Untersuchungen zur Bedeutung modaler und amodaler Perzepte in der visuellen Wahrnehmung

Eine erstaunliche Leistung des Wahrnehmungssystems besteht darin, trotz fragmentierter und unvollständiger sensorischer Informationen im proximalen Reiz, beispielsweise aufgrund von Verdeckung, stabile und vollständige Perzepte hervorzubringen. Die Wahrnehmung von Verdeckung kann hierbei sog. amodale Ergänzungen und Vervollständigungen von Objekten hervorrufen und die Interpretation einer Szene dadurch erheblich beeinflussen. Im Gegensatz zu amodalen Vervollständigungen verdeckter Objekte und Objektbereiche werden perzeptuelle Vervollständigungen unverdeckter Objekte und Objektbereiche als modal bezeichnet. Die Arbeit beginnt mit einer theoretischen Analyse der Dichotomie von modaler und amodaler Vervollständigung, die sich auf phänomenologische Beobachtungen und vorliegende empirische Befunde stützt. Dabei wird gezeigt, dass diese traditionelle Dichotomie nicht nur unvollständig, sondern auch inkonsistent ist. Das Kriterium der phänomenalen Präsenz, durch die sich modale und amodale Perzepte qualitativ voneinander unterscheiden sollen, erscheint aus theoretischer Sicht willkürlich und aus phänomenologischer Sicht ungerechtfertigt und daher insgesamt fragwürdig. Die dichotome Unterscheidung zwischen modalen und amodalen Perzepten ist außerdem häufig mit der impliziten Annahme verknüpft, modale Perzepte repräsentierten geometrisch-optische Sichtbarkeit und amodale Perzepte geometrisch-optische Unsichtbarkeit aufgrund von Verdeckung. Diese Annahme einer direkten Kopplung von phänomenaler Sichtbarkeit/Verdeckung an geometrisch-optische Sichtbarkeit/Verdeckung erweist sich jedoch unter empirischen Gesichtspunkten als fragwürdig, da einerseits auch bei phänomenaler Verdeckung modale Attribute wahrgenommen und sogar in ihrer Ausprägung beeinflusst werden können, andererseits wiederum auch unverdeckte Figuren einen amodalen Charakter besitzen können. Anknüpfend an diese theoretischen Überlegungen wird die Unangemessenheit der klassischen Dichotomie von modaler und amodaler Wahrnehmung und insbesondere die damit verbundene direkte Kopplung von phänomenaler an geometrisch-optische Sichtbarkeit/Verdeckung in mehreren empirischen Untersuchungen aufgezeigt. Der experimentelle Fokus liegt hierbei exemplarisch auf dem Nachweis des Einflusses von perzeptueller Verdeckung auf zwei explizit modale Aspekte der Wahrnehmung, nämlich auf die wahrgenommene Glattheit von Scheinbewegung und auf die phänomenale Sichtbarkeit von Objektoberflächen bei Verdeckung. In fünf Experimenten zum sog. Glättungseffekt wird nachgewiesen, dass die wahrgenommene Glattheit von ansonsten ruckartiger Scheinbewegung verbessert werden kann, wenn die räumlichen und ggf. auch zeitlichen Lücken in diskreten Bewegungsreizen durch geeigente Verdeckungshinweise maskiert werden. Der Effekt lässt sich erklären, wenn man annimmt, dass die ansonsten unerklärbaren Lücken im Reiz, die die Bewegungswahrnehmung beeinträchtigen, ganz oder teilweise kompensiert werden können, wenn diese vom visuellen System ursächlich auf Verdeckung zurückgeführt und somit plausibel "erklärt" werden können. Diese Erklärung greift konstruktivistische Theorievorstellungen auf, wonach Wahrnehmung intelligentem Problemlösen ähnelt (Rock, 1983, 1997) und auf abduktiven Inferenzprozessen basiert (Mausfeld, 2011a). In zwei weiteren Experimenten wird das sog. Sichtbarkeitsparadoxon nachgewiesen und seine Ausprägung quantifiziert. Das Paradoxon besteht darin, dass Teile der verdeckten Bereiche eines partiell verdeckten Objekts trotz deutlich erkennbarer Verdeckung gleichzeitig phänomenal sichtbar erscheinen. Dieser vermeintliche Widerspruch lässt sich auflösen, wenn man die klassische Annahme einer direkten Kopplung von phänomenaler Sichtbarkeit an geometrisch-optische Sichtbarkeit aufgibt und statt dessen die phänomenale Sichtbarkeit eines perzeptuellen Attributs (wie Form, Kontur, Oberflächenfarbe) als Repräsentation der Schlüssigkeit und Klarheit der sensorischen Hinweise für eine spezifische perzeptuelle Deutung hinsichtlich dieses Attributs auffasst, also als eine Art Maß für die Verlässlichkeit der Deutung. Aus dieser Perspektive lassen sich auch andere vermeintlich paradoxe Beobachtungen wie beispielsweise der amodale Charakter der verdeckten Bewegung beim Tunneleffekt (Burke, 1952) oder die partiell modale Ergänzung bei der Verdeckungstäuschung (Kanizsa, 1979; Palmer et al., 2007) in konsistenter Weise erklären. Die beschriebene Perspektive ist zwar zum jetzigen Zeitpunkt empirisch nur schwach gestützt, bietet aber eine vielversprechende Grundlage für weitere empirische Forschungen zur Bedeutung modaler und amodaler Perzepte. Zusammenfassend stellen die Ergebnisse dieser Arbeit die traditionelle Vorstellung über modale und amodale Perzepte, die eng an geometrisch-optische Sichtbarkeit/Verdeckung geknüpft ist, in Frage. Die vorgeschlagene alternative Perspektive, die die Bedeutung dieser Konzepte für die abduktiven Inferenzprozesse in der Wahrnehmung hervorhebt, steht mit den experimentellen Befunden dieser Arbeit in Einklang. Eine gründlichere Prüfung, inwieweit bestehende Hypothesen und Erklärungen für Vervollständigungsphänomene mit den neugewonnenen Erkenntnissen verträglich sind, erscheint deshalb lohnenswert.One of the most intriguing characteristics of the perceptual system is that it is able to produce stable, continuous, and complete percepts even if the sensory information available in the proximal stimulus is fragmented or incomplete, for instance due to occlusion. The perception of occlusion can provoke so-called amodal completion of objects, which may influence the interpretation of a scene substantially. In contradistinction to amodal completions, perceptual completions of non-occluded objects and object regions are termed modal. This dissertation begins with a theoretical analysis of the dichotomy of modal and amodal completion that draws on phenomenological observations and existing empirical findings. It is shown that the traditional dichotomy is both incomplete and inconsistent. In addition, it is argued that the claim that phenomenal presence can be used as a criterion to distinguish between modal and amodal percepts is theoretically ad-hoc and inconsistent with phenomenology and therefore highly questionable. A further issue is that the dichotomous distinction between modal and amodal percepts is often linked with the implicit assumption that modal percepts represent geometric-optical visibility, whereas amodal percepts represent geometric-optical invisibility due to occlusion. There are, however, two empirical findings that speak against this assumption of a direct coupling of phenomenal visibility/occlusion and geometric-optical visibility/occlusion: First, modal aspects of the percept are not only available in the case of direct visibility but may also be present under phenomenal occlusion. Moreover, the modal characteristics of a percept may even be influenced by occlusion cues. Second, amodal characteristics of a percept do not only occur in the case of occlusion. Instead, the perceptual presence of non-occluded figures may, under appropriate conditions, also have an amodal character, even though the figures are phenomenally clearly visible. Based on these theoretical considerations, several experiments were conducted with the aim to provide direct empirical evidence for the claim that the classical dichotomy of modal and amodal perception is inadequate, including the aforementioned "coupling assumption" of phenomenal and geometric-optical visibility/occlusion, which is often associated with the classical dichotomy. These experiments investigate the effect of perceptual occlusion on two explicitly modal aspects of perception, namely on the perceived smoothness of apparent motion and on the phenomenal visibility of object surfaces under occlusion. In a series of five experiments on the so-called smoothing effect it is shown that the perceived smoothness of apparent motion can be considerably improved when the spatial gaps (and temporal gaps, if applicable) in discrete motion stimuli are masked with appropriate occlusion cues. The effect can be explained if one assumes that the—otherwise inexplicable—gaps in the stimulus, which typically impair the perception of motion, are (partly) compensated whenever the visual system can "explain" the gaps in a plausible manner by attributing them to an external occluder rather than to the moving object itself. This explanation ties in with theoretical ideas of constructivists claiming that perception is a process similar to intelligent problem solving (Rock, 1983, 1997) and that it is based on abductive inferences (Mausfeld, 2011a). In two further experiments the existence of the so-called visibility paradox is rigorously demonstrated and the strength of the paradoxical effect is quantified. The paradox consists of the observation that portions of a partially occluded object are, at the same time, perceived as occluded and as phenomenally visible. This seeming contradiction may be resolved if one gives up the classical assumption of a direct coupling of phenomenal visibility and geometric-optical visibility. Instead, one might conceive of the phenomenal visibility of a particular attribute (such as shape, contour, surface color) as a representation of the conclusiveness of the sensory evidence underlying perceptual inferences regarding this attribute, i. e., as reflecting the reliability of the perceptual construction. From this perspective a number of other seemingly paradoxical observations can also be explained in a consistent manner, e. g., the amodal character of hidden movement reported in Burke’s (1952) study on the tunnel effect and the partial modal completion observed in the occlusion illusion (Kanizsa, 1979; Palmer et al., 2007). Admittedly, this perspective has currently only weak empirical support, but it seems to provide a promising basis for future empirical research on the meaning of modal and amodal percepts. In summary, the results of this dissertation challenge the traditional conception of modal and amodal percepts, which is closely tied to geometric-optical visibility/occlusion. The proposed alternative view that emphasizes the role of these concepts in abductive inferences seems to be in line with the results of the experiments reported in this work. Therefore it seems worthwhile to scrutinize in a more comprehensive way to what extent existing hypotheses and explanations of completion phenomena are compatible with the findings presented here

A study on the material properties of novel PEGDA/gelatin hybrid hydrogels polymerized by electron beam irradiation

Gelatin-based hydrogels are highly desirable biomaterials for use in wound dressing, drug delivery, and extracellular matrix components due to their biocompatibility and biodegradability. However, insufficient and uncontrollable mechanical properties and degradation are the major obstacles to their application in medical materials. Herein, we present a simple but efficient strategy for a novel hydrogel by incorporating the synthetic hydrogel monomer polyethylene glycol diacrylate (PEGDA, offering high mechanical stability) into a biological hydrogel compound (gelatin) to provide stable mechanical properties and biocompatibility at the resulting hybrid hydrogel. In the present work, PEGDA/gelatin hybrid hydrogels were prepared by electron irradiation as a reagent-free crosslinking technology and without using chemical crosslinkers, which carry the risk of releasing toxic byproducts into the material. The viscoelasticity, swelling behavior, thermal stability, and molecular structure of synthesized hybrid hydrogels of different compound ratios and irradiation doses were investigated. Compared with the pure gelatin hydrogel, 21/9 wt./wt. % PEGDA/gelatin hydrogels at 6 kGy exhibited approximately up to 1078% higher storage modulus than a pure gelatin hydrogel, and furthermore, it turned out that the mechanical stability increased with increasing irradiation dose. The chemical structure of the hybrid hydrogels was analyzed by Fourier-transform infrared (FTIR) spectroscopy, and it was confirmed that both compounds, PEGDA and gelatin, were equally present. Scanning electron microscopy images of the samples showed fracture patterns that confirmed the findings of viscoelasticity increasing with gelatin concentration. Infrared microspectroscopy images showed that gelatin and PEGDA polymer fractions were homogeneously mixed and a uniform hybrid material was obtained after electron beam synthesis. In short, this study demonstrates that both the presence of PEGDA improved the material properties of PEGDA/gelatin hybrid hydrogels and the resulting properties are fine-tuned by varying the irradiation dose and PEGDA/gelatin concentration

Weak electron irradiation suppresses the anomalous magnetization of N-doped diamond crystals

Several diamond bulk crystals with a concentration of electrically neutral single substitutional nitrogen atoms of ≲80 ppm, the so-called C or P1 centers, are irradiated with electrons at 10 MeV energy and low fluence. The results show a complete suppression of the irreversible behavior in field and temperature of the magnetization below 30 K, after a decrease in ≲40 ppm in the concentration of C centers produced by the electron irradiation. This result indicates that magnetic C centers are at the origin of the large hysteretic behavior found recently in nitrogen-doped diamond crystals. This is remarkable because of the relatively low density of C centers, stressing the extraordinary role of the C centers in triggering those phenomena in diamond at relatively high temperatures. After annealing the samples at high temperatures in vacuum, the hysteretic behavior is partially recovered

Transparent Low Molecular Weight Poly(Ethylene Glycol) Diacrylate-Based Hydrogels as Film Media for Photoswitchable Drugs

Hydrogels have shown a great potential as materials for drug delivery systems thanks to their usually excellent bio-compatibility and their ability to trap water-soluble organic molecules in a porous network. In this study, poly(ethylene glycol)-based hydrogels containing a model dye were synthesized by ultraviolet (UV-A) photopolymerization of low-molecular weight macro-monomers and the material properties (dye release ability, transparency, morphology, and polymerization kinetics) were studied. Real-time infrared measurements revealed that the photopolymerization of the materials was strongly limited when the dye was added to the uncured formulation. Consequently, the procedure was adapted to allow for the formation of sufficiently cured gels that are able to capture and later on to release dye molecules in phosphate-buffered saline solution within a few hours. Due to the transparency of the materials in the 400–800 nm range, the hydrogels are suitable for the loading and excitation of photoactive molecules. These can be uptaken by and released from the polymer matrix. Therefore, such materials may find applications as cheap and tailored materials in photodynamic therapy (i.e., light-induced treatment of skin infections by bacteria, fungi, and viruses using photoactive drugs)

Monitoring of the Degree of Condensation in Alkoxysiloxane Layers by NIR Reflection Spectroscopy

This paper introduces a novel analytical approach for monitoring the degree of condensation of thin siloxane films, which is potentially suitable for in-line process control during the deposition of such layers, e.g., to polymer films. Near-infrared (NIR) reflection spectroscopy in combination with chemometric methods was used as a process monitoring tool. The state of the formation of the inorganic Si–O–Si network in partially condensed 3-methacryl­oxypropyl­trimeth­oxysilane batches was analyzed by inverse gated ²⁹Si NMR spectroscopy. Results were expressed in terms of different relative ratios of the Tⁱ species (i.e., structures with different numbers of Si–O–Si units per Si atom). These data were used for calibration of the NIR method, which was applied to thin layers printed on a polymer foil with a thickness of ∼2.2 g m^–2. The root-mean-square error of prediction (RMSEP) for the determination of the ratio of the Tⁱ species from the NIR spectra was found to be less than 3%. The error of the reference data from ²⁹Si NMR spectroscopy is 4%, which results in an overall error of 5%. Moreover, the thickness of siloxane layers was determined by this method in a range from 2.5 to 5.5 g m^–2 using gravimetry for calibration (prediction error ∼0.3 g m^–2)

Electron-Beam-Initiated Crosslinking of Methacrylated Alginate and Diacrylated Poly(ethylene glycol) Hydrogels

An ideal wound dressing not only needs to absorb excess exudate but should also allow for a moist wound-healing environment as well as being mechanically strong. Such a dressing can be achieved by combining both a natural (alginate) and synthetic (poly(ethylene glycol) polymer. Interestingly, using an electron beam on (meth)acrylated polymers allows their covalent crosslinking without the use of toxic photo-initiators. The goal of this work was to crosslink alginate at different methacrylation degrees (26.1 and 53.5% of the repeating units) with diacrylated poly(ethylene glycol) (PEGDA) using electron-beam irradiation at different doses to create strong, transparent hydrogels. Infrared spectroscopy showed that both polymers were homogeneously distributed within the irradiated hydrogel. Rheology showed that the addition of PEGDA into alginate with a high degree of methacrylation and a polymer concentration of 6 wt/v% improved the storage modulus up to 15,867 ± 1102 Pa. Gel fractions > 90% and swelling ratios ranging from 10 to 250 times its own weight were obtained. It was observed that the higher the storage modulus, the more limited the swelling ratio due to a more crosslinked network. Finally, all species were highly transparent, with transmittance values > 80%. This may be beneficial for the visual inspection of healing progression. Furthermore, these polymers may eventually be used as carriers of photosensitizers, which is favorable in applications such as photodynamic therapy

Electron-Beam-Initiated Crosslinking of Methacrylated Alginate and Diacrylated Poly(ethylene glycol) Hydrogels

An ideal wound dressing not only needs to absorb excess exudate but should also allow for a moist wound-healing environment as well as being mechanically strong. Such a dressing can be achieved by combining both a natural (alginate) and synthetic (poly(ethylene glycol) polymer. Interestingly, using an electron beam on (meth)acrylated polymers allows their covalent crosslinking without the use of toxic photo-initiators. The goal of this work was to crosslink alginate at different methacrylation degrees (26.1 and 53.5% of the repeating units) with diacrylated poly(ethylene glycol) (PEGDA) using electron-beam irradiation at different doses to create strong, transparent hydrogels. Infrared spectroscopy showed that both polymers were homogeneously distributed within the irradiated hydrogel. Rheology showed that the addition of PEGDA into alginate with a high degree of methacrylation and a polymer concentration of 6 wt/v% improved the storage modulus up to 15,867 ± 1102 Pa. Gel fractions > 90% and swelling ratios ranging from 10 to 250 times its own weight were obtained. It was observed that the higher the storage modulus, the more limited the swelling ratio due to a more crosslinked network. Finally, all species were highly transparent, with transmittance values > 80%. This may be beneficial for the visual inspection of healing progression. Furthermore, these polymers may eventually be used as carriers of photosensitizers, which is favorable in applications such as photodynamic therapy

Transparent Low Molecular Weight Poly(Ethylene Glycol) Diacrylate-Based Hydrogels as Film Media for Photoswitchable Drugs

Hydrogels have shown a great potential as materials for drug delivery systems thanks to their usually excellent bio-compatibility and their ability to trap water-soluble organic molecules in a porous network. In this study, poly(ethylene glycol)-based hydrogels containing a model dye were synthesized by ultraviolet (UV-A) photopolymerization of low-molecular weight macro-monomers and the material properties (dye release ability, transparency, morphology, and polymerization kinetics) were studied. Real-time infrared measurements revealed that the photopolymerization of the materials was strongly limited when the dye was added to the uncured formulation. Consequently, the procedure was adapted to allow for the formation of sufficiently cured gels that are able to capture and later on to release dye molecules in phosphate-buffered saline solution within a few hours. Due to the transparency of the materials in the 400–800 nm range, the hydrogels are suitable for the loading and excitation of photoactive molecules. These can be uptaken by and released from the polymer matrix. Therefore, such materials may find applications as cheap and tailored materials in photodynamic therapy (i.e., light-induced treatment of skin infections by bacteria, fungi, and viruses using photoactive drugs)