Thermal image data of at least a region of a face of a person is provided. The thermal image data is transformed to blood flow rate data which includes a slope representative of the change of blood flow rate over time. The slope is compared to a slope threshold to determine a physiological state of the person. For example, it may be determined whether the person is deceptive or non-deceptive, e.g., deceptive with respect to an elicited response from the person.