A common framework for visual crowding in typical and amblyopic vision

Amblyopia is a developmental disorder of vision characterised by reduced acuity in one eye despite optical correction. When associated with strabismus, foveal vision is impaired by crowding: objects that are readily recognised in isolation become indistinguishable in clutter. In typical vision, crowding is minimal in the fovea and increases in the visual periphery. According to pooling accounts, the increase of crowding in the periphery arises from the integration of adjacent objects to promote perceptual homogeneity where sampling is insufficient and neurons have large receptive fields. It is unclear whether amblyopic crowding represents the same process. In this thesis I characterise amblyopic crowding, and investigate whether it can be understood within the same pooling framework as peripheral crowding. First, I show that amblyopic crowding systematically shifts the appearance of crowded objects to promote perceptual homogeneity, matching the perceptual effects in the periphery. A model simulating pooled responses of populations of visual neurons accurately characterises these effects in amblyopic and peripheral crowding, suggesting a common underlying mechanism. Second, I investigate the pattern of amblyopic crowding across the visual field and its neural correlates. I show that amblyopic crowding is elevated relative to typical vision in both fovea and periphery. At a group level, the increase of crowding in amblyopia and the typical periphery matches the increase of fMRI population receptive field (pRF) estimates in V1, V2, and V3, but at an individual level there is no correlation. Finally, I investigate the effects of higher-level grouping processes by examining whether uniformity in global clutter configuration modulates amblyopic crowding. I find that in most cases, clutter disrupts recognition in amblyopia regardless of global configuration, suggesting that contrary to the periphery, amblyopic crowding is largely unaffected by higher-level grouping processes. Therefore, on the whole pooling provides a successful framework for both amblyopic and peripheral crowdin

Crowding changes appearance systematically in peripheral, amblyopic, and developing vision

Visual crowding is the disruptive effect of clutter on object recognition. Although most prominent in adult peripheral vision, crowding also disrupts foveal vision in typically developing children and those with strabismic amblyopia. Do these crowding effects share the same mechanism? Here we exploit observations that crowded errors in peripheral vision are not random: Target objects appear either averaged with the flankers (assimilation) or replaced by them (substitution). If amblyopic and developmental crowding share the same mechanism, then their errors should be similarly systematic. We tested foveal vision in children aged 3 to 8 years with typical vision or strabismic amblyopia and peripheral vision in typical adults. The perceptual effects of crowding were measured by requiring observers to adjust a reference stimulus to match the perceived orientation of a target “Vac-Man” element. When the target was surrounded by flankers that differed by ± 30°, all three groups (adults and children with typical or amblyopic vision) reported orientations between the target and flankers (assimilation). Errors were reduced with ± 90° differences but primarily matched the flanker orientation (substitution) when they did occur. A population pooling model of crowding successfully simulated this pattern of errors in all three groups. We conclude that the perceptual effects of amblyopic and developing crowding are systematic and resemble the near periphery in adults, suggesting a common underlying mechanism

The Role of Emotional Expression and Eccentricity on Gaze Perception

The perception of another’s gaze direction and facial expression complements verbal communication and modulates how we interact with other people. However, our perception of these two cues is not always accurate, even when we are looking directly at the person. In addition, in many cases social communication occurs within groups of people where we can’t always look directly at every person in the group. Here, we sought to examine how the presence of other people influences our perception of a target face. We asked participants to judge the direction of gaze of the target face as either looking to their left, to their right or directly at them, when the face was viewed on its own or viewed within a group of other identity faces. The target face either had an angry or a neutral expression and was viewed directly (foveal experiment), or within peripheral vision (peripheral experiment). When the target was viewed within a group, the flanking faces also had either neutral or angry expressions and their gaze was in one of five different directions (from averted leftwards to averted rightwards in steps of 10°). When the target face was viewed foveally there was no effect of target emotion on participants’ judgments of its gaze direction. There was also no effect of the presence of flankers (regardless of expression) on the perception of the target gaze. When the target face was viewed peripherally, participants judged its direction of gaze to be direct over a wider range of gaze deviations than when viewed foveally, and more so for angry faces than neutral faces. We also find that flankers (regardless of emotional expression) did not influence performance. This suggests that observers judge that angry faces were looking at them over a broad range of gaze deviations in the periphery only, possibly resulting from increased uncertainty about the stimulus

Crowding for faces is determined by visual (not holistic) similarity: Evidence from judgements of eye position

Crowding (the disruption of object recognition in clutter) presents the fundamental limitation on peripheral vision. For simple objects, crowding is strong when target/flanker elements are similar and weak when they differ – a selectivity for target-flanker similarity. In contrast, the identification of upright holistically-processed face stimuli is more strongly impaired by upright than inverted flankers, whereas inverted face-targets are impaired by both – a pattern attributed to an additional stage of crowding selective for “holistic similarity” between faces. We propose instead that crowding is selective for target-flanker similarity in all stimuli, but that this selectivity is obscured by task difficulty with inverted face-targets. Using judgements of horizontal eye-position that are minimally affected by inversion, we find that crowding is strong when target-flanker orientations match and weak when they differ for both upright and inverted face-targets. By increasing task difficulty, we show that this selectivity for target-flanker similarity is obscured even for upright face-targets. We further demonstrate that this selectivity follows differences in the spatial order of facial features, rather than “holistic similarity” per se. There is consequently no need to invoke a distinct stage of holistic crowding for faces – crowding is selective for target-flanker similarity, even with complex stimuli such as faces

ESMO Consensus conferences:guidelines on malignant lymphoma. part 2: marginal zone lymphoma, mantle cell lymphoma, peripheral T-cell lymphoma

<p>To complement the existing treatment guidelines for all tumour types, ESMO organizes consensus conferences to focus on specific issues in each type of tumour. In this setting, a consensus conference on the management of lymphoma was held on 18 June 2011 in Lugano, next to the 11th International Conference on Malignant Lymphoma. The conference convened similar to 30 experts from all around Europe, and selected six lymphoma entities to be addressed; for each of them, three to five open questions were to be addressed by the experts. For each question, a recommendation should be given by the panel, referring to the strength of the recommendation based on the level of evidence. This consensus report focuses on the three less common lymphoproliferative malignancies: marginal zone lymphoma, mantle cell lymphoma, and peripheral T-cell lymphomas. A first report had focused on diffuse large B-cell lymphoma, follicular lymphoma, and chronic lymphocytic leukaemia.</p>