Journal ofNeurology, Neurosurgery, and Psychiatry 1982;45:975-978 Summation of rapid tactile stimuli in parietal lobe disease

SUMMARY The perception threshold for trains of rapid tactile pulses, applied to the index finger, has been measured in patients with parietal lobe lesions and in patients with median nerve lesions. The former patients had increased perception thresholds for single tactile pulses on the abnormal side. With successively prolonged pulse trains, the threshold decreased exponentially to reach a stable level after 150-400 ms. In contrast, the median nerve patients had increased perception thresholds for tactile pulses irrespective of pulse train duration. Lesions of the parietal lobe cause a disturbance of somatic sensation which mainly affects discriminative sensory functions, ' for example position sense, the ability to localise touch and the ability to identify objects by palpation, that is stereognosis. This contrasts with the primary sensory modalities which are far less impaired. The detection threshold for passive touch is often raised, both in man2 and in monkeys,3 but not to the extent that one sees afte