Visual Circuits

National Eye Institute

Investigators

Linked publications & trials

Abstract

Introduction Work in the Visual Circuits Sect addresses a major significant challenge in systems neuroscience – to understand how circuits distributed across the brain accomplish higher-order visual functions, including attention, perception and cognition. Because of the dynamic and distributed nature of the brain circuits involved, it is broadly recognized that many bottlenecks in our understanding can only be overcome by testing interactions across neurons and circuits. Accordingly, our approach often involves causally perturbing a specific brain region or circuit while measuring activity in other brain locations, and interpreting the changes in brain signaling in the context of a well-defined visual behavioral task. As an integral part of our research approach, we apply computational models to verify whether the principles we have identified can be generalized to other conditions, and to formulate new hypotheses about the functional organization of these brain circuits. Standard models of these visual functions emphasize the role of the cerebral cortex. In contrast, our results demonstrate that higher-order visual functions are built on top of conserved subcortical circuits in the superior colliculus (SC), thalamus and basal ganglia, that play a central role in action selection. Our aims are to understand the operation of the subcortical structures and how they interact with the cortex, with the long-term goal of identifying the detailed neuronal circuit so that more specific therapeutic interventions can be developed. 1) Computational modeling of visual object recognition If you asked a vision scientist which areas in the brain are most responsible for our remarkable ability to recognize faces and other visual objects, most would probably answer the temporal lobe of the cerebral cortex, especially the infero-temporal (IT) cortex. We recently performed computational modeling of visual object processing that raises fundamental questions about this assumption. While we agree that IT cortex is important for fine discriminations, and it is accordingly dominated by foveal visual inputs, our results show that there is more to object recognition than fine discrimination. Importantly, foveation of an object of interest usually requires recognizing its presence in the periphery, and then moving the eyes to place that image on the central, foveal portion of the visual field. Our results support the view that IT plays a secondary role in such peripheral recognition, and other visual areas might instead be more critical. To investigate how signals carried by early visual processing areas (such as LGN and V1) could be used for object recognition in the periphery, we focused here on the task of distinguishing faces from non-faces. We tested how sensitive various models were to nuisance parameters, such as changes in scale and orientation of the image, and the type of image background. We found that a model of V1 simple or complex cells could provide quite reliable information, resulting in performance better than 80% in realistic scenarios. An LGN model performed considerably worse. Because peripheral recognition is both crucial to enable fine recognition (by bringing an object of interest on the fovea), and probably sufficient to account for a considerable fraction of our daily recognition-guided behavior, we think that the current focus on area IT and foveal processing is too narrow. Rather than a traditional hierarchical system with IT-like properties as its primary aim, our computational results show that visual object recognition should be seen as a parallel process, with high-accuracy foveal modules operating in parallel with lower-accuracy and faster modules that can operate across the visual field. 2) Optogenetic manipulation of higher-order visual functions Optogenetics make it possible to manipulate the activity of neurons using light, and since the introduction of this innovative technique, remarkable advances have been made in dissecting the complex interactions between brain and behavior. This method also holds the promise for potential clinical applications in humans, but this prospect has undermined by some serious technical limitations. Specifically, the success of optogenetics has been most notably observed in the animal model in which it had been pioneered, the rodent, whereas progress in the nonhuman primate (NHP) has been more limited. This is primarily due to the substantially larger volume of NHP brains, the efficacy with which viral vectors are able to deliver transgenes of interest, and the complexity of the primate immune system and its response to viral. The challenge to successfully apply optogenetic techniques in the NHP is therefore ongoing, and is an important one, because successful application of optogenetics in the NHP will lead to a better understanding of the primate nervous system and in the longer term, could be leveraged for the treatment of neurological and psychiatric disorders. To address these challenges, we recently tested whether brief optogenetic suppression of the superior colliculus (SC) in primates can change performance in a covert attention We used a visual attention task that required the subject to detect and report a stimulus change at a cued location via joystick release, while ignoring changes at an irrelevant, uncued location. When the cued location was positioned in the portion of the visual field matching the response fields of transduced neurons in the SC, transient light delivery coincident with the stimulus change altered the subject’s detection performance, significantly changing their performance in the task. When the cued location was elsewhere, performance was unaltered, indicating that the effect was spatially specific and not a motor deficit. Performance on tests with only one stimulus were also unaltered, indicating that the effect depended on selection amongst distractors rather than a low-level visual impairment. Psychophysical analyses revealed that optogenetic suppression increased perceptual thresholds, but only for locations matching the transduced site. These data show that optogenetic manipulations can cause brief and spatially specific changes in a higher-order visual task requiring covert attention. These results demonstrate the feasibility of using optogenetic and related methods to address higher-order visual functions in the primate, including those involved in visual cognition.