Glutamatergic thalamopallial projections in the pigeon identified by retrograde labeling and expression of vGluT2 mRNA

A neocortical hypothesis as to homology of certain nuclear components of the avian brain proposes that the entopallium and field L2 are homologous to layer 4 of mammalian extrastriate and auditory neocortex, respectively. However, the hypothesis lacks support from the neurochemistry of thalamopallial projections. We investigated whether these projections are glutamatergic by injecting cholera toxin B into either the entopallium or field L2 in combination with in situ hybridization. Retrogradely labeled neurons in nucleus rotundus and nucleus ovoidalis were found to express vesicular glutamate transporter 2 mRNA, showing that the thalamopallial projections are glutamatergic. The results are consistent with the neocortical hypothesis.

Layer 4C in monkey V1 may linearize the output of the LGN

In primates, most LGN fibers terminate in cortical layer 4C, an anatomically prominent structure of unexplained function. We hypothesize that the enormous number of cells in layer 4C of monkey primate visual cortex functions as a neural network "hidden layer" that inverts distortions introduced by transmitting visual signals through the LGN. This hypothesis helps explain how simple cells respond (quasi-) linearly to visual inputs in spite of nonlinearities present in LGN responses. Linearization averts prematurely discarding visual information, in keeping with the role of primary visual cortex as the source of raw visual information to the rest of the brain.