The world of touch--from evoked potentials to conscious perception.

Microelectrode recordings have enabled several maps of the body surface to be recognized in the mammalian somatosensory cortex. The maps appear to represent increasingly complex levels of analysis of the sensory message. At present the prevailing opinion is that the different components of the ERPs (event related potentials) represent sequential steps in such an analysis, and such an interpretation is supported by the enhancement of the ERPs when attention is paid to a somatic stimulus. However, there are a number of critical observations which are inconsistent with this view and suggest that the ERP enhancement may be an epiphenomenon. An alternative explanation for the ERPs is that they reflect discharges from the non-specific thalamic nuclei, and are essentially similar to the long latency responses which can be recorded from the cortex during sleep or anaesthesia. Lastly, a hypothesis is proposed for the neuronal events in the somatosensory cortex which culminate in a conscious perception. In this "RULER" model, the deep pyramidal neurones read out the sensory information which has been retained in the apical dendrites of more superficial cells, and do so at the end of successive "time-chunks".