Functional Ultrasound Imaging of the Ferret Early Visual System

Wentao Hu

Supervised by Marvin Doyley

601 Computer Studies Building

Vision is one of the primary senses of humans and other animals. By transforming and decoding visual information, one can perform essential tasks, including environment perception, visual cognition, visual memory, etc. Good visual representation relies on functional organization and mechanisms in the visual system. In highly visual animals, like primates and carnivores, visual information is captured by different photoreceptors in the retina and transformed from optical signal to electrical signal. The electric signal is further relayed to different regions in the visual thalamus, the lateral geniculate nucleus (LGN) by three types of retinal ganglion cells (RGC). Neurons in different LGN regions are projected to their afferent zones in different parts of the primary visual cortex (V1). Retina, LGN, and V1 composed the early visual system. The three distinct processing pipelines from retina to LGN to V1 are also known as parallel streams. As three parallel streams are segregated functionally and anatomically from the retina to V1, neurons in each visual area also showed a cellular topography, that maps from the visual space to its corresponding neurons, known as retinotopy. The parallel streams and retinotopy in the early visual system provided a concise and efficient signal transmission from lower to higher visual area.

The encoded visual information gets more complex as it relays from lower to higher visual area. Neurons in LGN acted like dot detectors: they preferred center-surrounded visual stimulation. Whereas neurons in V1 preferred visual stimulation with different axial orientations, like an edge detector. V1 neurons that have the same orientation preference clustered in columns through cortical layers from the pial surface to white matter. In layers 3 and 4, neurons with different orientation preferences are organized in a pinwheel fashion. However, neurons in the deepest portion of a column, layer 6, don’t always share the same orientation preference as neurons in the more superficial portion of a column. Additionally, the columnar organization in ventral V1 is not investigated as the multi-neuron cellular imaging is not accessible to the back part of the brain. Aside from the feedforward signal transmission, another mechanism, corticogeniculate (CG) feedback, also modulates the visual information in the early visual system. Even though the visual information in LGN is governed by its feedforward input from RGC, CG neurons can also refine the responses of LGN neurons onto which they project, including sharpening the spatial and temporal preference, reducing the response latency, and increasing the information capacity and spike- timing precision. However, the CG feedback properties are mostly studied with electrophysiology with a couple of pairs of LGN and CG neurons in V1. As CG neurons are mostly located in layer 6 of V1, the spatial arrangement of their projected LGN neurons has not been fully investigated. Additionally, the modulatory effect of these CG neurons on their LGN neurons has not been evaluated at a populational level.