Development of functional connections between thalamic fibres and the visual cortex of the wallaby revealed by current source density analysis in vivo

Functional development of thalamic input to the cortex in anaesthetised wallaby pouch young between postnatal day 25 (P25) and P153 has been studied by electrical stimulation of the optic nerve, current source density (CSD) analysis, and histologic identification of recording sites. Conduction in the optic nerve was recorded prior to P39, by which time responses from the superior colliculus appeared. No evoked potential of cortical origin was recorded until P46, even though thalamic fibres grew into the cortical plate from P15. The first cortical synaptic responses were recorded at the margin of the subplate and the developing cortical plate, where cells that later comprise the adult layer 6 settle. At about P66, an additional short-latency, superficial response appeared, coinciding with the formation of layer 4. The deep response was retained in layer 6. Evoked activity in the presumed layer 4 was found progressively deeper in the cortex over the next few weeks, which would be expected from the addition of layer 3 above it. By P113, a new sink was added superficial in the cortex. Thalamocortical connections follow the same deep-to-superficial order in development as the cellular layers of the cortex. (C) 2000 Wiley-Liss, Inc.