The mammalian superior colliculus (SC) is regarded as a mid-brain center for visual orientation behaviors. The SC includes two important layers; the superficial layer (SGS) receives visual inputs and the intermediate layer (SGI) sends motor outputs, both of which have mutual connections. Although it is generally understood that the pathway from the SGS to SGI is involved in visuomotor transformation for orientation behaviors (Sparks 1986), there are little evidence in support of the role of this pathway. Recently interlaminar excitation was reported in the pathway from SGI to SGS using optical imaging (Vokoun et al. 2010). The functional properties of the pathway, however, is not yet clear. In this study, we focused on the opposite pathway (from the SGI to the SGS), which also produced interlaminar excitation (Vokoun et al. 2010). We also confirmed the existence of the interlaminar excitation from SGI to SGS with some developmental considerations (Morita et al. 2014). In this study, we attempted to evaluate the excitability of the pathway from SGS to SGI based on the level of excitation in the pathway from SGI to SGS. We applied a single pulse of electrical stimulation (500 μA) within either SGS or SGI, and observed spread of activation. We compared the responses between these two neural pathways; the excitation in the pathway from SGS to SGI was clearly weaker than that from SGI to SGS even when near-by SGS area was strongly activated by SGS stimulation. In addition, SGS response appeared 4 –5 ms SGI after stimulation.
These results suggest that the pathway from SGS to SGI is normally suppressed probably because the SC can control generation of too much reflexive action, giving chance of involvement of higher cognitive areas in visual orientation behaviors.
This asymmetric response should be also useful to avoid uncontrollable oscillation of mutual excitatory connections between SGS and SGI that forms positive feedback loop, although the functional role of the pathway from SGI to SGS is still unknown.