Functional Organisation of the Human Visual Cortex

In this thesis, we examined the monkey cortical regions involved in processing of color, visual motion information, and the recognition of actions done by others. The aim was to gain better insight in the functional organization of the monkey visual cortex using in-house developed functional imaging techniques. Two different functional imaging techniques were used in these studies, the double-label deoxyglucose technique (DG) and functional magnetic resonance imaging (fMRI) in the awake monkey (Chapter 2). Both techniques allow to obtain an overview of stimulus-related neural activity throughout the whole brain, integrated over a limited amount of time. The results of the color experiments (Chapter 3) clearly showed that color related information is processed within a group of areas belonging to the ventral stream, which is involved in the perception of objects. Color-related metabolic activity was observed in visual areas V1, V2, V3, V4 and inferotemporal cortex (area TEO and TE). These findings set to rest the longstanding controversial claims that color would be processed almost selectively in one extrastriate visual area (V4) (Zeki SM, Brain Res 1973 53: 422-427). These results also show the usefulness of whole brain functional mapping techniques, as a complimentary approach to single cell measurements. In Chapter 4, we investigated which regions in the superior temporal sulcus (STS) of the monkey are involved in the analysis of motion. While the caudal part of the STS has been studied extensively, including area MT/V5 and MST, little is known about motion sensitivity in more anterior-ventral STS regions. Using fMRI, we were able to localize and delineate six different motion sensitive regions in the STS. One of these regions, that we termed 1st (lower superior temporal), had not been described so far. We were able to further characterize the six motion sensitive regions, using a wide variety of motion-sensitivity tests. The results of the latter tests suggested that motion related information might be processed along a second pathway within the STS, in addition to the MT-MST path (which is involved in the perception of heading). This second pathway, which includes the more rostral motion sensitive STS regions (FST, 1st and STPm) is possibly involved in the visual processing of biological movements (movements of animate objects) and actions. Finally, we investigated how and where in the monkey brain visual information about actions done is processed (Chapter 5 and 6). We found (Chapter 5) that, in agreement with earlier single unit results, the observation of grasping movements activates several regions in the premotor cortex of the monkey. Remarkable is that these premotor regions predominantly have a motor function, coding different types of higher order motor acts (for instance grasping of an object). These results are in agreement with earlier suggestions that we are able to understand actions done by others, because observation of a particular motor act activates our own motor representation of the same act. Furthermore, these studies suggested that within the frontal cortex of the monkey, there is a distinction between context-dependent (a person grasping) and more abstract (a hand grasping) action representations. In Chapter 6 we studied two other regions which are involved in the processing of visual information of actions done by others, the superior temporal sulcus (STS) and the parietal cortex. In the parietal cortex, we found a similar distinction between context-dependent and more abstract action representations as observed in prefrontal cortex. These results suggest that the parietal cortex is not only involved in the visual control of action planning, but also in the visual processing of actions performed by others. Based upon anatomical connections between the STS, parietal and frontal regions and motion-, form- and action-related functional properties of the former regions, we tentatively suggest how information about actions done by others might be sent from the STS to the frontal cortex along three different pathways. The latter working hypothesis will be tested in the future by additional fMRI control experiments and by combining fMRI, inactivation and microstimulation experiments while monkeys perform grasping tasks and/or view actions performed by others.

Many complex aspects of human visual perception and recognition take place in the ventral temporal cortex; however, the functional organization of this region remains unknown. Much of the brain has been found to consist of discrete cortical areas which are distinguishable by various aspects of cortical organization, including functional properties. In the ventral temporal cortex, only a handful of visual category-selective areas have been found which collectively constitute a minority of the region. This thesis addresses two issues regarding the ventral temporal cortex. The first is an investigation of the development of known category-selective regions through a review of neural plasticity and an empirical study of a subject with a unique developmental history. The second is an approach to functionally parcellating the ventral temporal cortex, which surveys other approaches to parcellating the ventral temporal cortex and then introduces a novel data-driven method for identifying cortical functional organization.

Despite the fact that the visual system has been the most well-studied of all sensory systems, many questions remain in regard to its structure and function in both human and animal models. While a basic blueprint of the visual system exists across all animal species that sets in place the basic structure prior to the onset of visual experience, it has been well established that this system becomes fine-tuned through experience early in life. Using a variety of techniques and animal models, this dissertation addresses some questions regarding the functional organization and the effect of visual deprivation on the visual cortex of several animal models. Rodents offer several advantages for studying various aspects of the development, organization and plasticity of the visual system. An important model for studies of visual cortex plasticity is the system of ocular dominance columns (ODC, aggregates of cells with the same eye preference), which have been extensively studied in many carnivores and primates, but have been thought not to exist in rodents. Our lab recently reported the existence of ODCs in pigmented, Long Evans rats (Laing et al. 2015), but previous reports in albino rats (Diao et al., 1983) point to differences in the binocularity of certain regions of primary visual cortex (V1) and in the role that callosal connections may have in these differences. To explain these strain differences, we hypothesized that albino rats, unlike Long Evans rats, do not have ODCs, and that callosal connections in V1 of albino rats are not patchy, as they are in Long Evans rats. In the first chapter of this dissertation, we present anatomical and electrophysiological experiments supporting our prediction that input from both eyes intermix in the binocular region of V1 in albino rats, without segregating into ODCs, and that callosal connections in albino rats are homogeneously distributed in V1. In the second chapter, we explore the effect of loss of vision during early development on the surface area of V1. Using histological methods as well as MRI techniques, we examined how the reduction in mature brain surface area varies with age when blindness occurs in rats, ferrets and humans. To compare data across species, we translated the post-conception ages of each species to a common neurodevelopmental event-time scale. We predicted that the critical period for the effect of blindness on the area of V1 ends at a common developmental event-time across species. Our results support our prediction, and also show that the critical period for the effect of blindness on V1 surface area ends well before the visual cortex reaches its normal, mature size. Much of the research on the organization and function of visual cortex is presently carried out in mice. While a present advantage of mice is the possibility of using genetic tools, a disadvantage is the small size of their brain and visual cortex. In the third chapter, we use multiple anatomical tracers to explore the number, arrangement and internal topographic organization of extrastriate visual areas in the rabbit, whose brain is about 60 times larger than the mouse brain. Our results show that the visual cortical plan in rabbits closely resembles the plan in mice and rats, suggesting that the rodent plan may be more general, encompassing Lagomorphs and possibly other orders. Our study also underscores the usefulness of the rabbit as an alternative model to rats and mice for projects benefiting from a larger brain.

This comprehensive monograph updates progress in understanding children's language learning and its pathologies. It stresses the neurologic basis of normal language acquisition and the consequences of a variety of disorders using such tools as detailed analysis of language comprehension, production and use, as well as functional brain imaging and electrophysiology. It also underlines the import6ance of subcortical circuitry and inner speech and reviews the unfolding or regression of language of language in focal brain lesions, autism, Williams syndrome and developmental disorders of oral and written language.

Building on the successful formula of the first edition, Martin Tovée offers a concise but detailed account of how the visual system is organised and functions to produce visual perception. He takes his readers from first principles; the structure and function of the eye and what happens when light enters, to how we see and process images, recognise patterns and faces, and through to the most recent discoveries in molecular genetics and brain imaging, and how they have uncovered a host of new advances in our understanding of how visual information is processed within the brain. Incorporating new material throughout, including almost 50 new images, every chapter has been updated to include the latest research, and culminates in helpful key points, which summarise the lessons learnt. This book is an invaluable course text for students within the fields of psychology, neuroscience, biology and physiology.