Fig. 1.

Differences in processing time are very difficult to distinguish from differences in intensity of activation. The thick black line reflects data generated for four trials that vary in processing times (as listed in the figure) through convolution with a standard SPM hemodynamic response function. The dashed gray line reflects the results of fitting a model to those data in which differences between trials are based on the intensity of the activation, using a fixed duration of 400 ms (with the fitted intensity for each trial presented as “beta” in the figure). The black and dashed gray lines are almost indistinguishable (difference between these functions is plotted in blue), highlighting the fact that the effects of processing time and activation intensity are for practical purposes indistinguishable, both resulting in similarly increased activation. Code to generate this figure is available from https://github.com/poldrack/rtmodel. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)