The Effects of Presentation Rate During Word and Pseudoword Reading: A Comparison of PET and fMRI

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Abstract
The effect of stimulus rate and its interaction with stimulus type on brain activity during reading was investigated using functional magnetic resonance imaging (fMRI). This (i) enabled the segregation of brain regions showing differential responses, (ii) identified the optimum experimental design parameters for maximizing sensitivity, and (iii) allowed us to evaluate further the sources of discrepancy between positron emission tomography (PET) and fMRI signals. The effect of visual word rate has already been investigated in a previous PET study. However, rate effects can be very different in PET and fMRI, as seen in previous studies of auditory word processing. In this work, we attempt to replicate rate-sensitive activations observed with PET using fMRI. Our objective was to characterize the discrepancies in regionally specific rate-sensitive effects between the two imaging modalities. Subjects were presented with words and pseudowords at varying rates while performing a silent reading task. The analysis specifically identified regions showing (i) an effect of stimulus rate on brain activity during reading; (ii) modulation of this effect by word type; and (iii) increased activity during reading relative to rest, but with no dependence on stimulus rate. The results identified similar effects of rate for words and pseudowords (no interactions between rate and word type reached significance). Irrespective of word type, strong positive linear effects of rate (i.e., activity increasing with rate) were detected in visual areas, right superior temporal gyrus, and bilateral precentral gyrus. These findings replicate the results of the previous PET study, confirming that activation in regions associated with visual processing and response generation increases with the number of stimuli. Likewise, we detected rate-independent effects reported in the previous PET study in bilateral anterior middle temporal, inferior frontal, and superior parietal regions. These results differentiate the functionally specific responses in rate-dependent and rate-independent areas. However, for negative effects of rate, fMRI did not replicate the effects seen in PET, suggesting some form of hemodynamic "rectification." The discussion focuses on differences between evoked rCBF and BOLD signals.