Characteristics of biological substances, such as cerebral cortex matter, are sensed. According to an example embodiment, the present invention is directed to a negligibly-intrusive, multi-layer integrated circuit arrangement for monitoring activity of an area of a cerebral cortex that would normally be covered by an anatomical layer. The multi-layer integrated circuit arrangement includes an optics layer located outside the cerebral cortex area that includes an emitter and a detector. The optics layer is adapted for implantation in the anatomical layer and for sensing at least one brain-activity parameter. The multi-layered integrated circuit arrangement also includes a data-processing layer that includes a digital-processing circuit that is adapted for assimilating neural data in response to the optics layer sensing at least one brain-activity parameter.