The RULER Model. Is This How the Somatosensory Cortex Works?

Despite a wealth of information, it is still not known how neurones in the different neocortical layers interact to produce a conscious perception. We now put forward a model for the somatosensory cortex in which a touch is perceived whenever superficial cortical pyramidal cells (in layers II and III) are made to discharge by a recurrent input from deep pyramidal neurones (in layer V). The superficial cells act as biological amplifiers and the number discharging will depend both on the strength of the message from the thalamus and on the variable background depolarisation of their apical dendrites. The recurrent volley arises in the layer V neurones at the end of an IPSP (inhibitory postsynaptic potential), which itself follows an excitatory response induced by the incoming thalamic signal; the IPSP is generated by local basket cells. The duration of the initial excitation--IPSP--late excitation sequence corresponds to a time chunk, that is, the period over which neural activity is integrated to produce a perception. During the time chunk, the superficial cortical pyramids, unlike the deeper ones, can accumulate information as subthreshold excitatory postsynaptic potentials (EPSPs). The relative time at which the information arrived in the cortex is roughly coded by the gradient of EPSPs among cells in an axis perpendicular to the cortical surface. Although developed for the somatosensory cortex, the basic features of the model may well apply to other sensory receiving areas of the cortex.