Making sense: Determining the parameter space of electrical brain stimulation.

Conscious sensory experiences are the bountiful subject of countless works of art, music, poetry, and other creative endeavors. The subject of how these experiences emerge is no less compelling. Perceptual qualia arise from the electrical activity of one (1) or, likelier, many (2) neurons in the brain. This observation has been most widely studied in visual cortex (3), where a number of papers have described the causal relationship between neural activity and perceptual behaviors, namely, detection and discrimination of electrically activating, spatially discrete neural populations. Topographic organization, as seen not only in the retinotopy of visual cortex but in the tonotopy of auditory cortex and the somatotopy of somatosensory cortex, renders all (especially primary) sensory areas potent model systems for determining the quantitative relationship between neural activity and perception. In select human subjects, where it is possible to ascertain directly the relationship between neuronal selectivity and sensory qualia, topography (or perhaps attendant columnar microstructure) appears to be critical. Rudimentary (single electrode and brief, unpatterned pulse train) stimulation of a spatially contiguous population of neurons generates percepts in areas with exquisite retinotopy, for example, but not in areas where visual representations become more spatially diffuse (4). Electrical stimulation of clustered representations of sensory space may be more readily interpretable by the experimental subject, because it more closely mimics the natural sensory activation of neurons (but refer to 5). Similarly, in primary somatosensory cortex, it is thought that detection and discrimination of electrical pulse trains is facilitated by its modular organization and is indistinguishable from mechanical sensory stimuli (6, 7). In areas downstream of primary sensory cortices, activity may naturally be more spatially and …