David Bourget (Western Ontario)
David Chalmers (ANU, NYU)
Rafael De Clercq
Jack Alan Reynolds
Learn more about PhilPapers
Consciousness and Cognition 13 (3):626-645 (2004)
Libet discovered that a substantial duration (> 0.5-1.0 s) of direct electrical stimulation of the surface of the somatosensory cortex at threshold currents is required before human subjects can report that a conscious somatosensory experience had occurred. Using a reaction time method we confirm that a similarly long stimulation duration at threshold currents is required for activation of elementary visual experiences (phosphenes) in human subjects following stimulation of the surface of the striate cortex. However, the reaction times for the subject to respond to the cessation of the visual experience after the end of electrical stimulation could be as brief as 225-242 ms. We also carried out extensive studies in cats under a variety of anesthetic conditions using the same electrodes and parameters of stimulation employed in the human studies to study the patterns of neuronal activity beneath the stimulating surface electrode. Whereas sufficiently strong currents can activate neurons within milliseconds, stimulating currents close to threshold activate sustained neural activity only after at least 350-500 ms. When currents are close to threshold, some neurons are inhibited for several hundreds of millisecond before the balance between inhibition and excitation shifts towards excitation. These results suggest that the prolonged latencies, i.e., latencies beyond 200-250 ms, for the emergence of conscious experience following direct cortical stimulation result from a delay in the sustained activation of underlying cortical neurons at threshold currents rather than being due to any unusually long duration in central processing time. Intracellular records from cortical neurological cells during repetitive electrical stimulation of the surface of the feline striate cortex demonstrate that such stimulation induces a profound depolarizing shift in membrane potential that may persist after each stimulus train. Such a depolarization is evidence that extracellular K+ concentrations have increased during electrical stimulation. Such an increase in extracellular K+ progressively increases cortical excitability until the threshold for sustained activation of cortical neurons is reached and then exceeded. Consequently, the long latency for threshold activation of cortical neurons depends upon a dynamically increasing cortical facilatory process that begins hundreds of milliseconds before there is sustained activation of such neurons. In some cases, this facilatory process must overcome an initial stimulus-induced inhibition before neuronal firing commences
|Keywords||*Brain Stimulation *Reaction Time *Stimulus Duration *Visual Cortex Visual Perception|
|Categories||categorize this paper)|
|Through your library||Configure|
References found in this work BETA
No references found.
Citations of this work BETA
No citations found.
Similar books and articles
Morten Overgaard, Jorgen Feldbaek Nielsen & Anders Fuglsang-Frederiksen (2004). A TMS Study of the Ventral Projections From V1 with Implications for the Finding of Neural Correlates of Consciousness. Brain and Cognition 54 (1):58-64.
Juha Silvanto, Nilli Lavie & Vincent Walsh (2005). Double Dissociation of V1 and V5/MT Activity in Visual Awareness. Cerebral Cortex 15 (11):1736-1741.
Michael Niedeggen, Petra Wichmann & Petra Stoerig (2001). Change Blindness and Time to Consciousness. European Journal of Neuroscience 14 (10):1719-1726.
R. Nijhawan & B. Khurana (2000). Conscious Registration of Continuous and Discrete Visual Events. In Thomas Metzinger (ed.), Neural Correlates of Consciousness. MIT Press.
Bernard J. Baars, Thomas Zoega Ramsoy & Steven Laureys (2003). Brain, Conscious Experience, and the Observing Self. Trends in Neurosciences 26 (12):671-5.
Daniel Alroy (1995). Inner Light. Synthese 104 (1):147-160.
Gilles Pourtois, Michael De Pretto, Claude-Alain Hauert & Patrik Vuilleumier (2006). Time Course of Brain Activity During Change Blindness and Change Awareness: Performance is Predicted by Neural Events Before Change Onset. Journal of Cognitive Neuroscience 18 (12):2108-2129.
Susan Pockett (2006). The Great Subjective Back-Referral Debate: Do Neural Responses Increase During a Train of Stimuli? Consciousness and Cognition 15 (3):551-559.
David A. Oakley & Patrick Haggard (2006). The Timing of Brain Events: Authors' Response to Libet's 'Reply'. Consciousness and Cognition 15 (3):548-550.
Added to index2009-01-28
Total downloads4 ( #198,532 of 1,088,404 )
Recent downloads (6 months)1 ( #69,601 of 1,088,404 )
How can I increase my downloads?