Techniques of operating an AR system include determining hand gestures formed by a user based on a sequence of two-dimensional images through skin of the user's wrist acquired from a near-infrared camera. Specifically, an image capture device disposed on a band worn around a user's wrist includes a source of electromagnetic radiation, e.g., light-emitting diodes in the infrared (IR) wavelength band that emit the radiation into the user's wrist and an IR detector which produces the sequence of two-dimensional images of a region within a dermal layer in the user's wrist. From this sequence, gesture detection circuitry determines values of a biological flow metric, e.g., a change in perfusion index (PI) between frames of the sequence, based on a trained model that generates the metric from the sequence. Finally, the gesture detection circuitry maps the values of the biological flow metric to specific hand/finger movements that determine a gesture.