Influence of stimulus length on directional bias of complex cells in cat striate cortex.

Visual cortical cells respond optimally to an oriented bar moving either in one unique direction or in directions 180 degrees apart. Length-dependence of this direction selectivity was investigated in the striate cortex of lightly anaesthetized cats. Approaching half of all complex cells showed some lability in their direction selectivity. The incidence was highest in standard and intermediate (length-summating) complex cells, less amongst special complex cells (those with only localized summation) and least amongst end-stopped cells, especially those of the special category. By contrast, direction selectivity of simple cells was length-independent. No correlation between a cell's overt selectivity (i.e. bidirectional, direction-biased or direction-selective) and its lability with bar length or polarity of contrast (light/dark) was evident. Moreover, since individual neurons amongst a population of complex cells could exhibit either increase, decrease, or no change of direction selectivity with length, it is unlikely that length per se can be coded by direction-selectivity.