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Evaluating threshold and supra-threshold performance using compound gratings
Get PDFΗ ερμηνεία της ευαισθησίας φωτεινής αντίθεσης έχει επιτευχθεί με τη χρησιμοποίηση απλών gratings που διεγείρουν μερικώς συντονισμένους νευρικούς μηχανισμούς. Προκειμένου να μελετηθούν αλληλεπιδράσεις μεταξύ τέτοιων καναλιών πρέπει να χρησιμοποιηθούν πιο περίπλοκα ερεθίσματα. Το κίνητρο για την πραγματοποίηση της παρούσας εργασίας ήταν να προσεγγίσει την ευαισθησία φωτεινής αντίθεσης χρησιμοποιώντας ευρύ φάσμα χωρικών συχνοτήτων από τη μεριά των υψηλών χωρικών συχνοτήτων της καμπύλης ευαισθησίας φωτεινής αντίθεσης. Η περιγραφή του σχήματος και του μεγέθους των υποκείμενων μηχανισμών του οπτικού συστήματος ήταν ένα άλλο θέμα το οποίο μελετήθηκε σε αυτή την εργασία. Επιπλέον, αξιολογήθηκε η αναγνωρισημότητά τους σε sub-threshold και supra-threshold συνδυασμούς contrast.
Μεθοδολογία
Για τα πειράματα χρησιμοποιήθηκαν μια οθόνη υψηλής ανάλυσης FD Trinitron® SONY© GDM F520 CRΤ, μια κάρτα γραφικών VSG 2/5 (Cambridge Research Systems©), ένα ασύρματο τηλεχειριστήριο CB6 (Cambridge Research Systems©) και ένα ειδικά σχεδιασμένο για παραγωγή οπτικών ερεθισμάτων λογισμικό (NewRT, Dr Neil Parry©). Οι παρατηρητές που συμμετείχαν στα πειράματα ήταν 5, όλοι με πρότερη εμπειρία σε ψυχοφυσικές μετρήσεις
Τρία πειράματα διεξήχθησαν. Τα πρώτα δυο σχεδιάστηκαν για να αξιολογηθεί η απόδοση του οπτικού συστήματος χρησιμοποιώντας σύνθετα gratings σε threshold contrast. H κύρια χωρική συχνότητα ήταν πάντα 4c/deg ενώ η χωρική συχνότητα του δευτερεύοντος grating ήταν 4.5, 5, 6, 8, 12, 16, 24c/deg που αντιστοιχεί σε λόγο χωρικών συχνοτήτων 1.125, 1.25, 1.5, 2, 3, 4, 6. Το τρίτο πείραμα σχεδιάστηκε για να εξεταστεί η συσχέτιση μεταξύ νευρώνων καθώς η φωτεινή αντίθεση των δυο συνιστωσών του σύνθετου grating έπαιρνε τιμές τόσο μικρότερες όσο και μεγαλύτερες τις ουδούς.
Αποτελέσματα
Τα σύνθετα gratings βρέθηκαν να εμφανίζουν ελαφρώς πιο αυξημένη ευαισθησία από ότι τα απλά κατά έναν πολλαπλασιαστικό παράγοντα x1.15 (το οποίο σημαίνει διαφορά 1.2dB). Για λόγο χωρικών συχνοτήτων 1 (τα συνιστώσα gratings ήταν της ίδιας συχνότητας) η ευαισθησία ήταν αυξημένη (σε σχέση με τα απλά gratings) κατά 6dB (μέγιστη νευρωνική άθροιση) ενώ μειωνόταν σε 2-4dB για λόγο χωρικών συχνοτήτων μεταξύ 1 και 2. Όταν ο λόγος χωρικών συχνοτήτων ήταν μεγαλύτερος από 2 δεν υπήρχε νευρωνική αλληλεπίδραση και η ευαισθησία ήταν αυξημένη εξαιτίας μόνο του probability summation, κατά ένα παράγοντα μεταξύ 1.15 και 1.345.
Το εύρος του νευρωνικού καναλιού που εμφανίζει αυξημένη ευαισθησία στη χωρική συχνότητα των 4 κύκλων ανά μοίρα μετρήθηκε στο FWHM και βρέθηκε να είναι μεταξύ 0.4 και 0.6 οκτάβων για 2 παρατηρητές. Επιπλέον, όταν η χωρική συχνότητα του δευτερεύοντος grating ήταν χαμηλότερη από 4c/deg το μέγεθος της νευρωνικής αλληλεπίδρασης ήταν υψηλότερο, υποδηλώνοντας ένα μη συμμετρικό νευρωνικό κανάλι.
Όταν τα δυο συνιστώσα gratings είχαν contrast πάνω από την ουδό, η υψηλής χωρικής συχνότητας συνιστώσα ‘καταπιέζει’ το grating χαμηλής χωρικής συχνότητας και εμφανίζεται περισσότερο ανιχνεύσιμο.
Συζήτηση
Τα σύνθετα gratings είναι περισσότερο ανιχνεύσιμα. Το εύρος της αυξημένης ευαισθησίας εξαρτάται από το λόγο των χωρικών συχνοτήτων των συνιστωσών τους και εμφανίζεται μέγιστη (6dB) όταν οι συνιστώσες έχουν την ίδια χωρική συχνότητα και μειώνεται σε 2-4dB όταν ο λόγος χωρικών συχνοτήτων είναι περίπου 2. Η αύξηση της ευαισθησίας οφείλεται σε νευρωνικές αλληλεπιδράσεις. Για λόγο χωρικών συχνοτήτων μεγαλύτερο από 2, η ευαισθησία είναι αυξημένη κατά 1-2dB, που αντιστοιχεί σε probability summation. Υπάρχουν στοιχεία ότι τα νευρωνικά κανάλια που είναι συντονισμένα σε χωρικές συχνότητες δεν είναι συμμετρικά. Τέλος, υπάρχουν 6
ενδείξεις ότι η συνιστώσα υψηλής χωρικής συχνότητας είναι περισσότερο ανιχνεύσιμη όταν και οι δυο συνιστώσες έχουν supra-threshold contrast.Most of the understanding regarding visual contrast perception has been achieved using simple gratings, which stimulate spatially tuned neuronal mechanisms. In order to study any interaction between these channels more elaborated stimuli need to be used. The incentive of the present study was to assess the contrast sensitivity using a wide range of spatial frequencies at the high spatial frequency side of the contrast sensitivity function. The description of the form and the extent of the underlying mechanisms of the human visual system was another issue studies in the present work. Moreover, their recognition was evaluated at sub-threshold and supra-threshold combinations.
Methodology
For the experiments a high resolution FD Trinitron® SONY© GDM F520 CRT monitor, a VSG 2/5 (Cambridge Research Systems©) graphic card, a CB6 wireless remote control (Cambridge Research Systems©) and a specially developed software for stimuli generation (NewRT, Dr Neil Parry©) were used. Five observers, all with prior psychophysical experience, participated in the experiments.
Three sets of experiments were conducted. The first two experiments were designed to evaluate threshold performance of compound gratings. The main spatial frequency component was always 4c/deg and the spatial frequency of the secondary component was 4.5, 5, 6, 8, 12, 16 and 24 which corresponds at a spatial frequency ratio of 1.125, 1.25, 1.5, 2, 3, 4 and 6. The third experiment was conducted to examine the interaction between the spatial frequency tuned channels while the contrast of the two components of the compound gratings ranged from sub-threshold to supra-threshold.
Results
Compound gratings were found to be slightly more sensitive by a factor of x1.15 (which corresponds to an average 1.2dB difference) compared to
the sensitivity of individual gratings (while the two components were at equal contrast above their respective thresholds). For spatial frequency ratio equal to one (the components were of same frequency), the sensitivity was higher (compared to the single grating) by 6dB (maximum summation), decreasing to 2-4 dBs for ratios between 1 and 2. When the spatial frequency ratio was > 2 there were no channel interactions and the sensitivity is solely increased due to probability summation, by a factor between 1.15 and 1.345.
The bandwidth of the spatial frequency tuned channel selective at 4c/deg was found to be at FWHM about 0.4 to 0.6 octaves for the two subjects. Moreover, when the secondary component was of spatial frequency lower than 4c/deg, the magnitude of neuronal summation was higher, suggesting a non-symmetric neuronal channel.
When the two components were at supra-threshold contrast, the high spatial frequency component depresses the low spatial frequency component and appears more detectable.
Discussion
Compound gratings appear more detectable. The amount of increased sensitivity depends on the ratio between the spatial frequencies of the components, being maximum (6dB), when the components are of equal spatial frequency, decreasing to 2-4 dBs when the ratio is about 2. The increased sensitivity is due to neuronal summation. For ratios higher than 2 the sensitivity is increased about 1-2dB, corresponding to probability summation. There is evidence for asymmetric spatial-frequency tuned neuronal channels. Finally, there are indications that the high spatial frequency component is more detectable when the two components are at supra-threshold contrast
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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