A comparison of somatotopic organization in sensory neocortex of newborn kittens and adult cats

To determine the state of functional development in the newborn kitten's somatic sensory system, the organization of mechanoreceptive projections to the sensorimotor cortex was compared to that of the adult Cat. Microelectrode mapping procedures were used. Projections from all contralateral body surfaces to the primary somatomotor cortex (SmI) are present at birth and respond to mechanical stimulation of the receptors. The somatotopic organization of these projections in the newborn kitten is similar to that in the adult cortex with respect to the cortical region receiving projections from each part of the body and to the detailed arrangement of the projections within each of these cortical subdivisions. The relative sizes of peripheral receptive fields, and the intensity of stimulation effective for eliciting a response were similar for projections in SmI cortex of both kittens and adults. At both ages receptive field sizes decreased as their locations approached the distal portion of the limbs or rostral part of the face. In adults and newborns, over 75% of the neuronal responses were elicited by gentle bending of the hairs or light touch to the glabrous skin surfaces. Other similarities between adult and newborn sensorimotor cortexes included: (a) receptive fields of projections to SmI cortex were of fixed, local field type; (b) projections to SmII cortex responded to mechanical stimulation of the receptors; (c) ipsilateral as well as contralateral body surfaces were represented in SmII cortex; (d) the columnar arrangement of neurons and their receptive fields were apparent in the SmI cortex; (e) the coronal sulcus formed a division between the representations of the forepaw and face. Differences between newborn kittens and adult cats included: (a) shorter latency from electrical stimulation of the skin to a SmI cortical response in adults; (b) projections to SmI cortex having “disjunctive” receptive fields were not found in newborn kittens but existed in the adults; (c) the diversity of receptive field types found in neurons of the adult postcruciate MsI cortex was not found in newborn kittens; (d) newborn subjects displayed less variability in the somatotopic organization of projections and less overlap in the receptive fields of projections to SmI cortex. It is suggested that the SmI cortex develops as a point-to-point reflection of the distribution of mechanosensitive receptors in the body and that the complexities in this organization seen in the adult cortex occur during postnatal development.