Browsing by Subject "Contrast"

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1980 yıllarının devrimi: Bilkent Grafik Tasarımı
(Bilkent University, 2022) Çakmakcı, İnci Lizge; Uyanık, Ömer Faruk; Kayaselçuk, Ege Mehmet; Acar, Ada; Sağlık, Kadir Doğa
Grafik tasarım sanatı günümüzün önemli iletişim yöntemleri arasındadır. Osmanlıya uzansa da Türk grafik tasarım sanatının tam olarak bir devrim yaşaması ve zincirini kırıp gelişmeye başlaması geç olmuştur. Bu yazıda öncelikli olarak Türk grafik sanatının geçmişten günümüze gelişimi anlatılmıştır. Sonrasında, Grafik tasarımın Osmanlının son dönemlerindeki ve Cumhuriyetten sonraki süreçte halka, akademiye etkisi ve özellikle günümüzde nasıl bir teknolojik değişime uğradığı ele alınmıştır. Son olarak ise üniversite söz konusu olduğunda adı önemli bir yer tutan Bilkent’in grafik tasarım bölümü ve tarihi anlatılmıştır. Bilkent’in Türkiye adına grafik tasarımdaki yeri ve önemi açıklanarak yazı sonlandırılmıştır.
Open Access
Context-dependent lightness affects perceived contrast
(Elsevier Ltd, 2016) Pamir, Zahide; Boyacı, Hüseyin
Perceived contrast of a grating varies with its background (or mean) luminance: of the two gratings with the same photometric contrast the one on higher luminance background appears to have higher contrast. Does perceived contrast also vary with context-dependent background lightness even when the luminance remains constant? We investigated this question using a stimulus in which two equiluminant patches ("context squares", CSs) appear different in lightness. First we measured the lightness effect in a behavioral experiment. After ensuring that it was present for all participants, we conducted perceived contrast experiments, where participants judged the contrast of rectified incremental and decremental square-wave gratings superimposed on the CSs. For the incremental gratings participants' settings were significantly different for the two CSs. Specifically, perceived contrast was higher when the gratings were placed on the context square that was perceived lighter. In a follow-up experiment we measured perceived contrast of rectified gratings on isolated patches that differed in luminance. The pattern of results of the two experiments was consistent, demonstrating that possibly shared mechanisms underpin the effects of background luminance and context-dependent lightness on perceived contrast. © 2016 Elsevier Ltd.
Open Access
The effect of context-dependent lightness on contrast detection and identification, and its neural correlates
(2017-10) Karatok, Zahide Pamir
Perceived contrast of a grating varies with its background (or mean) luminance: of the two gratings with the same photometric contrast the one on higher luminance background appears to have higher contrast. On the other hand, context often causes a large perceived difference between equiluminant regions (e.g., simultaneous brightness contrast). Does perceived contrast also vary with contextdependent background lightness even when the luminance remains constant? In this study, the effect of context-dependent lightness on contrast perception was investigated using psychophysical and functional magnetic resonance imaging (fMRI) methods. First, we measured appearance judgments of participants and demonstrated that context-dependent lightness of background in uences the perceived contrast of rectified gratings. Perceived contrast of gratings superimposed on equiluminant but perceptually lighter background is higher compared to ones on perceptually darker backgrounds. However, this pattern is valid only for incremental, not for decremental contrast. Literature indicates a significant difference between visual processing near and above threshold. Also, behaviorally it has been shown that appearance and threshold tasks are mediated by different mechanisms. Therefore, here, we also measured the effect of context-dependent lightness on contrast detection and discrimination thresholds using a 2-IFC procedure. Results indicate that both detection and discrimination thresholds are lower for the gratings superimposed on perceptually lighter backgrounds. Differently from the appearance results, the effect was observed both for incremental and decremental contrast. In an fMRI study, we investigated whether activity in any brain region correlates with background-lightness-dependent contrast perception. Although our stimulus was physically identical, we observed difference in BOLD response within pre-defined region of interests (ROIs) in different visual areas. Both for incremental and decremental contrast, activation, especially in V1, was greater when the grating was superimposed on lighter background for all the contrast levels tested. Variation in V1 activity with varying contrast links better with the detection and discrimination thresholds than the appearance results. Therefore, this study might offer a neural evidence for dissociation between the mechanisms underlying detection (threshold) and identification (appearance) measures. However, the relationship between the threshold and fMRI data does not really agree with the previous findings in literature. These results indicate that the neural activation caused by the detection mechanism may change depending on the absolute or perceived value of the contrast.
Open Access
The effect of orientation-related prior probability information on contrast perception
(2019-07) Nazlı, İlayda
It is known that visual perception is the product of sensory input and prior probability information. Previous studies support well that expectation influence recognition and decision-making; however, we have limited knowledge about how expectation influences low level visual processing. In the current study, we examine the effect of expectation on early visual processing. That is, we provide task-irrelevant expectation cue related to the orientation of target Gabor grating, and we ask participants to indicate the spatial location of target grating while systematically manipulating its contrast level. In addition, we examine how different expectation validities (i.e. 75%, 50%, 100%) and neutral expectation cue affect visual perception. In Experiment 1, where the orientation of expectation cue and target gratings are vertical or horizontal, contrast threshold is lower in congruent trials at 75% validity condition. In Experiment 2, where the orientation of cue grating implies specific range (i.e. ±15 around 0 or 90), contrast threshold is lower in congruent trials at 75% and 50% validity condition. In Experiment 3B, where the orientation of expectation cue and target Gabor grating are within wider range (i.e. ±30 around 0 or 90), contrast threshold is lower in congruent trials at 75% validity condition. In Experiment 3A, where the orientation of expectation cue and target Gabor grating are within a specific range (i.e. ±15 around 0 or 90) contrast threshold is lower in congruent trials at 75% and 50% validity condition. In Experiment 3B, where the orientation of expectation cue and target Gabor grating are within wider range (i.e. ±30 around 0 or 90), contrast threshold is lower in congruent trials at 75% validity condition.
Open Access
Orhan Pamuk'un Kar'ında epigrafik ilişkiler
(2007) Riley, Nathaniel Brann
Orhan Pamuk’s political novel Snow’s multifaceted provocativeness is a conscious strategy of literary contrast designed to question all manner of prejudice. By attracting attention to the inaccuracy of stereotypical labels, the identity-related contradictions in the novel’s characters invite the reader to originality and individualism. The book’s illustration of opposites like white-black and heaven-hell using blurry shades of gray and pastels, as well as the snowflake diagram’s multidimensional symmetry, indicate the fundamental irreducibility of reality’s complexity. Within this context, the epigraphs quoted from Robert Browning’s “Bishop Blougram’s Apology”, Stendhal’s La Chartreuse de Parme (The Charterhouse of Parma), Dostoyevsky’s The Possessed (or The Demons or The Devils), and Joseph Conrad’s Under Western Eyes all bear significance for Snow. When the partly concrete, partly abstract similarities between Snow and its epigraphical sources are taken as the point of departure, several apparently significant structural contrasts stand out. Ultimately, this strategy of contrast can be evaluated as pluralist and antiOrientalist, as opposed to prejudiced and racist as widely supposed. Thus, more than mere deferential “tips of the hat” to the masters of the political novel genre, Snow’s epigraphs may be considered intertextual replies, parallels or parodies.
Open Access
The role of contrast and size in motion perception: behavioral and neuroimaging study of center-surround interactions in primary visual cortex (V1) and middle temporal area (MT+)
(2018-09) Er, Görkem
Behavioral experiments have demonstrated that observers' ability to discriminate the drift direction of a grating improves as its size increases if the grating has a low contrast, and deteriorates if it has a high contrast [1]. It has been proposed that receptive field organization in middle temporal (MT+) visual area underlies this counter-intuitive perceptual effect. Supporting evidence for this proposal has been provided in literature [2]. However, previous studies have not unequivocally showed that MT+ is the sole area whose activity underlies the perceptual effect. Here, we investigate the activity patterns of primary visual cortex (V1) and middle temporal (MT+) in response to drifting Gabor patches in differing size and contrast levels to elucidate the neural region involved in size-contrast interaction in motion perception. We first replicated the findings in the literature with a behavioral experiment, where small or large (1.67 and 8.05 degrees of visual angle) drifting gratings with either low (2%) or high (99%) contrast levels were presented at the periphery (centered 9.06 degrees of visual angle to left and right of fixation). We measured the duration thresholds (79%) for accurately discriminating the drift direction of gratings for eleven participants using an adaptive staircase and two-alternative forced choice (2AFC) design. In line with previous literature, we observed that increasing the size of the low-contrast stimuli resulted in decreased discrimination threshold, while for high-contrast stimuli, increasing the size resulted in increased discrimination threshold. In the second stage of the study, six observers participated in a block design fMRI study with the same spatial configuration and contrast levels used in the behavioral experiment. We first identified the region of interests (ROI) for visual area V1 and MT+ separately for all participants. Then, we identified a "sub-ROI" that corresponds to the region that was selectively responsive to the small sized stimuli (1.67 degrees) using an independent localizer. With this setup, we allowed for both large and small sized gratings to stimulate the sub-ROI throughout the entire scan. Therefore, changes in Blood Oxygenated Level Dependent (BOLD) response at the sub-ROI in response to large compared to small sized gratings indicated the in uence of the surrounding region to the center of the gratings. In area MT+, we observed that increasing the size of the grating increases the BOLD activity if the stimuli have low contrast, compared to high contrast. In other words, surrounding region had a facilitative in uence to the group of MT+ neurons encoding the center of the stimuli if the stimuli had low contrast. This neuronal facilitation observed with the neuroimaging data explain the enhancement of the performance with increasing the size of the low-contrasted stimuli observed at the behavioral experiment. In V1, however, increasing the size of the high-contrasted gratings increased the BOLD activity, compared to activity evoked by increasing the size of the low-contrasted gratings. On the whole, we show that center-surround interaction in V1 and MT were differentiated in response to peripherally viewed drifting Gabor patches at differing contrast and size levels, hence we provide further evidence that the perceptual size-contrast interaction effect is likely to originates at cortical area MT+.