Table 1.

Covariates used in modeling fMRI data

	Covariate Models	Covariate Relates to Internal Clock
S1 onset	Mean effect of S1 stimulus onset	Temporal attention: nonspecific effects of orienting to the stimulus and attending to time independent of the duration during which a putative accumulator operates.
S1 duration	Mean centered effect of the S1 stimulus duration	Accumulator: neural response that is linearly related to the duration of the S1 stimulus, therefore accumulation variability uniquely explained by the duration being timed.
S2 onset	Mean effect of S2 stimulus onset	Nuisance variable: nonspecific effects of orienting which are unable to be dissociated from ITI related neural activity.
S2 duration	Mean centered effect of the S2 stimulus duration	Nuisance variable: undifferentiated neural response related to accumulation, comparison, decision uncertainty and motor preparation.
Difficulty	Mean accuracy across subjects for each timing trial	Comparator: neural response where intensity neural activity is related to the difficulty of the comparison.

Covariates used in modeling our fMRI data, and our interpretations of the activation patterns related to the covariates of interest. By design, the three covariates of interest, S1, S1 duration, and difficulty, were orthogonal to each other.