Roger Watt

Do the eyes have it? Cues to the direction of social attention

The face communicates an impressive amount of visual information. We use it to identify its owner, how they are feeling and to help us understand what they are saying. Models of face processing have considered how we extract such meaning from the face but have ignored another important signal - eye gaze. In this article we begin by reviewing evidence from recent neurophysiological studies that suggests that the eyes constitute a special stimulus in at least two senses. First, the structure of the eyes is such that it provides us with a particularly powerful signal to the direction of another persons gaze, and second, we may have evolved neural mechanisms devoted to gaze processing. As a result, gaze direction is analysed rapidly and automatically, and is able to trigger reflexive shifts of an observers visual attention. However, understanding where another individual is directing their attention involves more than simply analysing their gaze direction. We go on to describe research with adult participants, children and non-human primates that suggests that other cues such as head orientation and pointing gestures make significant contributions to the computation of anothers direction of attention.

The computation of orientation statistics from visual texture.

This paper examines how observers estimate the overall orientation of spatially disorganised textures containing variable orientation. Experiments used asymmetrical distributions of orientations to separate the predictions from different models of average orientation estimation. Stimuli were composed of two spatially intermingled sets of oriented patches, each set having Gaussian distributed element orientation. The threshold separation of the means of the two sets was determined for a variety of tasks. Discrimination of these textures from a reference composed of two sets with the same mean orientation was well predicted by discrimination of orientation variability. A single interval judgement of which set contained more elements required a greater separation of the set orientations and suggested that the sets must be resolved in the orientation domain for independent representation of their properties. That resolution is required to perform this task further suggests that orientational skew is not coded. Threshold offsets for judgement of average orientation were re-expressed as shifts of four candidate features for coding the central tendency of texel orientations. Comparison with similar thresholds for single distributions of orientations indicated that average orientation is assigned to the centroid of a set of orientation measures.

The function of dynamic grouping in vision

In this review we consider the logic and the evidence relating to the issue of dynamic grouping in human vision. Dynamic grouping is required when the visual system is creating novel descriptions, either because it is dealing with novel stimuli or it is providing information for novel purposes. In such cases, dynamic grouping provides a mechanism for discovering regularities in the data. We consider a number of examples, including grouping of visual information into surface descriptions and contour descriptions. The main issues that arise concern the configurability of the process and the effects of the propagation of local configurations. We then turn to the complex issue of visual search. Visual search allows the experimenter to establish something of the nature of pre-attentive visual descriptions and how these differ from attentive descriptions. We describe the basic results of visual search experiments in terms of the type of grouping involved. Finally, we consider the hypothesis that dynamic grouping is signalled by neuronal synchrony.

A Psychological Investigation of Meaning in Music

Music has a strong effect on peoples mental state and behaviour. This effect can be at a simple motor level, or it can be at a more complex level of arbitrary association or it can be at a much more complex cognitive and representational level. This latter case is of specific interest because it implies that, to some degree, music can be said to have a content that is not musical. Thus, when music is taken to depict a rough sea, then something in the music causes that: the music contains something that is being taken to signify roughness-of-sea. The existence of a large, natural yet arbitrary vocabulary (as is demonstrably the case for language) to relate musical expressions to non-musical events/objects seems implausible. In this paper, the possibility is explored that the vocabulary of musical expression concerns psychological aspects of people. Thus music, it is hypothesised, can express male/female-ness and good/evil-ness and happy/sad-ness and so on. Data obtained in a novel paradigm designed to test this hypothesis are described. Substantial support for the hypothesis is found: the implications of this are discussed.

Families of models for gabor paths demonstrate the importance of spatial adjacency

This paper reports psychophysical and modelling results concerning the contour-detection paradigm of D. J. Field, A. Hayes, and R. F. Hess (1993). We measured psychophysically the maximum tolerable contour curvature (path angle) as a function of contour length. We compared these data to the predictions of an association field (D. J. Field et al., 1993) model based on the relative positions and mutual orientations of nearby elements and to models that explicitly link adjacent elements into chains and characterize each chain by its sequence of contour bends. For every stimulus, a large set of chains is produced and the target identified as the chain with the lowest maximum bend. We tested two different types of linking process: isotropic (linking one element to any other nearby) and anisotropic (linking one element to any others nearby along the orientation of its axis). All of these models can account for our data. Moreover, we show that the pattern of results due to path angle is principally a product of the distribution of spurious contours in the randomly oriented background. Given that some of the models do not embody constraints of orientation relationships between linked elements, this finding shows the importance for early vision in deciding which local elements are to be associated.

A Computational Examination of Image Segmentation and the Initial Stages of Human Vision

A novel approach to the question of image segmentation is considered. Instead of relying on edge-detection mechanisms to encircle the image of an object, it is proposed that the general Gestalt-like properties of images of objects can be used. These manifest themselves as particularly simple properties at relatively coarse spatial scales when the image is filtered with orientation-selective filters. The first part of the paper is concerned with a computational analysis of this proposal. A key issue is the question of how the information in filtered images is extracted. A simple primitive parametric description of zero-bounded blobs is used in this paper. It is shown how such a scheme can support readily the clustering of features with Gestalt-like properties. In the second part of the paper, two general difficulties for vision that are particularly severe for the present scheme are considered. The first of these are the effects of strongly asymmetric illumination. The second are the effects of using a cluttered scene. In each case the nature of the problem is examined and the nature of the computational solution considered.

The Combination of Filters in Early Spatial Vision: A Retrospective Analysis of the Mirage Model

Since the discovery of spatial-frequency-tuned channels in the visual system, most theories attempting to account for pattern encoding have assumed that the filters can be independently accessed and flexibly combined. We review here an alternative model, ‘MIRAGE’, in which the filters are inflexibly combined before pattern analysis. In the MIRAGE model the half-wave rectified outputs of all spatial-frequency channels are combined before locating spatial zero-bounded regions in the neural image, which serve as the spatial primitives for pattern analysis. We describe the evidence that led to this model, and review recent evidence on the rules of filter combination.

The Perception of Tempo in Music

Tempo is one factor that is frequently associated with the expressive nature of a piece of music. Composers often indicate the tempo of a piece of music through the use of numerical markings (beats min−1) and subjective terms (adagio, allegro). Three studies were conducted to assess whether listeners were able to make consistent judgments about tempo that varied from piece to piece. Listeners heard short extracts of Scottish music played at a range of tempi and were asked to make a two-alternative forced choice of “too fast” or “too slow” for each extract. The responses for each study were plotted as proportion of too fast responses as a function of tempo for each piece, and cumulative normal curves were fitted to each data set. The point where these curves cross 0.5 is the tempo at which the music sounds right to the listeners, referred to as the optimal tempo. The results from each study show that listeners are capable of making consistent tempo judgments and that the optimal tempo varies across extracts. The results also revealed that rhythm plays a role, but not the only role in making temporal judgments.

A role for eyebrows in regulating the visibility of eye gaze direction

The human eye is unique amongst those of primates in having white sclera against which the dark iris is clearly visible. This high-contrast structure makes the gaze direction of a human potentially easily perceptible to others. For a social creature such as a human, the ability to perceive the direction of anothers gaze may be very useful, since gaze usually signals attention. We report data showing that the accuracy of gaze deviation detection is independent of viewing distance up to a certain critical distance, beyond which it collapses. This is, of itself, surprising since most visual tasks are performed better at closer viewing distances. Our data also show that the critical distance, but not accuracy, is affected by the position of the eyebrows so that lowering the eyebrows reduces the critical distance. These findings show that mechanisms exist by which humans could expand or restrict the availability of their gaze direction to others. A way to regulate the availability of the gaze direction signal could be an advantage. We show that an interpretation of eyebrow function in these terms provides a novel explanation for several well-known eyebrow actions, including the eyebrow flash.

The utility of image descriptions in the initial stages of vision: A case study of printed text

Vision research has made very substantial progress towards understanding how we see. It is one area of psychology where the three-way thrust of behavioural measurements (psychophysics), brain imaging, and computational studies have been combined quite routinely for some years. The purpose of this paper is to demonstrate a relatively unusual form of computational modelling that we characterise as involving image descriptions. Image descriptions are statements about structures in images and relationships between structures. Most modelling in vision is either conceived in fairly abstract terms, or is done at the level of images. Neither is entirely satisfactory, and image descriptions are a simple formulation of age-old ideas about a Vocabulary of image features that are detected and parameterized from actual digital images. For our example, we use the domain of the visual perception of printed text. This is an area that has been characterized by thorough, robust psychophysical experiments. The fundamental requirements of visual processing in this domain are: grouping of some parts if the image into words; at the same time segmenting words from each other. We show how these are readily understood in terms of our model of image descriptions, and show quantitatively that typographical practice, refined over centuries, is about optimum for the visual system at least as represented by our model. In addition, we show that the same notion of image descriptions could, in principle, support word recognition in certain circumstances.