A Note on Object Class Representation and Categorical Perception

We present a novel scheme ("Categorical Basis Functions", CBF) for object class representation in the brain and contrast it to the "Chorus of Prototypes" scheme recently proposed by Edelman. The power and flexibility of CBF is demonstrated in two examples. CBF is then applied to investigate the phenomenon of Categorical Perception, in particular the finding by Bulthoff et al. (1998) of categorization of faces by gender without corresponding Categorical Perception. Here, CBF makes predictions that can be tested in a psychophysical experiment. Finally, experiments are suggested to further test CBF.

A Detailed Look at Scale and Translation Invariance in a Hierarchical Neural Model of Visual Object Recognition

The HMAX model has recently been proposed by Riesenhuber & Poggio as a hierarchical model of position- and size-invariant object recognition in visual cortex. It has also turned out to model successfully a number of other properties of the ventral visual stream (the visual pathway thought to be crucial for object recognition in cortex), and particularly of (view-tuned) neurons in macaque inferotemporal cortex, the brain area at the top of the ventral stream. The original modeling study only used ``paperclip'' stimuli, as in the corresponding physiology experiment, and did not explore systematically how model units' invariance properties depended on model parameters. In this study, we aimed at a deeper understanding of the inner workings of HMAX and its performance for various parameter settings and ``natural'' stimulus classes. We examined HMAX responses for different stimulus sizes and positions systematically and found a dependence of model units' responses on stimulus position for which a quantitative description is offered. Interestingly, we find that scale invariance properties of hierarchical neural models are not independent of stimulus class, as opposed to translation invariance, even though both are affine transformations within the image plane.

The Individual is Nothing, the Class Everything: Psychophysics and Modeling of Recognition in Obect Classes

Most psychophysical studies of object recognition have focussed on the recognition and representation of individual objects subjects had previously explicitely been trained on. Correspondingly, modeling studies have often employed a 'grandmother'-type representation where the objects to be recognized were represented by individual units. However, objects in the natural world are commonly members of a class containing a number of visually similar objects, such as faces, for which physiology studies have provided support for a representation based on a sparse population code, which permits generalization from the learned exemplars to novel objects of that class. In this paper, we present results from psychophysical and modeling studies intended to investigate object recognition in natural ('continuous') object classes. In two experiments, subjects were trained to perform subordinate level discrimination in a continuous object class - images of computer-rendered cars - created using a 3D morphing system. By comparing the recognition performance of trained and untrained subjects we could estimate the effects of viewpoint-specific training and infer properties of the object class-specific representation learned as a result of training. We then compared the experimental findings to simulations, building on our recently presented HMAX model of object recognition in cortex, to investigate the computational properties of a population-based object class representation as outlined above. We find experimental evidence, supported by modeling results, that training builds a viewpoint- and class-specific representation that supplements a pre-existing repre-sentation with lower shape discriminability but possibly greater viewpoint invariance.

Computational Models of Object Recognition in Cortex: A Review

Understanding how biological visual systems perform object recognition is one of the ultimate goals in computational neuroscience. Among the biological models of recognition the main distinctions are between feedforward and feedback and between object-centered and view-centered. From a computational viewpoint the different recognition tasks - for instance categorization and identification - are very similar, representing different trade-offs between specificity and invariance. Thus the different tasks do not strictly require different classes of models. The focus of the review is on feedforward, view-based models that are supported by psychophysical and physiological data.

Modeling Invariances in Inferotemporal Cell Tuning

In macaque inferotemporal cortex (IT), neurons have been found to respond selectively to complex shapes while showing broad tuning ("invariance") with respect to stimulus transformations such as translation and scale changes and a limited tuning to rotation in depth. Training monkeys with novel, paperclip-like objects, Logothetis et al. could investigate whether these invariance properties are due to experience with exhaustively many transformed instances of an object or if there are mechanisms that allow the cells to show response invariance also to previously unseen instances of that object. They found object-selective cells in anterior IT which exhibited limited invariance to various transformations after training with single object views. While previous models accounted for the tuning of the cells for rotations in depth and for their selectivity to a specific object relative to a population of distractor objects, the model described here attempts to explain in a biologically plausible way the additional properties of translation and size invariance. Using the same stimuli as in the experiment, we find that model IT neurons exhibit invariance properties which closely parallel those of real neurons. Simulations show that the model is capable of unsupervised learning of view-tuned neurons. The model also allows to make experimentally testable predictions regarding novel stimulus transformations and combinations of stimuli.

Generalization over contrast and mirror reversal, but not figure-ground reversal, in an "edge-based

Baylis & Driver (Nature Neuroscience, 2001) have recently presented data on the response of neurons in macaque inferotemporal cortex (IT) to various stimulus transformations. They report that neurons can generalize over contrast and mirror reversal, but not over figure-ground reversal. This finding is taken to demonstrate that ``the selectivity of IT neurons is not determined simply by the distinctive contours in a display, contrary to simple edge-based models of shape recognition'', citing our recently presented model of object recognition in cortex (Riesenhuber & Poggio, Nature Neuroscience, 1999). In this memo, I show that the main effects of the experiment can be obtained by performing the appropriate simulations in our simple feedforward model. This suggests for IT cell tuning that the possible contributions of explicit edge assignment processes postulated in (Baylis & Driver, 2001) might be smaller than expected.

Categorization in IT and PFC: Model and Experiments

In a recent experiment, Freedman et al. recorded from inferotemporal (IT) and prefrontal cortices (PFC) of monkeys performing a "cat/dog" categorization task (Freedman 2001 and Freedman, Riesenhuber, Poggio, Miller 2001). In this paper we analyze the tuning properties of view-tuned units in our HMAX model of object recognition in cortex (Riesenhuber 1999) using the same paradigm and stimuli as in the experiment. We then compare the simulation results to the monkey inferotemporal neuron population data. We find that view-tuned model IT units that were trained without any explicit category information can show category-related tuning as observed in the experiment. This suggests that the tuning properties of experimental IT neurons might primarily be shaped by bottom-up stimulus-space statistics, with little influence of top-down task-specific information. The population of experimental PFC neurons, on the other hand, shows tuning properties that cannot be explained just by stimulus tuning. These analyses are compatible with a model of object recognition in cortex (Riesenhuber 2000) in which a population of shape-tuned neurons provides a general basis for neurons tuned to different recognition tasks.

Investigating shape representation in area V4 with HMAX: Orientation and Grating selectivities

The question of how shape is represented is of central interest to understanding visual processing in cortex. While tuning properties of the cells in early part of the ventral visual stream, thought to be responsible for object recognition in the primate, are comparatively well understood, several different theories have been proposed regarding tuning in higher visual areas, such as V4. We used the model of object recognition in cortex presented by Riesenhuber and Poggio (1999), where more complex shape tuning in higher layers is the result of combining afferent inputs tuned to simpler features, and compared the tuning properties of model units in intermediate layers to those of V4 neurons from the literature. In particular, we investigated the issue of shape representation in visual area V1 and V4 using oriented bars and various types of gratings (polar, hyperbolic, and Cartesian), as used in several physiology experiments. Our computational model was able to reproduce several physiological findings, such as the broadening distribution of the orientation bandwidths and the emergence of a bias toward non-Cartesian stimuli. Interestingly, the simulation results suggest that some V4 neurons receive input from afferents with spatially separated receptive fields, leading to experimentally testable predictions. However, the simulations also show that the stimulus set of Cartesian and non-Cartesian gratings is not sufficiently complex to probe shape tuning in higher areas, necessitating the use of more complex stimulus sets.

On the difficulty of feature-based attentional modulations in visual object recognition: A modeling study.

Numerous psychophysical experiments have shown an important role for attentional modulations in vision. Behaviorally, allocation of attention can improve performance in object detection and recognition tasks. At the neural level, attention increases firing rates of neurons in visual cortex whose preferred stimulus is currently attended to. However, it is not yet known how these two phenomena are linked, i.e., how the visual system could be "tuned" in a task-dependent fashion to improve task performance. To answer this question, we performed simulations with the HMAX model of object recognition in cortex [45]. We modulated firing rates of model neurons in accordance with experimental results about effects of feature-based attention on single neurons and measured changes in the model's performance in a variety of object recognition tasks. It turned out that recognition performance could only be improved under very limited circumstances and that attentional influences on the process of object recognition per se tend to display a lack of specificity or raise false alarm rates. These observations lead us to postulate a new role for the observed attention-related neural response modulations.

INFO2009 - Group 14: DRM & Copyright

A short video about DRM & Copyright (with chins).

Programa de enseñanzas de la Universidad de Salamanca sobre India e Irán a través de Internet.

El Experto Universitario en Lenguas y Culturas de India e Irán se desarrolla en la Universidad de Salamanca, por iniciativa del 'Equipo Indoiranístico' del departamento de Filología Clásica. Su dirección incluye a la directora , Ana Agud; al diseñador y coordinador informático, César Fidalgo; al secretario, Íñigo Eguaras (becario de investigación). Los materiales lectivos son suministrados por profesores pertenecientes unos al propio Equipo Indoiranístico y otros a diversas instituciones nacionales y extranjeras. Intervienen los siguientes profesores: Ana Agud, Alberto Cantera, Iñigo Eguaras, Fernando López Serrano (Universidad de Salamanca); Maximilian Magrini-Kunze, Eva Fernández del Campo y Eva Borreguero (Universidad Complutense); Francisco Rubio (Universidad de Saarbrücken); Abraham Vélez de Cea (Universidad de Georgetown); Zácara (Universidad Autónoma de Madrid); Said Hushanguí (Embajada de Irán); María Jesús Merinero (Universidad de Extremadura). Cada uno prepara y suministra sus materiales desde su institución de origen. El proyecto cumple dos objetivos: el puramente académico de impartir, por primera vez en España y en castellano, un programa de estudios completo sobre dos importantes países asiáticos y sus culturas; el de poner a punto un diseño didáctico que permita volcar la totalidad de los contenidos lectivos en documentos asequibles a través de la red, diseñándolos de forma que se utilicen las modernas tecnologías de programación y comunicación en orden a optimizar el aprendizaje. El trabajo académico es desarrollado por los profesores y consiste en la elaboración de materiales lectivos en soporte informático (temas teóricos y todo tipo de ejercicios prácticos y de autoevaluación), que se remiten periódicamente al diseñador para su maquetación y eventual procesamiento informático en orden a dotarlos de características didácticas apropiadas al entorno de red. El diseñador los procesa y los cuelga en la red. El diseñador aporta una labor de diseño informático avanzado que se detalla en la memoria, y que arroja materiales didácticos muy eficaces para el aprendizaje. Los alumnos están en conexión entre sí y con el diseñador-coodinador y los profesores mediante un grupo de correo electrónico, de este modo plantean sus problemas técnicos (que son atendidos por el diseñador), académicos (que son atendidos por el secretario) y científicos (que son atendidos por los profesores). El grupo contribuye a que se genere una intensa comunicación, prácticamente diaria, entre los alumnos mismos, que en la práctica interactúan entre sí más que en la enseñanza presencial convencional. Las cuestiones que se suscitan permiten a los profesores y al diseñador un feedback constante. Al grupo llegan también informaciones sobre todo tipo de eventos de interés para los alumnos (conferencias, congresos, cursos de especialización) suministrados por todos los participantes. A los materiales didácticos de la asignatura se añaden archivos documentales de otros tipos que ponen a disposición de los estudiantes por una parte la investigación reciente de los profesores (archivos de artículos y publicaciones de los profesores), y por la otra todo tipo de materiales e información adicional sobre los países que se estudian (archivos de imágenes, de prensa, de información sobre viajes, de cocina, etc.). El proyecto está ya en su tercera edición. Dado que se trata de un 'título propio' de contenido cultural, que no proporciona acceso al mercado laboral (ya que todavía no hay demandas de este tipo), se comprueba que una proporción no trivial de los matriculados no realiza las evaluaciones y por lo tanto no obtiene el título, pero expresan su enorme satisfacción por la calidad de los materiales ofrecidos, y algunos incluso han llegado a matricularse de una nueva edición para poder acceder a las mejoras y ampliaciones de los materiales. Las asignaturas lingüísticas son las que más contribuyen al abandono, ya que las lenguas orientales clásicas son realmente muy difíciles. Este es el motivo por el que la dirección se plantea la posibilidad de suprimir su obligatoriedad. Los alumnos valoran muy positivamente la presentación de los materiales lingüísticos, de notable refinamiento técnico, así como la abundancia y solvencia científica de los temas de las asignaturas culturales. Se quejan de la falta de un soporte informático más eficáz en la propia universidad, para el acceso a estos materiales. De hecho el coordinador ha optado por soluciones de red alternativas a la plataforma de la universidad, y ésta se utiliza como depósito de materiales a largo plazo. En los alumnos que realizan las evaluaciones se advierte en general una buena recepción de los contenidos. Cada vez son más los que acceden a este programa por razones devocionales (en los estudios de la India hay un público de este tipo que intenta acercarse, pero que en general no puede con el nivel científico del programa, lo que es causa de abandono). La participación en este programa ha empezado a generar doctorandos y becarios (en la actualidad trabajan en la órbita del mismo cuatro, y está en trámite una quinta beca). Los temas se suministran en PDF, con todo tipo de ayudas informáticas (presentaciones en FLASH, archivos de audio, links hacia todo tipo de informaciones adicionales, en particular gráficas). Se trabaja con textos preparados por los propios profesores, imágenes tanto propias como de la bibliografía y archivos de audio. Todos los materiales son inéditos y exclusivos del programa conformando un bloque bibliográfico único en su género y muy voluminoso; de hecho, el programa sólo obliga a cursar veinte créditos, pero la oferta se acerca a los setenta. El objetivo a medio plazo es convertir este programa en un segundo ciclo como título propio de la Universidad de Salamanca, acumulable a otras titulaciones. El programa es por lo tanto el primer paso para la introducción de estos estudios en nuestro país, y se ha diseñado como enseñanza on line porque la necesidades del país en cuanto a información sobre India e Irán no pueden resolverse por la vía convencional de los estudios presenciales, dada la escasez y dispersión de los especialistas, de modo que cualquier enseñanza oficial sobre esta temática tendrá que plantearse inevitablemente como enseñanza virtual.

Geoffrey Hill and performative utterance

Utterance of a sentence in poetry can be performative, and explicitly so. The best-known of Geoffrey Hill’s critical essays denies this, but his own poetry demonstrates it. I clarify these claims and explain why they matter. What Hill denies illuminates anxieties about responsibility and commitment that poets and critics share with philosophers. What Hill demonstrates affords opportunities for mutual benefit between philosophy and criticism.