Cortical Circuits for Classical and Extra-Classical Receptive Field Interactions in Visual Cortex

University Of Utah, Salt Lake City UT

Investigators

Abstract

A prominent organizational feature of the visual cerebral cortex of primates is the existence of multiple areas interconnected by a dense network of feedforward and feedback connections, and the presence of long horizontal axons linking distant loci within any given visual cortical area. The long-term goal of the present research is to disentangle the relative roles of these different systems of neuronal connections in the generation of specific visual cortex neuron responses, and ultimately in visual perception. A basic feature of neurons at the early stages of visual cortical processing is that they respond to the presentation of specific visual stimuli within a localized region of space, called the neuron's receptive field (RF). Presentation of similar stimuli outside the neuron's RF, typically does not evoke a response from the neuron. Feedforward connections are thought to underlie the responses of visual cortex neurons to localized imaged features (such as the specific orientation of line segments), i.e. to stimuli confined to the neuron's RF. More recently, it has been demonstrated that visual cortex neurons respond not only to details within their RF, but also to the context in which they appear. In other words, neuronal responses are affected by the perceptual organization of the visual scene as a whole. For example, the responses of neurons in visual cortical areas V1 and V2 to stimuli within their RF can be modulated (i.e. facilitated or suppressed) by stimuli lying outside their RF, i.e. in the RF's surround, even though stimulation of the surround region alone does not evoke a response from the cells. Specifically, the response to a bar stimulus presented in the RF at the orientation preferred by the cell is enhanced by the simultaneous presentation of similarly oriented and co-aligned bars in the surround. However, surrounding bars of orthogonal orientation to that in the RF have no effect on the cell's response. This property of visual cortical neurons (defined as surround modulation) is thought to represent the neural mechanism by which fragmented line segments are integrated to form perceptually complete contours. These phenomena require fast communication between distant parts of the visual image. Horizontal and/or feedback connections are thought to carry out the long distance computations underlying the perceptual integration of oriented line segments into contours, surround modulation of RF responses, and possibly visual attention. The goal of this application is to disentangle the relative roles of feedforward, feedback and horizontal connections in the generation of single neurons' responses within and outside the RF. As an initial step towards this goal, the present research is designed to investigate how the spatial extent and organization of the different types of connections within and between primate areas V1, V2 and V3 relate to the spatial dimensions and organization of single V1, V2 and V3 cells' RF and RF's surround. Each system of connections will be visualized using neuroanatomical labeling methods, by making small injections of a neuroanatomical tracer in macaque V1, V2 or V3. The extent of the connections in visual space will be determined and compared with the spatial dimensions of electrophysiologically measured RF and surround field of neurons at the injected cortical sites. These studies will provide anatomical and physiological constrains necessary to rule out potential mechanisms and substrates for surround modulation and perceptual integration of contours, while also identifying potential candidates. Additionally, they will identify rules of connectivity for three different visual cortical areas. Similar rules for different areas will reveal fundamental principles of organization for these connectional systems in visual cortex and, possibly, in other sensory cortices, with important functional and developmental implications. This research will also provide a wealth of quantitative anatomical and physiological data essential to the generation of biologically-based theoretical models of interactions within and beyond the RF of visual cortex neurons. These data will be disseminated and shared with students and faculty of mathematical biology, both through weekly discussion meetings, and through active collaborations and laboratory rotations offered to the students. The P.I. is an adjunct member of an active IGERT training grant in mathematical biology funded by NSF, whose central goal is to promote interactions between mathematicians and biologists; this proposal will thus help strengthen and meet the aims of that grant. It will also help strengthen Systems Neuroscience, at the University of Utah, by promoting collaborations between neuroscientists in different departments within and outside the University of Utah. Finally, the proposal will promote training and education, and broaden the participation of underrepresented groups, by funding two graduate students and one post-doctoral fellow belonging to minority groups.