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Different theoretical models have tried to investigate the feasibility of recurrent neural mechanisms for achieving direction selectivity in the visual cortex. The mathematical analysis of such models has been restricted so far to the case of purely linear networks. We present an exact analytical solution of the nonlinear dynamics of a class of direction selective recurrent neural models with threshold nonlinearity. Our mathematical analysis shows that such networks have form-stable stimulus-locked traveling pulse solutions that are appropriate for modeling the responses of direction selective cortical neurons. Our analysis shows also that the stability of such solutions can break down giving raise to a different class of solutions ("lurching activity waves") that are characterized by a specific spatio-temporal periodicity. These solutions cannot arise in models for direction selectivity with purely linear spatio-temporal filtering.

Alteration of one sensory system can have striking effects on the processing and organization of the remaining senses, a phenomenon known as cross-modal plasticity. The goal of this thesis was to understand the circuit basis of this form of plasticity. I established the mouse as a model system for studying cross-modal plasticity by comparing population activity in visual cortex between animals reared in complete darkness from birth (DR) to those housed in a normal light/dark environment (LR). I found that secondary visual cortex (V2L) responds much more strongly to auditory stimuli in DR than LR. I provide evidence that there is a sensitive period for cross-modal responses that ends in early adulthood. I also show that exposure to light later in life reduces V2L auditory activity to LR levels. I recorded single units to show that there is a higher percentage of auditory responsive neurons in DR V2L. In collaboration with Lia Min in Michela Fagiolini’s laboratory, we discovered that this was associated with an increase in the number of projections from auditory thalamus and auditory cortex. We also provide evidence that V2L is multimodal from birth and becomes less so with visual experience. I examined several molecular pathways that are affected by dark-rearing to see if they are involved in cross-modal plasticity. I found that Nogo receptor (NgR), Lynx1, and Icam5 signaling all play a fundamental role in controlling the duration of plasticity. I also show that the hyperconnectivity in NgR -/- and DR mice leads to an increase in multisensory enhancement. In primary visual cortex, cross-modal influences were much weaker. Similar to V2L, the distribution of cell types was affected by NgR signaling. I also found that both the range of cross-modal influence and its sign (excitatory or inhibitory) is dependent on visual experience. Finally, I show that NgR signaling and the maturation of inhibitory circuits affect these two properties. Together, these results provide evidence of the molecular mechanisms underlying cross-modal plasticity. We believe that this will further our knowledge of how to improve rehabilitation strategies after loss of a sensory system.

Mammalian striate and circumstriate cortical neurons have long been understood as coding spatially localized retinal luminance variations, providing a basis for computing motion, stereopsis, and contours from the retinal image. However, such perceptual attributes do not always correspond to the retinal luminance variations in natural vision. Recordings from area 17 and 18 neurons revealed a specialized nonlinear processing stream that responded to stimulus attributes having no corresponding luminance variations. This nonlinear stream acts in parallel to the conventional luminance processing of single cortical neurons. The two streams were consistent in their preference for orientation and direction of motion, but distinct in processing spatial variations of the stimulus attributes. The ensemble of these neurons provides a combination of stimulus attributes with and without corresponding luminance variations.

This study comprised three questions, each posed with the intention of exploring the expression of the neurofilament protein (NF) and mRNA levels in the monkey lateral geniculate nucleus (LGN) and visual cortex. (1) Literature indicates NFs are differentially expressed across the two visual pathways that originate from magnocellular and parvocellular layers in LGN. The results herein suggest that this difference continues at the level of gene transcription for the neurofilament heavy gene. (2) NF expression, assessed by SMI-32 (an antibody that recognizes an epitope of the non-phosphorylated NFs), was investigated as a function of developmental changes occurring during the critical period in area V1 of the vervet monkey visual cortex. The findings indicate that the M pathway develops before the P pathway. (3) The effect of eye enucleation on long-term changes in NF expression was investigated. The results do not support the idea that NF are activity-dependent.

This thesis explores the visual functions of the right anterior temporal cortex of the human brain. In Part 1, 92 patients with unilateral temporal- or frontal-lobe excisions and 35 normal control subjects were tested under two experimental conditions (global, local) of a reaction-time task, employing hierarchically structured letters or designs as stimuli. In both versions, the right temporal-lobe group was less affected than other groups by interference from the global aspect of the stimulus. These findings support the hypothesis that the right temporal lobe contributes to global visual processing. In Part 2, the ability to learn a cue-system for discriminating between two targets against a background of visually similar items was examined in 107 patients with unilateral temporal- or frontal-lobe excisions and 37 control subjects, using three versions of a visual-cue learning task. With letters and nonsense syllables, all groups took longer to complete the task when the background information was changed after three learning trials. With abstract designs, only patients with right temporal-lobe lesions failed to show this interference effect after three learning trials, but did so after six. Hence, it is argued that the right temporal lobe plays a role in visual pattern-discrimination learning.

No description available.

Thesis (Ph. D.)--University of Washington, 2004. / Vita. Includes bibliographical references (leaves 186-193).

Thesis (Ph. D.)--University of Chicago, Dept. of Mathematics, August 2000. / Includes bibliographical references. Also available on the Internet.

Thesis (Ph.D.)--University of Alabama at Birmingham, 2009. / Title from PDF title page (viewed on July 19, 2010). Includes bibliographical references.