The response characteristics of single-units in the nucleus of the basal optic root (nBOR) of the pigeon accessory optic system (AOS) were investigated using standard extracellular techniques. The receptive fields (RFs) were large (20-115 degrees long) and elliptical and were found throughout the contralateral visual field with the exception of the red field. The RFs did not have inhibitory surrounds and there was no evidence of retinotopic organization. Most neurons responded to small moving spots although the optimal stimulus was wholefield motion of a particular direction. Analysis of 166 single-units showed that neurons preferring upward, downward and backward (nasal to temporal) motion were equally abundant (32.5, 32.5 and 31% respectively), while less than 5% preferred forward (temporal to nasal) motion. Mapping studies demonstrated that UP units were located in the dorsal portion of the nucleus; DOWN units were found ventral to UP units; BACK units were found along the ventral surface of the nucleus; and FORWARD units were found in the posterior-dorsolateral margin of the nucleus. Most cells were excited by wholefield motion in the preferred direction and inhibited by motion approximately 180 degrees in the opposite direction, however, some cells lacked the excitatory component while other lacked the inhibitory component. Neurons were grouped into six categories based on the relative contributions of excitation and inhibition. These results are compared with investigations of the AOS of other vertebrates.