Micro-probing enables fine-grained mapping of neuronal populations using fMRI

The characterization of receptive field (RF) properties is fundamental to understanding the neural basis of sensory and cognitive behaviour. The combination of non-invasive imaging, such as fMRI, with biologically inspired neural modelling has enabled the estimation of population RFs directly in humans. However, current approaches require making numerous a priori assumptions, so these cannot reveal unpredicted properties, such as fragmented RFs or subpopulations. This is a critical limitation in studies on adaptation, pathology or reorganization. Here, we introduce micro-probing (MP), a technique for fine-grained and largely assumption free characterization of multiple pRFs within a voxel. It overcomes many limitations of current approaches by enabling detection of unexpected RF shapes, properties and subpopulations, by enhancing the spatial detail with which we analyze the data. MP is based on tiny, fixed-size, Gaussian models that efficiently sample the entire visual space and create fine-grained probe maps. Subsequently, we derived population receptive fields (pRFs) from these maps. We demonstrate the scope of our method through simulations and by mapping the visual fields of healthy participants and of a patient group with highly abnormal RFs due to a congenital pathway disorder. Without using specific stimuli or adapted models, MP mapped the bilateral pRFs characteristic of observers with albinism. In healthy observers, MP revealed that voxels may capture the activity of multiple subpopulations RFs that sample distinct regions of the visual field. Thus, MP provides a versatile framework to visualize, analyze and model, without restrictions, the diverse RFs of cortical subpopulations in health and disease.</p

Studying Cortical Plasticity in Ophthalmic and Neurological Disorders:From Stimulus-Driven to Cortical Circuitry Modeling Approaches

Unsolved questions in computational visual neuroscience research are whether and how neurons and their connecting cortical networks can adapt when normal vision is compromised by a neurodevelopmental disorder or damage to the visual system. This question on neuroplasticity is particularly relevant in the context of rehabilitation therapies that attempt to overcome limitations or damage, through either perceptual training or retinal and cortical implants. Studies on cortical neuroplasticity have generally made the assumption that neuronal population properties and the resulting visual field maps are stable in healthy observers. Consequently, differences in the estimates of these properties between patients and healthy observers have been taken as a straightforward indication for neuroplasticity. However, recent studies imply that the modeled neuronal properties and the cortical visual maps vary substantially within healthy participants, e.g., in response to specific stimuli or under the influence of cognitive factors such as attention. Although notable advances have been made to improve the reliability of stimulus-driven approaches, the reliance on the visual input remains a challenge for the interpretability of the obtained results. Therefore, we argue that there is an important role in the study of cortical neuroplasticity for approaches that assess intracortical signal processing and circuitry models that can link visual cortex anatomy, function, and dynamics

Simultaneous estimation of population receptive field and hemodynamic parameters from single point BOLD responses using Metropolis-Hastings sampling.

We introduce a new approach to Bayesian pRF model estimation using Markov Chain Monte Carlo (MCMC) sampling for simultaneous estimation of pRF and hemodynamic parameters. To obtain high performance on commonly accessible hardware we present a novel heuristic consisting of interpolation between precomputed responses for predetermined stimuli and a large cross-section of receptive field parameters. We investigate the validity of the proposed approach with respect to MCMC convergence, tuning and biases. We compare different combinations of pRF - Compressive Spatial Summation (CSS), Dumoulin-Wandell (DW) and hemodynamic (5-parameter and 3-parameter Balloon-Windkessel) models within our framework with and without the usage of the new heuristic. We evaluate estimation consistency and log probability across models. We perform as well a comparison of one model with and without lookup table within the RStan framework using its No-U-Turn Sampler. We present accelerated computation of whole-ROI parameters for one subject. Finally, we discuss risks and limitations associated with the usage of the new heuristic as well as the means of resolving them. We found that the new algorithm is a valid sampling approach to joint pRF/hemodynamic parameter estimation and that it exhibits very high performance

Goals and information processing in human decisions

We do not make decisions in the void. Every day, we act in awareness of our context, adjusting our objectives according to the situations we find. Operating effectively under multiple goals is fundamental for appropriate learning and decision-making, and deficiencies in this capacity can be at the core of mental disorders such as anxiety, depression, or post-traumatic stress disorder. In this thesis, I present studies I conducted to investigate how goals impact different stages of the decision process, from simple perceptual choices to subjective value preferences. Previous studies have described how animals assess alternatives and integrate evidence to make decisions. Most of the time, the focus of this work has been on simplified scenarios with single goals. In this thesis, my experiments tackle the issue of how people adjust information processing in tasks that demand more than one objective. Through various manipulations of the behavioural goals, such as decision framing, I show that (i) attention and evidence accumulation, (ii) brain representations, and (iii) decision confidence were all affected by context changes. Using behavioural testing, computational models, and neuroimaging I show that goals have a crucial role in evidence integration and the allocation of visual attention. My findings indicate that brain patterns adapt to enhance goal-relevant information during learning and the valuation of alternatives. Finally, I report the presence of goal-dependent asymmetries in the generation of decision confidence, overweighting the evidence of the most-relevant option to fulfil the goal. In conclusion, I show how the entire process is highly flexible and serves the behavioural demands. These findings support the reinterpretation of some perspectives, such as reported biases and irrationalities in decisions, as attributes of adaptive processing towards goal fulfilment

Ventral occipito-temporal cortex function and anatomical connectivity in reading

Previous functional neuroimaging studies of reading in skilled readers, acquired dyslexia and developmental dyslexia have all shown that the left ventral occipito-temporal cortex (vOT) is involved in visual word recognition. Specifically, a region in the left posterior occipito-temporal sulcus lateral to fusiform gyrus and medial to inferior temporal gyrus has been reported to play an important role. However, the precise functional contribution of this area in reading is yet to be fully explored. In this thesis, I empirically evaluated a claim that vOT responds not only to bottom-up processing demands of the visual stimuli but is also influenced by automatic, top-down non-visual processing demands, as proposed by the Interactive Account of vOT functioning. The first part of this thesis investigated the functional properties of vOT during reading, using functional magnetic resonance imaging. In the first project, the top-down influences on vOT were investigated, teasing apart visual and non-visual properties of written stimuli. In the second project, using the Japanese orthography I disentangled a word’s lexical frequency from the frequency of its visual form – an important distinction for understanding the neural information processing in regions engaged by reading and further explored the interactive nature of the vOT responses. The second part then investigated the anatomical basis of these functional interactions between vOT and other cortical regions. I used diffusion-weighted magnetic resonance imaging and tractography, the only method currently available to identify and measure white matter fibre pathways non-invasively and in vivo. My research has demonstrated that vOT integrates bottom-up visual information and top-down predictions from regions encoding non-visual attributes of the stimulus in an interactive fashion. It also illustrated the putative anatomical basis for functional connectivity during reading, which is consistent with the parallel cortical visual pathways seen in other primates. Altogether, the results provide strong support for the Interactive Account

Learning spaces:built, natural and digital considerations for learning and learners

Learning spaces: Built, natural and digital considerations for learning and learner