Previous research has indicated that tympanic membrane temperature (TMT) is inversely correlated with cerebral blood flow and change in cerebral blood flow is a neurological measure of mental workload. We investigated whether TMT changed as a function of time on task and type of picture exposure (nature vs. urban) when participants performed two sessions of the original Sustained Attention to Response Task (SART: Robertson et al., 1997) that were separated by exposure to the pictures. Tympanic membrane temperature was recorded for both ears before and after each SART. We found a trend for declining temperature with time in the left ear and not the right ear. This finding is inconsistent with previous research that associated right-hemisphere dominance for vigilance tasks and suggested that SART is not a vigilance task. Further, the temperature asymmetries correlated with SART performance, which is in accordance with research by Helton (2010).