In this study we present an experiment investigating the reconfiguration process elicited by the task switching paradigm in synaesthesia. We study the time course of the operations involved in the activation of photisms. In the experimental Group, four digit-color synaesthetes alternated between an odd-even task and a color task (to indicate the photism elicited by each digit). In both tasks, the target stimuli were numbers between 1 and 9 written in white. One of the control groups ran the same tasks but this time with colored numbers (Naïve Control Group). The results of these studies showed the expected pattern for the control group in the case of regular shift: a significant task switch cost with an abrupt offset and a cost reduction in long RSI. However for the experimental group, we found switch cost asymmetry in the short RSI and non-significant cost in the long RSI. A second control group performed exactly the same tasks as the experimental group (with white numbers as targets and a second imaginary color task) -Trained Control Group-. We found no cost for this second control group. This means that the cost of mental set reconfiguration between numbers (inducers) and their photisms (concurrent sensations) occurs, that there is a specific cost asymmetry (from photisms to inducers) and that this cost cannot be explained by associative learning. The results are discussed in terms of exogenous and endogenous components of mental set reconfiguration.