Superimposed inhibitory surrounds underlying Saliency-based Stimulus Selection in avian Midbrain isthmi pars magnocellularis

In the avian midbrain network, bottom-up spatial attention is directed by saliency-based stimulus selection. However, it remains unclear whether the isthmi pars magnocellularis (Imc), the first site in the midbrain network to represent stimulus selection, can represent stimulus salience and what is the mechanism by which the midbrain network computes salience. Here, we used two separate translational motion stimuli as the main stimulation protocols and conducted in vivo electrophysiological experiments in the pigeon’s Imc. By combining bio-plausible model validation, we found two types of inhibitory surrounds of the Imc neuron receptive field, homogenous inhibitory surrounds (HIS) and non-homogenous inhibitory surrounds (non-HIS), and elucidated the mechanism by which both arise. While HIS is local and depends on stimulus feature similarity, which can be used to compute stimulus saliency, non-HIS is global and doesn’t depend on stimulus feature similarity, which can be used to compute stimulus selection. Moreover, the superimposition of HIS and non-HIS modulates the neural response of Imc. The two inhibitory surrounds of Imc identified in this study more clearly elucidate the full process of achieving bottom-up stimulus selection based on saliency in the midbrain network and show that Imc is a nucleus that can represent both stimulus saliency and stimulus selection.