92 research outputs found
A functional model for primary visual cortex
Many neurons in mammalian primary visual cortex have properties such as sharp tuning for contour orientation, strong selectivity for motion direction, and insensitivity to stimulus polarity, that are not shared with their sub-cortical counterparts. Successful models have been developed for a number of these properties but in one case, direction selectivity, there is no consensus about underlying mechanisms. This thesis describes a model that accounts for many of the empirical observations concerning direction selectivity. The model comprises a single column of cat primary visual cortex and a series of processing stages. Each neuron in the first cortical stage receives input from a small number of on-centre and off-centre relay cells in the lateral geniculate nucleus. Consistent with recent physiological evidence, the off-centre inputs to cortex precede the on-centre inputs by a small interval (~4 ms), and it is this difference that confers direction selectivity on model neurons. I show that the resulting model successfully matches the following empirical data: the proportion of cells that are direction selective; tilted spatiotemporal receptive fields; phase advance in the response to a stationary contrast-reversing grating stepped across the receptive field. The model also accounts for several other fundamental properties. Receptive fields have elongated subregions, orientation selectivity is strong, and the distribution of orientation tuning bandwidth across neurons is similar to that seen in the laboratory. Finally, neurons in the first stage have properties corresponding to simple cells, and more complex-like cells emerge in later stages. The results therefore show that a simple feed-forward model can account for a number of the fundamental properties of primary visual cortex
A functional model for primary visual cortex
Many neurons in mammalian primary visual cortex have properties such as sharp tuning for contour orientation, strong selectivity for motion direction, and insensitivity to stimulus polarity, that are not shared with their sub-cortical counterparts. Successful models have been developed for a number of these properties but in one case, direction selectivity, there is no consensus about underlying mechanisms. This thesis describes a model that accounts for many of the empirical observations concerning direction selectivity. The model comprises a single column of cat primary visual cortex and a series of processing stages. Each neuron in the first cortical stage receives input from a small number of on-centre and off-centre relay cells in the lateral geniculate nucleus. Consistent with recent physiological evidence, the off-centre inputs to cortex precede the on-centre inputs by a small interval (~4 ms), and it is this difference that confers direction selectivity on model neurons. I show that the resulting model successfully matches the following empirical data: the proportion of cells that are direction selective; tilted spatiotemporal receptive fields; phase advance in the response to a stationary contrast-reversing grating stepped across the receptive field. The model also accounts for several other fundamental properties. Receptive fields have elongated subregions, orientation selectivity is strong, and the distribution of orientation tuning bandwidth across neurons is similar to that seen in the laboratory. Finally, neurons in the first stage have properties corresponding to simple cells, and more complex-like cells emerge in later stages. The results therefore show that a simple feed-forward model can account for a number of the fundamental properties of primary visual cortex
Towards building a more complex view of the lateral geniculate nucleus: Recent advances in understanding its role
The lateral geniculate nucleus (LGN) has often been treated in the past as a linear filter that adds little to retinal processing of visual inputs. Here we review anatomical, neurophysiological, brain imaging, and modeling studies that have in recent years built up a much more complex view of LGN . These include effects related to nonlinear dendritic processing, cortical feedback, synchrony and oscillations across LGN populations, as well as involvement of LGN in higher level cognitive processing. Although recent studies have provided valuable insights into early visual processing including the role of LGN, a unified model of LGN responses to real-world objects has not yet been developed. In the light of recent data, we suggest that the role of LGN deserves more careful consideration in developing models of high-level visual processing
Multiplexed computations in retinal ganglion cells of a single type
In the early visual system, cells of the same type perform the same computation in different places of the visual field. How these cells code together a complex visual scene is unclear. A common assumption is that cells of a single-type extract a single-stimulus feature to form a feature map, but this has rarely been observed directly. Using large-scale recordings in the rat retina, we show that a homogeneous population of fast OFF ganglion cells simultaneously encodes two radically different features of a visual scene. Cells close to a moving object code quasilinearly for its position, while distant cells remain largely invariant to the object's position and, instead, respond nonlinearly to changes in the object's speed. We develop a quantitative model that accounts for this effect and identify a disinhibitory circuit that mediates it. Ganglion cells of a single type thus do not code for one, but two features simultaneously. This richer, flexible neural map might also be present in other sensory systems
Impact of the pulvinar on the ventral pathway of the cat visual cortex
Signals from the retina are relayed to the lateral geniculate nucleus from which they are sent to the primary visual cortex. At the cortical level, the information is transferred across several visual areas in which the complexity of the processing increases progressively. Anatomical and functional evidence demonstrate the existence of two main pathways in visual cortex processing distinct features of the visual information: the dorsal and ventral streams. Cortical areas composing the dorsal stream are implicated mostly in motion processing while those comprising the ventral stream are involved in the processing of form and colour. This classic view of the cortical functional organization is challenged by the existence of reciprocal connections of visual cortical areas with the thalamic nucleus named pulvinar. These connections allow the creation of a trans-thalamic pathway that parallels the cortico-cortical communications across the visual hierarchy.
The main goal of the present thesis is twofold: first, to obtain a better comprehension of the processing of light increments and decrements in an area of the cat ventral stream (area 21a); second, to characterize the nature of the thalamo-cortical inputs from the cat lateral posterior nucleus (LP) to area 21a.
In study #1, we investigated the spatiotemporal response profile of neurons from area 21a to light increments (brights) and decrements (darks) using a reverse correlation analysis of a sparse noise stimulus. Our findings showed that 21a neurons exhibited stronger responses to darks with receptive fields exhibiting larger dark subfields. However, no differences were found between the temporal dynamics of brights and darks. In comparison with the primary visual cortex, the dark preference in area 21a was found to be strongly enhanced, supporting the notion that the asymmetries between brights and darks are transmitted and amplified along the ventral stream.
In study #2, we investigated the impact of the reversible pharmacological inactivation of the LP nucleus on the contrast response function (CRF) of neurons from area 21a and the primary visual cortex (area 17). The thalamic inactivation yielded distinct effects on both cortical areas. While in area 17 the LP inactivation caused a slight decrease in the response gain, in area 21a a strong increase was observed. Thus, our findings suggest that the LP exerts a modulatory influence on the cortical processing along the ventral stream with stronger impact on higher order extrastriate areas.
Taken together, our findings allowed a better comprehension of the functional properties of the cat ventral stream and contributed to the current knowledge on the role of the pulvinar on the cortico-thalamo-cortical processing of visual information.Les signaux provenant de la rĂ©tine sont relayĂ©s dans le corps gĂ©niculĂ© latĂ©ral oĂč ils sont envoyĂ©s au cortex visuel primaire. Lâinformation passe ensuite Ă travers plusieurs aires visuelles oĂč la complexitĂ© du traitement augmente progressivement. Des donnĂ©es tant anatomiques que fonctionnelles ont dĂ©montrĂ© lâexistence de deux voies principales qui traitent diffĂ©rentes propriĂ©tĂ©s de lâinformation visuelle : les voies dorsale et ventrale. Les aires corticales composant la voie dorsale sont impliquĂ©es principalement dans le traitement du mouvement tandis que les aires de la voie ventrale sont impliquĂ©es dans le traitement de la forme et de la couleur. Cette vision classique de lâorganisation fonctionnelle du cortex est toutefois remise en question par lâexistence de connections rĂ©ciproques entre les aires corticales visuelles et le pulvinar, un noyau thalamique. En effet, ces connections permettent la crĂ©ation dâune voie trans-thalamique parallĂšle aux connections cortico-corticales Ă travers la hiĂ©rarchie visuelle.
Le but principal de la prĂ©sente thĂšse consiste en deux volets : le premier est dâobtenir une meilleure comprĂ©hension du traitement des incrĂ©ments et dĂ©crĂ©ments de la lumiĂšre dans une aire de la voie ventrale du chat (aire 21a); le second est de caractĂ©riser la nature des inputs
thalamo-corticaux du noyau latĂ©ral postĂ©rieur (LP) Ă lâaire 21a chez le chat.
Dans lâĂ©tude #1, nous avons investiguĂ© le profil spatiotemporel des rĂ©ponses des neurones de lâaire 21a aux incrĂ©ments (blancs) et dĂ©crĂ©ments (noirs) de lumiĂšre en utilisant lâanalyse de corrĂ©lation inverse dâun stimulus de bruit Ă©pars. Les neurones de lâaire 21a ont rĂ©pondu plus fortement aux stimuli noirs, en montrant des champs rĂ©cepteurs avec des sous-champs noirs plus larges. Cependant, aucune diffĂ©rence nâa Ă©tĂ© trouvĂ©e en ce qui concerne les dynamiques temporelles des rĂ©ponses aux blancs et aux noirs. En comparaison avec le cortex visuel primaire, la prĂ©fĂ©rence aux stimuli noirs dans lâaire 21a sâest avĂ©rĂ©e fortement augmentĂ©e. Ces donnĂ©es indiquent que les asymĂ©tries entre les rĂ©ponses aux blancs et aux noirs sont transmises et amplifiĂ©es Ă travers la voie ventrale.
Dans lâĂ©tude #2, nous avons investiguĂ© lâimpact de lâinactivation pharmacologique rĂ©versible du noyau LP sur la fonction de rĂ©ponse au contraste (CRF) des neurones de lâaire 21a et du cortex visuel primaire (aire 17). Lâinactivation a eu diffĂ©rents effets dans les deux aires corticales. Alors que, dans lâaire 17, lâinactivation du LP a causĂ© une lĂ©gĂšre rĂ©duction du gain de la rĂ©ponse, une forte augmentation a Ă©tĂ© observĂ©e dans lâaire 21a. Ainsi, nos rĂ©sultats suggĂšrent que le LP exerce une influence modulatrice dans le traitement cortical Ă travers la voie ventrale avec un impact plus important dans des aires extrastriĂ©es de plus haut niveau.
Nos rĂ©sultats ont permis dâavoir une meilleure comprĂ©hension des propriĂ©tĂ©s fonctionnelles de la voie ventrale du chat et de contribuer Ă enrichir les connaissances actuelles sur le rĂŽle du pulvinar dans le traitement cortico-thalamo-cortical de lâinformation visuelle
Unified developmental model of maps, complex cells and surround modulation in the primary visual cortex
For human and animal vision, the perception of local visual features can depend on
the spatial arrangement of the surrounding visual stimuli. In the earliest stages of visual
processing this phenomenon is called surround modulation, where the response of
visually selective neurons is influenced by the response of neighboring neurons. Surround
modulation has been implicated in numerous important perceptual phenomena,
such as contour integration and figure-ground segregation. In cats, one of the major
potential neural substrates for surround modulation are lateral connections between
cortical neurons in layer 2/3, which typically contains âcomplexâ cells that appear to
combine responses from âsimpleâ cells in layer 4C. Interestingly, these lateral connections
have also been implicated in the development of functional maps in primary
visual cortex, such as smooth, well-organized maps for the preference of oriented lines.
Together, this evidence suggests a common underlying substrate the lateral interactions
in layer 2/3âas the driving force behind development of orientation maps for
both simple and complex cells, and at the same time expression of surround modulation
in adult animals. However, previously these phenomena have been studied
largely in isolation, and we are not aware of a computational model that can account
for all of them simultaneously and show how they are related. In this thesis we resolve
this problem by building a single, unified computational model that can explain the
development of orientation maps, the development of simple and complex cells, and
surround modulation.
First we build a simple, single-layer model of orientation map development based
on ALISSOM, which has more realistic single cell properties (such as contrast gain
control and contrast invariant orientation tuning) than its predecessor. Then we extend
this model by adding layer 2/3, and show how the model can explain development of
orientation maps of both simple and complex cells. As the last step towards a developmental
model of surround modulation, we replace Mexican-hat-like lateral connectivity
in layer 2/3 of the model with a more realistic configuration based on long-range
excitation and short-range inhibitory cells, extending a simpler model by Judith Law.
The resulting unified model of V1 explains how orientation maps of simple and
complex cells can develop, while individual neurons in the developed model express
realistic orientation tuning and various surround modulation properties. In doing so,
we not only offer a consistent explanation behind all these phenomena, but also create
a very rich model of V1 in which the interactions between various V1 properties can
be studied. The model allows us to formulate several novel predictions that relate the variation of single cell properties to their location in the orientation preference maps
in V1, and we show how these predictions can be tested experimentally. Overall,
this model represents a synthesis of a wide body of experimental evidence, forming a
compact hypothesis for much of the development and behavior of neurons in the visual cortex
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A comparative study of cortical computations in the mammalian visual cortex
textA common feature of all mammals is the cerebral cortex, which is essential for higher-order functions and processing information to generate motor actions. While cortical circuits exhibit a striking uniformity in anatomical organization, it is unknown whether these circuits preform similar computations across mammalian species. In this dissertation I compare the emergence of two computations in the primary visual cortex (V1) of carnivores and rodents. A cortical computation is a transformation in neural representation, such that the spiking output of a cortical neuron exhibits a selectivity not present in the inputs from upstream neurons. Here I explore two computations: orientation selectivity, the preference of neurons for oriented edges in the visual world, and binocularity, the integration of signals from the two eyes. In the first section, I compare the emergence of orientation selectivity in the early visual pathway of mouse and cat. Recordings from thalamic relay cells and V1 neurons in both species reveal orientation selectivity in mouse V1 is not emergent, and could be inherited subcortically. In a second set of experiments, I measure orientation selectivity and the organization of V1 orientation preference in a grasshopper mouse with predatory behavior, compared to the scavenger lab mouse. Here I find the same functional properties. In the second section, I focus on the integration of ocular inputs in V1 of mouse and cat. I first compare disparity selectivity in cats, where convergence of ocular inputs has long been established, with mice, where ocular integration had not previously been investigated. Similar to cats, mouse V1 neurons were sensitive to binocular disparity, albeit to a lesser degree, and could be described by a linear feed-forward model. I next explore the disruption of binocular disparity tuning in both animals. In cats, strabismus induced during development causes increased monocularity in V1 and a loss of disparity selectivity. In mice, monocular deprivation causes increased ocular input, which also manifests as decreased disparity selectivity. Finally, I explore how excitatory and inhibitory neurons in mouse V1 integrate binocular signals. Paravalbumin-expressing inhibitory interneurons are more binocular but less disparity tuned than surrounding cortical neurons, providing a canonical mechanism explaining loss of disparity selectivity in both carnivores and rodents.Neuroscienc
Development and encoding of visual statistics in the primary visual cortex
How do circuits in the mammalian cerebral cortex encode properties of the sensory
environment in a way that can drive adaptive behavior? This question is fundamental
to neuroscience, but it has been very difficult to approach directly. Various computational
and theoretical models can explain a wide range of phenomena observed in the
primary visual cortex (V1), including the anatomical organization of its circuits, the
development of functional properties like orientation tuning, and behavioral effects
like surround modulation. However, so far no model has been able to bridge these
levels of description to explain how the machinery that develops directly affects behavior.
Bridging these levels is important, because phenomena at any one specific
level can have many possible explanations, but there are far fewer possibilities to
consider once all of the available evidence is taken into account.
In this thesis we integrate the information gleaned about cortical development, circuit
and cell-type specific interactions, and anatomical, behavioral and electrophysiological
measurements, to develop a computational model of V1 that is constrained
enough to make predictions across multiple levels of description. Through a series
of models incorporating increasing levels of biophysical detail and becoming increasingly
better constrained, we are able to make detailed predictions for the types of
mechanistic interactions required for robust development of cortical maps that have
a realistic anatomical organization, and thereby gain insight into the computations
performed by the primary visual cortex.
The initial models focus on how existing anatomical and electrophysiological knowledge
can be integrated into previously abstract models to give a well-grounded and
highly constrained account of the emergence of pattern-specific tuning in the primary
visual cortex. More detailed models then address the interactions between specific
excitatory and inhibitory cell classes in V1, and what role each cell type may play
during development and function. Finally, we demonstrate how these cell classes
come together to form a circuit that gives rise not only to robust development but
also the development of realistic lateral connectivity patterns. Crucially, these patterns
reflect the statistics of the visual environment to which the model was exposed
during development. This property allows us to explore how the model is able to
capture higher-order information about the environment and use that information to
optimize neural coding and aid the processing of complex visual tasks.
Using this model we can make a number of very specific predictions about the
mechanistic workings of the brain. Specifically, the model predicts a crucial role of
parvalbumin-expressing interneurons in robust development and divisive normalization,
while it implicates somatostatin immunoreactive neurons in mediating longer
range and feature-selective suppression. The model also makes predictions about the
role of these cell classes in efficient neural coding and under what conditions the
model fails to organize. In particular, we show that a tight coupling of activity between
the principal excitatory population and the parvalbumin population is central
to robust and stable responses and organization, which may have implications for
a variety of diseases where parvalbumin interneuron function is impaired, such as
schizophrenia and autism. Further the model explains the switch from facilitatory to
suppressive surround modulation effects as a simple by-product of the facilitating
response function of long-range excitatory connections targeting a specialized class
of inhibitory interneurons. Finally, the model allows us to make predictions about the
statistics that are encoded in the extensive network of long-range intra-areal connectivity
in V1, suggesting that even V1 can capture high-level statistical dependencies
in the visual environment.
The final model represents a comprehensive and well constrained model of the
primary visual cortex, which for the first time can relate the physiological properties
of individual cell classes to their role in development, learning and function. While
the model is specifically tuned for V1, all mechanisms introduced are completely
general, and can be used as a general cortical model, useful for studying phenomena
across the visual cortex and even the cortex as a whole. This work is also highly
relevant for clinical neuroscience, as the cell types studied here have been implicated
in neurological disorders as wide ranging as autism, schizophrenia and Parkinsonâs
disease
Temporal information processing across primary visual cortical layers in normal and red light reared tree shrews.
Visual neuroscience research has benefitted from decades of efforts of comparative studies of different species, since exploring and understanding the diversity of functional properties of visual system in different species has helped us identify both general organization rules and unique traits of certain species. In this study, spatio-temporal receptive fields (STRFs), together with some other functional properties (etc. stimulus preference to different visual stimuli, orientation tuning, temporal frequency tuning and the F1/F0 ratio of responses to sine-wave grating stimuli), of primary visual cortex (V1) cells were measured in normally reared and red-light reared tree shrews (Tupaia), a species considered the closest non-primate relative to human being. All data were sampled in anesthetized animals using extracellular recording techniques. In the current study, a diversity of STRFs structures were found in tree shrew V1, and the STRFs found were classified into two categories, Type I receptive fields (RFs) that had spatially discontinuous on- and off-regions, or had spatio-temporal inseparable RFs, and Type II RFs that had spatially overlapped circular or elliptical on- and off- regions, and spatio-temporal separable RFs. Spatial and temporal profile analysis indicated this Type I and Type II classification did not correspond to simple and complex RF types previously described in primates and carnivores. It was also found in the current study that the linear prediction based on STRFs did not predict temporal frequency tuning, orientation tuning or the F1/F0 ratio very well in tree shrew V1. In tree shrew V1, both low-pass and band-pass cells for temporal frequency were found, and the proportion of cells with different types of tuning curves also differed across layers, resulting in a low-pass filter between layer II/II and layer IV. Last but not least, it was found in this study that red light rearing after birth changes the population stimulus preference in layer IV in tree shrew V1
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