Operant conditioning of single unit activity in parietal cortex.

Two rhesus monkeys were trained to control firing patterns of single neurons in parietal cortex (areas 1, 2, 3, 5, 7) using an operant task previously applied to the study of precentral units. Twenty-four of 56 (43%) postcentral cells were controlled in contrast to 71 of 136 (52%) precentral units from these and 4 other rhesus monkeys. In addition, monkeys were able to drive precentral units to more sustained tonic firing rates than they could parietal units. An analysis of interspike interval (ISI) distributions showed that, in contrast to precentral units with modal ISIs of 25-50 ms, 50% of parietal units have modal ISIs of 2 ms. Such short ISIs may account for fewer postcentral units reaching control criteria for this particular operant task. Other factors that may contribute to the reduced control of postcentral cells are discussed, particularly the more complex afferent connections to parietal units when compared to precentral pyramidal tract neurons. The data indirectly support conclusions from previous studies that imply that operant control of cortical units is peripherally mediated and does not primarily involve a 'central' or 'open loop' system.

Comparison of percutaneous radiofrequency gangliolysis and microvascular decompression for the surgical management of tic douloureux.

Forty-two patients with tic douloureux underwent posterior fossa craniectomy and microvascular decompression (MVD) or partial rhizotomy of the trigeminal nerve and were followed an average of 25 months after operation. Thirty-six patients were found to have anatomical distortions of the nerve by an artery, vein, bony prominence, or a combination of factors, and 30 patients (83%) of this groups hav remained pain-free postoperatively. Six patients had no discernible pathological condition at the time of operation and underwent partial trigeminal rhizotomy. No patient underwent repeated MVD or rhizotomy, although 4 patients whose pain recurred after MVD underwent rhizotomy at a second operation. Eight of the 10 patients treated by rhizotomy are currently pain-free. The overall success rate of the entire group is 90%; 2% experienced a complication, and there was 1 perioperative death. Seventy-eight patients with tic douloureux who underwent 92 percutaneous radiofrequency trigeminal gangliolysis (PRTG) procedures were evaluated on average of 56 months postoperatively. Sixty-eight per cent of these patients when evaluated 1 year postoperatively were pain-free. However, only 35% of the PRTG procedures resulted in continued pain relief 5 years after operation. Twelve of the 78 patients (15%) required repeat gangliolysis because of recurrent tic pain. Considering all 78 patients treated with 92 PRTG procedures, 64% were pain-free at follow-up examination. PRTG was associated wtih an 8% risk of complications, which included anesthesia dolorosa, corneal anesthesia with keratitis, and significant facial paresthesias. Both PRTG and MVD have advantages. MVD should be considered because: (a) it attacks what is believed to be the primary etiology of tic douloureux, (b) the trigeminal nerve is preserved, (c) postoperative pain relief dose not depend upon the production of sensory deficit, and (d) it may have a greater potential for producing long-lasting pain relief. However, PRTG has other advantages: (a) it avoids the risks of craniectomy, (b) it is repeated easily if tic pain recurs, (c) morbidity is minimal and there is essentially no risk of mortality, and (d) it is much less expensive.

Intracranial pressure changes in brain-injured patients requiring positive end-expiratory pressure ventilation.

In 16 patients with severe head injury and 2 patients with subarachnoid hemorrhage, positive end-expiratory pressure (PEEP) ventilation was required to maintain adequate oxygenation. The effects of PEEP on intracranial pressure (ICP) were evaluated with respect to the volume-pressure response (VPR), an indicator of intracranial compliance, and the static lung compliance (CL). Based on these parameters: (a) All 11 patients with a normal VPR (less than 2 torr) had no significant change in ICP with PEEP therapy. (b) All 5 patients with abnormal VPR and a normal CL (greater than 30 ml/cm H2O) had significantly increased ICP or decreased cerebral perfusion pressure while on PEEP. (c) Two patients with both an abnormal VPR and a decreased CL had no significant change in their ICP with PEEP. (d) Significant elevations of ICP were also seen in 4 patients with abnormal VPRs concurrent with the rapid reduction or withdrawal of PEEP ventilation. Thus, the VPR is an accurate predictor of the effects of PEEP on the ICP. In addition, however, decreased lung compliance may buffer these effects in patients who have decreased intracranial compliance. We propose that ICP monitoring and intracranial compliance determination are necessary in the management of brain-injured patients requiring PEEP ventilation.