The shape of the fMRI response in primary sensori-motor regions to varying rates of a simple reaction time task

ABSTRACTS

The Shape of the fMRI Response in Primary Sensori-Motor Regions to Varying Rates of a Simple Reaction Time Task E Zarahn, GK Aguirre, JA Detre, DC Alsop, M D'Esposito Departments of Neurology and Radiology, Universityof Pennsylvania, Philadelphia PA Introduction: The purpose of this experiment was to characterize the changes in time integrated fMRI signal intensity evoked over a range of motor response rates. These effects were studied in the primary sensorimotor strip, a brain region known to be involved in response execution and feedback. In particular, the form of the response (linear or not linear with respect to the motor response rate) was of interest. Method: 6 normal subjects (3 M, 3F and 1 subject scanned twice, thus 7 runs total) were scanned using gradient echo, echoplanar fMRI (1.5T, TR=2 sec, voxel size=3.75 x 3.75 x 5.00 mm, 16 axial slices, total scan time 5 min 20 sec). Subjects performed a simple reaction task in which they responded to a flashing white circle (300 ms/flash) with a button press with a consistent finger of their choice of their right hand. The average rates (with pseudorandom ISI's) of the circle flashes varied, with a particular rate for a 20 sec epoch. The average flash rates of 0, 91, .2, .4, .6, .8, 1, and 1.25 Hz were presented twice (once in each of two, 160 sec blocks) in pseudorandom order

To avoid biasing the analysis towards those regions which do respond in a linear manner with response rate, the lowest 4 and highest 4 rates were used to create a dichotomous 'on-off' reference function which was used to probe each subject's data. This reference function will detect signal changes monotonic with response rate. In each run a single, contiguous 3-D region comprised of voxels with an R>.2 (a high, arbitrary threshold) located either over the left central sulcus or on the posterior bank of the left precentral gyrus was chosen. The time series data for each region were averaged across 20 sec epochs (time integration), averaged across the two blocks, and normalized by subtracting out the mean signal and dividing by the slope of the best fit line against response rate (yielding a set of data with slope=l). This normalization does not introduce artifact into the shape of the fMRI response, but simply standardizes the scale of response across runs. The data were then averaged at each response rate across runs, and the shape of the fMRI response to motor response rate observed. Results: The time integrated BOLD signal responded linearly over the middle range of response rates (from .1 to .8 Hz) and also evidenced the non-linearities of a threshold (between 0 and .1 Hz) and saturation (.8 t o 1.25

Hz).

Conclusion: If the neural activity associated with each motor response is itself not independent of average response rate, then the non-linearities could be due in part to the system mediating the transfer of neural activity into motor responses. If, however, it is assumed that each motor response is associated with a certain amount of time integrated neural activity in the specified brain regions, independent of average rate of response, then these non-linearities could be attributed to the system mediating the transfer of neural activity into the BOLD response. Under this same assumption, the presence of the linear portion of the fMRI response would show that there is a range over which the time integrated fMRI signal is approximately a linear measure of time integrated neural activity

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Each point is the average across 7 scans of the normalized fMRI signal. The error bars denote -+ l SEM.. The actual values along the y-axis are not m e a n i n g f u l . Only the shape of the response is of interest9