1,720,958 research outputs found
The effect of context on contour integration: evidence from psychophysical and clinical studies
Full text linkMuch research over the last decade has examined how the visual system links estimates of local image-structure into global, complex forms (e.g. spatially extended contours). Closely related to this issue is a second outstanding research-topic, namely the effect of context on visual processing. Here we bring together these two themes by addressing how contextual information influences the processing of contours (which requires the integration of local elements into a global structure). The key finding, consistently replicated in the two psychophysical studies we conducted, is a robust effect of context on contour localisation. In particular contextual information in the immediate surround of the contour can help but also hinder contour-localisation. A particular importance is given to the source of the effects of context on contour-localisation. Specifically we analyse whether these effects can be accounted for by local uncertainty (about the orientation of each contour-element) or whether also inhibitory interactions between global structures are involved. The results are interpreted in the light of the current models of contour integration. In particular, the finding of a robust effect of context on contour-localisation is used to compare and further constrain these models. Finally, also two clinical studies are presented, where we investigate contour integration and context processing in older observers and in schizophrenia patients. The results of these clinical studies suggest the possibility that deficits specifically shown by some populations (e.g. poor contour detection in schizophrenia) actually reflect an abnormal processing of context.Nell’ultimo decennio, diverse ricerche hanno analizzato come il sistema visivo integri stime di strutture locali di un’immagine in forme globali complesse (ad esempio contorni spazialmente estesi). Altro interessante tema di ricerca, strettamente relato al precedente, riguarda l’effetto del contesto sul processamento visivo. In tale lavoro di tesi si cerca di collegare questi due temi di ricerca, analizzando come l’informazione contestuale influenzi il processamento di contorni (che richiede l’integrazione di elementi locali in una struttura globale). Il risultato chiave evidenziato da entrambi gli studi psicofisici condotti in tale lavoro di tesi è un consistente effetto del contesto sulla localizzazione di contorni. Nello specifico, l’informazione contestuale nelle immediate vicinanze di un contorno può promuovere la localizzazione del contorno stesso, ma anche interferire con essa. Particolare importanza è data all’origine degli effetti del contesto sulla localizzazione di contorni. Analizziamo, in particolare, se l’incertezza locale (circa l’orientamento di ciascun elemento del contorno) possa rendere conto di tali effetti o se siano coinvolte anche interazioni inibitorie tra strutture globali. I risultati sono interpretati alla luce degli attuali modelli di integrazione di contorni. In particolare, il risultato di un robusto effetto del contesto sulla localizzazione di contorni è utilizzato per confrontare e vincolare ulteriormente tali modelli. Infine, sono anche presentati due studi clinici, in cui sono analizzati integrazione di contorni e processamento del contesto nell’invecchiamento e nella schizofrenia. I risultati di tali studi clinici suggeriscono la possibilità che deficits specifici di determinate popolazioni cliniche (ad esempio deficitaria detezione di contorni nella schizofrenia) in realtà riflettano anormale processamento del contesto
Contextual influences in texture-segmentation: Distinct effects from elements along the edge and in the texture-region
Full text linkBoth neurophysiological and psychophysical evidence suggest a strong influence of context on texturesegmentation.
Here we extend and further analyse this issue, with a particular focus on the underlying
mechanism. Specifically, we use a texture-edge discrimination task and separately investigate the effect
of elements far from and along the edge. Consistent with previous studies, we report both an iso-near
contextual effect – whereby performance is better if elements along the edge are iso-oriented compared
to ortho-oriented to the edge – as well as an ortho-far effect – whereby discrimination is higher when
elements far from the edge are orthogonal to the edge. We found that backward mask, which is known
to interrupt re-entrant processing from extrastriate areas, only interferes with the iso-near effect whereas
perturbing orientation, position or contrast polarity of elements far from the edge only abolishes the
ortho-far effect. This suggests that feedback processes may be involved in the iso-near effect. Instead,
the ortho-far effect may be accounted for by recurrent interactions among 1st order filters
Effects of aging on visual contour integration and segmentation.
Full text linkPURPOSE:
Perception of circular disconnected contours requires the integration of relevant local orientation information across space and the suppression of irrelevant orientations. Using a detection of deviation from circularity (DFC) task, the present study examined whether the efficiency of either integrative or suppressive visual mechanisms, or both, declines with age.
METHODS:
Younger and older observers' sensitivities in detecting the DFC of a contour formed by Gabors were compared in three conditions: when all elements were oriented tangentially to the contour, with and without the presence of randomly oriented background noise; and when they had alternated tangential and orthogonal orientations, without background noise.
RESULTS:
In agreement with previous studies, the authors found that younger observers were not impaired in the mixed condition with respect to the tangential condition, suggesting the involvement of a high-level mechanism responding to the global closure information provided by tangential local orientations, even if they are interspersed with orthogonal ones. Instead, older observers were specifically impaired in the mixed condition, suggesting a reduced capability of suppressing nontangential information along the contour, and were also less efficient in suppressing irrelevant orientations in the background.
CONCLUSIONS:
These results support the suggestion that, whereas integrative mechanisms are not affected by age, suppressive mechanisms are
Positional noise in Landolt-C stimuli reduces spatial resolution: A study with younger and older observers
No full textIn the present study we examined the effect of positional noise on spatial resolution in younger and older observers. We used a yes/no discrimination task in which observers indicated whether the size of two gaps in a Landolt-C-like contour was the same or not. The proportion of trials observers perceived one gap larger was measured when gaps-position was fixed (low positional noise) and random (high positional noise). Specifically, we compared, across conditions and groups, the values of threshold, lower and upper asymptote of the psychometric function. In the younger group, noise does not prevent detection of gap-size difference although sensitivity is lower, as revealed by higher threshold and lower upper asymptote, i.e., the proportion of responses “I see a larger gap” at the largest gap-size difference (asymptotic performance). In the older group detection is prevented, as revealed by threshold, lower and upper asymptote data. This may be because, at stimulus onset, high positional noise has associated coarse filter analysers averaging across the two gaps, which cannot be switched off
The role of crowding in contextual influences on contour integration
Full text linkDakin and Baruch (2009) investigated how context influences contour integration, specifically reporting that near-perpendicular surrounding-elements reduced the exposure-duration observers required to localize and determine the shape of contours (compared to performance with randomly oriented surrounds) while near-parallel surrounds increased this time. Here, we ask if this effect might be a manifestation of visual crowding (the disruptive influence of "visual clutter" on object recognition). We first report that the effect generalizes to simple contour-localization (without explicit shape-discrimination) and influences tolerance to orientation jitter in the same way it affects threshold exposure-duration. We next directly examined the role of crowding by quantifying observers' local uncertainty (about the orientation of the elements that comprised our contours), showing that this largely accounts for the effects of context on global contour integration. These findings support the idea that context influences contour integration at a predominantly local stage of processing and that the local effects of crowding eventually influence downstream stages in the cortical processing of visual form
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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