Why does licking a 9-Volt battery elicits a taste sensation? Can taste be stimulated by electrical stimulation?
It typically generates electrotactile sensations, like champagne bubbles. However, some subjects do report metallic taste sensations when using the device. In fact, Lawless et al. (2005)* investigated the electrically evoked metallic-taste in response to anodic stimulation of the tongue directly and confirmed the anecdotal reports obtained with the BrainpPort device that electric stimuli evoke metallic-taste sensations.
Physiologically, taste receptor cells sense certain compounds (salts, acids etc.) due to activation of receptor molecules on the cell's surface. In turn, activation of receptor molecules depolarizes the cell membrane which eventually lead to neurotransmitter release and activation of secondary neurons that transmit the signal up to the brain (Fig. 2). Electrical stimuli bypass the receptor molecules and directly depolarize the taste receptor cells.
Given the anatomy of the taste receptor system in the tongue (Fig. 2), I am inclined to believe that the taste receptors are stimulated directly, and not the secondary neurons, because they are situated in deeper layers.
Note that taste receptors do not sense electrons. Rather, larger ions or molecules may stimulate them and cause a depolarization of the receptor cell, in turn activating the secondary neuron via a chemical synapse.