Object similarity affects the perceptual strategy underlying invariant visual object recognition in rats

In recent years, a number of studies have explored the possible use of rats as models of high-level visual functions. One central question at the root of such an investigation is to understand whether rat object vision relies on the processing of visual shape features or, rather, on lower-order image properties (e.g., overall brightness). In a recent study, we have shown that rats are capable of extracting multiple features of an object that are diagnostic of its identity, at least when those features are, structure-wise, distinct enough to be parsed by the rat visual system. In the present study, we have assessed the impact of object structure on rat perceptual strategy. We trained rats to discriminate between two structurally similar objects, and compared their recognition strategies with those reported in our previous study. We found that, under conditions of lower stimulus discriminability, rat visual discrimination strategy becomes more view-dependent and subject-dependent. Rats were still able to recognize the target objects, in a way that was largely tolerant (i.e., invariant) to object transformation; however, the larger structural and pixel-wise similarity affected the way objects were processed. Compared to the findings of our previous study, the patterns of diagnostic features were: (i) smaller and more scattered; (ii) only partially preserved across object views; and (iii) only partially reproducible across rats. On the other hand, rats were still found to adopt a multi-featural processing strategy and to make use of part of the optimal discriminatory information afforded by the two objects. Our findings suggest that, as in humans, rat invariant recognition can flexibly rely on either view-invariant representations of distinctive object features or view-specific object representations, acquired through learning

Characterization of visual object representations in rat primary visual cortex

For most animal species, quick and reliable identification of visual objects is critical for survival. This applies also to rodents, which, in recent years, have become increasingly popular models of visual functions. For this reason in this work we analyzed how various properties of visual objects are represented in rat primary visual cortex (V1). The analysis has been carried out through supervised (classification) and unsupervised (clustering) learning methods. We assessed quantitatively the discrimination capabilities of V1 neurons by demonstrating how photometric properties (luminosity and object position in the scene) can be derived directly from the neuronal responses

Astrocyte calcium dysfunction causes early network hyperactivity in Alzheimer's disease

Dysfunctions of network activity and functional connectivity (FC) represent early events in Alzheimer's disease (AD), but the underlying mechanisms remain unclear. Astrocytes regulate local neuronal activity in the healthy brain, but their involvement in early network hyperactivity in AD is unknown. We show increased FC in the human cingulate cortex several years before amyloid deposition. We find the same early cingulate FC disruption and neuronal hyperactivity in AppNL-F mice. Crucially, these network disruptions are accompanied by decreased astrocyte calcium signaling. Recovery of astrocytic calcium activity normalizes neuronal hyperactivity and FC, as well as seizure susceptibility and day/night behavioral disruptions. In conclusion, we show that astrocytes mediate initial features of AD and drive clinically relevant phenotypes

High-Level Vision in Rodents

The human visual system is able to recognize objects despite tremendous variation in their appearance due to changes in size, position, viewpoint, illumination, etc. This ability, referred to as invariant object recognition, requires the construction of representations that are tolerant to these identity-preserving transformations. This forms the basis of higher visual processing, but its neural underpinnings are poorly understood. Rodents have been the animal model of choice for many research topics, e.g. learning, memory, spatial navigation, drug development, and all have experienced great advances thanks to methodological innovations like two-photon calcium imaging, optogenetics and genetic manipulation most often applied in rodents. This dissertation explores the possibility to extend the use of rodents to studies of high-level vision, where traditionally nonhuman primates (e.g. macaques) are the model of choice. Rodents have been largely overlooked because their brains are often assumed to lack advanced visual processing machinery. However, recent studies have shown a surprising degree of functional specialization in at least nine visual areas that surround rodent primary visual cortex (V1). At the same time carefully designed behavioral studies have indicated that they are capable to solve complex visual tasks. These arguments are further discussed in Chapter 1, together with an overview of the main concepts and state-of-the-art in the field of high-level vision. In Chapter 2, we investigated how simple, parametric stimuli are represented by the rat visual system. After being trained on a simple discrimination between two stimuli ( prototypes ), they effortlessly generalized to new stimuli that varied quite a lot in the orthogonal direction in stimulus space. It did not matter to the rats whether the category boundary between classes was aligned with the dimensions of the stimulus space (orientation and spatial frequency). A markedly different result is obtained when humans perform a similar task. We conclude that rats are simply computing the similarity between each of the newly presented stimuli and the prototypes they had learned in a previous phase, a strategy that is sometimes more optimal than the rule-based strategy used by human participants. Chapter 3 is dedicated to the shape discrimination experiment that was published in Current Biology (Vermaercke & Op de Beeck, 2012). One of the main achievements of this study is that we successfully applied a multivariate paradigm called Bubbles to behavioral research in rats. The Bubbles paradigm has mostly been used in humans and monkeys. It involves the partial occlusion of stimuli used in different trials. By applying reverse correlation to correct and incorrect trials, this technique allows the visualization of the crucial information that drives task performance. We used this paradigm in a discrimination task that required rats to tolerate changes in stimulus position and were able to pinpoint the underlying strategy used by rats. We performed a similar experiment with human subjects, which allowed us to directly compare both species. We concluded that rats were using a less complicated, but still a quite flexible strategy. In Chapter 4, we recorded single unit responses for a stimulus set of six shapes. Using the responses of populations of neurons in five areas along the cortical what pathway, we could show how the representation of these shapes changed gradually. By comparing responses to shapes at two positions within each cell s receptive field, we noticed that populations in higher areas are more tolerant to these position changes. This could be considered a neural substrate for solving the task of Chapter 3. We also compared the neuronal similarities between shapes (related to the representational space containing these shapes) in different areas to behavioral discrimination performance and found good correlations in higher areas, whereas V1 responses were only correlated to physical differences between the shapes. Chapter 5 summarizes the main results of the studies presented in this dissertation and finally we conclude that rodents are a promising model of high-level vision, if their limitations are taken into account. This conclusion is also supported by recent studies that have tried to answer the same question, which clearly indicates the great importance of this work. Indeed, being able to investigate (rudimentary) forms of high-level vision in an animal model that allows the use of new techniques to study neural processes at the system level, will be a huge step towards understanding the computational underpinnings of complex visual processing in the human brain.nrpages: 153status: publishe

K0-NAA for the Determination of Trace Elements in a Synthetic Material: a Collaborative Study

As part of an interlaboratory comparison in order to certify a synthetic material to be used for the validation of the proper implementation of k0-NAA a collaborative trial was organised. Based on this collaborative study, reproducibility relative standard deviations valued ranged typically from 2 to 5%. These values were compares with the Horwitz criterium leading to HORRAT values in most cases well below 1, although two exceeding 1, depending on the element to be determined. It could be concluded that k0-NAA is a very precise method, but that still some inconsistencies for certain elements remain.JRC.D.8-Food safety and qualit

Report A Multivariate Approach Reveals the Behavioral Templates Underlying Visual Discrimination in Rats

Summary Although rodents are the first-choice animal model in the life sciences, they are rarely used to study higher visual functions. It is unclear to what extent rodents follow complex visual strategies to solve visual object recognition and discrimination tasks Results and Discussion Behavioral Templates in Rats Five rats were trained to discriminate a square from a triangle. Although object recognition in general is a well-studied capacity in rats We were able to extract templates that identified stimulus locations affecting task performance (Cluster test with threshold set at 2.7, see To what degree can performance be explained by a simple strategy such as ''the square is at the side that is brighter at the bottom''? We modeled this ''bright below = square'' strategy by comparing the average luminance pattern of both stimuli, weighted with a vertical gradient decreasing from bottom to top (see A Flexible, Context-Dependent Strategy Is Revealed by Conditional Templates A common characteristic of this subset of trials on which a simple ''bright below = square'' strategy fails, is that the bottom of the shapes was covered by bubbles. Given that in these trials the bubbles mostly cover the significant areas in the behavioral templates, we next investigated which image areas were being used by the animals. Hence we constructed ''conditional'' templates using only the subset of the trials on which the ''bright below = square'' strategy fails. As shown i

A Multivariate Approach Reveals the Behavioral Templates Underlying Visual Discrimination in Rats

Although rodents are the first-choice animal model in the life sciences, they are rarely used to study higher visual functions. It is unclear to what extent rodents follow complex visual strategies to solve visual object recognition and discrimination tasks [1–5]. We report the performance of rats in a visual discrimination task applying the multivariate ‘‘Bubbles’’ paradigm previously used in highly visual species such as humans, monkeys, and pigeons [6–8]. We demonstrate a relationship between accuracy and local occlusion of stimuli by bubbles, as such revealing the strategies or ‘‘templates’’ that underlie visual discrimination behavior. Performance was guided by relatively simple, screencentered templates as well as more adaptive templates reflecting context dependency and tolerance for changes in stimulus position. These findings demonstrate the complexity of visual strategies followed by rats and reveal interesting similarities (e.g., potential for position tolerance) as well as differences (overall efficiency of visual processing) compared to primates. In conclusion, this study illustrates the feasibility of investigating visual cognition in rats with multivariate behavioral paradigms, with the ultimate aim to use a comparative approach to explore the anatomical and neurophysiological basis of vision, also for those visual abilities that are traditionally studied in humans and monkeys.status: publishe