The length summation properties of layer VI cells in the visual cortex and hypercomplex cell end zone inhibition

Layer VI of the visual cortex has been considered to be dominated by cells with very long receptive fields, typically summing to 8° or more. We have re-examined this issue in a series of experiments in which the length tuning profiles of layer VI cells in the cat visual cortex have been quantitatively determined. Responses were assessed to optimally oriented bars of light of varying length drifted over the receptive field. The lengths were varied on a randomised interleaved sequence. Although our data confirm the presence of long field cells in layer VI, only 24% of a population of 119 cells had fields greater than 6° in length. Fields greater than 8° were only seen in 17% of cells. 61% of the population of cells had fields showing summation to 4° or less with a mean length of 2.8° (+ / -0.15 sem). In this "short field" group, 18% had fields of 1° or less. We observed 7 cells with rapid initial spatial summation up to 1°, followed by clear end zone inhibition. It has been recently suggested on the basis of localised inactivation experiments, that layer VI cells with long (> 8°) fields may provide the drive to inhibitory interneurones in layer IV generating hypercomplex cell end zone inhibition. This observation is difficult to equate with evidence indicating that hypercomplex cell end zone inhibition reflects a mechanism showing maximal summation at lengths in the region of 2.8°. The present data show layer VI to possess cells capable of providing the drive for hypercomplex cell end zone inhibition in layer IV. © 1991 Springer-Verlag.