Whole-cell patch recording were made with dye-filled electrodes from layer IV in slices of the ferret striate cortex. Projections from the thalamus and layer VI provide most of the extralaminar input to layer IV. Interactions between these two pathways are thought to play a role in the generation of suppressive non-linearities such as end-inhibition. Thus, synaptic responses evoked by stimulating each pathway individually were compared with those produced by activating both projections together. 2. Spiny stellate cells are the majority population in layer IV and were the most frequently patched neurons (n = 23); all fired adapting trains of large, fast action potentials. About half of those tested (n = 13) were progressively inhibited by strengthening the stimulus to layer VI, while the rest became more excited. For the former, the response evoked by stimulating both pathways in coincidence was often more hyperpolarizing than would have been predicted by summing the responses to either projection alone (n = 4). Hence, the inputs from the thalamus and layer VI are integrated by circuits that can produce strong and non-linear inhibition. 3. Recordings from various basket cells, which are inhibitory, have provided a first view of the suppressive circuits in layer IV (n = 5). Two cells were excited by stimulation of both pathways. The remaining three cells were mainly excited by activation of thalamic afferents but were largely inhibited by stimulation of layer VI. Thus, inhibition seen at the level of the spiny stellate cells could result from two mechanisms operating via presynaptic smooth cells: convergent excitation provided by both ascending pathways on the one hand, and a push-pull relationship between pathways on the other.
