The ascending sensory projections from the dorsal thalamus to the pallium are one of the most distinctive features of the amniote brain. In mammals, these projections can be broadly classified into two main types: (1) those arising from "core" neurons, mostly located in the central region of sensory nuclei, which relay topographic inputs to the intermediate layers of primary sensory cortices, forming a reciprocal circuit with the reticular thalamic nucleus (RTN); and (2) those originating from "matrix" neurons, located in the periphery of sensory nuclei, which project to superficial layers of primary and higher order cortices. To date, both the reciprocal core-RTN circuitry and the matrix projections have only been described in mammals. In this study, we re-examined the organization of thalamic projections to the visual dorsal ventricular ridge (DVR) in pigeons using neural tracing techniques. In addition to the well-known projection from the thalamic nucleus rotundus (Rt) to the ventral layer of the visual DVR (entopallium), we described a reciprocal projection pattern between the Rt and the RTN. Moreover, we identified a second ascending thalamopallial projection, arising from neurons surrounding the Rt and terminating primarily in the intermediate layer of the visual DVR. These neurons form a continuous "matrix" of cells within the dorsal thalamus, which as a group also give rise to axons targeting several associative pallial areas. Furthermore, we found that this "matrix" region receives descending inputs from the motor arcopallium. Collectively, these findings indicate that the avian thalamopallial system is more complex than previously recognized and thus more comparable to its mammalian counterpart.
Fernández et al. (Wed,) studied this question.