Littérature scientifique sur le sujet « Visual regularities »

Neural representation of auditory regularities can be probed using the MMN, a component of ERPs generated in the auditory cortex by any violation of that regularity. Although several studies have shown that visual information can influence or even trigger an MMN by altering an acoustic regularity, it is not known whether audiovisual regularities are encoded in the auditory representation supporting MMN generation. We compared the MMNs elicited by the auditory violation of (a) an auditory regularity (a succession of identical standard sounds), (b) an audiovisual regularity (a succession of identical audiovisual stimuli), and (c) an auditory regularity accompanied by variable visual stimuli. In all three conditions, the physical difference between the standard and the deviant sound was identical. We found that the MMN triggered by the same auditory deviance was larger for audiovisual regularities than for auditory-only regularities or for auditory regularities paired with variable visual stimuli, suggesting that the visual regularity influenced the representation of the auditory regularity. This result provides evidence for the encoding of audiovisual regularities in the human brain.

For our adaptive behavior in a dynamically changing environment, an essential task of the brain is to automatically encode sequential regularities inherent in the environment into a memory representation. Recent studies in neuroscience have suggested that sequential regularities embedded in discrete sensory events are automatically encoded into a memory representation at the level of the sensory system. This notion is largely supported by evidence from investigations using auditory mismatch negativity (auditory MMN), an event-related brain potential (ERP) correlate of an automatic memory-mismatch process in the auditory sensory system. However, it is still largely unclear whether or not this notion can be generalized to other sensory modalities. The purpose of the present study was to investigate the contribution of the visual sensory system to the automatic encoding of sequential regularities using visual mismatch negativity (visual MMN), an ERP correlate of an automatic memory-mismatch process in the visual sensory system. To this end, we conducted a sequential analysis of visual MMN in an oddball sequence consisting of infrequent deviant and frequent standard stimuli, and tested whether the underlying memory representation of visual MMN generation contains only a sensory memory trace of standard stimuli (trace-mismatch hypothesis) or whether it also contains sequential regularities extracted from the repetitive standard sequence (regularity-violation hypothesis). The results showed that visual MMN was elicited by first deviant (deviant stimuli following at least one standard stimulus), second deviant (deviant stimuli immediately following first deviant), and first standard (standard stimuli immediately following first deviant), but not by second standard (standard stimuli immediately following first standard). These results are consistent with the regularity-violation hypothesis, suggesting that the visual sensory system automatically encodes sequential regularities. In combination with a wide range of auditory MMN studies, the present study highlights the critical role of sensory systems in automatically encoding sequential regularities when modeling the world.

AbstractWhile temporal expectations (TE) generally improve reactions to temporally predictable events, it remains unknown how the learning of temporal regularities (one time point more likely than another time point) and explicit knowledge about temporal regularities contribute to performance improvements; and whether any contributions generalise across modalities. Here, participants discriminated the frequency of diverging auditory, visual or audio-visual targets embedded in auditory, visual or audio-visual distractor sequences. Temporal regularities were manipulated run-wise (early vs. late target within sequence). Behavioural performance (accuracy, RT) plus measures from a computational learning model all suggest that learning of temporal regularities occurred but did not generalise across modalities, and that dynamics of learning (size of TE effect across runs) and explicit knowledge have little to no effect on the strength of TE. Remarkably, explicit knowledge affects performance—if at all—in a context-dependent manner: Only under complex task regimes (here, unknown target modality) might it partially help to resolve response conflict while it is lowering performance in less complex environments.

One of the deepest insights in neuroscience is that sensory encoding should take advantage of statistical regularities. Humans’ visual experience contains many redundancies: Scenes mostly stay the same from moment to moment, and nearby image locations usually have similar colors. A visual system that knows which regularities shape natural images can exploit them to encode scenes compactly or guess what will happen next. Although these principles have been appreciated for more than 60 years, until recently it has been possible to convert them into explicit models only for the earliest stages of visual processing. But recent advances in unsupervised deep learning have changed that. Neural networks can be taught to compress images or make predictions in space or time. In the process, they learn the statistical regularities that structure images, which in turn often reflect physical objects and processes in the outside world. The astonishing accomplishments of unsupervised deep learning reaffirm the importance of learning statistical regularities for sensory coding and provide a coherent framework for how knowledge of the outside world gets into visual cortex.

Students in technical fields use visual as well as verbal modes to express their meaning, employing ways of expressing meaning that are useful later in their professional lives. This study investigates visual meaning in student Builders’ Diaries, journals that are written by carpentry trainees to provide a record of their learning. In professional carpentry practice, Diaries function as a record of building work and are used in planning, billing and record-keeping. For this study, a corpus of 43 Builders’ Diaries, written by apprentices working in industry and by trainees in an educational institution, were analyzed. Findings reveal the role of visual meaning in the Builders’ Diary in developing the professional identity of the students. Compositional regularities were found, including regularities in image–image and image–text relations. These regularities suggest the extent to which our participants, who have no formal training in design, participate in culturally shared understandings of visual meaning.

Patterns of visual objects, streams of sounds, and spatiotemporal events are just a few examples of the structures present in a variety of sensory inputs. Amid such variety, numerous regularities can be found. In order to handle the sensory processing, individuals of each species have to be able to rapidly track these regularities. Statistical learning is one of the principal mechanisms that enable to track patterns from the flow of sensory information, by detecting coherent relations between elements (e.g., A predicts B). Once relevant structures are detected, learners are sometimes required to generalize to novel situations. This process can be challenging since it demands to abstract away from the surface information, and extract structures from previously-unseen stimuli. Over the past two decades, researchers have shown that statistical learning and generalization operate across domains, modalities and species, supporting the generality assumption. These mechanisms in fact, play a crucial role in organizing the sensory world, and developing representation of the environment. But when and how do organisms begin to track and generalize patterns from the environment? From the overall existing literature, very little is known about the roots these mechanisms. The experiments described in this thesis were all designed to explore whether statistical learning and generalization of visual patterns are fully available at birth, using the newborn domestic chick (Gallus gallus) as animal model. This species represents an excellent developmental model for the study of the ontogeny of several cognitive traits because it can be tested soon after hatching, and allows complete manipulation of pre- and post-natal experience. In Chapter 2, four statistical learning experiments are described. Through learning-by-exposure, visually-naive chicks were familiarized to a computer-presented stream of objects defined by a statistical structure; in particular, transitional (conditional) probabilities linked together sequence elements (e.g., the cross predicts the circle 100% of the times). After exposure, the familiar structured sequence were compared to a random presentation (Experiment 1) or a novel, structured combination (Experiment 2) of the familiar shapes. Chicks successfully differentiated test sequences in both experiments. One relevant aspect of these findings is that the learning process is unsupervised. Despite the lack of reinforcement, the mere exposure to the statistically-defined input was sufficient to obtain a significant learning effect. Two additional experiments have been designed in order to explore the complexity of the patterns that can be learned by this species. In particular, the aim of Experiments 3 and 4 was to investigate chicks’ ability to discriminate subtle differences of distributional properties of the stimuli. New sequences have been created; the familiar one was formed by a pairs of shapes that always appear in that order whereas the unfamiliar stimulus was formed by shapes spanning the boundaries across familiar pairs (part-pairs). Unfamiliar part-pairs were indeed created by joining the last element of a familiar pair and the first element of another (subsequent) familiar pair. The key difference among pairs and part-pairs lied on the probabilistic structure of the two: being formed by the union of two familiar elements, part-pairs were experienced during familiarization but with a lower probability. In order to distinguish test sequences, chicks needed to detect a very small difference in conditional probability characterizing the two stimuli. Unfortunately, the animals were unable to differentiate test sequences when formed by 8 (Experiment 3) or 6 (Experiment 4) elements. My final goal would have been to discover whether chicks are effectively able to pick up transitional probabilities or whether they simply track frequencies of co-occurrence. In Experiments 1 and 2, since the frequency of appearance of each shape was balanced across stimuli, it was impossible to tell if chicks detected transitional probabilities (e.g., X predicts Y) or frequencies of co-occurrence (e.g., X and Y co-occur together, but any predictive relation characterize them) among elements. However, since the animals did not succeed in the first task, being unable to discriminate pairs vs. part-pairs, data are inconclusive as regards to this issue. Possible explanations and theoretical implications of these results are provided in the final chapter of this thesis. In Chapter 3, the two studies described were aimed at testing newborn chicks’ capacities of generalization of patterns presented as stings of visual tokens. For instance, the pattern AAB can be defined as “two identical items (AA) followed by another one, different from the formers (B)”. Patterns were presented as triplets of simultaneously-visible shapes, arranged according to AAB, ABA (Experiment 5), ABB and BAA (Experiment 6). Using a training procedure, chicks were able to recognize the trained regularity when compared to another (neutral) regularity (for instance, AAB displayed as cross-cross-circle vs. ABA displayed as cross-circle-cross). Chicks were also capable of generalizing these patterns to novel exemplars composed of previously-unseen elements (AAB vs. ABA implemented by hourglass-hourglass-arrow vs. hourglass-arrow-hourglass). A subsequent study (Experiment 6) was aimed at verifying whether the presence/absence of contiguous reduplicated elements (in AAB but not in ABA) may have facilitated learning and generalization in previous task. All regularities comprised an adjacent repetition that gave the triplets asymmetrical structures (AAB vs. ABB and AAB vs. BAA). Chicks discriminated pattern-following and pattern-violating novel test triplets instantiating all regularities employed in the study, suggesting that the presence/absence of an adjacent repetition was not a relevant cue to succeed in the task. Overall, the present research provides new data of statistical learning and generalization of visual regularities in a newborn animal model, revealing that these mechanisms fully operate at the very beginning of life. For what concerns statistical learning, day-old chicks performed better than neonates but similar to human infants. As regards to generalization, chicks’ performance is consistent to what shown by neonates in the linguistic domain. These findings suggest that newborn chicks may be predisposed to track visual regularities in their postnatal environment. Despite the very limited previous experience, after a mere exposure to a structured input or a 3-days training session, significant learning and generalization effects have been obtained, pointing to the presence of early predispositions serving the development of these cognitive abilities.<br>Il mondo sensoriale è composto da un insieme di regolarità. Sequenze di sillabe e note musicali, oggetti disposti nell’ambiente visivo e sequenze di eventi sono solo alcune delle tipologie di pattern caratterizzanti l’input sensoriale. La capacità di rilevare queste regolarità risulta fondamentale per l’acquisizione di alcune proprietà del linguaggio naturale (ad esempio, la sintassi), l’apprendimento di sequenze di azioni (ad esempio, il linguaggio dei segni), la discriminazione di eventi ambientali complessi come pure la pianificazione del comportamento. Infatti, rilevare regolarità da una molteplicità di eventi permette di anticipare e pianificare azioni future, aspetti cruciali di adattamento all’ambiente. Questo meccanismo di apprendimento, riportato in letteratura con il nome di statistical learning, consiste nella rilevazione di distribuzioni di probabilità da input sensoriali ovvero, relazioni di dipendenza tra i suoi diversi componenti (ad esempio, X predice Y). Come illustrato nell capitolo introduttivo della presente ricerca, nonostante si tratti di uno dei meccanismi responsabili dell’apprendimento del linguaggio naturale umano, lo statistical learning non sembra essersi evoluto in modo specifico per servire questa funzione. Tale meccanismo rappresenta un processo cognitivo generale che si manifesta in diversi domini sensoriali (acustico, visivo, tattile), modalità (temporale oppure spaziale-statico) e specie (umana e non-umane). La rilevazione di pattern gioca quindi un ruolo fondamentale nell’elaborazione dell’informazione sensoriale, necessaria ad una corretta rappresentazione dell’ambiente. Una volta apprese le regolarità e le strutture presenti nell’ambiente, gli organismi viventi devono saper generalizzare tali strutture a stimoli nuovi da un punto di vista percettivo, ma rappresentanti le stesse regolarità. L’aspetto cruciale della generalizzazione è quindi la capacità di riconoscere una regolarità familiare anche quando implementata da nuovi stimoli. Anche il processo di generalizzazione ricopre un ruolo fondamentale nell’apprendimento della sintassi del linguaggio naturale umano. Ciò nonostante, si tratta di un meccanismo dominio-generale e non specie-specifico. Ciò che non risultava chiaro dalla letteratura era l’ontogenesi di entrambi i meccanismi, specialmente nel dominio visivo. In altre parole, non era chiaro se le abilità di statistical learning e generalizzazione di strutture visive fossero completamente sviluppate alla nascita. Il principale obbiettivo degli esperimenti condotti in questa tesi era quindi quello di approfondire le origini di visual statistical learning e generalizzazione, tramite del pulcino di pollo domestico (Gallus gallus) come modello animale. Appartenendo ad una specie precoce, il pulcino neonato è quasi completamente autonomo per una serie di funzioni comportamentali diventando il candidato ideale per lo studio dell’ontogenesi di diverse abilità percettive e cognitive. La possibilità di essere osservato appena dopo la nascita, e la completa manipolazione dell’ambiente pre- e post- natale (tramite schiusa e allevamento in condizioni controllate), rende il pulcino un’ottimo modello sperimentale per lo studio dell’apprendimento di regolarità. La prima serie di esperimenti illustrati erano allo studio di statistical learning (Chapter 2). Tramite un paradigma sperimentale basato sull’apprendimento per esposizione (imprinting filiale), pulcini neonati naive dal punto di vista visivo, sono stati esposti ad una video-sequenza di elementi visivi arbitrari (forme geometriche). Tale stimolo è definito da una struttura “statistica” basata su transitional (conditional) probabilities che determinano l’ordine di comparsa di ciascun elemento (ad esempio, il quadrato predice la croce con una probabilità del 100%). Al termine della fase di esposizione, i pulcini riuscivano a riconoscere tale sequenza, discriminandola rispetto a sequenze non-familiari che consistevano in una presentazione random degli stessi elementi (ovvero nessun elemento prediceva la comparsa di nessun altro elemento; Experiment 1) oppure in una ricombinazione degli stessi elementi familiari secondo nuovi pattern statistici (ad esempio, il quadrato predice la T con probabilità del 100% ma tale relazione statistica non era mai stata esperita dai pulcini; Experiment 2). In entrambi gli esperimenti i pulcini discriminarono la sequenza familiare da quella non-familiare, dimostrandosi in grado di riconoscere il struttura statistica alla quale erano stati esposti durante la fase d’imprinting. Uno degli aspetti più affascinanti di questo risultato è che il processo di apprendimento è non-supervisionato ovvero nessun rinforzo era stato dato ai pulcini durante la fase di esposizione. Successivamente, sono stati condotti altri due esperimenti (Experiments 3 and 4) con l’obbiettivo di verificare se i pulcini fossero in grado di apprendere regolarità più complesse di quelle testate in precedenza. In particolare, il compito che dovevano svolgere i pulcini consisteva nel differenziare una sequenza familiare strutturata similmente a quella appena descritta e una sequenza non-familiare composta da part-pairs ovvero coppie di figure composte dall’unione dell’ultima figura componente una coppia familiare e la prima figura componente un’altra coppia familiare. Essendo formate dall’unione di elementi appartenenti a coppie familiari, le part-pairs venivano esperite dai pulcini durante la fase di familiarizazzione ma con una probabilità più bassa rispetto alle pairs. La difficoltà del compito risiede quindi nel rilevare una sottile differenza caratterizzante la distribuzione di probabilità dei due stimoli. Sfortunatamente i pulcini non sono stati in grado di discriminare le due sequenze ne quando composte da 8 elementi (Experiment 3) ne da 6 (Experiment 4). L’obbiettivo finale di questi due esperimenti sarebbe stato quello di scoprire il tipo di regolarità appresa dai pulcini. Infatti, negli esperimenti 1 e 2 i pulcini potrebbero aver discriminato sequenze familiari e non familiari sulla base delle frequenze di co-occorrenza delle figure componenti le coppie familiari (ad esempio, co-occorrenza di X e Y) piuttosto che sulle probabilità condizionali (ad esempio, X predice Y). Tuttavia, non avendo superato il test presentato negli esperimenti 3 e 4, la questione riguardante quale tipo di cue statistico viene appreso da questa specie rimane aperta. Possibili spiegazioni e implicazioni teoriche di tale risultato non significativo sono discusse nel capitolo conclusivo. Il secondo gruppo di esperimenti condotti nella presente ricerca riguarda l’indagine del processo di generalizzazione di regolarità visive (Chapter 3). Le regolarità indagate sono rappresentate come stringhe di figure geometriche organizzate spazialmente, i cui elementi sono visibili simultaneamente. Ad esempio, la regolarità definita come AAB viene descritta come una tripletta in cui i primi due elementi sono identici tra loro (AA), seguiti da un’altro elemento diverso dai precedenti (B). I pattern impiegati erano AAB, ABA (Experiment 5) ABB e BAA (Experiment 6) e la procedura sperimentale utilizzata prevedeva addestramento tramite rinforzo alimentare. Una volta imparato a riconoscere il pattern rinforzato (ad esempio, AAB implementato da croce-croce-cerchio) da quello non rinforzato (ad esempio, ABA implementato da croce-cerchio-croce), i pulcini dovevano riconoscere tali strutture rappresentate da nuovi elementi (ad esempio, clessidra-clessidra-freccia vs. clessidra-freccia-clessidra). Gli animali si dimostrarono capaci di generalizzare tutte le regolarità a nuovi esemplari delle stesse. L’aspetto più importante di questi risultati è quanto dimostrato nell’esperimento 6, il cui obbiettivo era quello di indagare le possibili strategie di apprendimento messe in atto dagli animali nello studio precedente. Infatti, considerando il confronto AAB vs. ABA, i pulcini potrebbero aver riconosciuto (e generalizzato) il pattern familiare sulla base della presenza di una ripetizione consecutiva di uno stesso elemento (presente in AAB ma non in ABA, dove lo stesso elemento A è ripetuto e posizionato ai due estremi della tripletta). Nell’esperimento 6 sono state quindi confrontate regolarità caratterizzate da ripetizioni: AAB vs. ABB e AAB vs. BAA. I pulcini si mostrarono comunque in grado di distinguere le nuove regolarità e di generalizzare a nuovi esemplari, suggerendo come tale abilità non sia limitata a un particolare tipo di configurazione. Complessivamente, i risultati ottenuti nella presente ricerca costituiscono la prima evidenza di statistical learning e generalizzazione di regolarità visive in un modello animale osservato appena dopo la nascita. Per quanto riguarda lo statistical learning, i pulcini dimostrano capacità comparabili a quelle osservate in altre specie animali e agli infanti umani ma apparentemente superiori a quelle osservate nel neonato. Ipotesi e implicazioni teoriche di tali differenze sono riportate nel capitolo conclusivo. Per quanto riguarda i processi di generalizzazione, la performance dei pulcini è in linea con quanto dimostrato dai neonati umani nel dominio linguistico. Alla luce di questi risultati, è plausibile pensare che il pulcino si biologicamente predisposto ad rilevare regolarità caratterizzanti il suo ambiente visivo, a partire dai primi momenti di vita.

In everyday situations humans are continuously confronted with complex and cluttered visual environments that contain a large number of objects. Despite this complexity, performance in real-life tasks is surprisingly efficient. As a novel explanation for this efficiency, we propose that the brain uses typical regularities between objects (e.g., lamps are typically appearing above dining tables) to group these objects to reduce complexity and thereby facilitate behavioral performance. In a series of experiments, we show that object regularities reduce competitive interactions in visual cortex, and we relate this benefit to improved detection of target objects among regular distracter groups. Furthermore, we show that this inter-object grouping also enhances performance in visual working memory and determines how fast objects enter visual awareness in the first place. Altogether, our findings demonstrate that inter-object grouping effectively reduces the number of competing objects and thus can facilitate perception in cluttered, but regular environments.

Image-based estimation of camera motion, known as visual odometry (VO), plays a very important role in many robotic applications such as control and navigation of unmanned mobile robots, especially when no external navigation reference signal is available. The core problem of VO is the estimation of the camera’s ego-motion (i.e. tracking) either between successive frames, namely relative pose estimation, or with respect to a global map, namely absolute pose estimation. This thesis aims to develop efficient, accurate and robust VO solutions by taking advantage of structural regularities in man-made environments, such as piece-wise planar structures, Manhattan World and more generally, contours and edges. Furthermore, to handle challenging scenarios that are beyond the limits of classical sensor based VO solutions, we investigate a recently emerging sensor — the event camera and study on event-based mapping — one of the key problems in the event-based VO/SLAM. The main achievements are summarized as follows. First, we revisit an old topic on relative pose estimation: accurately and robustly estimating the fundamental matrix given a collection of independently estimated homograhies. Three classical methods are reviewed and then we show a simple but nontrivial two-step normalization within the direct linear method that achieves similar performance to the less attractive and more computationally intensive hallucinated points based method. Second, an efficient 3D rotation estimation algorithm for depth cameras in piece-wise planar environments is presented. It shows that by using surface normal vectors as an input, planar modes in the corresponding density distribution function can be discovered and continuously tracked using efficient non-parametric estimation techniques. The relative rotation can be estimated by registering entire bundles of planar modes by using robust L1-norm minimization. Third, an efficient alternative to the iterative closest point algorithm for real-time tracking of modern depth cameras in ManhattanWorlds is developed. We exploit the common orthogonal structure of man-made environments in order to decouple the estimation of the rotation and the three degrees of freedom of the translation. The derived camera orientation is absolute and thus free of long-term drift, which in turn benefits the accuracy of the translation estimation as well. Fourth, we look into a more general structural regularity—edges. A real-time VO system that uses Canny edges is proposed for RGB-D cameras. Two novel alternatives to classical distance transforms are developed with great properties that significantly improve the classical Euclidean distance field based methods in terms of efficiency, accuracy and robustness. Finally, to deal with challenging scenarios that go beyond what standard RGB/RGB-D cameras can handle, we investigate the recently emerging event camera and focus on the problem of 3D reconstruction from data captured by a stereo event-camera rig moving in a static scene, such as in the context of stereo Simultaneous Localization and Mapping.

The visual identification of inconsistencies in patterns is an area in computing that has been understudied. While pattern visualisation exposes the relationships among identified regularities, it is still very important to identify inconsistencies (irregularities) in identified patterns. The significance of identifying inconsistencies for example in the growth pattern of children of a particular age will enhance early intervention such as dietary modifications for stunted children. It is described in this chapter, the need to have a system that identifies inconsistencies in identified pattern of a dataset. Also, techniques that enable the visual identification of inconsistencies in patterns such as fault tolerance and colour coding are described. Two approaches are presented in this chapter for visualising inconsistencies in patterns namely; visualising inconsistencies in objects with many attribute values and visual comparison of an investigated dataset with a case control dataset. These approaches are associated with tools which were developed by the authors of this chapter: Firstly, ConTra which allows its users to mine and analyse the contradictions in attribute values whose data does not abide by the mutual exclusion rule of the dataset. Secondly, Datax which mines missing data; enables the visualisation of the missingness and the identification of the associated patterns. Finally, WellGrowth which explores Children’s growth dataset by comparing an investigated dataset (data obtained from a Primary Health Centre) with a case control dataset (data from the website of World Health Organisation). Instances of inconsistencies as discovered in the explored datasets are discussed.

Is minimal selfhood a build-in feature of our experiential life (Gallagher 2005; Zahavi 2005, 2014; Legrand 2006) or a later socio-culturally determined acquisition, emerging in the process of social exchanges and mutual interactions (Fonagy et al. 2004; Prinz 2012; Schmid 2014)? This chapter, building mainly on empirical research on affective touch and interoception, argues in favor of a reconceptualization of minimal selfhood that surpasses such debates, and their tacitly “detached,” visuo-spatial models of selfhood and otherness. Instead, the relational origins of the self are traced on fundamental principles and regularities of the human embodied condition, such as the amodal properties that govern the organization of sensorimotor signals into distinct perceptual experiences. Interactive experiences with effects on “within” and “on” the physical boundaries of the body (e.g., skin-to-skin touch) are necessary for such organization in early infancy when the motor system is not as yet developed. Therefore, an experiencing subject is not primarily understood as facing another subject “there.” Instead, the minimal self is by necessity co-constituted by other bodies in physical contact and proximal interaction.

Research in motion perception has concentrated on recognizing regularities in images due to rigid motion. However, rigid motion is only one type of motion that occurs in our world. There is also elastic motion, fluid motion, and turbulent flow. Examples of turbulent flow include clouds, waves, boiling water, rustling leaves, or flags fluttering in the wind. Fully developed turbulent flow is completely chaotic and incoherent motion. Yet, it has a regular statistical structure underlying the apparent chaos. These regularities are due to the coherence of the physical process which generates turbulence. By using image data to characterize the parameters of this process we can make useful predictions and inferences: solid/fluid, viscosity, mean flow velocity. This paper develops a fractal-based model of turbulent flow, demonstrates that turbulence can be recognized visually, and suggests how the model can be used to make inferences about the flowing fluid. The model is implemented by testing for a fractal scale-invariant regularity across space and time using linear spatiotemporal bandpass filters. These filters can also be used to measure optical flow and are similar to receptive fields in the visual cortex.

Natural visual stimuli are multidimensional signals encompassing parameters of space, time, and color. Information in these signal dimensions is redundant because natural images exhibit some spatial, temporal, and chromatic regularities. Further, these signal dimensions are not independent. The incoming visual stimulus is processed and coded in the retina by spatially organized center-surround chromatically antagonistic receptive fields with complex temporal characteristics. The basic hypothesis is that the purpose of retinal signal processing is to reduce signal redundancy and to efficiently code visual information before transmission to higher stages of the visual system. Under this hypothesis it is shown that retinal coding is designed to obtain optimality in coding, considering the relative amount of information in each dimension, corresponding to a high bit rate in the spatial dimension and a low bit rate in the chromatic dimension. The coding in these dimensions achieves favorable image degradation minimizing sensitivity to picture-to-picture variations, considering the computational load required from the retina.

The human visual system is remarkably good at making accurate and reliable interpretations of the world from incomplete or noisy retinal image data. It does this by exploiting implicit knowledge of the statistical regularities of both objects and images. By quantifying the statistical uncertainty inherent in a visual task, ideal observer models specify a limit on the best performance for that task. Comparisons of human and ideal performance, often made by measures of efficiency, can be used to quantify information utilization. Historically, this technique has been developed most fully in the context of visual sensitivity and the early coding of image information. Recently, the ideal observer has been extended to problems of image understanding. I will present an overview of the ideal observer in visual perception research with examples drawn from our work at early and late levels of visual processing. For example, at the image level, human discrimination efficiencies for fractal images are relatively best for fractal dimensions near those of natural image ensembles. At the object level, human classification efficiency for 3D shaded "wire" objects is high enough to exclude generalized 2D template models of recognition and indicates particularly efficient processing of mirror symmetric and coplanar 3D objects.