Plasticité de la réponse aux orientations dans le cortex visuel primaire du chat par la méthode d'imagerie optique intrinsèque

Dans le cortex visuel primaire du chat (aires 17 et 18), les neurones répondant aux orientations présentes dans l’environnement (comme le contour des objets) sont organisés en colonnes perpendiculaires à la surface du cortex. Il a précédemment été montré qu'un changement drastique des orientations présentes dans l’environnement change la réponse des neurones. Par exemple, un neurone répondant à des orientations horizontales pourra répondre, après apprentissage d'un nouvel environnement, à des orientations obliques. Nous avons voulu, dans cette thèse, suivre les changements de propriétés de populations entières de neurones suite à ce type d'apprentissage. A cet effet, nous avons utilisé la technique d'imagerie optique des signaux intrinsèques, qui permet de mesurer l'activité d'une surface de cortex en utilisant le signal BOLD (blood-oxygen-level dependent). Cette thèse s'articule sur trois axes : l'effet de l'apprentissage au niveau local, l'effet de l’apprentissage à l'échelle de l'aire cérébrale, et la modélisation de l’apprentissage. Dans la première partie, nous avons comparé les changements d’orientations des neurones en fonction du gradient d’orientation local. Ce gradient est fort quand deux neurones voisins ont des orientations très différentes, et faible quand leurs orientations sont semblables. Les résultats montrent que plus les neurones sont entourés de neurones aux orientations différentes, plus l'apprentissage change leur réponse à l’orientation. Ceci suggère que les connexions locales ont une influence déterminante sur l'ampleur de l’apprentissage. Dans la deuxième partie, nous avons comparé le changement d’orientation des neurones des aires 17 et 18 avant et après apprentissage. Les résultats ne sont pas notablement différents entre les aires 17 et 18. On peut toutefois noter que les changements d’orientations dans l’aire 18 ont des amplitudes plus variables que dans l’aire 17. Ceci peut provenir du fait que l’aire 18 reçoit des afférences plus variées que l’aire 17, notamment une afférence directe des cellules Y du CGLd (Corps Genouillé Latéral dorsal). Dans la troisième partie, nous avons modélisé l'apprentissage expérimentalement observé à l'aide de réseaux de neurones utilisant un apprentissage Hebbien (cartes auto-organisatrices). Nous avons montré que le « feedback » des aires supérieures vers le cortex visuel primaire était souhaitable pour la conservation de la sélectivité à l'orientation des neurones. De manière générale, cette thèse montre l'importance des connexions locales dans la plasticité neuronale. Notamment, elles garantissent un apprentissage homéostatique, c'est-à- dire conservant la représentativité des orientations au niveau du cortex. De manière complémentaire, elle montre également l’importance des aires supérieures dans le maintient à long terme des orientations apprises par les neurones lors de l'apprentissage.In the cat primary visual cortex (areas 17 and 18), neurons responding to orientations in the environment (such as the outline of objects) are organized in columns perpendicular to the cortical surface. It was previously shown that a drastic change in orientations in the environment changes the response of neurons. For example, a neuron responding to a horizontal orientation will respond, after learning a new environment, to an oblique orientation. In this thesis, we seek to follow the changes of properties of large populations of neurons due to this type of learning. To this end, we used the intrinsic signals optical imaging technique, which measures the activity of a cortical surface using the BOLD (blood-oxygen-level dependent) signal. This thesis follows three axes: the effect of learning at the local level, the effect of learning at the visual area scale, and the modeling of learning. In the first part, we compared the changes in orientation of neurons according to the local gradient of orientation. This gradient is strong when two neighboring neurons have very different orientations, and weak when their orientations are similar. The obtained relation between the gradient and the magnitude of change in orientation shows that when neurons are increasingly surrounded by neurons with different orientations, they change their response to orientation to a greater extent. This suggests that local connections have a decisive influence on the extent of learning. In the second part, we followed the change in the orientation of neurons in the areas 17 and 18, before and after learning. The results are not significantly different between area 17 and area 18. However, it is noteworthy that orientation changes in area 18 are more variable in amplitude than in area 17. This may be because area 18 receives more diverse inputs than area 17, including a direct input from dLGN (dorsal Lateral Geniculate Nucleus) Y cells. In the third part, we modeled the experimentally observed learning with neural networks using a Hebbian learning rule (networks are self-organizing maps). We have shown that feedback from higher areas to the primary visual cortex was desirable for the neurons orientation selectivity conservation. Overall, this thesis shows the importance of local connections in neuronal plasticity. In particular, they guarantee a homeostatic learning, i.e. maintaining the representativeness of orientations in the cortex. In a complementary manner, it also shows the importance of the superior areas in the conservation of learned orientations

The health effects of Sure Start

From lagging well behind most European countries in the early 1990s, the UK is now one of the highest spenders on the under-5s in Europe (OECD, 2014). One of the biggest programmes for this age group is Sure Start. It offers families with children under the age of 5 a ‘one-stop shop’ for childcare and early education, health services, parenting support, and employment advice, with the aim of improving children’s school readiness, health, and social and emotional development

How can we increase girls’ uptake of maths and physics?

There is a large gender gap in the likelihood of taking maths and physics at A-level, even among high-achieving pupils. In partnership with the STEM Skills Fund, we conducted a study to understand thebarriers that stop girls from taking maths and physics at A-level. This took the form of a small-scale randomised control trial in which girls in Year 11 who were predicted to achieve at least grade 7 (equivalent to at least grade A) in maths, physics or combined science GCSE were offered financial support in return for applying to study physics or maths A-level. As part of this trial, we surveyed 266 girls, as well as a senior staff member across 40 schools, about girls’ A-level subject choices and what drives them. We also conducted four focus groups with 6-8 girls in schools in Bolton, Hull, Birmingham and Portsmouth to discuss these reasons in more detail. This report details our findings

Identifying Sibling Influence on Teenage Substance Use

A number of studies have found substantial correlations in risky behavior between siblings, raising the possibility that adolescents may directly influence the actions of their brothers or sisters. We assess the extent to which correlations in substance use and selling drugs are due to causal effects. Our identification strategy relies on panel data, the fact that the future does not cause the past, and the assumption that the direction of influence is from older siblings to younger siblings. Under this assumption along with other restrictions on dynamics, one can identify the causal effect from a regression of the behavior of the younger sibling on the past behavior and the future behavior of the older sibling. We also estimate a joint dynamic model of the behavior of older and younger siblings that allows for family specific effects, individual specific heterogeneity, and state dependence. We use the model to simulate the dynamic response of substance use to the behavior of the older sibling. Our results suggest that smoking, drinking, and marijuana use are affected by the example of older siblings, but most of the link between siblings arises from common influences.

Estimating the Production Function for Human Capital: Results from a Randomized Control Trial in Colombia

We examine the channels through which a randomized early childhood intervention in Colombia led to significant gains in cognitive and socio-emotional skills among a sample of disadvantaged children. We estimate production functions for cognitive and socio-emotional skills as a function of maternal skills and child’s past skills, as well as material and time investments that are treated as endogenous. The effects of the program can be fully explained by increases in parental investments, which have strong effects on outcomes and are complementary to both maternal skills and child’s past skills

How does the association between special education need and absence vary overtime and across special education need types?

School absenteeism is a significant social and public health problem, and it has considerable negative consequences on the development of children and adolescents not only in the short term but also in the long term. We investigated special education needs (SEN) as a risk factor for absenteeism. For 418,455 mainstream secondary school students from 151 local authorities in England, multilevel linear regression models were run to investigate the association between SEN, SEN types and absenteeism during their secondary school period from year 7 to year 11. Local authority level variation was also investigated. Adolescents with SEN were more likely to be absent than their peers without SEN. Of adolescents with SEN, those with physical disability, followed by those with behavioural, emotional and social difficulties had the highest rates of absenteeism. Absenteeism rates increased as adolescents grew older. The association between absenteeism and having any SEN varied substantially across Local authorities. The results suggest that early interventions/preventative measures could mitigate loss of schooling due to absence. Moreover, the substantial variation in attendance for children across different local authorities suggest that there may be scope for local authorities to influence absence rates among adolescents