NON-LENGTH-TUNED CELLS IN LAYER-II III AND LAYER-IV OF THE VISUAL-CORTEX - THE EFFECT OF BLOCKADE OF LAYER-VI ON RESPONSES TO STIMULI OF DIFFERENT LENGTHS/

We have previously shown, using a local inactivation technique, that l ayer VI provides a facilitatory input to the majority of hypercomplex cells located in layer IV above, and hence to layers II/III, which in many cases enhances length selectivity. However, many cells in these l ayers are not tuned for stimulus length, being equally responsive to l ong and short stimuli. Thus it is important to know whether layer VI c an influence the responses of these cells. We have now used a similar paradigm of iontophoretic application of GABA to examine the effect of blockade of layer VI on the length tuning profiles of these cells in layers II-IV. During the blockade of layer VI, the most common effect, seen in 41% of the cells, was inhibition of visual responses, (i.e. c ommensurate with loss of a facilitatory input). An increase in respons e magnitude was found in 21% of the population, and responses were una ffected in 36% of cells tested. This suggests that the predominant inf luence of local regions of layer VI on this cell type, located in laye rs II/III and IV, is facilitatory, with a smaller proportion of cells receiving an inhibitory input. Such effects were seen even with the sh ortest lengths tested, suggesting once more that elements of layer VI are responsive to stimuli much shorter than was previously accepted. T hus these data suggest that layer VI plays a role in the generation of the response dynamics of non-length-tuned cells in overlying layers I I/III and IV.