Manifestation of some physiological responses (e.g., stress, mental workload, or a headache) may involve the emergence of asymmetric thermal patterns on the face. Thus, thermal measurements of the face that are indicative of thermal asymmetry can be useful to detect such physiological responses. In one embodiment, a system includes first and second inward-facing head-mounted thermal cameras (CAM1 and CAM2, respectively) that are located less than 15 cm from the user's face, which take thermal measurements of regions on the right and left sides of the face (THROI1 and THROI2, respectively) of the user. The symmetric overlapping between the regions on the right and left sides (ROI1 and ROI2, respectively) is above 60%, and CAM1 and CAM2 do not occlude ROI1 and ROI2. Optionally, the system includes a computer that detects a physiological response based on thermal asymmetry between THROI1 and THROI2.