Physiology of perception - Cortical stimulation and recording in humans

Objectives. 1) To determine the effect of stimulus train duration (TD) on s ensory perception using direct stimulation of somatosensory and visual cort ices. 2) To investigate the occurrence of evoked potentials in response to stimulation that is subthreshold for perception. Back ground: Studies of th e mechanisms of conscious perception using direct cortical stimulation and recording techniques are rare. The clinical necessity to implant subdural e lectrode grids in epilepsy patients undergoing evaluation for surgery offer s an opportunity to examine the role of stimulus parameters and evoked pote ntials in conscious perception. Methods: Subjects included epilepsy patient s with grids over somatosensory or occipital cortex. Single pulses (100 mic roseconds) and stimulus trains were applied to electrodes, and thresholds f or perception were found. Evoked potentials were recorded in response to pe ripheral stimulation at intensities at, above, and below sensory threshold. Results: During cortical stimulation, sensory threshold changed little for stimulus trains of 250 milliseconds and longer, but increased sharply as T D decreased below this level. Primary evoked activity was recorded in respo nse to peripheral stimulations that were subthreshold for conscious percept ion. Conclusions: The results confirm a previous report of the effects of s timulus TD on sensory threshold. However, no motor responses occurred follo wing somatosensory stimulation with short trains, as previously reported. T he TD threshold pattern was similar in visual cortex. In agreement with the previous report, early components of the primary evoked response were not correlated with conscious sensory awareness.