Frédéric J.A.M. Poirier

A biologically plausible model of human shape symmetry perception.

Symmetry is usually computationally expensive to encode reliably, and yet it is relatively effortless to perceive. Here, we extend F. J. A. M. Poirier and H. R. Wilsons (2006) model for shape perception to account for H. R. Wilson and F. Wilkinsons (2002) data on shape symmetry. Because the model already accounts for shape perception, only minimal neural circuitry is required to enable it to encode shape symmetry as well. The model is composed of three main parts: (1) recovery of object position using large-scale non-Fourier V4-like concentric units that respond at the center of concentric contour segments across orientations, (2) around that recovered object center, curvature mechanisms combine multiplicatively the responses of oriented filters to encode object-centric local shape information, with a preference for convexities, and (3) object-centric symmetry mechanisms. Model and human performances are comparable for symmetry perception of shapes. Moreover, with some improvement of edge recovery, the model can encode symmetry axes in natural images such as faces.

Object perception and masking: Contributions of sides and convexities

Object perception uses a variety of visual cues, including shape cues derived from sides and convexities. Two recent masking studies using radial frequency patterns have argued, respectively, for a predominant role of convexity [Habak, C., Wilkinson, F., Zakher, B., & Wilson, H. R. (2004). Curvature population coding for complex shapes in human vision. Vision Research, 44 (24), 2815-2823] or side information [Hess, R. F., Wang, Y. -Z., & Dakin, S. C. (1999). Are judgements of circularity local or global? Vision Research, 39, 4354-4360]. Here we resolve the controversy by separating the masks into their parts (e.g., convexities and sides), and measuring the relative masking influences of the different mask components. We found that both side and convexity information contribute to masking. However, masking due to side information was much less dependent on alignment compared to masking due to convexities. This supports a theory where convexities constitute a prime source of information for shape processing, and sides do also contribute but to a smaller extent.

Perceptive fields of saliency.

Visual saliency plays an important role in early vision. Humans automatically orient to salient information, via covert attentional shifts and overt eye movements. Here, we measured saliency using a novel psychophysical method. The stimulus was a grid of colored oriented lines. Line luminance varied continuously over the image. Using a mouse, participants adjusted line luminance at locations in the image, until all lines appeared homogeneously luminant. Participants tended to increase (or decrease) luminance of lines where perceptually salient information was absent (or present), thus line luminance setting correlates with perceived saliency. Perceptually less salient image regions were more homogenous in color and orientation, consistent with iso-feature suppression. Perceptual fields of contextual modulation are derived, showing increased saliency surrounding color and/or orientation changes, increased saliency for collinear and end-stopping lines, and a nonlinear integration of saliencies across dimensions. It took 3 or more surround items identical to a target to generate a measurable inhibitory effect, beyond which every additional identical item had a monotonic effect. These novel findings allow a revision of current models of visual saliency. In particular, we found evidence of sustained saliency. Moreover, this new method is sensitive within the normal functioning range, unlike most current research methods.

Clouds are not normal occluders, and other oddities: More interactions between textures and lightness illusions

Identical textured disks can appear white or black depending on the luminance properties of the surrounding textured region (B. L. Anderson & J. Winawer, 2005, 2008). This occurs when the stimulus is perceptually segmented in three layers: (1) a uniform foreground disk, (2) a uniform background surface, and (3) a cloud-like layer that covers parts of the foreground and background regions. However, local occlusion cues fail to predict the pattern of data observed, suggesting that in some cases a different strategy may be adopted depending on texture characteristics (F. J. A. M. Poirier, 2009). Here, we produced a variety of stimuli using three different textures and several luminance configurations (including the White and inverse White configurations and the Anderson-Winawer illusion), for which participants reported the perceived characteristics of the central disk (e.g., lightness, transparency, whether the disk was textured). The results show several interactions between textures and luminance configurations, which we account for using mathematical models of previously documented strategies. We show how the strategies chosen depend on an interaction between texture properties and luminance configuration.

Optimal faces for gender and expression: A new technique for measuring dynamic templates used in face perception

Facial expressions are important for human communications. Face perception studies often measure the impact of major degradation (e.g., noise, inversion, short presentations, masking, alterations) on natural expression recognition performance. Here, we introduce a novel face perception technique using rich and undegraded stimuli. Participants modified faces to create optimal representations of given expressions. Using sliders, participants adjusted 53 face components (including 37 dynamic) including head, eye, eyebrows, mouth, and nose shape and position. Data was collected from six participants and 10 conditions (six emotions + pain + gender + neutral). Some expressions had unique features (e.g., frown for anger, upward-curved mouth for happiness), whereas others had shared features (e.g., open eyes and mouth for surprise and fear). Happiness was different from other emotions. Surprise was different from other emotions except fear. Weighted sum morphing provides acceptable stimuli for gender-neutral and dynamic stimuli. Many features were correlated, including (1) head size with internal feature sizes as related to gender, (2) internal feature scaling, and (3) eyebrow height and eye openness as related to surprise and fear. These findings demonstrate the methods validity for measuring the optimal facial expressions, which we argue is a more direct measure of their internal representations.

Head-Eye Coordination Increases with Age and Varies across Countries

Purpose Head movements in older people may contribute to their dizziness and equilibrium problems. Head gain is the ratio of head movement to total movement (head + eye) when executing a saccade to an eccentric target. Two studies have investigated the relationship between head gain and age but have provided conflicting results. Methods We report head gain data collected from research laboratories and optician stores. Our sample sizes are much larger (n = 657 for laboratory, n = 64,458 for optician stores), permitting more detailed analyses. Results The head-eye coefficient, expressed as 100 times the square root of head gain, was bimodal with one mode of primarily eye movers and one mode of eye-and-head movers. Head-eye coefficient increased with age and was invariant with eye correction and gender. We also found an effect of nation that seemed associated with gross domestic product or by latitude (in the northern hemisphere) and log population density. Discussion Assuming that head movements and visual distortions contribute to dizziness and equilibrium problems, our study suggests that customizing eyewear based on age and country may help in reducing the prevalence of problems associated with head and/or eye movements.

Subjectively homogeneous noise over written text as a tool to investigate the perceptual mechanisms involved in reading.

In an effort to understand the factors influencing text legibility in natural reading, we adapted the visual spread method (Poirier, Gosselin, & Arguin, 2008) to natural text. Stimuli were sentences conforming to MNREAD standards (Legge, Ross, Luebker, & LaMay 1989) mixed with dynamic probabilistic noise-i.e., each pixel in the image is associated with a probability that its polarity is inverted on a given refresh cycle of the display screen. Noise level varied continuously over the image as initially determined by Gaussian-filtered noise. Participants adjusted noise levels in the text using the mouse until the text appeared homogenously noisy. We assume that participants increased (or decreased) noise at locations where stimulus features were easy (or difficult) to encode and thus that local noise settings correlate with legibility. Data from 11 participants and 30 sentences revealed interesting effects, demonstrating the validity of the method for assessing the impact of various factors on noise resistance in natural text. For example, participants increased noise over (a) spaces and adjacent letters, (b) the second half of words, (c) words with more orthographic neighbors but fewer phonological neighbors, (d) less useful word types, (e) less complex letters, and (f) diagnostic letters (a novel metric). Our observations also offer significant insights on constraints acting upon letter identification as well as on higher-level processes that are involved in reading.

Visual spread: A new method for quantifying context effects in early vision

Introduction: Context effects are common in perception, performance, and cognition. For example, context effects modulate visual saliency, which in turn guides both attention shifts and eye movements. Using a novel psychophysical method to measure saliency, we derive perceptual fields of contextual modulation. Methods: The stimulus is a grid of rightor left-oblique red or green lines on a black background. Line luminance varies continuously over the image, which participants (N = 10) adjust towards equiluminance, increasing or decreasing local luminance using a mouse. Assuming that participants compensate for saliency, local luminance setting correlates negatively with local saliency. Results: Perceptually salient image regions are heterogeneous in color and orientation, indicative of short-range iso-feature inhibition. Predictors of local saliency include: (1) enhancement near color differences, (2) inhibition of parallel orientations, especially over short distances and for non-continuous lines, (3) enhancement of orthogonal orientations, especially for end-stopping lines, (4) inhibitory effects of same-color and same-orientation are additive, and (5) enhancement effects of different-color and differentorientation sum with reduced gain. On average, these effects account for 60% of the variance in the data. Discussion: This new method is ideal for testing clinical populations because: (1) 1–2 h of testing is often sufficient, (2) it is sensitive within the normal functioning range, where most current research methods produce ceiling effects and flat reaction time functions, and (3) it generates estimates of several effects simultaneously. Moreover, although here we used a simple stimulus to validate the method, it can be generalized to other stimuli (e.g. reading, visual textures, natural images).