The anatomical basis of somaesthetic temporal discrimination in humans.

Somaesthetic temporal discrimination (STD) is the ability to perceive as separate two successive somaesthetic stimuli applied to the same or different parts of the body. Paired electrical stimuli were applied to the index finger, using different time-intervals, to study the STD threshold (STDT) in 84 normal subjects and 51 patients with focal cerebral lesions. Abnormal STDT values were found on the affected side of patients with a lesion of the primary somatosensory cortex, and internal capsulethalamus. Lesions which did not produce sensory impairment but caused abnormal STDT were located in the posterior parietal cortex, the head of the caudate nucleus, the putamen, the medial thalamus and the lenticular nucleus. Frontal, temporal and occipital cortex lesions did not produce any abnormality in the STDT, but one patient with a bilateral lesion of the supplementary motor area (SMA) had abnormal STDT. These results indicate that normal perception of two somaesthetic stimuli as separate in time depends not only upon the integrity of the somatosensory pathway and primary somaesthetic cortex, but also of the posterior parietal cortex, SMA and subcortical structures such as the striatum and thalamus.