as energy expenditure does not seem to have changed substantially over the past 30  years , an increased energy intake is believed to be the main factor explaining the current obesity epidemic .
it is generally assumed that energy intake has changed because of changes in the environment : compared to the 1980s , western(ized ) environments are now characterized by an abundant availability of highly rewarding , generally high - fat / high - sugar foods , which are advertised aggressively .
that is , everyone in the western world is now exposed continuously and heavily to food - related cues .
food responsiveness is regulated in a brain network , consisting of mesolimbic and frontal brain regions , which is referred to as the reward system .
communication within the brain reward network happens through neurotransmitters such as dopamine , which is thought to be primarily involved in appetitive (  wanting  ) responses to rewarding stimuli [ 3 ] .
several addiction theories acknowledge that a drug - related cue reactivity , and particularly the subjective experience of craving , is a central feature of addiction , involved in the inability of addicted persons to stop the drug use and in relapse .
one of these theories is the incentive sensitization theory [ 5 , 6 ] . in short , according to this theory the repeated exposure to drugs leads to a process of incentive sensitization in the dopamine - driven brain reward system . through a process of associative conditioning , not only drugs themselves , but all stimuli associated with drug use acquire incentive salience qualities .
that is , the mere exposure to drug - related stimuli elicits the release of dopamine in the brain reward system , and because of this these stimuli become attention - grabbing , induce subjective craving , and ultimately elicit approach behavior .
according to this theory , an attention bias to drug - related cues is , as much as subjective craving , a central feature of addiction , and is also directly related to dopamine activity in the reward system .
a wealth of research shows that addicted persons ( to all kinds of drugs ) are generally characterized by an attention bias for relevant drug - related stimuli , which is associated with subjective craving and predicts relapse [ 7 , 8 ] .
there are similarities between addictive behavior and overeating and obesity . as addicts , persons with an overeating problem report cravings for , generally high - calorie , food and loss of control over eating . for many people , it is difficult to resist palatable , high - calorie foods , although the negative consequences of indulging in these foods are well known .
relapse rates are equally high in weight loss programs as in addiction treatment programs [ 9 , 10 , 11 ] . on a neurobiological level
, several studies point to similar deficits in dopamine - based reward system functioning in persons with an addiction and an overeating problem / obesity [ 12 , 13 ] .
according to some obesity researchers , neurocognitive addiction models , such as the incentive sensitization theory , might be applicable to obesity [ 3 ] .
this means that , similar to addiction , an attention bias to rewarding foods might play an important role in the development and maintenance of overeating behavior and weight gain / obesity . during the last few years
the relationship between attention bias to food - related stimuli and obesity has received increasing attention .
this review gives a critical overview of studies concerning the attentional processing of food cues in obese and overweight individuals over the past 3  years ( 20092011 ) .
the general hypothesis of these studies is that overweight / obese individuals demonstrate an enhanced attention bias to highly rewarding , generally high - calorie foods , and that this attention bias is associated with subjective ratings of food craving .
table  1 gives an overview of studies that have directly compared , otherwise healthy , overweight / obese and normal weight persons with regard to food - related attention bias .
various tasks and techniques were used to assess two different components of attention to food - related stimuli : the initial orientation of attention ( ie , the immediate and automatic detection of food - related stimuli ) and the maintenance of attention ( ie , the difficulty to disengage attention from food - related stimuli ) .
interestingly , several studies found evidence for an enhanced oriented attention to , particularly high - calorie , food stimuli in overweight / obese persons . this was found by castellanos et al .   and werthmann et al . , using an eye - tracking procedure , by nijs et al .
[ 16 ] , using a visual probe task , and by nijs et al .   using the amplitude of the p200 event - related potential ( erp
also interesting is that in some studies evidence was found for a reduced maintenance of attention , that is , an avoidance of or a shift away from high - calorie food - related stimuli in overweight / obese persons .
[ 16 ] , using the amplitude of the p300 erp , and by werthmann et al . , who investigated the duration of initial eye fixations to food - related stimuli .
an approach - avoidance tendency or a motivational ambivalence toward high - calorie food stimuli in overweight / obese persons makes sense , as these persons might experience a conflict between the desire to eat ( as reflected in a strong initial orientation toward food ) and the desire to lose weight ( as reflected in a subsequent shift away from food ) . a motivational ambivalence is also seen , for instance , in unsuccessful dieters , self - reported chocolate cravers , and alcoholics who try to abstain from alcohol , suggesting that it might be a typical response to desired but
substances.table 1a summary of food - related attention bias studies in ob / ow visual probe task individuals between 20092011studystudy groupsmeasure of attentionstimuliattention parameters ( oriented / maintained attention)hunger / satiety interventionmain findings : difference between ow / ob and nw subjectsmain findings : positive association between attention bias and subjective cravingcastellanos et al .
18 ob and 18 nw womenet during performance of a vptpairs of pictures of hc / lc foods and non - food itemset direction biashunger ( > 8  h )  +  satiety ( shake 400  kcal ; within - subjects design)et direction  +  duration bias : direction / duration bias  ~  hungeret duration biashunger : ob  =  nwvpt rt bias 2,000  mssatiety : reduced in nw ; retained in obwerthmann et al .
22 ow / ob and 29 nw womenet during performance of a vptpairs of pictures of high - fat foods and non - food itemset direction biasrestrain from eating 2  h prior to testing ; satiated with a lunch ( 400500  kcal ) before testinget direction bias : ow / ob  >  nwin ow / ob : et duration bias  ~  cravinget first fixation duration biaset first fixation duration bias : ow / ob  <  nwet duration biasvpt rt duration bias 2,000  msnijs et al .
[ 16]26 ow / ob and 40 nw womenet , vpt , erppairs of pictures of hc foods and non - food itemset direction biashunger ( 17  h )  +  satiety ( shake 600  kcal ; between - subjects design)rt bias 100  ms : ow / ob  >  nw , especially in hungerin nw : p300 bias  ~  hungeret duration biasp300 bias : in ow / ob : vpt direction bias  ~  hungervpt rt direction bias 100  mshunger : ow / ob  <  nwvpt rt duration bias 500  msp300 erp biassatiety : ow / ob  >  nwnijs et al .
20 ob and 20 nw men and womenerp during a food - modified stroop taskwords , referring to hc foods and non - food itemsp200 erplight meal 2  h prior to testingp200 bias : ob  >  nwin ob : stroop rt bias  ~  cravingin nw : p200/p300
15 long - term successful weight loss maintainers , 14 ob , 19 nwfood - modified stroop taskwords , referring to hc and lc foodsstroop rtfasting state ( > 4  h)stroop rt interference hc words : successful dieters slower than ob / nw  loeber et al . 20 ob and 20 nw men and womenvptpairs of pictures of foods and non - food itemsvpt rt bias 50  msrestrain from eating 3  h prior to testinggraham et al .
15 ow / ob and 21 nw womenetpairs of pictures of hc sweet foods ; hc savory foods ; lc foodset direction bias  direction bias lc food : ow / ob  >  nw  et duration biasbmi body mass index ; erp event - related potentials ; et eye tracking ; hc high - calorie ; lc low - calorie ; nw normal weight ( bmi 18.525  kg / m ) ; ob obese ( bmi  >  30 
kg / m ) ; ow overweight ( bmi 2530  kg / m ) ; rt reaction time ; vpt visual probe task a summary of food - related attention bias studies in ob / ow visual probe task individuals between 20092011 bmi body mass index ; erp event - related potentials ; et eye tracking ; hc high - calorie ; lc low - calorie ; nw normal weight ( bmi 18.525  kg / m ) ; ob obese ( bmi  >  30 
kg / m ) ; ow overweight ( bmi 2530  kg / m ) ; rt reaction time ; vpt visual probe task however , as the results in table  1 reveal , the approach - avoidance idea is not supported by all studies .
results have been inconsistent and seem to depend on methodological choices and the study design .
there is a large variety in study designs , which makes it rather difficult to compare directly results of different studies .
moreover , one could raise questions on the validity of some of the used attention measures . in this review
some issues are discussed , which could be taken into account when interpreting attention studies .
in the studies summarized in table  1 attention for food - related stimuli is derived from behavior in reaction time tasks , eye tracking , electrocortical activity ( erps ) , or a combination of these measures . among reaction time tasks the food - modified stroop task and visual probe task were used , being classic attention tasks that have been applied in various other areas of attention research [ 7 , 21 ] . in the food - modified stroop task attention bias
is derived from delayed reaction times in the color naming of food - related as compared to non - food - related words . using a food - modified stroop task , nijs et al
.   found no differences between weight groups with regard to the food - related interference effect .
no differences between obese and  always  normal weight participants were found in the study by phelan et al .  
as well , although in this study slower reaction times to food words were found in a group of long - term weight loss maintainers with a history of overweight / obesity .
these data suggest that an enhanced food - related stroop interference might not be associated with overweight / obesity per se , but rather with attempts to restrict and control food intake . as discussed by phelan et al . , it is unclear which index of information processing is measured with the food - related stroop task .
delayed reaction times might reflect an emotional distraction by the content of food - related stimuli , because these are desired or perceived as threatening , or they could reflect an avoidance of food stimuli , because these produce a conflict between the desire to eat and the desire to maintain cognitive control .
this might even differ between groups : for instance , nijs et al .   found strong positive associations between stroop interference scores and self - reported desire to eat in the obese group , but not in the normal weight group .
there is also debate whether stroop interference effects reflect a fast automatic or a slower strategic process [ 2325 ] . in a food - modified visual probe task
participants have to respond to a probe appearing at the position of one of two pictures ( generally one food - related and one non - food - related ) , which were displayed simultaneously .
faster reactions occur when attention is already drawn to the position where the probe appears . with the visual probe task it should be possible to investigate the different processes of initial orientation and maintenance of attention , simply by adjusting the presentation duration of stimulus pairs ( respectively , < 200  ms and > 200  ms ) .
however , some researchers question the usefulness of a visual probe task as an index of maintained attention .
that is , when picture pairs are shown for a duration longer than 200  ms [ 14 , 15 , 16 ] , this gives participants the opportunity to shift their attention back and forth between the pair of stimuli . as reaction times
are measured on just one specific point in time ( ie , after the picture pair has disappeared ) , an  attention bias  could as well reflect the coincident direction of the eyes to one of the stimuli at that moment .
this makes it hard to interpret results from studies using the visual probe task with longer presentation durations , and it might explain why no weight group differences were found in maintained attention to food - related stimuli using such visual probe tasks [ 14 , 15 , 16 ] . in general ,
the visual probe task is acknowledged to be a valid measure of attentional orientation when a short presentation time ( < 200  ms ) of stimulus pairs is applied . with a stimulus presentation duration of 100  ms , nijs et al .
[ 16 ] found evidence for an enhanced attention orientation to high - calorie food pictures in overweight / obese participants , but this was not corroborated by loeber et al .
various other study design  related factors might underlie these inconsistent results , such as differences in the hunger / satiety state of participants of both studies .
the recording of electrocortical brain activity ( electroencephalography / erps ) during exposure to relevant stimuli is generally seen as a more direct measure of information processing . the amplitude of early ( eg , p200 ) and later ( eg , p300 and the following late positive potential , lpp ) erp components is thought to reflect the intensity by which cognitive / attentional resources are allocated to stimuli in early and later stages of information processing , and is modulated by the emotional relevance of stimuli [ 27 , 28 ] . from addiction research , there is a general idea that particularly late erps are indices of selective and motivated attention , reflecting activity in the underlying reward system , as these have been found to be associated with subjective drug craving scores [ 8 ] . in eating - related research
it was also demonstrated that particularly late erps elicited by food - related stimuli are enlarged when ( normal weight ) participants are in a state of hunger ( ie , in a state of enhanced motivation to eat ) compared to a state of satiety .
however , in one of our studies it was demonstrated that the reverse pattern might be true in overweight / obese persons : p300  bias  was strongly reduced , and even almost absent in overweight / obese participants who were food - deprived for more than 17  h compared to overweight / obese participants who were satiated [ 16 ] .
it was concluded that p300 , as lpp , amplitude is sensitive to cognitive strategies , and the results were interpreted as an attempt to avoid attending to food stimuli in food - deprived overweight women . in a following study , we found differences between weight groups with regard to the p200 amplitude to food - related words , which is suggestive of an enhanced ( more automatic ) processing of food stimuli in overweight persons in early stages of information processing .
the application of erps to study the processing of food - related information is still novel , and more research is necessary to study the modulation of erps by weight group and hunger state .
probably the most direct measure of attention is the recording of eye movements during the exposure to two or more stimuli .
four studies employed eye tracking and similar attention parameters ( ie , a gaze direction and duration bias as indices of , respectively , oriented and maintained attention ) .
castellanos et al .   found evidence for an enhanced direction and duration bias to low - calorie and high - calorie food pictures in obese participants relative to normal weight participants , but only after satiation .
[ 16 ] found no differences in a ( high - calorie ) food - related direction and duration bias between weight groups and hunger / satiety conditions .
reported an enhanced direction bias ( but no duration bias ) to high - calorie food pictures in satiated overweight / obese persons relative to normal weight persons .
finally , graham et al .   only found an enhanced direction bias to low - calorie ( but not high - calorie ) food stimuli in an overweight / obese compared to a normal weight group .
these results demonstrate that not only the choice of attention task and parameters is an important factor , influencing study results , but also other differences in study designs might account for this .
there were , for instance , differences across studies with regard to the gender of participants ( males / females or only females , also see ) , the food deprivation level of participants ( ie , the degree of hunger / satiety ) , the satiation procedure ( eg , milkshake vs normal meal ) , the general study design of hunger / satiety intervention studies ( eg , within - subjects vs between - subjects design ) , the time of day when the experiment was conducted , the choice ( eg , words vs pictures ) and matching of food - related and control stimuli ( table  1 ) . some of these factors have also been found to influence findings on substance - related attention in addiction research [ 7 , 8 ] .
interestingly , studies using several methods for measuring attention underline the statement that different methodologies yield different results .
[ 16 ] investigated correlations between attention - related outcomes of a visual probe task , eye tracking , and the p300 erp component within one and the same study sample .
this lack of a clear relationship between various attention - related measures has been reported before in addiction [ 8 , 33 ] , as well as eating - related literature .
in circa half of the studies differences in attention bias to food were investigated between overweight ( body mass index [ bmi ]  >  25  kg / m ) and normal weight participants , whereas in the other half obese participants ( bmi  >  30 25 
it is likely that addiction - like deficits in reward system functioning , and consequently an enhanced food cue - responsiveness , are particularly present in persons with a severe overeating / weight problem . for instance , one of the first neuroimaging studies , reporting on altered dopamine functioning in obesity , was conducted in participants with a bmi of 40  kg / m or more . in the same vein , research in addicted persons suggests that the intensity of a substance - related attention bias is directly proportional to the quantity and the frequency of the substance use .
none of the mentioned studies has investigated differences between overweight and obese persons in food - related attention .
moreover , it might even be the case that enhanced attention bias to food is not characteristic for all obese persons , but rather for a subgroup of obese persons , for instance persons who have more serious overeating episodes with clear loss of control over eating , such as binge eaters .
there is increasing consensus that binge eating disorder might be closer related to addiction than obesity per se [ 3537 ] . in line with this point of view ,
svaldi et al .   found , using an erp methodology , that women with binge eating disorder demonstrate enhanced attentional engagement to specifically high - calorie food pictures compared to healthy overweight controls .
further , attention research in patients with eating disorders also suggests that enhanced attention bias to food is particularly seen in those patients characterized by binge eating .
in addition , obese bingeing women have also been found to report more food craving than obese non - bingeing women after being exposed to food stimuli . for future attention studies ,
it might thus be recommended to focus on differences between obese bingeing , obese non - bingeing , overweight , and normal weight participants . on the other hand
, evidence suggests that enhanced attention bias to food is not exclusively seen in persons with overweight or an eating problem , but is also present in normal weight persons with an eating style that might render them vulnerable to become obese or develop an eating disorder in future .
for instance , enhanced attention bias to food stimuli has been reported in external eaters [ eg , 41 , 42 ] , in successful and unsuccessful dieters ( eg , [ 18 , 43 ] ) , and in chocolate cravers .
interestingly , some researchers have started to investigate causal or directional relationships between enhanced food responsiveness and weight / eating problems .
for instance , calitri et al .   found that a cognitive bias to unhealthy food words in a food - related stroop task predicted an increase in bmi over a 1-year period , whereas a cognitive bias to healthy food words was associated with a decrease in bmi .
in line with addiction research , several studies investigated associations between attention bias scores and subjective craving or hunger ratings . whether positive associations are found again seems to depend on study design and methods , as well as weight group .
[ 16 ] found positive correlations between erp measures and subjective craving ( as well as acute calorie intake ) in normal weight participants , but not in overweight participants .
this supports the idea that erps are an adequate index of food motivation in normal weight persons , but not in overweight . in overweight participants , however , positive correlations were found between behavioral outcomes of attention ( ie , visual probe task , stroop task ) and craving , which was not seen in normal weight participants [ 16 , 17 ] .
reported general positive correlations between eye - tracking direction / duration bias and subjective hunger , whereas werthmann et al .   only found a positive correlation between direction bias and subjective food craving , and this only in the overweight / obese group
this implies that some attention measures might be adequate indices of food motivation in one group of persons , but not in another group .
one problem , perhaps explaining inconsistent correlational findings , might be the use of self - report questionnaires .
it is well known that obese persons tend to under - report , to a greater extent than normal weight persons , eating- and weight - related issues , such as their body weight   and food intake .
obese individuals might also tend to ( purposively or not ) under - report feelings of hunger or a desire to eat , perhaps because of social pressures or feelings of shame .
researchers should always keep in mind that any self - report measure is unlikely to be a pure reflection of the subjective state it aims to measure , and that this might distort study results .
[ 16 ] added a bogus taste task to their attention study , to assess direct calorie intake of participants during the exposure to high - calorie foods .
the assessment of direct food intake in this way might be a more straightforward measure of motivational food responsiveness than self - report questionnaires . in the study by nijs et al .
[ 16 ] , only in the hunger condition overweight / obese participants consumed significantly more calories than normal weight participants .
this was interpreted as overweight / obese persons being particularly hyperresponsive to food in a state of hunger
. however , werthmann et al .   did find differences between weight groups after satiation , with overweight / obese persons eating more calories than normal weights .
as mentioned earlier , differences in study findings might be the result of the satiation procedure or other methodological choices .
the incentive sensitization theory implies that the intensity of attention to reward - related stimuli is directly related to dopamine - based activity in the reward system .
although various studies have found evidence for altered reward functioning in obese persons compared to normal weight persons ( eg , [ 12 , 13 , 49 ] ) , few studies have investigated whether attention bias scores are directly related to activity in reward - related brain regions . to our knowledge ,
only one study combined the measurement of food - related attention in a behavioral task with functional neuroimaging ( functional mri [ fmri ] ) .
[ 50 ] examined attentional bias in adolescent girls ranging from lean to obese using a food - modified attention network task .
outcomes of the behavioral task correlated positively with bmi , and were thus indicative for an enhanced orientation and reallocation of attention to food stimuli in persons with a higher bmi .
moreover , it was found that bmi correlated positively with activation in brain regions related to attention and particularly food reward , during initial orientation and reallocation of attention to appetizing food images .
in addition , greater activation of the lateral orbitofrontal cortex ( eg , involved in the evaluation of the reward value of a stimulus and decision making ) during initial orientation to appetizing food cues predicted increase in bmi over a 1-year follow - up .
in recent food - related attention bias studies , an interesting pattern of attention to food is seen in overweight / obese individuals , which is different from normal weight individuals : after an enhanced initial automatic orientation of attention to high - calorie stimuli ( suggestive of an approach response ) , overweight / obese persons tend to show a more strategic attentional disengagement from these stimuli ( suggestive of an avoidance response ) . however , as was discussed in this paper , study results are contradictory .
this is possibly due to the use of different methodologies for measuring attention , which makes it difficult to directly compare study results .
therefore , it is too premature to draw straightforward conclusions on the question whether overweight / obese individuals demonstrate enhanced attention to food cues and to what degree this influences further food responsiveness , such as food craving and food intake .
although attention research is complex and challenging , it is of great importance to further investigate key issues , such as the identification of those ( obese ) persons in whom an attention bias to food is particularly present and problematic ( eg , binge eaters ) , the exact circumstances in which an enhanced attention to food might be particularly present and problematic ( eg , hunger / satiety or stress / emotions ) , the directional / causal relationships between food - related attention bias and food intake / weight gain , and the underlying neurobiology of food - related attention .
this knowledge may be important for developing or improving prevention or treatment programs ( eg , based on attention retraining ) , which could support obesity - prone or obese individuals in dealing with the overwhelming exposure to food cues in daily life .
 the incentive sensitization model of obesity hypothesizes that obese individuals in the western world have acquired an enhanced attention bias to food cues , because of the overwhelming exposure to food . 
 this article gives an overview of recent studies regarding attention to food and obesity . 
 in general , an interesting approach - avoidance pattern in food - related attention has been found in overweight / obese individuals in a number of studies . 
 however , it should be noted that study results are contradictory . 
 this might be due to methodological issues , such as the choice of attention measurements , possibly tapping different underlying components of information processing . 
 although attention research is challenging , researchers are encouraged to further explore important issues , such as the exact circumstances in which obese persons demonstrate enhanced attention to food , the directional relationship between food - related attention bias , overeating and weight gain , and the underlying involvement of the reward system . 
 knowledge on these issues could help improve treatment programs . 
Introduction
Attention Bias Studies in Obese Versus Normal Weight Individuals
Results Depend on Methodological Choices
Attention to Food Characteristic for All Overweight/Obese Persons?
Associations Between Attention Bias and Subjective Craving/Hunger
Attention Bias as a Marker of Altered Reward Functioning
Conclusions