Reversing the Standard Neural Signature of the Word–Nonword Distinction

Abstract

The distinction between letter strings that form words and those that look and sound plausible but are not meaningful is a basic one. Decades of functional neuroimaging experiments have used this distinction to isolate the neural basis of lexical (word-level) semantics, associated with areas such as the middle temporal, angular, and posterior cingulate gyri that overlap the default-mode network. In two functional magnetic resonance imaging (fMRI) experiments, a different set of findings emerged when word stimuli were used that were less familiar (measured by word frequency) than those typically used. Instead of activating default-mode network areas often associated with semantic processing, words activated task-positive areas such as the inferior prefrontal cortex and supplementary motor area, along with multi-functional ventral occipito-temporal cortices related to reading, while nonwords activated default-mode areas previously associated with semantics. Effective connectivity analyses of fMRI data on less familiar words showed activation driven by task-positive and multi-functional reading-related areas, while highly familiar words showed bottom-up activation flow from occipito-temporal cortex. These findings suggest functional neuroimaging correlates of semantic processing are less stable than previously assumed, with factors such as word frequency influencing the balance between task-positive, reading-related, and default-mode networks. More generally, this suggests results of contrasts typically interpreted in terms of semantic content may be more influenced by factors related to task difficulty than is widely appreciated.

Awake fMRI Reveals Brain Regions for Novel Word Detection in Dogs

“…In humans, real words typically result in more activation than pseudowords, evoking activity in what has been called a general semantic network ( Binder et al, 2009 ). Although such activations are generally bilateral, they tend to lateralize more to the left and cluster around the angular gyrus, but that anatomical location is ill-defined and is sometimes called the TPJ or temporal-parietal-occipital cortex (TPO) ( Graves et al, 2017 ). It is clear that the nature of the task and the relative frequency of words and pseudowords can influence the relative activation to words and pseudowords.…”

Duality of Function: Activation for Meaningless Nonwords and Semantic Codes in the Same Brain Areas

“…1 with Fig. 2 A , a binarized version of nonword activations from Graves et al 2017 , their Fig. 1 A ).…”

“…But we also know that differences in task difficulty can influence results that depend on contrasting different task conditions. Indeed, a recent study demonstrated a new level of influence of task difficulty effects, where differences in task difficulty led to a reversal of the typical pattern of activation for contrasting meaningful words and meaningless nonwords ( Graves et al 2017 ). Areas that typically respond to semantic information were found to activate more for meaningless nonwords than meaningful words.…”

“…Recently, we attempted to replicate the Binder et al (2009) meta-analysis finding in a single study ( Graves et al 2017 ). It included two semantic contrasts that were well-represented in that meta-analysis: words – pseudowords (lexicality contrast), and high – low imageability words (imageability contrast).…”

“…The participants in that study also showed poorer performance, in terms of RT and accuracy, on the words than pseudowords (pronounceable but meaningless letter strings). This suggests that the unexpected activation of the TPN for words and DMN for pseudowords reflected differences in difficulty rather than semantic content ( Graves et al 2017 ). A major implication of this result is that activation in DMN areas for words that had been interpreted as reflecting semantic processing might instead reflect difficulty effects.…”