Metacontrast masking is processed before grapheme–color synesthesia

Abstract

We investigated the physiological mechanism of grapheme-color synesthesia using metacontrast masking. A metacontrast target is rendered invisible by a mask that is delayed by about 60 ms; the target and mask do not overlap in space or time. Little masking occurs, however, if the target and mask are simultaneous. This effect must be cortical, because it can be obtained dichoptically. To compare the data for synesthetes and controls, we developed a metacontrast design in which nonsynesthete controls showed weaker dichromatic masking (i.e., the target and mask were in different colors) than monochromatic masking. We accomplished this with an equiluminant target, mask, and background for each observer. If synesthetic color affected metacontrast, synesthetes should show monochromatic masking more similar to the weak dichromatic masking among controls, because synesthetes could add their synesthetic color to the monochromatic condition. The targetmask pairs used for each synesthete were graphemes that elicited strong synesthetic colors. We found stronger monochromatic than dichromatic U-shaped metacontrast for both synesthetes and controls, with optimal masking at an asynchrony of 66 ms. The difference in performance between the monochromatic and dichromatic conditions in the synesthetes indicates that synesthesia occurs at a later processing stage than does metacontrast masking.Keywords Visual perception . Visual awareness . Neural mechanisms . Metacontrast . Synesthesia After more than a century of being on the fringes of perceptual science (Galton, 1880), synesthesia has seen renewed interest in the past few years with the application of modern psychophysical methods (Cytowic, 2002;Ramachandran & Hubbard, 2001a, 2001b. Synesthesia is both involuntary and stable (Brang & Ramachandran, 2011;Cytowic & Eagleman, 2009); as a perception in one modality that occurs as a result of stimulation in another, it represents a failure of accurate perception of the properties of the world. In this way, synesthesia is a tool for uncovering perceptual mechanisms, which are often investigated by exploring the limits of perceptual capability. Investigating when and how perception breaks down (e.g., measuring thresholds) often informs researchers about the mechanisms of perception. The natural breakdown of correspondence between physical stimulation and perception can thus be informative about perceptual mechanisms.We can begin to locate the level of grapheme-color synesthesia in the brain by examining the point of perceptual breakdown using direct psychophysical methods. The best-documented type of synesthesia is grapheme-color synesthesia, in which letters and numbers evoke an idiosyncratic experience of color for each grapheme. In this study, we employed a metacontrast-masking paradigm to compare the performance of grapheme-color synesthetes and nonsynesthete control participants using both dichromatic and monochromatic stimuli. If both synesthetes and controls experienced stronger masking under monochromatic than under dic...

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“…It also differs from all other conditions by the number of sequentially presented visual transients: With an SOA of zero, there are two visual transients less, as the disks (target and distractors) and rings (masks) start and end at the same time. We expected a u-shaped distribution of the accuracy rates as a function of the different SOA steps (e.g., Alpern, 1953 ; Enns and Di Lollo, 2000 ; Tata, 2002 ; Boyer and Ro, 2007 ; Bacon et al, 2013 ; Agaoglu et al, 2018 ). We presented 24 stimuli, whereof only four (in the inner corners of the virtual square) were potential targets (highlighted by lines pointing toward these positions; Figure 1 ).…”

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“…That is, while A may elicit a red color experience for one grapheme-color synesthete, it may elicit a blue color experience for another grapheme-color synesthete (Grossenbacher and Lovelace, 2001). In contrast, mirrored sensory experiences seem rather systematic than idiosyncratic (for the use of “systematic” in relation to crossmodal correspondences or “weak synesthesia” see, Martino and Marks, 2001). That is, observed soft touch would result in perceived soft touch and observed strong touch would result in perceived strong touch.…”

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“…We manipulated three within-participant variables: SOA (five steps: 0 ms/85 ms/153 ms/221 ms/289 ms), Singleton Status (target singleton/distractor singleton/no singleton), and Mask Fit (tight fit/loose fit). For the variable SOA, we expected a U-shaped distribution of the accuracy rates depending on the different time intervals between disks and masks (e.g., Agaoglu et al, 2018;Alpern, 1953;Bacon et al, 2013;Baier et al, 2020;Boyer & Ro, 2007;Tata, 2002). To vary the singleton status, one of the four disks in the corners of the inner square shape (either the target or one of the other three stimuli at potential target positions) had a different color than the other 23 disks.…”

“…Francis, 2000). This results in a U-shaped function of discrimination accuracies depending on the SOA (e.g., Agaoglu et al, 2016, 2018; Alpern, 1953; Bacon et al, 2013; Baier et al, 2020; Enns & Di Lollo, 2000; Francis, 2000; Tata, 2002).…”