Understanding neural system dynamics through task modulation and measurement of functional MRI amplitude, latency, and width

Abstract

Estimates of hemodynamic amplitude, delay, and width were combined to investigate system dynamics involved in lexical decision making. Subjects performed a lexical decision task using word and nonword stimuli rotated 0 degrees, 60 degrees, or 120 degrees. Averaged hemodynamic responses to repeated stimulation were fit to a Gamma-variate function convolved with a heavyside function of varying onset and duration to estimate each voxel's activation delay and width. Consistent with prolonged reaction times for the rotated stimuli and nonwords, the motor cortex showed delayed hemodynamic onset for both conditions. Language areas such as the lingual gyrus, middle temporal gyrus, fusiform gyrus, and precuneus all showed delayed hemodynamic onsets to rotated stimuli but not to nonword stimuli. The inferior frontal gyrus showed both increased onset latency for rotated stimuli and a wider hemodynamic response to nonwords, consistent with prolonged processing in this area during the lexical decision task. Phonological processing areas such as superior temporal and angular gyrus showed no delay or width difference for rotated stimuli. These results suggest that phonological routes but not semantic routes to the lexicon can proceed regardless of stimulus orientation. This study demonstrates the utility of estimating hemodynamic delay and width in addition to amplitude allowing for more quantitative measures of brain function such as mental chronometry.

Figure 1

Mean reaction time for each condition. Error bars depict the SEM.

Figure 2

Warm colors (red/yellow) are areas where words are more than nonwords. Cool colors (blues) are areas where nonwords are more than words. The left hemisphere is toward the left margin.

Figure 3

Warm colors (red/yellow) are areas where nonrotated stimuli are more than rotated. Cool colors (blues) are areas where rotated stimuli are more than nonrotated. The left hemisphere is toward the left margin.

Figure 4

Graphs depicting the estimated hemodynamic impulse response functions for both the lexical (words vs. nonwords) and rotation effects (degree of rotation). The x axis depicts time in seconds, and the y axis depicts percent signal change. Error bars represent the SEM, and the left hemisphere is toward the left margin.