The detection threshold of a luminance-defined Gabor is improved by two high contrast, aligned flanking Gabors, an effect termed collinear facilitation. However, the neural basis of collinear facilitation is not well understood. This thesis focuses on a number of issues in collinear facilitation to better our understanding of its neural basis. (1) Cortical sites: the cortical site of collinear facilitation was investigated, and results showed that collinear facilitation is a purely monocular phenomenon. (2) Temporal properties: Collinear facilitation has fast dynamics for initiation and once collinear facilitation occurs it either decays slowly or is associated with a sustained detection. (3) Selectivity to other types of stimuli: chromatic stimuli (which isolated the S-cone opponent and the L/M cone opponent mechanisms) and 2nd order stimuli (a 2D white noise or ID noise multiplied with a Gabor envelope) were used and the results showed that collinear facilitation occurs in chromatic processing, and that some 2nd order stimuli also exhibit collinear facilitation. However, there was no interaction between luminance and chromatic systems nor between 1st and 2nd order mechanisms, suggesting independent processing streams for collinear facilitation. All of these results supported the conclusion that collinear facilitation is not a general property of cortical neurons in V1 since most V1 neurons are binocular, sensitive to both chromatic and achromatic stimuli and sensitive to both 1 st and 2nd order stimuli. Furthermore, the temporal properties of collinear facilitation suggest complex dynamic interactions, not simply explained by the passive propagation of long-range recurrent intra-cortical connections between flanks and target.

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William Eric Leifur Grimson. / Based on the author's thesis (Ph.D.--Massachusetts Institute of Technology) Includes indexes. / Bibliography: p. [247]-267.

Processing of visual stimuli in the vertebrate retina is complex and diverse. The retinal output to the higher centres of the nervous system, mediated by ganglion cells, consists of several different channels. Neurons in these channels can have very distinct response properties, which originate in different retinal pathways. In this work, the retinal origins and possible functional implications of the segregation of visual pathways will be investigated with a detailed, biologically realistic computational model of the retina. This investigation will focus on the two main retino-cortical pathways in the mammalian retina, the parvocellular and magnocellular systems, which are crucial for conscious visual perception. These pathways differ in two important aspects. The parvocellular system has a high spatial, but low temporal resolution. Conversely, the magnocellular system has a high temporal fidelity, spatial sampling however is less dense than for parvocellular cells. Additionally, the responses of magnocellular ganglion cells can show pronounced nonlinearities, while the parvocellular system is essentially linear. The origin of magnocellular nonlinearities is unknown and will be investigated in the first part of this work. As their main source, the results suggest specific properties of the photoreceptor response and a specialised amacrine cell circuit in the inner retina. The results further show that their effect combines in a multiplicative way. The model is then used to examine the influence of nonlinearities on the responses of ganglion cells in the presence of involuntary fixational eye movements. Two different stimulus conditions will be considered: visual hyperacuity and motion induced illusions. In both cases, it is possible to directly compare properties of the ganglion cell population response with psychophysical data, which allows for an analysis of the influence of different components of the retinal circuitry. The simulation results suggest an important role for nonlinearities in the magnocellular stream for visual perception in both cases. First, it will be shown how nonlinearities, triggered by fixational eye movements, can strongly enhance the spatial precision of magnocellular ganglion cells. As a result, their performance in a hyperacuity task can be equal to or even surpass that of the parvocellular system. Second, the simulations imply that the origin of some of the illusory percepts elicited by fixational eye movements could be traced back to the nonlinear properties of magnocellular ganglion cells. As these activity patterns strongly differ from those in the parvocellular system, it appears that the magnocellular system can strongly dominate visual perception in certain conditions. Taken together, the results of this theoretical study suggest that retinal nonlinearities may be important for and strongly influence visual perception. The model makes several experimentally verifiable predictions to further test and quantify these findings. Furthermore, models investigating higher visual processing stages may benefit from this work, which could provide the basis to produce realistic afferent input.