Left fusiform BOLD responses are inversely related to word-likeness in a one-back task

Abstract

Although its precise functional contribution to reading remains unclear, there is broad consensus that an activity in the left mid-fusiform gyrus is highly sensitive to written words and word-like stimuli. In the current study, we take advantage of a particularity of the Chinese writing system in order to manipulate word-likeness parametrically, from real characters, to pseudo-characters that vary in whether they contain phonological and semantic cues, to artificial stimuli with varying surface similarity to real characters. In a one-back task, BOLD activity in the left mid-fusiform was inversely related to word-likeness, such that the least activity was observed in response to real characters, and the greatest to artificial stimuli that violate the orthotactic constraints of the writing system. One possible explanation for this surprising result is that the short-term memory demands of the one-back task put more pressure on the visual system when other sources of information cannot be used to aid in detecting repeated stimuli. For real characters and, to a lesser extent for pseudo-characters, information about meaning and pronunciation can contribute to performance, whereas artificial stimuli are entirely dependent on visual information. Consistent with this view, functional connectivity analyses revealed a strong positive relationship between left mid-fusiform and other visual areas, whereas areas typically involved in phonological and semantic processing for text were negatively correlated with this region.

Figure 1

Examples of Chinese Components (A) and materials in current study (B). A) Components can contain only orthographic information (O), probabilistic information about semantics (S), or probabilistic information about phonology and semantics (PS). B) Combining two of three categories of components forms eight conditions varying in word-likeness: Real PS, OS, OO; Pseudo PS, OS, OO; RR and NN.

Figure 2

Behavioral performance on a lexical decision task collected outside the scanner (Panel A), and the one-back task performed during the imaging experiment (Panel B).

Figure 3

Different classes of stimuli (Real, Pseudo and Artifical) activated a very similar brain network as shown in (A) side-by-side comparisons of activation maps and (B) an overlap map.

Figure 4

Direct contrasts among three super-conditions stimuli (p < .05, corrected)

Figure 5

Beta weights for three regions (Angular Gyrus, AG; Superior/Middle Temporal Gyrus, STG/MTG; Supramarginal Gyrus, SMG) identified as "activated" in the Real > Artificial contrast depicted in Figure 4.

Figure 6

A) Beta weights and impulse response functions (as percent signal change) for three super-conditions (Real words, Pseudocharacters, and Artificial stimuli) in left fusiform. B) Beta weights for all eight conditions in the same region, * = p < .05, corrected.)

Figure 7

Analyses of the topography of sensitivity in left ventral temporal cortex following Vinckier et al., (2007). A) Gradient maps representing the proportion of activation of the least wordlike stimuli. B) Beta weights for three super-conditions varying in wordlikeness (from greatest to least) across six ROIs from anterior to posterior ventral temporal cortex.

Figure 8

Functional connectivity of left fusiform gyrus (p < .05, corrected).