Neuronal vs. Subjective Timing for a Conscious Sensory Experience

Abstract

In order to investigate a relation between conscious experience and specific kinds of neuronal activities, it is virtually necessary to study brain function intracranially in the awake, responsive human subject. Obviously, any such direct experimental investigation is subject to the severe limitations imposed by the rights of the subject and the ethical responsibility not to add unwarranted risks to the therapy. However, it has been possible to utilize gainfully the opportunities afforded by the surgical implantation of electrodes intracranially for therapeutic purposes^1,2, in informed and consenting human subjects.When electrode contacts are located in various parts of the cerebral somatosensory system, innocuous electrical stimulation procedures can be employed in a controlled fashion to manipulate and nvestigate neuronal function in a causative, rather than merely correlative, relationship to conscious sensory responses. A subdural stimulus to primary somatosensory cortex (S I) initiates an input different in its entry path and pattern from that generated by a peripheral sensory stimulus. Nevertheless, we have found it possible to elicit conscious sensory experiences with natural-like somatosensory qualities in most subjects by careful regulation of electrical stimulus parameters (particularly of intensity, train duration, and pulse frequency) to near-liminal values³,⁴,⁵. This is in contrast to the paresthesias more commonly reported in the pioneering studies of Penfield and others³.