Abstract
Optokinetic responses function to maintain retinal image stabilization by minimizing optic flow that occurs during self-motion. The hovering ability of hummingbirds is an extreme example of this behaviour. Optokinetic responses are mediated by direction-selective neurons with large receptive fields in the accessory optic system (AOS) and pretectum. Recent studies in hummingbirds showed that, compared to other bird species, (i) the pretectal nucleus lentiformis mesencephali (LM) is hypertrophied, (ii) LM has a unique distribution of direction preferences, and (iii) LM neurons are more tightly tuned to stimulus velocity. In this study, we sought to determine if there are concomitant changes in the nucleus of the basal optic root (nBOR) of the AOS. We recorded the visual response properties of nBOR neurons to largefield drifting random dot patterns and sine wave gratings in Anna's hummingbirds and zebra finches and compared these with archival data from pigeons. We found no differences with respect to the distribution of direction preferences: Neurons responsive to upwards, downwards and nasal-to-temporal motion were equally represented in all three species, and neurons responsive to temporal-to-nasal motion were rare or absent (<5%). Compared to zebra finches and pigeons, however, hummingbird nBOR neurons were more tightly tuned to stimulus velocity of random dot stimuli. Moreover, in response to drifting gratings, hummingbird nBOR neurons are more tightly tuned in the spatio-temporal domain. These results, in combination with specialization in LM, supports a hypothesis that hummingbirds have evolved to be "optic flow specialist" to cope with the optomotor demands of sustained hovering flight.