The lengthwise organization of receptive fields has been studied quantitatively using an optimally oriented narrow (≤0.29°) light-bar stimulus to prepare length-response curves from 60 orientation-sensitive striate cells (simple, 16; hypercomplex I, 24; complex, 6; hypercomplex II, 10; atypical, 4) from 10 anesthetized (N2O/O2) and paralyzed cats. Unilateral length-response curves from 11 cells (hypercomplex I, 9; hypercomplex II, 2) showed that, in every case, end-zone inhibitory areas were present at both ends of the discharge region. For any given hypercomplex I cell the two inhibitory areas differ somewhat in the strength of their inhibition but the lenghts of the areas are closely similar, the mean difference for the nine cells being only 0.1°. Like bilateral curves, the unilateral curves show that cells with the shortest optimal stimulus length tend to have the strongest end-zone inhibition. Bilateral length-response curves give a reasonably good approximation to the lengthwise organization of cells in the simple and complex families. Length-response curves for the non-preferred direction of stimulus motion were studied in 56 cells. Of the 11 simple cells that responded sufficiently in the nonpreferred direction to prepare a length-response curve, 5 gave curves of the hypercomplex type. For these five cells, the strength of the apparent inhibition in the nonpreferred direction was relatively weak (mean 41%, range 23-63%) but the optimal stimulus length (mean 2.6°, range 1.6-4.4°) was much shorter than it was for the preferred direction (mean 5.3°). It seems unlikely that this inhibition is due to the presence of direction-sensitive inhibitory end zones, but may possibly be due to the greater summation at longer stimulus lengths of the inhibitory mechanisms responsible for the direction selectivity of the discharge region. The end-zone inhibitory regions of hypercomplex I cells are usually not directionally sensitive and have properties that are very similar in the two directions of stimulus motion. Only two of 14 hypercomplex I cells showed a simple-type length-response curve in the nonpreferred direction. Four cells had atypical length-response curves. With continued lengthening of the stimulus, two cells showed a late onset of inhibition after a clear response plateau had been achieved, and two cells showed a late recovery of responsiveness after a period of inhibition. For any given cell in the simple or complex families, the length-response curves were similar whether the stimulus was a moving or a stationary flashing bar. The lengths of the discharge region and of the end-zone inhibitory areas were the same whichever stimulus was used for the length-response curve. Interactive effects of stimulus length and orientation on the responses of a hypercomplex I cell showed that, like simple cells, the orientation tuning curve progressively sharpens as stimulus lengthens. With hypercomplex II cells, the sharpness of the orientation tuning curve is relatively unaffected by stimulus length. Cells with receptive-field centers near or over the vertical midline may have the boundaries of their fields extending up to 4.1° into the 'wrong' hemifield. The fields that extend over the midline appear not to be truncated or to have their organization altered in any way.
