Lateralized reward-related visual discrimination in the avian entopallium

In humans and many other animals, the two cerebral hemispheres are partly specialized for different functions. However, knowledge about the neuronal basis of lateralization is mostly lacking. The visual system of birds is an excellent model in which to investigate hemispheric asymmetries as birds show a pronounced left hemispheric advantage in the discrimination of various visual objects. In addition, visual input crosses at the optic chiasm and thus testing of each hemisphere is easily accomplished. We aimed to find a neuronal correlate for three hallmarks of visual lateralization in pigeons: first, the animals learn faster with the right eye-left hemisphere; second, they reach higher performance levels under this condition; third, visually guided behavior is mostly under left hemisphere control. To this end, we recorded from the left and right forebrain entopallium while the animals performed a colour discrimination task. We found that, even before learning, left entopallial neurons were more responsive to visual stimulation. Subsequent discrimination acquisition recruited more neuronal responses in the left entopallium and these cells showed a higher degree of differentiation between the rewarded and the unrewarded stimulus. Thus, differential left-right responses are already present, albeit to a modest degree, before learning. As soon as some cues are associated with reward, however, this asymmetry increases substantially and the higher discrimination ratio of the left hemispheric tectofugal pathway would not only contribute to a higher performance of this hemisphere but could thereby also result in a left hemispheric dominance over downstream motor structures via reward-associated feedback systems.