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 rep resent increasingly complex levels of analysis of the sensory message. At p resent 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 cr itical 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 nu clei, and are essentially similar to the long latency responses which can b e recorded from the cortex during sleep or anaesthesia. Lastly, a hypothesi s is proposed for the neuronal events in the somatosensory cortex which cul minate in a conscious perception. In this "RULER" model, the deep pyramidal neurones read out the sensory information which has been retained in the a pical dendrites of more superficial cells, and do so at the end of successi ve "time-chunks".