Eye-Tracking Studies on Sustainable Food Consumption: A Systematic Literature Review

: In recent decades, sustainable products have been increasingly investigated using eye-tracking. Eye-tracking is applied to the record eye movements and visual attention of consumers as they search for food, and from this, conclusions can be drawn about their sustainable food consumption. To obtain a comparative overview of the previous utilization of eye-tracking in studies on sustainable food and consumption, a systematic literature search following the guidelines of the PRISMA statement was applied. In total, 38 eye-tracking studies were identiﬁed, with six main areas of investigation emerging: eye-tracking application, labeling, consumer attention, consumer choice and preference, consumer attitude and behavior, and willingness-to-pay. The review is aimed at both researchers and managers. In future research, the sample size of eye-tracking should be increased or focused on certain age groups in order to uncover sustainable consumer habits. In addition, ﬁeld and not just laboratory studies with eye-tracking need to be conducted. To obtain comparable results, it is necessary that researchers apply the same eye-tracking metrics and terms. Organic labels can inﬂuence consumer attention and purchase decisions if bottom-up factors of the labels are better aligned with other product information. Top-down factors, i


Introduction
Sustainable consumption is understood as a normative concept through which people, in the interest of environmental protection and ecological integrity, should try to reduce the footprints they leave behind through the misuse of resources [1].One possibility to minimize the impacts on the environment is by making consumers aware of sustainable food consumption or influencing their consumption behavior [2].Since consumers are confronted with a large amount of information at the point of sale that can capture their attention and interest [3,4], the question is whether and how to raise consumer awareness so that they buy sustainable food and ultimately promote sustainable food consumption.
To answer this, eye-tracking technology has been used in recent decades to examine consumers' visual attention and thus understand consumer preferences for food [5][6][7].Research studies about food using eye-tracking have mainly focused on the role and use of eco-labels and provide recommendations for label improvements [7][8][9][10].A major challenge, however, is to examine sustainable food consumption and the determinants of sustainable food purchasing decisions [11].Through a systematic literature review, this review shows how eye-tracking has been used to investigate consumers' sustainable food consumption, what factors influence sustainable food consumption, and highlights future research opportunities.In addition, possible applications of eye-tracking are presented in detail.This study therefore provides a comparative overview of previous studies that have investigated sustainable food consumption using eye-tracking, thus expanding the knowledge of the applications and results of previous eye-tracking studies.The following research questions are asked: what has been studied in the last 18 years using eye-tracking in terms of sustainable food consumption and how has it been investigated?
The attention consumers give to food depends on the stimuli that automatically act on them (bottom-up factors) and is also determined by their interests and goals (top-down factors) [6,[12][13][14][15][16]. Bottom-up and top-down processing occur simultaneously and interact with each other [17,18].Bottom-up factors act automatically on the consumer and include certain characteristics for processing basic stimuli such as size, color, or shape [13,17].Top-down factors are related to the person themselves [9].They include the consumer's voluntary search for and attention to specific product information [15] as well as the processing of individual experiences, motivations, and expectations [17].Both factors can be considered and analyzed with eye-tracking since they influence visual attention [6,18], and their analysis can lead to a better understanding of consumer decision-making processes [17].
Eye-tracking makes it possible to obtain information about consumer decision-making by studying consumers' search strategies based on their eye movements and visual attention to food during the purchase process [7,19,20].This involves constructing hypothetical purchase decisions using web-based, screen-based, or head-mounted eye-trackers in different test situations, i.e., test labs [17,21] or shopping environments [5,22].The eye-trackers are used to measure the eye movements and visual attention of test participants and to record which elements trigger stimuli or which elements the focus is on [17,23].Accordingly, eye-tracking is applied to determine the visual stimuli of food as well as the areas of interests (AOIs) with fixation times, fixation paths of the eye, and the percentage of fixated areas.In addition, the path of visual exploration up to the selection of a product can be recorded [24].
Organic products are products that are manufactured using environmentally friendly technologies and do not pose a risk to the environment [2,25,26].Promoting sustainable food is crucial for the conservation of natural resources and sustainable development [2], as is environmentally friendly consumer behavior.Sustainable consumption can be influenced by economics and politics as well as by technologies and the marketing mix of companies [2].In the food industry, consumers are confronted with a lot of information (often about food labeling) [7].When consumers engage with this information, and in particular environmental aspects, and this is then reflected in their purchase decisions, it is referred to as sustainable food consumption [2,27,28].To help consumers to choose sustainable food, the food industry provides information on production methods (e.g., organic farming, country-of-origin, genetic modification) and on ecological and ethical aspects of food production (e.g., carbon footprint, fair-trade, animal welfare) [7].The influence of this information on consumers and their consumption has been investigated using eyetracking [18].In order to obtain a comparative overview of the previous applications and their results on sustainable food consumption using eye-tracking, as well as to provide recommendations for future research, this study was conducted.
The results of this review suggest that both researchers and managers can gain a comprehensive picture of sustainable food consumption through the application of eyetracking.Researchers can base their future research on the conclusions mentioned below.They receive an overview of how eye-tracking is used in the context of sustainable food consumption, which eye-tracking measures are applied, and which sustainability labels are investigated with eye-tracking.In addition, previous research gaps are identified, e.g., conducting field research that concentrates on specific foods or consumer groups, which needs to be taken into account in future.By using previous studies to summarize what consumers paid attention to when buying sustainable food, i.e., how sustainable labels affect them, whether and how sustainable food is preferred and selected, what should be taken into account in terms of consumer attitudes, behavior, and willingness-to-pay, managers can draw conclusions to improve the marketing of sustainable foods.This can ultimately lead to an optimization of sales of sustainable food.The material and methods of the study are described in Section 2, followed by the results and discussion in Section 3. The study is concluded with recommendations for future research streams in Section 4.

Materials and Methods
The systematic literature review was conducted in accordance with the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [29,30].The checklist for the PRISMA guidelines can be found in the Supplementary Material.Two databases (Business Source Premier (BSP) and Web of Science (WoS)) were used to search for relevant literature.The final search was conducted on 23 October 2023 with the following search terms: (1) "eye-track*" OR "eye track*" OR "eye movement*" OR "eye gaze" OR "visuali*" OR "visual attention" AND (2) "organic" OR "sustainab*" OR "eco*" OR "environ*" OR "green*" OR "proenvironment*" OR "pro-environment*" AND (3) "consumer behav*" OR "purchas*" OR "shopp*" OR "customer*" OR "buy*" OR "consumer choice" AND (4) "food" OR "food*" OR "grocer*" OR "beverage" OR "eat*" OR "drink*" OR "product*".The wildcard (*) was used to include spelling variation and reduce the number of phrases while still providing a comprehensive search result.Figure 1 outlines the search strategy.
the study are described in Section 2, followed by the results and discussion in Section 3. The study is concluded with recommendations for future research streams in Section 4.

Materials and Methods
The systematic literature review was conducted in accordance with the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [29,30].The checklist for the PRISMA guidelines can be found in the Supplementary Material.Two databases (Business Source Premier (BSP) and Web of Science (WoS)) were used to search for relevant literature.The final search was conducted on 23 October 2023 with the following search terms: (1) "eye-track*" OR "eye track*" OR "eye movement*" OR "eye gaze" OR "visuali*" OR "visual attention" AND (2) "organic" OR "sustainab*" OR "eco*" OR "environ*" OR "green*" OR "proenvironment*" OR "pro-environment*" AND (3) "consumer behav*" OR "purchas*" OR "shopp*" OR "customer*" OR "buy*" OR "consumer choice" AND (4) "food" OR "food*" OR "grocer*" OR "beverage" OR "eat*" OR "drink*" OR "product*".The wildcard (*) was used to include spelling variation and reduce the number of phrases while still providing a comprehensive search result.Figure 1 outlines the search strategy.The first search for articles resulted in a total number of records of n = 909 (n = 229 BSP and n = 680 WoS).An article was included if it was (1) written in English, (2) published in a peer-reviewed journal, (3) a primary source (i.e., neither a conceptual paper nor a review), (4) published in the period 2005-2023, and (5) examining sustainable food consumption using eye-tracking.After removal of duplicates (n = 125), non-peer-reviewed journals (n = 19), and review papers (n = 33), the titles and abstracts of 732 articles were scanned (screening).In total, 681 articles were excluded and 51 full-text articles were assessed for eligibility.Thereafter, 13 full-text articles were excluded because they did not meet the indicated inclusion criteria.In total, 38 articles were included in the final analysis (see Figure 2, which shows the different stages of study selection for the systematic review following the PRISMA statement [29,30]).The first search for articles resulted in a total number of records of n = 909 (n = 229 BSP and n = 680 WoS).An article was included if it was (1) written in English, (2) published in a peer-reviewed journal, (3) a primary source (i.e., neither a conceptual paper nor a review), (4) published in the period 2005-2023, and (5) examining sustainable food consumption using eye-tracking.After removal of duplicates (n = 125), non-peer-reviewed journals (n = 19), and review papers (n = 33), the titles and abstracts of 732 articles were scanned (screening).In total, 681 articles were excluded and 51 full-text articles were assessed for eligibility.Thereafter, 13 full-text articles were excluded because they did not meet the indicated inclusion criteria.In total, 38 articles were included in the final analysis (see Figure 2, which shows the different stages of study selection for the systematic review following the PRISMA statement [29,30]).

Methodological Assessment
The 38 articles included in the final analysis were reviewed for following general information: authors, year of publication, journal, sampling country, food, sample size after eye-tracking and participant information, sustainable stimuli, apparatus, methodology, and measures.Table 1 shows the summary of all studies included in the systematic literature review.Because the focus of the review was on eye-tracking, all articles used at least eyetracking as method of data collection.Only three studies used the eye-tracking methodology alone.In all other studies (92.1%), at least one other method was used.In addition to eyetracking, questionnaires (

Methodological Assessment
The 38 articles included in the final analysis were reviewed for following genera formation: authors, year of publication, journal, sampling country, food, sample size eye-tracking and participant information, sustainable stimuli, apparatus, methodol and measures.Table 1 shows the summary of all studies included in the systematic l ature review.Because the focus of the review was on eye-tracking, all articles used at eye-tracking as method of data collection.Only three studies used the eye-tracking m odology alone.In all other studies (92.1%), at least one other method was used.In add to eye-tracking, questionnaires (26 studies), choice experiments (19 studies), interview studies), implicit association test (2 studies), experimental auction (1 study), observa (1 study), and face reader (1 study) were used as additional data collection methods.[29,30]).
The studies were conducted in 15 countries (see Figure 3).Of these, most studies were conducted in the USA (31.6%), followed by Germany (13.1%) and Italy (10.5%).The studies were conducted in 15 countries (see Figure 3).Of these, most studies were conducted in the USA (31.6%), followed by Germany (13.1%) and Italy (10.5%).The sample size reported in this review is the number of participants whose data were analyzed after the eye-tracking and is therefore smaller than the original sample size.If more than one study was reported in an article, only the number of participants in those studies in which eye-tracking was applied were reported.The sample size of eye-tracking participants varied from 17 to 434 participants.The price of organic products has a significant negative impact on the purchase of organic products, but visual attention (longer gaze at the organic area) increases the likelihood of a purchase.The level of knowledge correlates with organic purchases.For non-label users, the choice is almost random for products with organic labels.When organic food and healthy food are positively correlated, consumers pay more attention to organic food when assessing the healthiness of food.In this case consumers are more likely to focus on the organic information and more likely to choose products with an organic label.A larger and visually more eye-catching label significantly increases the fixation likelihood of that organic label.The consumers' fixation on the organic label decides whether they choose the product or not, same as the design of the organic label.

Visit duration
Visual attention to sustainable claims influences product choice.Higher visual attention is associated with a higher likelihood of food choice.The higher the price, the less likely the consumer is to choose the sustainable food.When consumers focus (fixate) on an attribute for longer, this leads to a higher preference for that attribute.Eye-tracking, questionnaire

Fixation duration
The perceived benefit and quality of recycled water has a positive effect on the population's willingness-to-purchase, while the perceived risk of recycled water influences the willingness-to-purchase negatively.The higher the visual attention to user comments, the more likely it is to stimulate and promote the public's perceived usefulness of recycled water.
The sample size reported in this review is the number of participants whose data were analyzed after the eye-tracking and is therefore smaller than the original sample size.If more than one study was reported in an article, only the number of participants in those studies in which eye-tracking was applied were reported.The sample size of eye-tracking participants varied from 17 to 434 participants.
The age-range of the eye-tracking participants was reported in 20 studies and was grouped as follows: 18-30 years (three studies), 19-48 years (three studies), 18-59 years (three studies), and 18-65 years (eleven studies).The average age of the participants was reported in 17 studies and was 32 years.Twelve articles focused on young people and nine of these recruited participants (students) from the university environment.The gender distribution was not the same across all studies (female 57% and male 43%).In nineteen studies, the proportion of women was higher, while in seven studies, the proportion of men was higher, and in three studies, there was an even gender distribution.Nine studies did not report on the gender distribution of their participants (see Table 2).Appendix A).The systematic literature search according to the defined criteria in the two databases shows that previous use of eye-tracking studies on sustainable food consumption was found from 2015 onwards with three publications.By 2018, this increased to six releases per year.After three years with a constant publication count of six (2018-2020), the number of publications declined and has since then remained stable at four publications per year (see Figure 4).Although eye-tracking was conducted long before 2015 [6], research interest in eye-tracking related to sustainable food consumption seems to have emerged only in the last decade (since 2015).By 2018, this increased to six releases per year.After three years with a constant publication count of six (2018-2020), the number of publications declined and has since then remained stable at four publications per year (see Figure 4).Although eye-tracking was conducted long before 2015 [6], research interest in eye-tracking related to sustainable food consumption seems to have emerged only in the last decade (since 2015).The most frequently examined single food in the studies reviewed were coffee and meat (both with four studies) (see Figure 5).The term "multiple" includes at least 3 foods and ranges up to 64 foods studied.Food causes different ecological footprints.Coffee, meat, cheese, and combinations of foods ("multiple") are among the most resource-inten- The most frequently examined single food in the studies reviewed were coffee and meat (both with four studies) (see Figure 5).The term "multiple" includes at least 3 foods and ranges up to 64 foods studied.Food causes different ecological footprints.Coffee, meat, cheese, and combinations of foods ("multiple") are among the most resource-intensive foods [62].Reducing their consumption is one of the best ways to reduce food-related greenhouse gases [62].The most frequently examined single food in the studies reviewed were coffee and meat (both with four studies) (see Figure 5).The term "multiple" includes at least 3 foods and ranges up to 64 foods studied.Food causes different ecological footprints.Coffee, meat, cheese, and combinations of foods ("multiple") are among the most resource-intensive foods [60].Reducing their consumption is one of the best ways to reduce food-related greenhouse gases [60].

Application of Eye-Tracking
The majority of eye-tracking was carried out in laboratory settings (89%).Only four eye-tracking studies were conducted in the field, i.e., directly in the supermarket.A total of 32 screen-based eye-trackers, 7 head-mounted eye-trackers, and 1 web-based eyetracker were used (see Figure 6).Two studies used two eye-trackers; specifically, Gidlöf et

Application of Eye-Tracking
The majority of eye-tracking was carried out in laboratory settings (89%).Only four eye-tracking studies were conducted in the field, i.e., directly in the supermarket.A total of 32 screen-based eye-trackers, 7 head-mounted eye-trackers, and 1 web-based eye-tracker were used (see Figure 6).Two studies used two eye-trackers; specifically, Gidlöf et al. [36] used one screen-based and one head-mounted eye-tracker and Orquin et al. [14] used two screen-based eye-trackers.Depend on the eye-tracker, different gaze sampling frequencies (in Hz) were used in the studies (see Figure 7).Head-mounted and web-based eye-trackers usually have a lower sampling rates, while screen-based eye-trackers have a higher sampling rate [63,64].Depend on the eye-tracker, different gaze sampling frequencies (in Hz) were used in the studies (see Figure 7).Head-mounted and web-based eye-trackers usually have a lower sampling rates, while screen-based eye-trackers have a higher sampling rate [63,64].Depend on the eye-tracker, different gaze sampling frequencies (in Hz) were used i the studies (see Figure 7).Head-mounted and web-based eye-trackers usually have lower sampling rates, while screen-based eye-trackers have a higher sampling rate [61,62 Different eye-tracking measures were applied in the studies (see Figure 8).Accord ingly, there is no uniform framework for eye-tracking.This finding is consistent with Bor gianni et al. [11], who recommended that AOIs in eye-tracking studies require the sam approaches and developed a framework for evaluating AOIs for sustainable products and designs.In addition, different terms are used in the reviewed studies for same measures such as fixation duration and fixation time.To achieve a consistent understanding in eye tracking research, a uniform definition and application of terms is advisable in the future In this study, the measures were transferred and are reported in a uniform system (se Appendix B) and can be used as a recommendation for future eye-tracking studies.Different eye-tracking measures were applied in the studies (see Figure 8).Accordingly, there is no uniform framework for eye-tracking.This finding is consistent with Borgianni et al. [11], who recommended that AOIs in eye-tracking studies require the same approaches and developed a framework for evaluating AOIs for sustainable products and designs.In addition, different terms are used in the reviewed studies for same measures, such as fixation duration and fixation time.To achieve a consistent understanding in eye-tracking research, a uniform definition and application of terms is advisable in the future.In this study, the measures were transferred and are reported in a uniform system (see Appendix B) and can be used as a recommendation for future eye-tracking studies.In total, 85 sustainability labels were investigated in the eye-tracking studies.Organic labels were the most frequently examined labels, followed by country-of-origin, carbon footprint, fair-trade, non-GMO/GMO-free, Rainforest Alliance, and region-of-origin labels (see Table 3).In total, 85 sustainability labels were investigated in the eye-tracking studies.Organic labels were the most frequently examined labels, followed by country-of-origin, carbon footprint, fair-trade, non-GMO/GMO-free, Rainforest Alliance, and region-of-origin labels (see Table 3).

Labeling
Consumers fixate on most sustainable labels [48].Moreover, if consumers pay attention to a sustainable label at least once, they will also spend a certain amount of time looking at it [46].While consumers pay attention to sustainable labels, according to Oselinsky et al. [46], labelling has no significant influence on food choices.Grebitus et al. [37], on the other hand, found that the labelling of food with organic labels has a significant and positive influence on consumer decisions.The majority of participants include sustainable labels in their assessment [49].If this is the case, logo and picture labels are preferred over text labels and receive greater visual attention [13,45].Sustainable labels should be designed with as little text as possible in order to achieve a high level of initial attention [22].A larger and visually more eye-catching sustainable label significantly increases the fixation likelihood of that label and the impact on consumer attention [50].Information, images, and slogans that are displayed at the consumer's eye-level are more likely to be fixated on by consumers.Consumers require less time and cognitive effort to view sustainable labels when they are large and highly visible [51].Larger sustainable labels, but not higher saliency, help consumers cognitively process the label [51].Increased salience alone does not increase fixation likelihood, i.e., the combination of size and saliency is crucial [50].
Sustainable labels are hardly noticed by consumers in competition with other product information [5].Food product appearance, price, and nutritional information are often fixated first and longest.Eco-labels are neither the first nor the longest viewed AOIs in the consumers' product evaluation process [5].Consumers prefer products with two organic labels over products with one label or no label [49].Accordingly, country-of-origin labels are an important factor in signaling sustainability [48].Carbon footprint labeling stands out for some consumers, especially when the size of the sustainable label is matched with other labels (e.g., price or nutrition labels) and it is displayed on the front of the product [33].When it comes to menu selection, carbon footprint and local farmer labels do not influence menu choices and capture little consumer attention [31].To increase consumer interest in organic labels, visual elements need to be considered [51].The design of organic labels can have a significant impact on consumer attention [50].The most important task of an organic food label is to attract visual attention in order to increase product selection [50,51].
The visual ecology of product packaging has a predictable structure that favors brandrelated elements [65].Product logo and brand images are more concise, larger, and centrally positioned than sustainability-related elements.Therefore, organic labeling is essential for organic packaging (longer dwell time and high number of fixations) and must be highlighted on the food packaging [54].The type of bottle determines the choice of the best place for the organic label [47].Since salience, size, and distance (bottom-up factors) increase the likelihood that consumers will fixate on a product [66], preference for brand-related elements leads to neglect of sustainable elements [14].Changes in visual ecology and design could therefore lead to greater consumer awareness towards sustainability information.

Consumer Attention
Consumers pay attention to various attributes when choosing food [34,59].They tend to quickly check all food alternatives, focus on a few food alternatives, and then compare key characteristics (e.g., price, animal feeding, and labels) [35].Sustainable labels (country-of-origin or organic labels) attract consumers visual attention [21,34,57], but onethird of the consumers are not interested or do not pay attention to them [21,34].Higher and lower prices receive more visual attention than medium prices [45].Visual attention is related to extrinsic cue words such as local and menu choice [34].Consumers who notice information displays at the point of sale spend more time looking at environmentally friendly products (e.g., certified coffee) than those who do not notice the information [22,39].Sustainable criterions are more valued by consumers who spend more time attending to and fixating them [34,57].Consumers who do not visually pay attention to sustainable logos are ignoring them [58].Priming can significantly increase consumers' visual attention to an environmentally friendly food product [39].
Consumers who favor text labels direct their gaze stronger and longer at the text labels than those consumers who favor logo labels [13].Country-of-origin logo labels tend to attract consumer attention quicker and have a longer eye-tracking time than text labels [42].Regardless of a packaging design, consumers spent most time looking at the organic label section [54].Since the color green is associated with organic and natural characteristics, green positively influences consumers' visual attention [39].Thus, the color of the sustainable label matters [54].
When consumers' visual attention is focused on organic labels, this has a significant impact on the purchase of organic foods [17].The longer consumers gaze on foods with organic labels (hormone-free and country-of-origin labels), the more likely they are to choose that product [37].Low-involvement products appear to attract less visual attention than high-involvement products [44].When consumers derive greater benefit from certain properties of a food product, they also pay more attention to them visually [44].Although visual attention influences the purchase decision process, Meyerding and Merz [44] find that different organic labels play a less important role in food purchase decision processes than expected.
In the study by Grebitus et al. [38], the label pesticide-free attracts the most visual attention, followed by the GMO-free label, and the region-of-origin label attracts the least.Consumers pay more attention to the first two labels when they are concerned about these production methods.Attention to the region-of-origin label depends on how strongly consumers associate this product with a particular region [38].The country-of-origin label is the label consumers most often look for when selecting food [59].The level of consumer ethnocentrism influences visual attention directed to country-of-origin labels [59].
Visual attention to sustainable food options is relative equal among hungry or satiated consumers [36], while visual intention is influenced by self-esteem and image congruence in food decision making [24].An increase in self-esteem can lead to better food choices [24,67].Consumers pay greater visual attention to the criteria they value higher [57].Unless it is a top-down situation, there is no significant relationship between visual attention and food choice [45].However, visual attention to nutritional and sustainable claims impacts food choice [60].
User comments indirectly influence the public's willingness to purchase recycled water because they influence the perceived benefits of recycled water reuse.The higher the visual attention to user comments, the more likely it is to stimulate and promote the public's perceived usefulness of recycled water [61].

Consumer Choice and Preference
Organic product labelling can play a role in decision making [21,51].Country-of-origin and organic labels have a significant positive effect on consumer choice [35].However, a third of consumers choose their meat without paying attention to its origin [35].In a store-setting environment, hungry or satiated consumers do not make different choices when it comes to eco-labelled foods [36].Cue words such as local seem not to be the main reason why consumers select menu items [34].Consumers prefer impeccable foods over suboptimal foods when shopping [40].However, when impeccable and suboptimal foods have differently designed price tags, there is a positive trend towards purchasing suboptimal foods [40].
Consumer choice is directly related to the attention consumers pay to sustainable irrigation labels [13].Consumers who are more likely to pay attention to organic information are also more likely to choose products with organic labels [49].Hormone-free and country-of-origin labels have statistically significant and positive effects on consumer choice [37].Consumers prefer local and organic products over non-local and non-organic products [42].Women are more likely to exhibit high self-esteem behavior and men are more likely to exhibit low self-esteem behavior, which means that woman are more likely to buy foods with sustainable labels than men [24].
The attention capture effect, i.e., the fixation of an organic label, determines whether the product is chosen or not, as with the design of the organic label [50,51].Price is the most important criteria that consumers consider when choosing food.Consumers who place more value on sustainability aspects and/or price also pay more attention to this information when making food choices [57].Consumer preference increases when a food product has an organic label [57,58].
Consumers who include organic labels in their decision process are moderately likely to choose food with one organic label and highly likely to choose food that has two organic labels [49].Consumers' visual attention to the certification program logo, coffee product name, or a promotional statement does not influence their food choice [56].Higher visual attention is associated with a higher likelihood of food choice [60].If the consumer spends more time on a particular attribute during the entire selection sequence, this leads to a higher evaluation of this attribute and, at the same time, to a preference for this attribute [60].

Consumer Attitude and Behavior
Consumers with a positive attitude towards sustainability spend more time searching for organic labels [33] and fixate on sustainable packaging and display elements longer [22].They are more likely to remember sustainable product features and individual display elements than consumers with a negative attitude towards sustainability [22].The amount of time consumers view logos and their spontaneous emotional response is dependent on consumers' implicit attitudes [55].Consumers with positive implicit attitudes towards sustainability recognize the recycling logo quicker and spend more time processing the label, which in turn leads to better emotional response attitudes [55].The longer consumers fixate an organic label, the stronger the connection between implicit attitudes and spontaneous emotional reaction [55].Nevertheless, there is no significant relationship between consumer attitude and overall proportion of time spent looking at the label [33].A positive attitude towards sustainability does not automatically lead to higher overall attention to the carbon footprint label.However, consumers with a positive implicit attitude to a low carbon footprint are more likely to fixate first on the carbon footprint label rather than other labels compared to consumers with a more negative implicit attitude towards carbon footprint [33].
If a consumer pays more attention to a sustainable attribute (e.g., organic, country-oforigin labels) of a food, it is because the consumer values this attribute more than other attributes [32].However, designing specific and colorful price tags can increase consumer awareness of organic food [40].This can overcome the barrier to sustainable purchasing behavior [68].The likelihood of choosing an organic food product decreases if consumers are unfamiliar with it.Visual attention likely counteracts this behavior [37].Consumers who have sustainable purchase intentions and generally look for environmentally friendly foods tend to take their environmental attitudes into account [39].Knowledge about sustainability correlates significantly with the purchase of organic food [17].
There are no significant differences in food-elicited emotions between organic and non-organic samples [41].Consumers tend to exaggerate their positive emotional attitudes towards organic over non-organic food and their negative emotional attitudes towards non-organic over organic food [41].When consumers are presented with two samples, non-organic versus organic, they have the same emotional attitudes.This can change if consumers know they are testing a non-organic or organic sample [41].
The presence of environmentally friendly aspects in organic foods does not lead to increased emotional involvement of consumers, i.e., organic foods do not arouse more curiosity and excitement among consumers than conventional products [43].The attitudebehavior gap shows that consumers' interest in sustainability issues does not reflect their consumption decisions [43].Consumers who know and understand sustainable labels pay more attention to them than consumers who do not [53].In addition, visual attention and positive purchase intention are stronger among consumers who understand the meaning or purpose of organic labels [53].Awareness and level of interest in sustainability issues of certified coffee and purchase experiences have no influence on consumer purchasing behavior [56].Since there is no statistical correlation between the duration of the logo fixation and the purchases, the logo does not appear to be a decisive factor in consumers' purchasing behavior [56].Consumers do not need to focus on some visual attributes to the same extent as other attributes because they already know them [58].
A majority of participants (54%) do not evaluate the product information of the foods they purchase, suggesting habitual shopping [5].This is also evident among consumers who prefer foods with organic labels.Organic labels are neither the first nor the longest observed attribute in the consumer product evaluation process [5].Consumers do not actively look for environmentally friendly information during their decision process [5].This means that consumers pay little attention to sustainable labels when buying food.To gain attention and influence consumer behavior, the visibility of sustainable labels should be improved.

Willingness-to-Pay (WTP)
Price is an important attribute for consumers and has a significant negative coefficient [35].This means that the price of organic food has a significant negative impact on the purchase of organic food [17,43,44].Sustainable labels (local or organic labels) cannot guarantee increases in sales [21], since they are commonly associated with higher prices [48].The higher the price, the less likely consumers are to buy organic food [60].However, when consumers look longer at the organic area, the likelihood of purchasing the organic food increases [17].Compared to a control group, the vast majority of consumers are willing to pay 15% or more for wine labeled "sustainable irrigation" [13].Other studies also show that consumers are willing to pay a premium after considering (viewing) sustainable production characteristics [13,38,39,52].In addition, they are also willing to pay a higher price for products with a logo label than for products with text labels or without a label [42].Images of forests on the labels of certified forest coffee attract the visual attention of consumers and additionally stimulate the actual purchase of certified forest coffee [56].However, information about the certification program displayed on the certified coffee has no additional purchase effect [56].Additionally, mood does not influence organic purchases significantly [22].
As consumer attention shifts from product claims and price to nutritional value and ingredients, consumers are less price sensitive and expect better product quality when purchasing eco-labeled food [5].Consumers are therefore willing to pay a premium for organic food if they attach importance to sustainability aspects and pay more visual attention to sustainability information [57].A positive attitude towards sustainability tends to increase the willingness-to-pay for organic food [22].Also, the perceived benefit and quality of recycled water has a positive effect on the population's willingness to purchase the product, while the perceived risk of recycled water influences the willingness-to-purchase negatively [61].

Conclusions
Following the search strategy (see Figure 1), studies investigating sustainable food consumption using eye-tracking were found from 2015 onwards, although the systematic literature search started from 2005.Accordingly, the research focus in this study is very current and needs to be pursued further to obtain relevant information on sustainable food consumption with eye-tracking.Eye-tracking plays an important role in food sustainability research [7], as evidenced by the number of publications found and analyzed in this systematic literature review.Since the review is limited to selected search terms and to two databases, the actual potential of the research area is not fully demonstrated.
The following should be taken into account in the future.As mentioned, the sample size varied from 17 to 434 participants.This represents a wide range of variation.In eye-tracking in particular, it appears that the number of participants is reduced because participants do not fulfill the prerequisite for eye-tracking or their eye-tracking data cannot be evaluated [69].Because it is easier to recruit participants from university environments, nine studies (24% of the studies) did so.This recruitment then only includes certain consumer group (35% of participants in the studies are younger (<30 years)).In order to be able to investigate larger sample sizes in everyday shopping, scientific personnel and appropriate financial resources are required for research organization and implementation.Only four studies use eye-tracking directly in the supermarket.This points to limitations of the reviewed studies, since the focus is on a specific consumer group (younger) and environment (university setting).It can be assumed that the results of young consumers or undergraduate students cannot be generalized.Therefore, in the future, the sample size should be increased or focused on specific age groups or social milieus in order to uncover, with eye-tracking, specific consumption habits with regard to sustainable food consumption.Consumer groups need be recruited from various life situations and social milieus, not only from the university environment.Despite the organizational and financial effort, field studies with eye-tracking must be carried out in real shopping situations and not just in the laboratory [70].
The selection of food examined in the studies is broad.It can be assumed that coffee and meat are the most frequently examined foods in these studies because coffee and meat have high carbon dioxide emissions [71].Future studies should specify why which foods are being examined or focus on foods that can make a significant contribution to sustainability.It might be helpful to focus on one food in each study to make the results comparable and also to provide a country comparison across different consumer groups.Food selection could be connected with the specific characteristics of consumers.Categories that arise from social milieus, such as age, gender, and income, could be taken into account in order to select foods specifically according to certain consumer behavior.As the sociodemographic characteristics of the eye-tracking participants were not specified in most of the studies, it was not possible to form meaningful categories regarding to this in this review.Future studies should specifically include the socio-demographic characteristics of participants after the eye-tracking is carried out so that it is possible to determine sustainable food consumption across different social milieus.Since different measures are used for eye-tracking, it is necessary to find in future a uniform application of the measurement variables so that equivalent results can be obtained.The attempt to generalize eye-tracking scenarios by Borgianni et al. [11] can lead in this direction (see also Appendix B).
Salience, size, and distance (so-called bottom-up factors), as well as the color of the organic label, increase the likelihood that consumers fixate on a food [16,54] and most likely influence the purchase decision [17,37].Particular food marketers and policy makers could use this finding to adjust label size and saliency relative to other elements to increase consumer attention toward organic food [50].Consumers who have a positive attitude towards sustainability [55] search for organic labels [33], fixate them long [22], and are more likely to buy organic food [57,58,60].Nevertheless, organic food does not arouse more curiosity and excitement among consumers than conventional food [43].To increase sustainable food consumption, consumers should be informed and educated about the importance of sustainable consumption and the meaning of organic labels [5,53,57].Retailers could influence consumer intent to purchase sustainable food by displaying relevant information, orienting consumers in-store, and offering an environmentally friendly product assortment [39].Since higher prices reduce the purchase probability of sustainable food [44], policy makers and mangers have to think about how sustainable food can be made affordable.Even if the different design of price tags shows a positive trend towards buying sustainable food [40], this cannot be an option to increase sustainable food consumption in the long run.[ 5,17,38,42,44,45,48,59,60]

4 Figure 2 .
Figure 2. Flow chart of the systematic literature review according to PRISMA (adapted from [29

Figure 3 .
Figure 3. Overview of the countries in which the eye-tracking studies were carried out.

Figure 3 .
Figure 3. Overview of the countries in which the eye-tracking studies were carried out.

Figure 4 .
Figure 4. Number of publications per year.

Figure 4 .
Figure 4. Number of publications per year.

Figure 4 .
Figure 4. Number of publications per year.

Figure 5 .
Figure 5. Overview of the foods investigated in the studies (n = 38).

Figure 5 .
Figure 5. Overview of the foods investigated in the studies (n = 38).

Figure
Figure Eye-trackers used in the reviewed studies.

Figure 6 .
Figure 6.Eye-trackers used in the reviewed studies.

Figure 7 .
Figure 7. Frequency and sampling rates (Hz) of the eye-trackers in all 38 studies.

Figure 7 .
Figure 7. Frequency and sampling rates (Hz) of the eye-trackers in all 38 studies.

Figure 8 .
Figure 8. Frequencies of the measures applied in the eye-tracking studies.

Figure 8 .
Figure 8. Frequencies of the measures applied in the eye-tracking studies.

Table 1 .
Summary of all studies included in the systematic literature review.

Table 2 .
Categories analyzed in the reviewed studies.
Six articles were published in Food Quality and Preference, four articles in Sustainability, and two articles each in Ecological Economics, Journal of Business Research, Journal of Cleaner Production, and Journal of Retailing & Consumer Studies.All the other 20 articles were published in various journals (see

Table 3 .
Sustainability labels examined in the studies using eye-tracking.

Table 3 .
Sustainability labels examined in the studies using eye-tracking.

Table A4 .
Cont.Total number of visits a participant makes to an AOI; • Time span between the start of the first eye movement inside the AOI to the end of the last eye movement in the same AOI; • Number of visits within each AOI; • A visit includes both saccades and fixations during the time the eyes first fixate on an AOI until they move out of it.Total length of time a participant spends in the AOI; • Total number of seconds that a participant looks at a particular AOI over the total time she or he is observing the presented slide; • Total time each participant visits each AOI; • Duration of all visits within an AOI; • Sum of visit durations of an active AOI.