A device for examining or treating living tissue by means of local heating of the tissue by absorbing electromagnetic radiation, with at least one radiation source emitting electromagnetic radiation, a control unit for controlling the irradiation parameters of the radiation source, and at least one FD-OCT apparatus with a light source emitting a measurement light for illuminating that tissue region in which the electromagnetic radiation is absorbed by the tissue, characterized by a computational unit for carrying out the following steps: determining the depth-resolved tissue velocity in the radiation direction of the measurement light at a predetermined measurement point of the tissue from the phase information from the FD-OCT interference light, integrating the established tissue velocity over time, differentiating the calculated time integral with respect to space, and displaying the spatial derivative as a function of space and time and/or feeding the spatial derivative as a function of space and time to an evaluation module and/or feeding the spatial derivative as a function of space and time to the control unit.
