A parsimonious computational model of visual target position encoding in the superior colliculus

International audienceThe superior colliculus (SC) is a brain-stem structure at the crossroad of multiple functional pathways. Several neurophysiological studies suggest that the population of active neurons in the SC encodes the location of a visual target to foveate, pursue or attend to. Although extensive research has been carried out on computational modeling, most of the reported models are often based on complex mechanisms and explain a limited number of experimental results. This suggests that a key aspect may have been overlooked in the design of previous computational models. After a careful study of the literature, we hypothesized that the representation of the whole retinal stimulus (not only its center) might play an important role in the dynamics of SC activity. To test this hypothesis, we designed a model of the SC which is built upon three well accepted principles: the log-polar representation of the visual field onto the SC, the interplay between a center excitation and a surround inhibition and a simple neuronal dynamics, like the one proposed by the dynamic neural field theory. Results show that the retino-topic organization of the collicular activity conveys an implicit computation that deeply impacts the target selection process

Studies on eye movements in Parkinson's disease

Heterogeneity in Parkinson’s Disease (PD) phenotype and genotype is probably the main reason why, despite the abundance of biomarkers, we still lack a robust method for diagnosis and prognosis, besides clinical evaluation. Subjective changes in vision and objective measures in eye movements have been extensively studied, but the results are mainly used to better understand the pathophysiology of PD and are not integrated into the clinical praxis. The aim of this doctoral project was to examine if eye movements could serve as useful biomarkers for PD diagnosis and prognosis, and investigate their association with motor function, cognition, and medication effect. In addition, we aimed to examine cognition in a group of patients with a rare metabolic disorder and prominent eye-movement difficulties, the Norrbottnian Gaucher Disease 3 (GD3). Saccades, reading, and sustained fixation were examined in PD patients and healthy controls (HC) in the first three studies. Recruitment took place at Karolinska University Hospital Huddinge for the first two studies, and for the third study at Academic Specialist Center in Stockholm. Three different eye trackers were used, a head-mounted and two screen based, and the assessments were performed in a clinical setting. In the first two studies patients were examined in ON and OFF medication status, in order to evaluate the role of levodopa. In study 1, we examined saccadic parameters in 20 HC and 40 PD patients; study 2 involved reading assessments for 13 HC and 19 PD patients; in study 3 we examined sustained fixation in 43 HC and 50 PD patients. Recruitment for study 4 took place at Sunderby Regional Hospital, in Luleå, and we examined 10 patients with the Norrbottnian type of GD3. Cognitive evaluation was done with the Repeatable Battery for Assessment of Neuropsychological Status (RBANS). PD participants had worse saccadic performance, a slower reading speed, and deficient fixation control. Saccadic gain was associated with motor performance, while latency was related to cognition. Levodopa had no effect on saccadic gain, it worsened latency for the horizontal visually guided saccades and ameliorated the latency of antisaccades, but not the error rate or reading performance. We assumed that reading difficulties were attributed to cognitive, rather than oculomotor deficits. Fixation was more easily interrupted in PD compared to HC, and PD participants’ pupils did not dilate to the same extent as HC, in response to the cognitive effort put during sustained fixation. In study 4 we found that patients with the Norrbottnian type of GD3 have an overall worse cognitive performance compared to that of healthy population, scoring worse in memory and attention tests, present however with preserved language and visuospatial skills. The eye-tracking studies led to the conclusion that this method could be integrated into the clinical praxis as part of the clinical evaluation. It is easy to perform and provides reliable results that enable the understanding of motor, cognitive, and behavioral changes in PD. In order to do so, we would need a common protocol of assessment, so that the results would be comparable between different populations. The last study identified RBANS as a useful and easy-to-use tool for the cognitive examination of Norrbottnian GD3 patients

A global framework for a systemic view of brain modeling

International audienceThe brain is a complex system, due to the heterogeneity of its structure, the diversity of the functions in which it participates and to its reciprocal relationships with the body and the environment. A systemic description of the brain is presented here, as a contribution to developing a brain theory and as a general framework where specific models in computational neuroscience can be integrated and associated with global information flows and cognitive functions. In an enactive view, this framework integrates the fundamental organization of the brain in sensorimotor loops with the internal and the external worlds, answering four fundamental questions (what, why, where and how). Our survival-oriented definition of behavior gives a prominent role to pavlovian and instrumental conditioning, augmented during phylogeny by the specific contribution of other kinds of learning, related to semantic memory in the posterior cortex, episodic memory in the hippocampus and working memory in the frontal cortex. This framework highlights that responses can be prepared in different ways, from pavlovian reflexes and habitual behavior to deliberations for goal-directed planning and reasoning, and explains that these different kinds of responses coexist, collaborate and compete for the control of behavior. It also lays emphasis on the fact that cognition can be described as a dynamical system of interacting memories, some acting to provide information to others, to replace them when they are not efficient enough, or to help for their improvement. Describing the brain as an architecture of learning systems has also strong implications in Machine Learning. Our biologically informed view of pavlovian and instrumental conditioning can be very precious to revisit classical Reinforcement Learning and provide a basis to ensure really autonomous learning

Autism Pathogenesis: The Superior Colliculus

After been exposed to the visual input, in the first year of life, the brain experiences subtle but massive changes apparently crucial for communicative/emotional and social human development. Its lack could be the explanation of the very high prevalence of autism in children with total congenital blindness. The present theory postulates that the superior colliculus is the key structure for such changes for several reasons: it dominates visual behavior during the first months of life; it is ready at birth for complex visual tasks; it has a significant influence on several hemispheric regions; it is the main brain hub that permanently integrates visual and non-visual, external and internal information (bottom–up and top–down respectively); and it owns the enigmatic ability to take non-conscious decisions about where to focus attention. It is also a sentinel that triggers the subcortical mechanisms which drive social motivation to follow faces from birth and to react automatically to emotional stimuli. Through indirect connections it also activates simultaneously several cortical structures necessary to develop social cognition and to accomplish the multiattentional task required for conscious social interaction in real life settings. Genetic or non-genetic prenatal or early postnatal factors could disrupt the SC functions resulting in autism. The timing of postnatal biological disruption matches the timing of clinical autism manifestations. Astonishing coincidences between etiologies, clinical manifestations, cognitive and pathogenic autism theories on one side and SC functions on the other are disclosed in this review. Although the visual system dependent of the SC is usually considered as accessory of the LGN canonical pathway, its imprinting gives the brain a qualitatively specific functions not supplied by any other brain structure