The subplate forms a transient circuit that is required for the development of axonal projections between thalamus and cerebral cortex. Subplate neurons are in a key, intermediate position to control the flow of information into the developing cortex when first spontaneous (prenatal) and then visual (postnatal) activity are present in visual cortex. The fact that ocular dominance columns (ODCs) emerge even before the onset of patterned visual experience, poses the question how subplate neurons influence the functional development of cortex. When subplate neurons are ablated after the arrival of thalamic inputs to layer 4 (P7-P10), ODCs do not form despite the robust presence of thalamic axons. In addition, functional orientation maps are disorganized and visual responses are weak (Kanold, Kara, Reid & Shatz, Abstr. SFN, 2001). Here we show by in vitro slice recordings and in vivo CSD measurements that such late ablation results in reduced efficacy of thalamocortical synaptic transmission 3 weeks later, whe...