourses of BOLD activity for Amharic character pairs that are all the same, 2 character different hard pairs, and 4 character different easy pairs. The RTs and BOLD activity increase with the number of characters that must be evaluated to make a matching decision, indicating character by character processing. Asterisks denote RT values that have differences with p < 0.05. Though not shown, consonant strings show a similar pattern of both RTs and BOLD activity. (C) Reaction times and timecourses of BOLD activity for word pairs that are all the same, 2 character different hard pairs, and 3 character different easy pairs. The RTs and BOLD activity are equivalent for the all same and all different easy pairs, indicating these stimuli are evaluated as a group. Asterisks denote RT values that have differences with p < 0.05. Though not shown, pseudowords, which contain all legal letter combinations, show a similar pattern of both RTs and BOLD activity. Figure adapted from Vogel et al. (2012b).

Figure 3

The VWFA has stronger RSFC correlations with regions of the dorsal attention network than “reading related” regions. (A) Seed map of voxels with the strongest RSFC correlations to the putative VWFA, as defined in a meta-analysis of single word reading studies. Positive correlations in RSFC are shown in warm colors, negative correlations shown in cool colors. The location of regions of the dorsal attention network are shown in green and regions thought to be used predominantly in reading, as defined by a meta-analysis of region activated by single word reading as well as a review of the literature, shown in blue. (B) RSFC Correlation coefficients between the VWFA and regions thought to be used predominantly in reading, shown in blue, and regions of the dorsal attention network, shown in green. Figure from Vogel et al. (2012a).

Figure 4

Regions thought to be used predominantly in reading do not form a reading community in large scale network analyses. (A) The network organization of all regions activated during reading. Communities were defined using two separate algorithms, the results for Infomap shown on the left and Modularity Optimization shown on the right. In each case, communities were defined across a range of RSFC correlation thresholds. Each RSFC correlation threshold corresponds to a column in the color plot. Each individual region is denoted by a row. The classification of each region is shown as the color of the row in each threshold column. The overall classification of each region, made by visual inspection of the community classification across thresholds and methods is shown in the far left column. The rows containing the regions thought of as relatively specific for reading are denoted by arrowheads to the right of the plots. (B) The location of all of the regions activated in a meta-analysis of reading studies, colored by overall community assignment made in the leftmost column of (A). Regions thought to be relatively specifically used for reading are denoted by number. Figure from Vogel et al. (2013).