Nonlinear Hebbian learning as a unifying principle in receptive field formation

The development of sensory receptive fields has been modeled in the past by a variety of models including normative models such as sparse coding or independent component analysis and bottom-up models such as spike-timing dependent plasticity or the Bienenstock-Cooper-Munro model of synaptic plasticity. Here we show that the above variety of approaches can all be unified into a single common principle, namely Nonlinear Hebbian Learning. When Nonlinear Hebbian Learning is applied to natural images, receptive field shapes were strongly constrained by the input statistics and preprocessing, but exhibited only modest variation across different choices of nonlinearities in neuron models or synaptic plasticity rules. Neither overcompleteness nor sparse network activity are necessary for the development of localized receptive fields. The analysis of alternative sensory modalities such as auditory models or V2 development lead to the same conclusions. In all examples, receptive fields can be predicted a priori by reformulating an abstract model as nonlinear Hebbian learning. Thus nonlinear Hebbian learning and natural statistics can account for many aspects of receptive field formation across models and sensory modalities

Early experience, binocular competition, and the sculpting of relay cell morphology in the cat lateral geniculate nucleus

Perturbations of early visual experience imposed by either monocular deprivation (MD) or artificially induced strabismus (squint) affect the morphological development of cells in the lateral geniculate nucleus (LGN) of cats. These morphological anomalies are thought to arise from competitive mechanisms which control many features of LGN cell development. Competition between the axon terminals of LGN cells innervated by the right and left eyes for establishing and maintaining synaptic contacts with cells in the visual cortex (i.e., binocular competition) is known to be a very important mechanism for controlling the development of LGN cells. Much of our present understanding regarding the importance of binocular competition is based on the effects of MD and squint on LGN somata. The objective of the present study was to further explore the role of binocular competition in the somatic development of LGN cells and extend these observations to determine the role of binocular competition in the dendritic development of LGN cells. Binocular competition was eliminated in some of the animals reared with either MD or squint by concomitant sagittal transection of the optic chiasm (OX), performed on either postnatal day 27 or 28

Weighting of binocular experience in visual cortical development

After birth the brain adapts to characteristics in the environment in order to optimise its resources with respect to the individual's circumstances. For instance, early monocular deprivation results in reduced cortical representation and visual acuity of the deprived eye. However, such a loss of visual function in one eye after only transient periods of compromised vision through injury or infection would seem to be maladaptive. I examined here whether cortical deprivation effects can be counteracted by daily periods of normal experience. Cats received variable daily regimens of monocular deprivation (by wearing a mask) and binocular exposure. Visual cortex function was subsequently assessed with optical imaging of intrinsic signals, visually evoked potentials, and extracellular electrophysiological recordings. Regardless of the overall length of visual experience, daily binocular vision for as little as 30 minutes, but no less, allowed normal ocular dominance and visual responses to be maintained despite several times longer periods of deprivation. Thus, the absolute amount of daily binocular vision rather than its relative share of the total daily exposure determined the outcome. When 30 minutes binocular exposure were broken up into two 15-minute blocks flanking the deprivation period, ocular dominance resembled that of animals with only 15 minutes binocular vision, suggesting that binocular experience must be continuous to be most effective. My results demonstrate that normal experience is clearly more efficacious in maintaining a binocular visual cortex than abnormal experience is in shifting the ocular dominance distribution. These findings con tribute to the larger debate about how much nature and nurture, respectively, contribute to the development of the brain they suggest that while experience plays a significant role, for some functions there may be an intrinsic bias towards a state that is optimally adapted to the most probable environment.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Weighting of binocular experience in visual cortical development.

After birth the brain adapts to characteristics in the environment in order to optimise its resources with respect to the individual's circumstances. For instance, early monocular deprivation results in reduced cortical representation and visual acuity of the deprived eye. However, such a loss of visual function in one eye after only transient periods of compromised vision through injury or infection would seem to be maladaptive. I examined here whether cortical deprivation effects can be counteracted by daily periods of normal experience. Cats received variable daily regimens of monocular deprivation (by wearing a mask) and binocular exposure. Visual cortex function was subsequently assessed with optical imaging of intrinsic signals, visually evoked potentials, and extracellular electrophysiological recordings. Regardless of the overall length of visual experience, daily binocular vision for as little as 30 minutes, but no less, allowed normal ocular dominance and visual responses to be maintained despite several times longer periods of deprivation. Thus, the absolute amount of daily binocular vision rather than its relative share of the total daily exposure determined the outcome. When 30 minutes binocular exposure were broken up into two 15-minute blocks flanking the deprivation period, ocular dominance resembled that of animals with only 15 minutes binocular vision, suggesting that binocular experience must be continuous to be most effective. My results demonstrate that normal experience is clearly more efficacious in maintaining a binocular visual cortex than abnormal experience is in shifting the ocular dominance distribution. These findings con tribute to the larger debate about how much nature and nurture, respectively, contribute to the development of the brain; they suggest that while experience plays a significant role, for some functions there may be an intrinsic bias towards a state that is optimally adapted to the most probable environment

Visual Cortex

The neurosciences have experienced tremendous and wonderful progress in many areas, and the spectrum encompassing the neurosciences is expansive. Suffice it to mention a few classical fields: electrophysiology, genetics, physics, computer sciences, and more recently, social and marketing neurosciences. Of course, this large growth resulted in the production of many books. Perhaps the visual system and the visual cortex were in the vanguard because most animals do not produce their own light and offer thus the invaluable advantage of allowing investigators to conduct experiments in full control of the stimulus. In addition, the fascinating evolution of scientific techniques, the immense productivity of recent research, and the ensuing literature make it virtually impossible to publish in a single volume all worthwhile work accomplished throughout the scientific world. The days when a single individual, as Diderot, could undertake the production of an encyclopedia are gone forever. Indeed most approaches to studying the nervous system are valid and neuroscientists produce an almost astronomical number of interesting data accompanied by extremely worthy hypotheses which in turn generate new ventures in search of brain functions. Yet, it is fully justified to make an encore and to publish a book dedicated to visual cortex and beyond. Many reasons validate a book assembling chapters written by active researchers. Each has the opportunity to bind together data and explore original ideas whose fate will not fall into the hands of uncompromising reviewers of traditional journals. This book focuses on the cerebral cortex with a large emphasis on vision. Yet it offers the reader diverse approaches employed to investigate the brain, for instance, computer simulation, cellular responses, or rivalry between various targets and goal directed actions. This volume thus covers a large spectrum of research even though it is impossible to include all topics in the extremely diverse field of neurosciences

Early cross-modal interactions and adult human visual cortical plasticity revealed by binocular rivalry

In this research binocular rivalry is used as a tool to investigate different aspects of visual and multisensory perception. Several experiments presented here demonstrated that touch specifically interacts with vision during binocular rivalry and that the interaction likely occurs at early stages of visual processing, probably V1 or V2. Another line of research also presented here demonstrated that human adult visual cortex retains an unexpected high degree of experience-dependent plasticity by showing that a brief period of monocular deprivation produced important perceptual consequences on the dynamics of binocular rivalry, reflecting a homeostatic plasticity. In summary, this work shows that binocular rivalry is a powerful tool to investigate different aspects of visual perception and can be used to reveal unexpected properties of early visual cortex

Epigenetic mechanisms in the normal brain development and in a neuropathological condition: the Rett syndrome

La regolazione della trascrizione genica avviene a diversi livelli, dal DNA alle proteine, per mezzo di svariati meccanismi. Il rimodellamento della cromatina è implicato nell’attivazione o nella repressione di sequenze all’interno dei cromosomi. Tale rimodellamento si attua tramite metilazione del DNA e modificazioni postraduzionali degli istoni, generalmente note come modificazioni epigenetiche. ella prima parte ci siamo concentrati sullo studio degli effetti delle modificazioni epigenetiche sullo sviluppo del sistema visivo, prendendolo come paradigma di sviluppo del sistema nervoso in condizioni normali. In questo contesto, abbiamo analizzato come i meccanismi epigenetici siano in grado di regolare la plasticità visiva anche nell’animale adulto. Nella seconda parte di questo studio ci siamo focalizzati su una malattia umana: la sindrome di Rett, un esempio di patologia del cervello causata da un’alterazione della regolazione dei meccanismi epigenetici