Information processing in the DG relies on the interactions between activity of excitatory and inhibitory neurons. Inhibitory neurons are not a homogeneous population. It is interesting to know how each subtype of inhibitory neurons control the activity of excitatory neurons. Traditional extracellular unit recording lacks the ability to identify neuronal types, despite its advantage in simultaneously detecting activity of multiple neurons. Here we used a transgenic mouse line that express Channelrhopsin-2 (ChR2), a light-activated ion channel, in a subtype of inhibitory neurons that also express parvalbumin (PV). By identifying their firing response to optical stimulation and firing during exploration, we were able to characterize the firing pattern of these neurons during spatial exploration of the animal. The result contributes to understanding how processing of spatial information is achieved in the dentate gyrus. Master of Science