Neuroanatomical Mapping of Projections from Superior Colliculus through the Mediodorsal Thalamic Nucleus to Frontal Orienting Field

Abstract

Decision making is a complex neurological process in which relevant multisensory information is transformed, in combination with prior knowledge, into meaningful information in the brain, which then dictates a particular external motor task (or lack thereof) and internal neural change. Psychophysical tasks have been developed to probe decision making, primarily in order to a) elucidate general models of decision making across several taxa, and b) to identify particular brain regions necessary for or involved in the process. These tasks have been well probed over several decades of experimentation. One model which is derived from such research is the accumulation of evidence model, which posits that incoming, relevant sensory information induces a biased random walk in the observer towards two or more possible decisions. The amount of evidence accumulated is thus directly proportional to how accurate the decision is, on average. Several brain regions have been identified which seem to play a pivotal role in decision making, and perhaps the accumulation of evidence. Of these areas, frontal eye field (FEF) (or the rat equivalent, frontal orienting field (FOF)) and superior colliculus (SC) seem to be highly important. A recent model of working memory in rats deduced that these areas most likely interact with one another to reinforce or inhibit neural subpopulations to arrive at a decision; however, only a descending, direct projection from FOF to SC has been elucidated in rats. A direct, disynaptic one, through the mediodorsal (MD) thalamic nucleus, has been observed in primates. Thus, this study was undertaken to identify whether this pathway exists in the rat. Further, the general afferents of FOF were elucidated due to the area’s recent delineation. Here, we show that FOF receives a wide range of sensory, motor, prefrontal, and limbic cortical and thalamic efferents. Further, a subpopulation of neurons was identified in MD which mediate the disynaptic pathway from SC through MD to FOF. It is concluded, then, that SC projects disynaptically to FOF and receives monosynaptic projections from FOF, supporting the aforementioned working memory model.