The lateral geniculate nucleus (LGN) of rabbits and rodents contains a population of neurons that show strong directional selectivity (DS) to visual stimulation. Although these LGN DS neurons are known to project to the primary visual cortex (V1), their synaptic targets, and their role in the synthesis of V1 receptive fields are unknown. Whereas in cats and primates, LGN neurons display little or no orientation and directional selectivity and these properties are thought to be largely synthesized within V1, in rabbits and rodents these properties could potentially be inherited, in part, from the LGN DS neurons. The proposed experiments will address this question, aiming to understand how LGN DS neurons contribute to the synthesis of the diverse receptive field seen in V1 neurons. This will be accomplished using two sets of complementary methods, in fully awake rabbits.
First (Aim 1), we will determine which layers of V1 receive a strong input from LGN DS neurons. We will do this using single-axon spike-triggered current source-density analysis, a method that provides a view of the laminar profile of the presynaptic (axonal) and monosynaptic local field potentials and currents generated by the spikes of single thalamocortical neurons within the topographically aligned region of recipient cortex.
Next (Aim 2), we will record the spike trains of both LGN DS neurons, and retinotopically aligned cortical neurons of different receptive field classes. We will determine which cortical neurons within the synaptic recipient zone of the LGN DS neuron receives synaptic input (using methods of extracellular cross-correlation), and how the directional preferences of the LGN DS neurons relate to the preferences of their synaptic targets. Direction and orientation selectivity are among the most salient response properties of visual cortical neurons and this project is aimed at understanding how these properties emerge.
The current work will have an important impact on our understanding of how complex properties seen in cortical neurons are synthesized and, more generally, it will contribute to our understanding of how motion is perceived by the mammalian visual system. A disruption in motion perception is associated numerous clinical disorders. A better understanding of the principals of motion perception will provide a basis for future clinical studies of related to mental health, visual and behavioral disorders.
