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 representing stimulus selection, can represent stimulus salience and what is the mechanism by which the midbrain network computes salience. Here, two separate translational motion stimuli were adopted as principal stimulation protocols, and in vivo electrophysiological experiments were performed in pigeons' Imc. By combining bio-plausible model validation, we found two types of inhibitory surrounds of the Imc neuron receptive field, namely homologous inhibitory surrounds (HIS) and non-homologous inhibitory surrounds (non-HIS), and we expounded the mechanisms of their emergence. HIS is local and dependent on stimulus feature similarity for computing stimulus saliency, whereas non-HIS is global and independent of stimulus feature similarity for computing stimulus selection. Furthermore, the superimposition of HIS and non-HIS modulates the neural responses in Imc. The two identified inhibitory surrounds of Imc in this study more precisely explicate the entire process of realizing bottom-up stimulus selection based on saliency in the midbrain network and indicate that Imc is a nucleus capable of representing both stimulus saliency and stimulus selection.