according to the dsm - iv manual ( american psychiatric association 
 ) ,   specific phobias are anxiety disorders that are 
 characterized by an excessive , unreasonable , and persistent fear 
 that is manifested by the presence or expectation of an object or 
 feared situation ( phobic situation ) . the manual states that 
 9% of the population suffers from specific phobias . spider phobia is one of the most common specific phobias ( bourdon 
 et al . ) . arachnophobic individuals develop an avoidance 
 behavior for all contexts related to the animal ( apa   ) . many patients are so afraid of being confronted by the phobic 
 object that they refuse to undergo any kind of therapy ( marks 
 ) . existing therapies range from those that confront the patient with 
 the real spider , such as  in vivo  exposure therapy ( ost 
 ) , to those that avoid this confrontation by requiring 
 the patient to imagine situations involving spiders ( hecker 
 ) . in between , several therapies try to minimize the 
 anxiety of the direct exposure by using computer simulations in 
 which either the patient himself ( garcia - palacios et al .   [ 6 , 7 ] ) or a  virtual  person guided by the patient ( gilroy et al .   [ 8 , 9 ] ) interacts with a  virtual  spider . the treatment proposed here ( slat : spiderless arachnophobia 
 therapy ) it is specifically oriented to those patients with 
 severe arachnophobia that would not undergo any kind of therapy 
 involving a spider . this treatment makes use of the idea that 
 aversive information does not need to be perceived consciously to 
 trigger an emotional response . nonconscious processing mechanisms 
 of emotionally relevant stimuli are sufficient to activate the 
 autonomic components of a phobic reaction ( hman and soares 
 [ 10 , 11 ] ) . from the neural point of view , fearful information 
 does not need to reach cortical levels to generate the typical 
 fear response . individuals with bilateral destruction of the 
 visual cortices exhibit amygdala responses to emotional faces even 
 when brain damage is recent so that cortical networks have had too 
 short time to reorganize ( pegna et al .   ) . in this case , 
 the amygdala activation requires mediation by thalamic 
 ( pulvinar nucleus ) or tectal ( superior colliculus ) areas ( morris 
 et al . ; pegna et al .   ) . , 
 responsible for recognizing fearful stimuli and triggering 
 subsequent autonomic responses such as increased heart rate , 
 respiration , and sweating ( ledoux ; doyre et al . 
 ) . according to these authors , when an aversive stimulus 
 arrives at the thalamus , it passes rough , almost archetypal 
 information , directly to the amygdala , producing a rapid response 
 to the possible danger . the therapy proposed in this paper makes use of these ideas by 
 presenting to the patient a collection of images that contain a 
 reduced subset of the features of a spider . figure 1 
  shows some of these images : the atomium of brussels in which the 
 spheres resembles the spider 's body , a carousel in which the seats 
 hang like the preys of a spider , a tripod whose legs are 
 articulated like spider 's legs , and so forth . these images , 
 sharing a limited subset of features of a spider , were called slat 
 images . after a preliminary presentation , only the images in which 
 the features of the spider appear in a subtler way are kept in the 
 final presentation . the images that evoke spider - related feelings 
 above a certain degree are discarded from the final therapeutic 
 set ( see section 2.3.2 ) . to avoid the patient 's 
 thoughts related to spiders while seeing the treatment 
 presentation , the patient is given a question that should be 
 answered at the end of the run , like  in how many images there is 
 a rounded object ? patients were recruited by means of advertisements in several 
 newspapers and on television . of the 160 volunteers that made 
 contact with us , 36 with symptoms of severe arachnophobia that 
 were reluctant to undergo other types of treatments they were then included in the study if 
 they ( 1 ) met dsm - iv criteria of specific phobia ( apa   ) 
 assessed by structured clinical interview for dsm - iv axis i 
 disorders ( scid ) , ( 2 ) had been phobic for at least ten 
 years ,   ( 3 ) did not have any neurological or psychiatric 
 problems , and ( 4 ) were classified as arachnophobes according to a 
 k - means multivariate analysis . four volunteers were excluded because of the three first criteria . 
 a further 6 were excluded because they had difficulty in coming on 
 a regular basis to the university to participate in the 
 experiments . regarding the last criterion , the k - means multivariate analysis 
 was conducted using as inputs the five measurements obtained from 
 a behavioral avoidance test ( bat ) and from the fear of spider 
 questionaire ( fsq ) ; see section 3 . these instruments 
 were applied to the remaining 26 volunteers , and to 29 nonphobic 
 control subjects recruited among the personnel and students of 
 so paulo university , so that the algorithm could establish 
 two well - defined clusters : the arachnophobic and the 
 nonarachnophobic cluster . after applying the k - means 
 multivariate analysis , one of the 26 volunteers was characterized as nonphobic 
 by the k - means analysis and was eliminated from the study 
 leaving 25 arachnophobic patients . the mean age and standard 
 deviation of the arachnophobic patients and controls were 
 31.3  7.4 and 32.6  8.2 years , respectively . the five measurements ( see the following section ) that were used 
 as inputs in the k - means algorithm were ( a ) the distance 
 tolerated to a real tarantula in a bat ; ( b ) the distance tolerated 
 to a photo of a tarantula in a bat ; ( c ) the subjective percentage 
 of anxiety according to the subjective units of disconfort scale 
 ( suds ) , using a real tarantula ; ( d ) the percentage of anxiety with 
 a photo of a spider ; ( e ) the numerical result of the fsq test . the chief advantage of the k - means algorithm is that it uses a multivariate 
 approach ( here , 5 measurements ) in order to separate phobic from nonphobic 
 subjects . this procedure is more robust than adopting only one measurement , 
 such as the bat or the result of the fsq , as conventionally used for separating 
 phobic from nonphobic subjects . it is also important to remark that the k - means 
 algorithm does not use any arbitrary parameter that can bias the results . 
 to assess the degree of spider phobia , three different instruments 
 were used . as described , the scid ( first et al .   ) was 
 used to produce a preliminary selection of participants . afterwards , the bat and the fsq provided the 5 measurements used 
 to evaluate if participants showed improvement . to verify that patients met dsm - iv criteria for specific phobias 
 ( 300.29 ) , all of them underwent an scid ( first et al . ) . the bat is a widely used measurement of clinical improvement in 
 specific phobias ( lang and lazovick ; lang et al . 
 ) . it consists of an artificial situation in which the 
 subject approaches the phobic object until discomfort sets in . the 
 experimenter measures the distance from the subject to the object 
 and assesses the subject 's anxiety level using , in our case , the 
 suds scale ( wolpe ) . these tests usually start at 5 
 meters from the real spider , but in this study the initial 
 distance was established as 25 meters because of the severity of 
 arachnophobia in our patients . the bat was performed in two stages : first with a photo of a 
 tarantula ( grammostola acteon , 20 cm ) and afterwards with a real 
 tarantula . in both cases the phobic object was placed at the end 
 of a 25-meter long corridor . before beginning the test , an 
 assistant read the instructions to the subject :  this is a 
 behavioral assessment test and is not part of the therapy . walk the farthest you are able to 
 approximate to the spider at the end of the corridor without 
 forcing yourself . when the subject stops less than one meter from the object , the 
 assistant says :  touch the photo  or  touch the cage  in the 
 case of the real tarantula . note that instead of asking the patient to approach as much as 
 possible to the spider , the patient is asked to approach to the 
 spider as much as possible without forcing himself . this kind of 
 suggestion guaranteed complying with the desire of patients of not 
 confronting in any way the phobic object . the bat was rated by measuring the distance from the subject to 
 the phobic object , starting at 25 meters . the bat score ranged 
 from 26 if the subject refused to do the test , to 1 , if the 
 subject opened the lid of the cage . when subjects stopped , the 
 assistant applies the suds by saying :  please , rate you anxiety 
 from 0% to 100% , 100% being the greatest fear you 
 have had in your life . the fear of spiders questionnaire ( fsq ) assesses the subjective 
 perception of spider fear ( szymanski and o'donohue ) . it 
 is composed of 18 questions rated on a 17 likert scale ( 1 = i 
 strongly disagree , 7 = i strongly agree ) . the fsq was able to 
 discriminate between phobics and nonphobics , f(1.111 ) = 5.99 , 
 p < .01 , f(1.76 ) = 13.28 , p < .01 , respectively ( szymanski and o'donohue    ) . it also provided evidence for the 
 improvement of phobic patients following a cognitive restructuring 
 treatment ( comparing pretest to posttest : t(37 ) = 4.38 , p < .01 , 
 t(79 ) = 5.09 , p < .01 , resp . ) . when applied to nontreated 
 subjects this instrument has an internal consistency of 0.92 with 
 a split half reliability of 0.89 . the presentation used in the slat consists of an initial set of 
 165 images , 124 of them having some features that resemble any of 
 the characteristics ( color , shape , texture , etc . ) of a spider and 
 were selected as explained in section 2.3.1 . examples include the image of a person with a rastafarian hair style , the 
 atomium of brussels , a carousel , and so forth . the remaining 41 images were neutral and were selected with the 
 purpose of making it more difficult for the subject to realize 
 there were slat images in the presentation . the placebo group presentation consisted of a sequence of images 
 without arachniform features . among the selected figures , there 
 were abstract or surreal paintings that might induce placebo 
 subjects to think there was something hidden in the figures . the features that were 
 selected in the images were related , for example , to the radial 
 symmetry of spiders , the design of their webs , their texture , the 
 way they articulate their legs , the hook - like shape of their 
 extremities , or the fact that they hang from a string . 
 for validating our selection , 43 nonarachnophobic persons were 
 asked to rate , on a 0 to 10 scale , the content of spider features 
 in all the images . not to bias the process of rating the images , 
 no instructions related to what features to consider in rating the 
 images it was necessary to establish a threshold in this scale for 
 separating slat images from neutral images . this threshold was 
 obtained by means of the bayes decision rule that yields a 
 threshold of 0.92 . images with a greater rate were classified as 
 slat images , and images with a lower rate were classified as 
 neutral . according to this rule , 8 of the figures initially 
 classified as slat images were neutral , and 3 neutral figures were 
 slat images . therefore , a total of 11 images were excluded from 
 the final therapeutic repertoire . to apply the bayes decision 
 rule , a histogram was created giving the probability of finding a 
 slat image inside intervals of 0.6 unit length in the 0 to 10 
  arachniform scale . we 
 replaced both histograms by two curves after smoothing the 
 histograms by using interpolation by splines . the intersection of 
 the two curves yielded the value of 0.92 that served to 
 discriminate between slat and neutral images . one of the assumptions that served to delineate the slat ( see 
 assumption ( a ) in section 4.1 ) deals with avoiding a 
 high activation in the neural circuits involved in fear . for this 
 reason , we elaborated a procedure to exclude from the final 
 therapeutic presentation those images that might produce 
 discomfort in the patients , keeping only the more comfortable 
 images that would probably not produce a high degree of activation 
 in these neural circuits . ( a ) once the entire set of figures had been shown to the patient in a preparatory 
 presentation , we asked the patient to see the figures once more and collaborate 
 with us to determine the adjusted duration , tad , of each one of the 
 images . the patient was instructed as follows :  each one of the following 
 images will be presented by default for 5 seconds . if you do not like the 
 image , press the  enter  button to pass 
 to the following image sooner . the sooner you press the button , the more 
 fearful we will understand the image to be for you .  ( b ) after seeing all images the subjects were asked : 


 which images , if any , are intolerable?which images are tolerable?which images are so nice that you might place them in your bedroom ? which images , if any , are intolerable ? which images are so nice that you might place them in your bedroom ?   with all this information , nine rules were applied to obtain the 
 final duration of each image , tad , in the 
 presentation . as some patients were faster than others in pressing 
 the  enter  button , the average time tm for each 
 subject served as the patient 's unit of time . in the following rules , times t0 , t1 , and so on were   set as arbitrary multiples of   tm . the adjusted 
 duration of each image , tad , was obtained by 
 multiplying the duration chosen by the subject in the preparatory 
 presentation , t , by a coefficient calculated as follows . we defined three thresholds : t0 = tm/5 , 
 t1 = tm/2 , 
 t2 = tm/3 . if t < t0 , the image was eliminated from the presentation.intolerable images with t < t1 were also eliminated.tad = 0.2  t in intolerable images with 
 t > t1.tad = t for tolerable images 
		 with t <   t2.tad = 1.5  t for tolerable images with 
 t1 > t   > t2 . tad = 1.8  t   for 
		 tolerable images with   t > t1.tad = 2  t in images deemed nice.other images , not included in previous groups , maintained their time t.to make the total presentation time equal to 12 minutes , each 
                 tad was multiplied by 12 and divided by the total duration ( in minutes ) of the presentation . tad = 1.5  t for tolerable images with 
 t1 > t   > t2 . tad = 1.8  t   for 
		 tolerable images with   t > t1 . other images , not included in previous groups , maintained their time t. to make the total presentation time equal to 12 minutes , each 
                 tad was multiplied by 12 and divided by the total duration ( in minutes ) of the presentation . this research was approved by the ethics committee on research of 
 the institute of psychology of the university so paulo . as mentioned in section 2.1 , of the 160 patients that 
 contacted us , 36 were interviewed and 25 were included in the 
 experiment . these patients signed forms , agreeing to participate 
 in either the placebo or treatment group , and allow the use of 
 collected data for research . patients were randomly divided into 
 two groups : treatment   ( n = 13 ) and placebo   ( n = 12 ) . 
 after adjusting the timing of the presentation , a personalized cd 
 was prepared for each patient . in the following session , this cd 
 was given to the patient . the patient was then instructed to run 
 the presentation twice a day at home preferably during moments in 
 which she / he was not tired or under stress . prior to each 
 presentation run , the patient was given one question to answer at 
 the end of the run . these questions were intended to 
 distract the patient from arachniform features in the images . examples include :  in how many images there is an animal ?  or 
  in how many images there is a rounded object ? when answering 
 the question , the patient was instructed to write , beside the 
 answer , the date and time she / he ran the presentation . every week these data were checked out in order to verify the rate of 
 cooperation of patients and to encourage noncooperative patients , 
 if any . in all subjects , the cooperation was satisfactory and no 
 statistics were deemed necessary to measure the rate of 
 cooperation . to assess progress during the treatment , placebo and treatment 
 subjects underwent the bat ( including the suds ) each week . in the 
 last week , the fsq was also applied . experiments were carried out 
 in three stages . in stage 1 , data collected during these first 
 four weeks were used to compare placebo and treatment groups . a 
 period of four weeks was established prior to the experiment with 
 the intention of minimizing the duration of the experiment in 
 order to avoid drop out . in stage 2 , the treatment group ( but not 
 the placebo ) was asked ( and luckily agreed ) to continue for two 
 more weeks to assess if this additional time might help the 
 treated group to achieve a more substantial recovery . they were 
 evaluated at the end of the 6th week . 
 in stage 3 , after the fourth week , placebo subjects were invited 
 to receive the slat . the ten subjects that were accepted were 
 treated for 6 weeks and evaluated after the 4th and 6th weeks . there were no difference between the placebo ( n = 12 ) and 
 treatment   ( n = 13 ) groups at the beginning of the study in the 
 following demographic and clinical variables : age , f(1,23 ) = 
 0.3315 , p = 0.5703 ; duration of phobia , f(1,23 ) = 3.8758 , 
 p = .0611 . no significant differences were found in behavioral 
 variables during the initial bat test with the real spider bat : 
 f(1,23 ) = 0.0015 , p = .9692 ; suds , f(1,23 ) = 0.0739 , 
 p = .7881 ; or with the spider photo bat , f(1,23 ) = 1.6764 , 
 p = .2082 ; suds , f(1,23 ) = 0.0003 , p = .9866 . no 
 significant difference was found in the subjective measure of fear 
 of spiders , fsq : f(1,23 ) = 0.020 , p = .8895 . 
 of the 13 treatment subjects , 3 refused to stay at any distance 
 from the real spider if the spider was visible . they received an 
 arbitrary score of 26 , one meter more than the maximum score of 25 
 meters used in the bat test . regarding the test with the spider 
 photo , one subject refused to stay at any distance in which he 
 could see the photo . analogously , we assigned a score of 26 meters 
 in the bat test to this subject . we emphasize that , different from 
 previous studies in which the initial distance of the bat test was 
 standardized to 5 meters , this distance was augmented to 25 meters 
 because of the desire of the patients not to confront the spider 
 in anyway . table 1 
  shows the mean and standard deviation ( in 
 parenthesis ) of the various groups evaluated . the percentage 
 improvement ( table 2 ) was calculated by dividing the 
 absolute improvement in each measure by the initial measure . after 
 4 weeks , the percentage improvement in all measurements was higher 
 in the treated than in the placebo group . during the presentation 
 of the real spider , the percentage improvement in the bat was more 
 than twice as high ( 61.6% versus 28.8% ) in the treated 
 than in the placebo group ( see table 2 ) . the suds was 
 more than six - fold ( 40.3% versus 5.9% ) higher . the same 
 measurements made with the spider photo yielded a percentage 
 improvement of 19.3% ( 66.6%47.3% ) in the bat and 
 32% ( 53%21% ) in the suds . differences between 
 placebo and treated groups were consistent throughout the four 
 weeks of the experimental procedure ( see evolution of measures in 
 figure 2 ) . improvement in the fsq was 13.1% ( 28.8%15.7% ) 
 higher in the treatment than in the placebo group . a 2 ( group )  5 ( times ) repeated measure multivariate 
 anova ( hair et al . ) was conducted to evaluate whether 
 the differences between placebo and treated groups were 
 significant . in this multivariate analysis , 4 simultaneous 
 variables were used : bat and suds for real spiders ; and bat and 
 suds for spider photo . by analyzing the results of the 
 multivariate anova , we conclude that the significant time effect 
 f(4,92 ) = 14.5475 , p < .0001 , and the significant group 
 effect f(1,23 ) = 4.5678 , the significantly different time - course of the 
 improvement in the two groups is also reflected in a significant 
 group  time effect f(4,92 ) = 4.4217 , p = .0026 . 
 in order to evaluate how the test with the real spider and the 
 test with the spider photo contribute to these results , a 2-group , 
 5 times , multivariate anova was performed , first with 
 the bat and suds of the real spider and then with the bat and suds 
 of the spider photo . the test with the real spider yielded a 
 significant group    time interaction : f(1,23 ) = 7.981610 , 
 p = .009598 , ms = 1369.772 while the test with the spider 
 photo yielded a moderate group  time interaction f(1,23 ) 
 = 2.908077 , p = .101608 , ms = 750.1708 . the fsq also yielded a 
 nonsignificant 2 ( groups ) 2 ( time = pretreatment versus 
 post treatment ) interaction f(1,23 ) = 1.833 , p = .188 . the 
 difference between bat and suds tests and the fsq test results after the four weeks in which placebo and treated subjects were 
 compared , treated subjects continued receiving the slat for two 
 more weeks , achieving 76.6% improvement in the bat and 
 45.6% in the suds with the real spider . with the spider 
 photo , there was an 88.5% improvement in the bat ; a 
 61.4% improvement in the suds , and a 40% improvement in 
 the fsq . the results of the treated placebo were consistent with the 
 results of the treatment group ( see tables   1 
  and   2 ) . 
 a six - month follow - up there 
 was 90.2% improvement in the treatment group in the bat test : 
 patients were capable of approaching a live tarantula at 2(3.9 ) 
 meters ( on average ) , six patients opened the lid of the tarantula 
 cage and , of these , three patients touched the tarantula 
 ( grammostola acteon , 14 cm , the initial one died ) . in the case of the follow - up study with the treated placebo 
 patients three of them opened the lid of the cage and two of them touched 
 the tarantula . a k - means multivariate cluster analysis was used to assess the 
 number of patients that made the transition from arachnophobic to 
 normal during treatment . five variables were used to characterize 
 each subject : bat and suds with real spider , bat and suds with 
 photo of a spider , and fsq . the algorithm was applied with these 
 five variables gathered from the 25 arachnophobes at the beginning 
 of treatment , and from 29 normal subjects recruited in the 
 university . the k - means algorithm was initially used to 
 eliminate nonphobic subjects from the group of volunteers , as 
 explained in section 2.1 . to calculate the percentages 
 of patients that migrated from arachnophobic to normal 
 along the different stages of the experimental procedure ( see 
 table 2 ) , the k - means algorithm was fed 
 with the scores of the participants in each one 
 of the stages ( bat spider , bat photo , suds spider , 
 suds photo , and fsq ) . 
 during the four weeks of treatment , 41.7% of 
 individuals in the treatment group and 25% of the placebo 
 group moved over to the normal condition . when the placebo group 
 was treated ,   50% fell in the normal group . a more substantial improvement was evident in the follow - up , six 
 months after the conclusion of treatment : 91.7% 
 of individuals in the treatment group and 90% 
 of the treated placebo group were classified as 
 nonarachnophobes . these results are discussed below . in this section , the following topics will be discussed : the hypothetical assumptions taken into   consideration to elaborate the therapy ; the neurocomputational background of the therapy ; the influence of the bat assessment test in the efficacy of slat ; the delay of improvement in the fsq ; the therapeutical limitations of the procedure ; suggestions for further studies . two hypothetical assumptions that are consistent with neurological 
 findings served to delineate the methodology of slat . the results 
 of the therapy , however , are not intended to assess the validity 
 of these preliminary assumptions , which would require much further 
 confirmation . ( a ) the first assumption is that some connections from thalamus to 
 amygdala are abnormally potentiated in phobic patients , possibly 
 because of a process in which a conditioned stimulus ( cs ) , the 
 phobic object , is associated with an unconditioned stimulus ( us ) 
 such as a loud sound or an acute pain . the possibility of plastic 
 changes taking place in the thalamo - amygdala pathway is supported 
 by the work of doyre et al . , in which they were 
 able to induce long - term potentiation ( ltp ) in thalamic and 
 cortical inputs to the amygdala in freely moving rats , 
 demonstrating that ltp in thalamic inputs is much more persistent 
 and long - lasting than ltp in cortical inputs . have further shown that 
 intralaminar thalamic neurons contribute to presynaptic plasticity 
 in the thalamo - amigdaloid pathway during fear conditioning . thalamic intralaminar neurons are also described as a locus of 
 functional cs - us convergence for fear conditioning to acoustic 
 stimuli ( cruikshank et al . ) . the possibility of altering 
 these circuits by means of either habituation to the spider or by 
 cognitive - behavioral therapy is also mentioned , for example , by 
 veltman et al . 
  and paquette et al . . 
 regarding the degree to which plastic changes would take place in 
 the thalamo - amygdaloid pathway , it is worth mentioning that 
 postsynaptic voltage value is critical to determining whether a 
 synapse is reinforced or depressed ( figure 3 ) . 
 according to figure 3 , postsynaptic if the value of postsynaptic depolarization is 
 greater than a threshold , called the ltp threshold , active 
 synapses are potentiated ( i.e. , increment their synaptic 
 connectivity or synaptic weight ) ; below this threshold they 
 are depressed ( artola and singer   ; 
 bear et al . ) ( these synapses experiment a decrement of 
 their synaptic connectivity or synaptic weight ) . if the 
 postsynaptic depolarization is very low , synaptic depression is 
 small or null . we conjectured that the effectiveness of slat depends on 
 activating neurons that project from thalamus to amygdala in such 
 a way that they are inside the depression interval . unfortunately , 
 depression intervals vary for each synapse according to a synaptic 
 property called metaplasticity . the same postsynaptic activity may 
 produce potentiation in one synapse and depression in another 
 while leaving a third unaltered . we were also unable to directly 
 evaluate the postsynaptic activity that a given slat figure 
 produced in these neurons . despite all these difficulties , we conjectured that the fear 
 reaction produced by slat figures was correlated to the 
 postsynaptic activity in neurons in the thalamo - amygdaloid 
 pathway . to avoid potentiation and favor depression , the duration of the remaining images 
 were adjusted so that comfortable images were exhibited during a 
 longer time and less comfortable images during a shorter 
 interval . ( b ) the second hypothetical assumption that served to delineate 
 slat is related to the nature of the archetypal information that , 
 according to ledoux , is relayed from the thalamus to the amygdala . morris et al . 
  found that the amygdala appears to sum , in 
 a nonlinear manner , individual responses to specific facial 
 features . a two - stage theory for facial perception of emotions was 
 proposed by de bonis et al . 
  and tested by morris et al . , 
  who concluded that  the perception of emotional 
 expressions depends on an initial processing of individual facial 
 features followed by a nonlinear association of the different 
 components .  according to weinberger and collaborators ( lennart 
 and weinberger ; 
  edeline and weinberger ) , the 
 thalamus is able to recognize features , augmenting its response to 
 a specific feature that was previously paired to a us . neurocomputational models ( pelez [ 32 , 
 33 ] ) are 
 consistent with the two - stage theory , conjecturing that the first 
 stage of the process , the preliminary processing of individual 
 features , is performed in the thalamus . according to these models , 
 in the thalamus each sensory pattern is represented as a vector 
 with components in a coordinate frame in which each axis 
 corresponds to a specific feature of the pattern . each one of 
 these axes / features corresponds to the output of a thalamic 
 reticular neuron . the output of these reticular neurons ( crabtree 
 and isaac ) is nonlinearly summed by intralaminar neurons 
 ( see figure 4 ) and if this sum exceeds a threshold , the 
 result is relayed to the amygdala . according to the computational 
 model , the set of axes / features created by the firing of reticular 
 neurons in the thalamus , constitute a code that identifies , in a 
 rough way , each input pattern . this code would correspond to the 
 rough , almost archetypal description of the aversive stimuli , 
 that , according to ledoux and colleagues ( ledoux ; 
 doyre et al . ) , is passed from the thalamus to the 
 amygdala . 
 according to the first assumption , a way of depressing 
 thalamo - amygdaloid synapses would be by avoiding high 
 post - synaptic potentials in thalamo - amygdaloid neurons by means of 
 reducing the intensity of phobic stimuli ( figure 3 ) . a 
 possible way of reducing this intensity would be by masking 
 or obscuring the phobic object . however , a masked or obscured 
 phobic object is still intense enough to fire the amygdala ( whalen 
 et al . ) and aversive for patients . instead of reducing the duration or intensity of spider images , we 
 propose to reduce the number of arachnoid features present in each 
 image . according to the second assumption , when the number of 
 arachniform features in the input pattern is reduced , the 
 activation of intralaminar neurons ( computing the sum of these 
 features ) is also reduced . this lower activation of intralaminar 
 neurons contributes to reduce the activation of the neurons in the 
 thalamo - amigdaloid axis , so that their synapses would undergo 
 depression instead of potentiation . therefore , when , instead of 
 the spider code , a code with a smaller repertoire of arachniform 
 features is relayed , neurons in the thalamo - amigdaloid pathway are 
 hypothetically less activated , their synapses more prompted to 
 undergo depression rather than potentiation . the purpose of this instruction 
 was to adhere , during the bat and suds tests , to the principles 
 that inspired the therapy , that is , to avoid any stimuli that 
 could contribute to enhance thalamo - amygdala connectivity . it could be argued that the bat assessment test could , by itself , 
 have a therapeutical effect over arachnophobia . this effect might 
 be thought to be responsible for the improvement observed in the 
 placebo group . however , as shown in section 3.2 , 
 improvement of patients in the treatment group was significantly 
 better than that of patients in the placebo group . many patients reported that they did not realize that they had 
 lost their fear of spiders until they were confronted to a real 
 spider during their daily life . they had the strange sensation of 
 not reacting with fear when , for the first time after treatment , 
 they saw a real spider . since during daily life , a real 
 confrontation with a spider is an unpredictable event , the 
 realization of having lost the fear varies from individual to 
 individual . the bat assessment test , independently of its possible 
 placebo effect , could contribute to accelerate this process of 
 realization . related to this , we observed that the improvement in the fsq was 
 delayed in comparison to the improvement in the automatic 
 responses measured by the bat and suds . this is consistent with 
 the reasonable supposition that patients did not realize that they 
 had lost their fear until they actually confronted a real spider 
 during their daily life situations . depending on the frequency 
 with which they actually confronted a spider in their daily lives , 
 the realization of recovery took a shorter or longer time in the 
 different patients . this fact was reflected in the follow - up study 
 that was carried out six months after the conclusion of the 
 treatment . although the 25 subjects that took part in the experiment came 
 from a very large sample of 160 arachnophobic volunteers , there 
 were no volunteers above the age of 46 . taking into account that 
 neural plasticity depends on age ( burke and barnes 
 ) and 
 that our experiments were not able to assess the therapeutic 
 effect of slat in elderly people , we suggest to apply the slat to 
 patients below the age of 46 , until performing an assessment with 
 older volunteers in the future . the 160 arachnophobic patients that contacted us were classified 
 in terms of their degree of arachnophobia . among the six with the 
 highest scores , three of them suffered thyroid hormone impairment . we wondered whether this coincidence might be a possible 
 psycho - somatic effect produced in the long run by arachnophobia . a 
 similar case of thyroid hormone alteration was found in the 
 literature ( friedman et al . 
 ) among women with 
 posttraumatic stress disorders . these considerations motivate a 
 study to assess the relationship between thyroid hormone 
 alteration and phobias . 
 according to our theoretical assumptions , the slat acts at 
 subcortical levels . neuroimaging studies could help to evaluate 
 this assumption by comparing the brain activation before and after 
 the slat . this study concluded that the 
 dorsolateral prefrontal cortex and the parahippocampal 
 gyrus diminished their activation significantly after 
 treatment with cognitive behavioral therapy .   in the case 
 of the slat , we expect that reduction of activity in the 
 dorsolateral prefrontal cortex and the parahippocampal 
 gyrus will be preceded by reduced activity of amygdala 
 and superior colliculus . this sequence would be consistent with 
 the fact that during the slat , improvement in the bat test 
 ( measuring automatic responses ) proceeded the improvement in the 
 fsq tests ( measuring cognitive variables related to fear of 
 spiders ) . a novel technique for treating spider phobia , that does not 
 require any use of spiders , was described and tested . in the slat , here described , each patient is given a personalized presentation 
 in a compact disk , containing a set of images that , although not 
 containing spiders , present subsets of spider characteristics . the 
 degree the most evocative images are excluded from the 
 personalized presentation whereas the less evocative images are 
 presented to the patient during a longer interval ( see 
 section 2.3.2 ) . regarding the subtlety of the images , 
 two treatment group patients declared that they thought they were 
 in the placebo group because their presentation caused no 
 discomfort at all . 
 to compare the evolution of the placebo and treatment groups , a 
 four - week experiment was designed . treatment and placebo groups 
 went through their corresponding presentation twice a day and came 
 once a week to the university to apply the bat and suds tests . to 
 carry out these tests , instead of encouraging the subjects to 
 approach as much as possible to a spider , they were told to 
 approach the spider , but without forcing themselves . they could 
 also refuse to do the test , which was the case of three treatment 
 subjects in their initial evaluation ( see section 3.1 ) . this kind of suggestion respects the desire of the subjects of not 
 confronting the spider in any way , and is coherent with the main 
 philosophy of the procedure , according to which the subtler the 
 better . the improvement in every measure of phobia was higher for 
 the treatment group than in the placebo group ( see tables 1 
  and 2 ) . moreover , the repeated measures 
 multivariate anova showed that the patients ' improvement was not 
 due to a placebo effect ( group  time interaction : 
 f(1,23 ) = 7.98 , p = .0096 ) . 
 in the follow - up study performed after six months , 91.7% of 
 the patients in the treatment group were classified as 
 nonarachnophobes by the k - means algorithm , six patients of this 
 group opened the lid of the tarantula cage , and , of these , three 
 touched the tarantula . 
 the therapy proposed here was aimed at subconscious , automatic 
 responses , while behavioral or psychoanalytic therapies emphasize 
 the rational control of fear reactions . according to ledoux , the alteration of fear behavior can be produced by the 
 cortical control of fear reactions without the actual deletion of 
 what ledoux calls fear memories  were intentionally 
 the targets of the therapy proposed in this paper . slat is particularly appropriate for , but not exclusive to , those 
 patients who , because of the severity of their arachnophobia or 
 whatever other reason , are unwilling to undergo therapies that 
 involve any real , imagined or virtual spider . the theoretical 
 basis of the therapeutic strategy was aiming to produce plastic 
 changes in the thalamo - amygdaloid circuit responsible for the 
 subconscious , automatic reactions triggered when the subject sees 
 a spider . the therapy might have been effective for other , 
 fortuitous , reasons , but the consistency with the theoretical 
 basis that motivated it ( sections 4.1 
  and 4.2 ) 
 is very encouraging , both from a practical point of view , 
 providing an additional strategy to deal with certain phobias , and 
 from a theoretical point of view , motivating further studies to 
 test these ideas .
we describe a new arachnophobia therapy that is specially suited for those individuals with severe arachnophobia who are reluctant to undergo direct or even virtual exposure treatments . in this therapy , patients attend a computer presentation of images that , while not being spiders , have a subset of the characteristics of spiders . 
 the atomium of brussels is an example of such an image . 
 the treatment group ( n = 13 ) exhibited a significant improvement 
 ( time    group interaction : p = .0026 ) when compared to the placebo group 
 ( n = 12 ) in a repeated measures multivariate anova . 
 a k - means clustering algorithm revealed that , after 4 weeks of treatment , 42% of the patients moved from the arachnophobic to the nonarachnophobic cluster . 
 six months after 
 concluding the treatment , a follow - up study showed a substantial consolidation of the recovery process where 92% of the arachnophobic patients moved to the nonarachnophobic cluster .