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The Influence of Visual Stimuli on Sniffing Behavior

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The Influence of Visual Stimuli on Sniffing Behavior The influence of visual stimuli on sniffing behavior. Masterthesis Student: G.D. Jongenelen (0358053) Supervisor: P.J. Bulsing The influence of visual stimuli on sniffing behavior Abstract Odor perception is influenced to an important extent by the expectations one has about the characteristics of that odor. Instead of focusing on the effect of cognitive processes on the final olfactory percept, the present experiment focused on the effect that cognitive processes – expectations- have on the early perceptual stage; the sniff. Visual stimuli, positive or negative pictures preceded the presentation of pleasant or unpleasant odors in mostly a congruent manner (positive – pleasant, negative – unpleasant) but sometimes in an incongruent manner (positive-unpleasant, negative – pleasant). Sniff vigor, total inhaled volume and sniff duration were expected to be mediated by expectation. The results showed that visual images have an influence on different sniff characteristics. The total inhaled volume of the odor increased when it was preceded by a positive picture compared to a negative picture, whatever odor was presented. Also, the sniff vigor slightly increased when the odor was preceded by a positive picture. People seem to use different ‘sniff patterns’ tailored to the odor they expect to perceive. 2 The influence of visual stimuli on sniffing behavior Introduction The chance that an odor will be detected, is influenced for a great part by the expectation one has about being exposed to an odor. Furthermore, expectations may have an influence on the fysiological response in odor perception. Engen (1972) showed that the likelihood for people to report an odor was significantly greater when smelling a substance that was colored, than when it was transparent, independent of whether an odor was present or not. The presence of a visual stimulus therefore seemed to induce the expectation about smelling an odor, and as a result increased the likelihood of participants reporting the presence of an odor. Additionally, the more frequently they perceived an odor in the past, the more likely they were to report the presence of an odor later, whether or not it was actually present. In other words, the chance of reporting an odor increased based on the participants expectation to perceive the odor more frequently. That expectation, besides odor detection, also influences odor identification, was demonstrated by Blackwell (1995). Here, participants had to recognize fruit odors when the substance was either colored differently or the same (e.g. yellow substance to represent lemon), from the fruit substance they smelled. This appeared to be significantly more difficult when the solution was colored differently than when it had the same color as the fruit. Also, when the intensities of the odor samples had to be arranged in terms of intensity (from strongest to weakest), this proved to be more difficult when the intensity of the substance color and the odor did not match. The visual cues seemed to distract the participants from making correct decisions about the odor, in both the odor detection and the odor intensity rating experiments. Not only us naïve individuals seem to be fooled by this ‘color’ effect, as even experts rely heavily on visual stimuli. The typical ‘red’ or ‘white’ wine judgments, that wine experts use to rate wines, are influenced by the color of the wine (Morrot et al. 2001). In the Morrot et 3 The influence of visual stimuli on sniffing behavior al. study, white wine was colored red with a non-odorous dye. Consequently, when the experts had to rate the ‘red white’ wine, they did so by using ‘red’ attributes. This showed that color can have a priming effect on odor perception. Furthermore, the integration of visual information, in this case color, seemed to occur in an unconscious way. In line with the previous studies, de Araujo et al. (2005) showed that words, instead of colors, can also influence odor perception. In their experiment, the odor cheddar cheese, was labeled by a visual word descriptor; either ‘cheddar cheese’ or ‘body odor’. Participants rated the odor as significantly more unpleasant when it was labeled ‘body odor’. Surprisingly, the same effect was noticeable when using a neutral odor (clean air). The effect of cognitive modulation was supported by fMRI data, which showed that activated brain regions were correlated with the participants’ pleasantness ratings. This activity thus seemed to be dependent on the word labels that were provided during the odor delivery. Distel and Hudson (2001) suggested that source information about a certain odor activates appropriate mental schemes, which lead to expectations about the odor. In their study, participants received twelve odors. In one condition, the source information (name of the odor) was known to the participants, and in the other condition it was not. The ratings of intensity, pleasantness and familiarity were enhanced when the name of the odor was provided. In a third condition, subjects were asked to report whether a given odor name matched their own perception. Ratings of intensity, pleasantness and familiarity were highest when they reported a good match. In this case, the authors speculate that people employ certain perceptual schemas which are activated in perceptual events. Perceptual schema’s can be described as the way in which objects are semantically related. For example, ovens and refrigerators are typically found in kitchens, barns and chickens on a farm and desks and computers in offices. This information is acquired during a lifetime of perceptual experience (Chun & Jiang, 1999). With respect to this description, it can be assumed that perceptual stimuli activate associations throughout different modalities associated with the perceived stimuli. For example, when observing freshly baked bread, the smell and taste of it are activated simultaneously. In the Distel and Hudson experiment, the high ratings in the good 4 The influence of visual stimuli on sniffing behavior match situation probably resulted from a good match between the neural activation of the perceptual schemas, and the actual perceptual input (knowing the source of the odor). Lower ratings, on the other hand, were explained as the result of an incomplete match, leading to reduced neural activity (e.g. not knowing the source of the odor). These findings led to the conclusion that cognitive processes, activated by certain information about the odor, influence odor perception. Gottfried and Dolan (2003) underlined the principle that congruency, or good matches, can facilitate odor perception. They offered picture-odor pairs, which could be semantically congruent (picture of a bus, with diesel odor) or incongruent (picture of cheese, with fish odor). An odor appeared to be detected significantly faster when the picture and the odor were a matched pair. Furthermore, the accuracy of detection responses (odor yes/no) also differed significantly, in the favor of the congruent pairs. Blackwell’s fruit odor experiment, described earlier, showed that it is difficult to ignore visual cues, and more importantly, that these cues seemed to dominate the eventual response. Blackwell (1995) compared this effect to the Stroop effect, where names of colors are printed in ink of a different color. In the Stroop task, higher perceptual processes (reading words) interfere with lower perceptual processes (naming ink colors), resulting in slower reaction times when asked to name the ink color. The same was the case in Blackwell’s study described above; the visual cues seemed to distract participants from making correct decisions about the odor. This, in turn can be integrated with Distel and Hudson’s mental schemata premise, because one process activates a certain set of semantic relations, which is hard to overrule by another process, in this case odor evaluations. So, like in the Stroop task; one of the processes had a dominant effect on the eventual response, in this case the knowledge of the odor source. The studies that have been discussed so far, all focus on the influence of cognitive processes on perception and subjective appreciation of odors. But, in order for a person to be able to smell, odorant transduction, the process of odorant binding at olfactory receptors, has to take place. Here, the first step is for odor molecules to find their way into the nasal cavity 5 The influence of visual stimuli on sniffing behavior where they bind to the olfactory epithelium, a tissue that lies several centimeters in the nasal cavity. In order for the molecules to reach the epithelium, an active process called sniffing has to take place. According to Comroe (1974) sniffing can be best described as “a mechanism for bringing ambient air into contact with the olfactory receptors in the nose without carrying the air (which may contain irritant, toxic materials) deep into the lung”. Olfaction thus actually consists of two parts; sniffing and smelling. The first concerns the airflow in the nostril, regardless of the presence of odor molecules. Secondly, smelling refers to the actual odor perception, regardless of airflow in the nostril. Although olfactory perception is usually assumed to reflect the ‘smelling’ part, it is largely dependent on the ‘sniffing’ part (Mainland & Sobel, 2006). Evidence for this statement lies, among others, in a study done by Bocca et al. (1965). Here, odor molecules were injected intravenously, so that odorants could reach the epithelium through the bloodstream in the absence of sniffing. They found that no odor was perceived, when participants did not sniff. Only when they could breathe normally through the nose or when odorless nitrogen was injected to mimic a sniff, the intravenous odor was perceived. This led them to conclude that the sniff, or mechanical stimulation of the epithelium, is necessary for olfactory perception. This necessity has also been shown by Bensafi et al. (2003). In their experiment, airflow was measured when participants had to imagine different stimuli across modalities, including olfaction (odor imagination).
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