Intraoperative monitoring (IOM) during spinal cord surgery are effectively obtained with percutaneous cortical stimulation. Ipsilateral MEPs have significantly larger amplitudes, and lower thresholds than contralateral MEPs. MEP onset latencies were not significantly different, suggesting transcallosal stimulation of the contralateral motor cortex resulted in ipsilateral MEPs as composites from early ipsilaterally conducted descending motor tracts (via corticoreticulospinal or corticopropriospinal pathways) and late transcallosally stimulated components. Cross scalp stimulation position is advocated to elicit larger amplitude MEPs when anaesthetic agents may cause suppression of cortical or spinal excitability. Although less focal in nature, cross scalp stimulation would be most suitable for infratentorial or spinal surgery. Mechanical compromise of ipsilaterally and contralaterally descending motor tracts suggest a critically large functional derangement enough to result in visible neurological deficits. Conversely, sole reduction of ipsilateral or contralateral MEP amplitudes may not have resulted in a significant dysfunction of the motor tracts to cause visible neurological deficit. Concurrent ipsilateral and contralateral MEP amplitude changes point to a definite surgical urgency. Ipsilateral MEPs are of value in reducing false positive observations during IOM. Comparison of IOM data between neurologically normal adolescent and degenerative scoliosis patients suggests inherent motor system dysfunction in the former. These novel findings will be discussed.