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Riani M, Atkinson AC, Torti F, Corbellini A. Robust correspondence analysis. J R Stat Soc Ser C Appl Stat 2022. [DOI: 10.1111/rssc.12580] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Affiliation(s)
- Marco Riani
- Dipartimento di Scienze Economiche e Aziendale and Interdepartmental Centre for Robust StatisticsUniversità di Parma Parma43100Italy
| | | | - Francesca Torti
- European CommissionJoint Research Centre (JRC) Ispra21027Italy
| | - Aldo Corbellini
- Dipartimento di Scienze Economiche e Aziendale and Interdepartmental Centre for Robust StatisticsUniversità di Parma Parma43100Italy
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Riani M, Atkinson AC, Corbellini A. Automatic robust Box–Cox and extended Yeo–Johnson transformations in regression. STAT METHOD APPL-GER 2022. [DOI: 10.1007/s10260-022-00640-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
AbstractThe paper introduces an automatic procedure for the parametric transformation of the response in regression models to approximate normality. We consider the Box–Cox transformation and its generalization to the extended Yeo–Johnson transformation which allows for both positive and negative responses. A simulation study illuminates the superior comparative properties of our automatic procedure for the Box–Cox transformation. The usefulness of our procedure is demonstrated on four sets of data, two including negative observations. An important theoretical development is an extension of the Bayesian Information Criterion (BIC) to the comparison of models following the deletion of observations, the number deleted here depending on the transformation parameter.
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Cozzini P, Cavaliere F, Spaggiari G, Morelli G, Riani M. Computational methods on food contact chemicals: Big data and in silico screening on nuclear receptors family. Chemosphere 2022; 292:133422. [PMID: 34971624 DOI: 10.1016/j.chemosphere.2021.133422] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/25/2021] [Revised: 12/20/2021] [Accepted: 12/22/2021] [Indexed: 06/14/2023]
Abstract
According to Eurostat, the EU production of chemicals hazardous to health reached 211 million tonnes in 2019. Thus, the possibility that some of these chemical compounds interact negatively with the human endocrine system has received, especially in the last decade, considerable attention from the scientific community. It is obvious that given the large number of chemical compounds it is impossible to use in vitro/in vivo tests for identifying all the possible toxic interactions of these chemicals and their metabolites. In addition, the poor availability of highly curated databases from which to retrieve and download the chemical, structure, and regulative information about all food contact chemicals has delayed the application of in silico methods. To overcome these problems, in this study we use robust computational approaches, based on a combination of highly curated databases and molecular docking, in order to screen all food contact chemicals against the nuclear receptor family in a cost and time-effective manner.
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Affiliation(s)
- Pietro Cozzini
- Molecular Modelling Lab, Department of Food and Drug, University of Parma, Parco Area Delle Scienze 17/A, 43124, Parma, Italy.
| | - Francesca Cavaliere
- Molecular Modelling Lab, Department of Food and Drug, University of Parma, Parco Area Delle Scienze 17/A, 43124, Parma, Italy.
| | - Giulia Spaggiari
- Molecular Modelling Lab, Department of Food and Drug, University of Parma, Parco Area Delle Scienze 17/A, 43124, Parma, Italy.
| | - Gianluca Morelli
- Department of Economics and Management and Interdepartmental Center of Robust Statistics, University of Parma, Via J. F. Kennedy 6, 43100, Parma, Italy.
| | - Marco Riani
- Department of Economics and Management and Interdepartmental Center of Robust Statistics, University of Parma, Via J. F. Kennedy 6, 43100, Parma, Italy.
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Abstract
A new methodology for constrained parsimonious model-based clustering is introduced, where some tuning parameter allows to control the strength of these constraints. The methodology includes the 14 parsimonious models that are often applied in model-based clustering when assuming normal components as limit cases. This is done in a natural way by filling the gap among models and providing a smooth transition among them. The methodology provides mathematically well-defined problems and is also useful to prevent us from obtaining spurious solutions. Novel information criteria are proposed to help the user in choosing parameters. The interest of the proposed methodology is illustrated through simulation studies and a real-data application on COVID data.
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Affiliation(s)
- Luis A. García-Escudero
- Department of Statistics and Operational Research and IMUVA, University of Valladolid, Valladolid, Spain
| | - Agustín Mayo-Iscar
- Department of Statistics and Operational Research and IMUVA, University of Valladolid, Valladolid, Spain
| | - Marco Riani
- Department of Economics and Management and Interdepartmental Centre of Robust Statistics, University of Parma, Parma, Italy
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Salini S, Laurini F, Morelli G, Riani M, Cerioli A. Covariance matrices of S robust regression estimators. J STAT COMPUT SIM 2021. [DOI: 10.1080/00949655.2021.1972300] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
Affiliation(s)
- S. Salini
- Department of Economics, Management and Quantitative Methods, University of Milan, Milano, Italy
| | - F. Laurini
- Department of Economics and Management, University of Parma, Parma, Italy
| | - G. Morelli
- Department of Economics and Management, University of Parma, Parma, Italy
| | - M. Riani
- Department of Economics and Management, University of Parma, Parma, Italy
| | - A. Cerioli
- Department of Economics and Management, University of Parma, Parma, Italy
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Abstract
The purpose of this paper is to show in regression clustering how to choose the most relevant solutions, analyze their stability, and provide information about best combinations of optimal number of groups, restriction factor among the error variance across groups and level of trimming. The procedure is based on two steps. First we generalize the information criteria of constrained robust multivariate clustering to the case of clustering weighted models. Differently from the traditional approaches which are based on the choice of the best solution found minimizing an information criterion (i.e. BIC), we concentrate our attention on the so called optimal stable solutions. In the second step, using the monitoring approach, we select the best value of the trimming factor. Finally, we validate the solution using a confirmatory forward search approach. A motivating example based on a novel dataset concerning the European Union trade of face masks shows the limitations of the current existing procedures. The suggested approach is initially applied to a set of well known datasets in the literature of robust regression clustering. Then, we focus our attention on a set of international trade datasets and we provide a novel informative way of updating the subset in the random start approach. The Supplementary material, in the spirit of the Special Issue, deepens the analysis of trade data and compares the suggested approach with the existing ones available in the literature.
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Affiliation(s)
- Francesca Torti
- European Commission, Joint Research Centre (JRC), Via Enrico Fermi 2749, 21027 Ispra, VA Italy
| | - Marco Riani
- Dipartimento di Scienze Economiche e Aziendali and Interdepartmental Centre for Robust Statistics, University of Parma, Via Kennedy 6, 43125 Parma, Italy
| | - Gianluca Morelli
- Dipartimento di Scienze Economiche e Aziendali and Interdepartmental Centre for Robust Statistics, University of Parma, Via Kennedy 6, 43125 Parma, Italy
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Affiliation(s)
- Anthony C. Atkinson
- Anthony C. Atkinson is Emeritus Professor, Department of Statistics, London School of Economics, London WC2A 2AE, United Kingdom
| | - Marco Riani
- Marco Riani is Full Professor, Dipartimento di Scienze Economiche e Aziendale and Interdepartmental Centre for Robust Statistics, Università di Parma, 43100 Parma, Italy
| | - Aldo Corbellini
- Aldo Corbellini is Senior Lecturer, Dipartimento di Scienze Economiche e Aziendale and Interdepartmental Centre for Robust Statistics, Università di Parma, 43100 Parma, Italy
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Di Lazzaro P, Atkinson AC, Iacomussi P, Riani M, Ricci M, Wadhams P. Statistical and Proactive Analysis of an Inter-Laboratory Comparison: The Radiocarbon Dating of the Shroud of Turin. Entropy (Basel) 2020; 22:e22090926. [PMID: 33286695 PMCID: PMC7597180 DOI: 10.3390/e22090926] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 07/26/2020] [Revised: 08/11/2020] [Accepted: 08/18/2020] [Indexed: 06/12/2023]
Abstract
We review the sampling and results of the radiocarbon dating of the archaeological cloth known as the Shroud of Turin, in the light of recent statistical analyses of both published and raw data. The statistical analyses highlight an inter-laboratory heterogeneity of the means and a monotone spatial variation of the ages of subsamples that suggest the presence of contaminants unevenly removed by the cleaning pretreatments. We consider the significance and overall impact of the statistical analyses on assessing the reliability of the dating results and the design of correct sampling. These analyses suggest that the 1988 radiocarbon dating does not match the current accuracy requirements. Should this be the case, it would be interesting to know the accurate age of the Shroud of Turin. Taking into account the whole body of scientific data, we discuss whether it makes sense to date the Shroud again.
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Affiliation(s)
- Paolo Di Lazzaro
- Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Dipartimento FSN, Centro Ricerche Frascati, via E. Fermi 45, 00044 Frascati, Italy
| | | | - Paola Iacomussi
- Istituto Nazionale di Ricerca Metrologica (INRIM), 00135 Torino, Italy;
| | - Marco Riani
- Dipartimento di Scienze Economiche e Aziendale and Interdepartmental Centre for Robust Statistics, Università di Parma, 43125 Parma, Italy
| | - Marco Ricci
- Independent Researcher, Via Fra Dolcino 19, 28100 Novara, Italy;
| | - Peter Wadhams
- Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK;
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Riani M, Atkinson AC, Corbellini A, Perrotta D. Robust Regression with Density Power Divergence: Theory, Comparisons, and Data Analysis. Entropy (Basel) 2020; 22:e22040399. [PMID: 33286173 PMCID: PMC7516876 DOI: 10.3390/e22040399] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 02/22/2020] [Revised: 03/26/2020] [Accepted: 03/26/2020] [Indexed: 11/16/2022]
Abstract
Minimum density power divergence estimation provides a general framework for robust statistics, depending on a parameter α , which determines the robustness properties of the method. The usual estimation method is numerical minimization of the power divergence. The paper considers the special case of linear regression. We developed an alternative estimation procedure using the methods of S-estimation. The rho function so obtained is proportional to one minus a suitably scaled normal density raised to the power α . We used the theory of S-estimation to determine the asymptotic efficiency and breakdown point for this new form of S-estimation. Two sets of comparisons were made. In one, S power divergence is compared with other S-estimators using four distinct rho functions. Plots of efficiency against breakdown point show that the properties of S power divergence are close to those of Tukey's biweight. The second set of comparisons is between S power divergence estimation and numerical minimization. Monitoring these two procedures in terms of breakdown point shows that the numerical minimization yields a procedure with larger robust residuals and a lower empirical breakdown point, thus providing an estimate of α leading to more efficient parameter estimates.
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Affiliation(s)
- Marco Riani
- Dipartimento di Scienze Economiche e Aziendale and Interdepartmental Centre for Robust Statistics, Università di Parma, l43125 Parma, Italy; (M.R.); (A.C.)
| | | | - Aldo Corbellini
- Dipartimento di Scienze Economiche e Aziendale and Interdepartmental Centre for Robust Statistics, Università di Parma, l43125 Parma, Italy; (M.R.); (A.C.)
| | - Domenico Perrotta
- European Commission, Joint Research Centre, 21027 Ispra, Italy
- Correspondence:
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Affiliation(s)
| | - Marco Riani
- Interdepartmental Centre for Robust Statistics of the University of Parma Italy
| | - Aldo Corbellini
- Interdepartmental Centre for Robust Statistics of the University of Parma Italy
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Affiliation(s)
- Andrea Cerioli
- Department of Economics and ManagementUniversity of Parma Parma Italy
| | - Alessio Farcomeni
- Department of Public Health and Infectious DiseasesSapienza University of Rome Rome Italy
| | - Marco Riani
- Department of Economics and ManagementUniversity of Parma Parma Italy
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Cerioli A, García-Escudero LA, Mayo-Iscar A, Riani M. Finding the Number of Normal Groups in Model-Based Clustering via Constrained Likelihoods. J Comput Graph Stat 2018. [DOI: 10.1080/10618600.2017.1390469] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Affiliation(s)
- Andrea Cerioli
- Dipart. di Scienze Economiche e Aziendali, Università di Parma, Parma, Italy
| | | | - Agustín Mayo-Iscar
- Dpto. de Estadística e I.O. and IMUVA, Universidad de Valladolid, Valladolid, Spain
| | - Marco Riani
- Dipart. di Scienze Economiche e Aziendali, Università di Parma, Parma, Italy
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Affiliation(s)
- Marco Riani
- Department of Economics and Management; University of Parma; Parma Italy
| | - Aldo Corbellini
- Department of Economics and Management; University of Parma; Parma Italy
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Omari A, Abaloune Y, Hanafi Y, Riani M, Reda K, Aoubaaz A. Endophtalmie endogène compliquant un ulcère variqueux de la jambe : à propos d’un cas. J Fr Ophtalmol 2018; 41:e85-e87. [DOI: 10.1016/j.jfo.2017.03.030] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2016] [Revised: 03/17/2017] [Accepted: 03/29/2017] [Indexed: 10/18/2022]
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Cerioli A, Riani M, Atkinson AC, Corbellini A. The power of monitoring: how to make the most of a contaminated multivariate sample. STAT METHOD APPL-GER 2017. [DOI: 10.1007/s10260-017-0409-8] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
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Affiliation(s)
| | - Marco Riani
- Dipartimento di Scienze Economiche e Aziendale, Università di Parma, Parma, Italy
| | - Andrea Cerioli
- Dipartimento di Scienze Economiche e Aziendale, Università di Parma, Parma, Italy
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Atkinson AC, Cerioli A, Riani M. Discussion of ‘Asymptotic Theory of Outlier Detection Algorithms for Linear Time Series Regression Models’ by Johansen and Nielsen. Scand Stat Theory Appl 2016. [DOI: 10.1111/sjos.12210] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Affiliation(s)
| | | | - Marco Riani
- Department of Economics; University of Parma
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Affiliation(s)
- Silvia Salini
- Department of Economics Management and Quantitative Methods; University of Milan; Milan Italy
| | - Andrea Cerioli
- Department of Economics; University of Parma; Parma Italy
| | | | - Marco Riani
- Department of Economics; University of Parma; Parma Italy
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Riani M, Le Jan S, Plee J, Durlach A, Bernard P, Antonicelli F. CXCL10, un marqueur de rechute dans la pemphigoïde bulleuse. Ann Dermatol Venereol 2014. [DOI: 10.1016/j.annder.2014.04.134] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
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Riani M, Plee J, Le Jan S, Durlach A, Bernard P, Antonicelli F. Régulation de l’expression de la MMP-9 par CXCL10 dans la pemphigoïde bulleuse. Ann Dermatol Venereol 2013. [DOI: 10.1016/j.annder.2013.09.623] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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Riani M, Cerioli A, Rousseeuw PJ. Special Issue on Robust Methods for Classification and Data Analysis. ADV DATA ANAL CLASSI 2010. [DOI: 10.1007/s11634-010-0071-6] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
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Piana M, Canfora M, Riani M. Role of noise in image processing by the human perceptive system. Phys Rev E Stat Phys Plasmas Fluids Relat Interdiscip Topics 2000; 62:1104-1109. [PMID: 11088566 DOI: 10.1103/physreve.62.1104] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/09/2000] [Indexed: 05/23/2023]
Abstract
Two psychophysics experiments are described, pointing out the significant role played by stochastic resonance in recognition of capital stylized noisy letters by the human perceptive apparatus. The first experiment shows that an optimal noise level exists at which the letter is recognized for a minimum threshold contrast. A simple two-parameter model that best fits the experimental data is also discussed. In the second experiment we show that a dramatically increased ability of the visual system in letter recognition occurs in an extremely narrow range of increasing noise. Possible interesting future investigations suggested by these experimental results and based on functional imaging techniques are discussed.
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Affiliation(s)
- M Piana
- INFM, Unità di Ricerca di Genova, I-16146 Genova, Italy
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Simonotto E, Spano F, Riani M, Ferrari A, Levrero F, Pilot A, Renzetti P, Parodi R, Sardanelli F, Vitali P, Twitty J, Chiou-Tan F, Moss F. fMRI studies of visual cortical activity during noise stimulation. Neurocomputing 1999. [DOI: 10.1016/s0925-2312(99)00042-9] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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Sannita W, Vitali P, Levrero F, Ferrari A, Pilot A, Parodi R, Renzetti P, Riani M, Sardanelli F, Simonotto F, Spano F. CHOLINERGIC BRAIN MODULATION and VISION: fMRI and electrophysiological pilot study of the effect of scopolamine in volunteers. Neuroimage 1998. [DOI: 10.1016/s1053-8119(18)31146-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
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Riani M, Simonotto E. Stochastic resonance in the perceptual interpretation of ambiguous figures: A neural network model. Phys Rev Lett 1994; 72:3120-3123. [PMID: 10056072 DOI: 10.1103/physrevlett.72.3120] [Citation(s) in RCA: 50] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
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Saviolo Negrin N, Riani M, Zanforlin M. On the neural model of the visual system in flies. ETHOL ECOL EVOL 1990. [DOI: 10.1080/08927014.1990.9525467] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
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Saviolo-Negrin N, Riani M, Zanforlin M. On flies' behavior with a visual excitatory-inhibitory stimulus. Percept Mot Skills 1990; 70:446. [PMID: 2342843 DOI: 10.2466/pms.1990.70.2.446] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Affiliation(s)
- N Saviolo-Negrin
- Dipartimento di Psicologia Generale, Università di Padova, Italia
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Abstract
The perspective reversals elicited by a set of drawings based on the Mach truncated pyramid are examined. We obtained each pattern of the set from the previous one by adding to it some graphic cues, which were easily integrated into one of the two competing interpretations, thus reducing step by step the ambiguity of the basic pattern. The phenomenological model, proposed to link the mean times of both alternative interpretations with their complexities, is in close agreement with our experimental data. Furthermore, different aspects of such data are well described by the model equations: The measure of the prevalence of the supported interpretation is well correlated with the difference in complexity between the two alternative interpretations; the two different trends of the mean time of the unfavored interpretation, found in the data obtained from different observers as a function of the various patterns of the set, are well fitted by the model without the need for any specific additional hypothesis.
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Abstract
In this work, the results of two experiments on ambiguous patterns are reported, which have been obtained by presenting a series of stimuli designed, in both cases, to reduce gradually the ambiguity of the patterns. Such reduction has been performed by respectively increasing or decreasing the amount of graphic details in the experiments. Data of both experiments show a lengthening of mean reversal time. The increase in the stability of one percept can be regarded as associated with the increasing difficulties encountered by an observer in organizing and restating the alternative "hypothesis" through the perceptual mechanisms. The loss of balance in the durations of percepts is discussed in terms of their different informational contents. Finally, in Exp. 1 an analysis is made to evaluate to what extent an addition of perceptual cues, designed to reinforce a three-dimensional interpretation of a pattern, can influence its figure-ground alternation.
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Abstract
The perceptual alternation of the Necker cube, in its stationary phase, is studied as a function of the stimulus luminance down to the extreme condition in which the cones are completely inactivated so that the pattern of neural excitation reaching the primary visual cortex is very different from the normal one. No evident effect of luminance was found under passive observation either on the reversal rate or on the features of data distributions. Moreover, the complementary situation in which perception is based only on the cones does not affect the phenomenon either. These results permit the assumption that the perceptual alternation in its stationary phase, is a passive and automatic process, that is affected neither by a satiation of the first levels of the visual system nor by the psychological attitude of the subject consequent on dazzling the retina.
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Borsellino A, Carlini F, Riani M, Tuccio MT, De Marco A, Penengo P, Trabucco A. Effects of visual angle on perspective reversal for ambiguous patterns. Perception 1982; 11:263-73. [PMID: 7167334 DOI: 10.1068/p110263] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
Abstract
Reversal rates of an ambiguous figure (the Necker cube) were studied for different pattern sizes covering a range of visual angles theta from approximately 1 to 62 deg. A large number of reversals was obtained for each observer and each pattern in order to examine the statistical distributions of reversal times. A pronounced flattening of the statistical distributions (represented throughout by a gamma distribution) and a growth of the mean duration of each percept, with increasing pattern size was found. A plateau in the range of theta between 5 and 20-30 deg was observed. For larger values of theta two kinds of observers have been identified: for 'fast' observers the inversion rate is little affected by theta, whilst for 'slow' observers, the mean reversal time increases strongly with theta. A tentative model, based on three different contributions to the duration of the alternation process, is proposed: a constant term, independent of theta, and two terms dependent on theta--a retinal term, and a cortical one. The last term is interpreted as due to the spreading of excitation with the characteristic of a filling-in process.
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