Social facilitation generally denotes increments in frequency / intensity of behaviors in the presence of conspecifics (Crawford 1939). In domestic chicks, foraging efforts are socially facilitated even without resource competition (Ogura & Matsushima 2011), suggesting that the facilitation could be separable from the behavioral processes responsible for reward gain and efforts.
As a step to reveal the neural substrates, we focused on the two major dopaminergic pathways, i.e., (1) ventral tegmental area (VTA) projecting to nucleus accumbens (NAc) and (2) substantia nigra (SN) projecting to medial and lateral striatum (MSt and LSt). Despite the functional dissociation reported in human (O'Doherty et al. 2004; but see Morris et al. 2006), few studies have been made in birds. We made localized electrolytic lesion or dopamine depletion in VTA / SN, and examined the effects on foraging effort and its social facilitation.
First of all, we revealed topography of the dopaminergic projections by tyrosine hydroxylase (TH) immunostaining. Microinfusion of 6-hydroxydopamine in SN caused impaired staining exclusively in LSt, MSt, arcopallium and globus pallidus. On the other hand, infusion that included VTA caused additional impairments in NAc core/shell, ventral pallidum and septum. Our results thus confirmed the homology of subpallial structures between birds and mammals (Metzger et al. 1996, Reiner et al. 2004).
Secondly, electrolytic lesion to MSt/NAc suppressed the basal foraging effort, whereas the social facilitation remained unimpaired. In contrast, electrolytic lesion to SN suppressed the social facilitation, but the activity in the single condition did not change. On the other hand, dopamine depletion had no effects on the running activities in both groups of chicks that received depletory injections in MSt/NAc and in SN. The neural substrates responsible for reward-based foraging effort could be doubly dissociated from those of the social facilitation.