Near infrared spectroscopy (NIRS) is now an established non-invasively clinical technique to detect changes in blood flow [1][2]. In recent studies, NIRS, is instead used to detect neuronal activities, consistent with evoked potentials, arising from changes in light scattering and diffusion related either to membrane potential variations and cell swelling [3]. These optical response, called fast NIRS, occurs with a latency time of an order of milliseconds after stimulus presentation. After the first studies performed with a continuous light system [4], we tested the ability to detect an optical response due to visual stimulation with a frequency modulated light source.