Personal factors of creativity: A second order meta-analysis

Both concepts share the generation of new ideas and practices. Innovation is the successful implementation of creative ideas ( Hennessey & Amabile, 2010 ) and in this sense is understood as a broad process which s the generation of ideas or creativity but also the application of these in real environments (at work, in education, etc.). Creativity is about absolute novelty, while in innovation relative newness may the application in a new or different way of products, procedures, or processes which have already been used in a different place ( Anderson, De Drew, & Nijstad, 2004 ).

The context in which creativity is developed is a further factor to take into account in its analysis ( Baas, De Dreu, & Nijstad, 2008; Davis, 2009; Hammond, Neff, Farr, Schwall, & Zhao, 2011; Hülsheger, Anderson, & Salgado, 2009b; Hunter, Bedell, & Mumford, 2007; Kozbelt, Beghetto, & Runco, 2010; Ma, 2009 ). We propose an interactional model which suggests that in order to innovation in organizations it is necessary to consider different levels: individual, group, and organizational ( Amabile, 1996; Csikszentmihalyi, 2011; Hülsheger et al., 2009b; Sternberg & Lubart, 1995 ). An organizational context rich in resources and with complex and autonomous roles reinforces creativity ( Costa, Páez, Sánchez, Gondim, & Rodríguez, 2014 ), while it is in the interaction with the personal factors of creativity (see Table 2) that innovation is reinforced.

Different authors (see for example Amabile, 1996; Basadur, Basadur, & Licina, 2012; Csikszentmihalyi, 2011; Ma, 2009 ) have proposed models of stages in the creative process, such as: 1) preparation stage, with immersion in the problems inherent in a task and/or in something that arouses curiosity; 2) incubation stage, with formulation of problems or conceptualization of hypothesis; 3) generation of solutions or assembling the pieces of a puzzle ( Csikszentmihalyi, 2011 ); 4) generation of criteria to evaluate appropriate solutions, involving communication, evaluation, modification and/or validation of ideas for the solution of the problem; and 5) ion, solution, and application, involving decision making or implementation of the proposed solution (product, ideas and practices) or innovation ( Amabile, 2013; Basadur et al., 2012; Hammond et al., 2011 ). This last stage can be d in a process of combining the categories or reorganizing knowledge depending on the complexity of the problem.

Prior knowledge will be decisive in the first and second stages, with divergent thinking (DT) playing an important role in stages three and four, and convergent thinking being essential for the final stage ( Csikszentmihalyi, 2011; Ma, 2009; Simonton, 2012 ). The meta-analyses have revealed that personality traits influence the generation of ideas more strongly, while the contexts in which they are developed have greater influence on the application stage ( Hammond et al., 2011).

Sternberg and Lubart (1995) propose that personality, intelligence, knowledge, thinking style, motivation, and environment are factors associated with creativity. While the first five are individual factors corresponding to the creative person, we also need to consider given characteristics such as age and gender, acquired characteristics such as education, and other individual attributes such as attitude, affect, and emotional intelligence ( Garaigordobil, 2003, p. 151; see Table 2 for definition and examples of items).

With respect to given characteristics, like gender and age, previous reviews suggest a positive relationship between being a woman and creativity. Narrative reviews ( Baer & Kaufman, 2008 ) find many results with no difference between genders, although some slightly favor being female as opposed to male. The results of one study of gender identity and creativity found that subjects who were both “feminine” or expressive and instrumental or “masculine” (androgynous) were more creative than people who scored low on both dimensions. Highlighting the importance of instrumentality, the participants who reported strongly instrumental or masculine gender characteristics showed greater creativity ( Stoltzfus, Nibbelink, Vredenburg, & Thyrum, 2011 ). In terms of the development of creativity related to age, some studies suggest a curvilinear progression, peaking between 30 and 40 year of age and declining after 40. Regarding acquired characteristics like education, it has been suggested that the level of education is associated with creativity, although it is not a determining factor beyond a certain level ( Artola et al., 2012).

With regard to individual traits associated with creativity, it has been proposed that favorable personality traits are creative self-concept (CSC) or creative personality and openness to experience (OE). The relationship with creativity is less clear than with other traits such as extraversion, agreeableness, neuroticism, and being conscientious (see for example Feist, 1998; Hülsheger, Anderson, & Salgado, 2009a ). Continuing with cognitive individual traits, intelligence is linked to creativity and, since it is not decisive beyond a certain level, there is an asymptotic relationship between intelligence and creativity ( Kim, 2005 ). As well as the role of general mental ability, some authors posit the existence of thinking styles and attitudes associated specifically with creativity, such as field independence, tolerance of ambiguity, perseverance in the face of frustration, relative disinterest in social approval, and a pro-risk attitude (see Feist, 1998; Hülsheger et al., 2009a ). It is specifically thought that creativity correlates positively with DT and weakly or negatively with convergent thinking. The former allows tasks with multiple solutions to be completed, and is characterized by high ideation, intuition, tolerance of ambiguity, and low evaluation, while the latter is found in the area of logical deduction, and is defined by evaluation, reasoning, adaptation, and intolerance of ambiguity ( Guilford, 1968; Ma, 2009 ). A further important skill is to distance thinking from the procedures normally applied in dealing with problems (for example making the unusual normal and the normal unusual), or when all else fails to try something counter-intuitive ( Amabile, 1996; Baas et al., 2008; Davis, 2009; Ma, 2009 ). Nevertheless, some authors submit that there are no cognitive styles generic to creativity and that these are specific to the area in question ( Hennessey & Amabile, 2010).

In addition to intelligence, personality traits, attitudes, and cognitive styles, motivation is a further individual factor which is important in creativity. It has been claimed that there is a positive relation between creativity, intrinsic motivation, and self-efficacy ( Artola et al., 2012; Guilford, 1968; Runco, 2014 ). It is expected that people are more likely to be creative when they are intrinsically motivated, i.e., by interest, pleasure, satisfaction, and the challenge of the task itself, more than by external pressure and reward ( Amabile, 1996 ). Self-efficacy refers to the perception that one has the capacity to behave in a way d to attain a goal and the beliefs about one's personal competences to be proactive in the generation of new ideas and the application of innovations. It is expected that high self-efficacy is associated with greater creativity ( Hülsheger, Anderson & Salgado, 2009a; Ma, 2009 ). The relation between extrinsic motivation and creativity is not clear, although some authors suggest a negative link ( Ma, 2009).

Finally, emotions and affectivity are individual factors relevant to creativity. In terms of the relation between affectivity and creativity, it is thought that emotions can change thinking and that knowing them can help one use them to process information in a better way, thus allowing emotional intelligence to further creativity ( Mayer & Salovey, 2007 ). Several authors propose that positive affect facilitates creativity ( Amabile et al., 2005; Fredrickson, 2009; Isen, Daubman, & Nowicki, 1987 ). There is also evidence to suggest a link between negative affect and psychopatholgy, given the fact that the percentage of disorders is high among some creative groups such as artists, although this association has also been questioned ( Feist, 1998; Ivcevic, Brackett, & Mayer, 2007; Ma, 2009 ).

On the basis of our review, the hypothesis is put forward in this study that creativity is associated with the female gender and androgyny, in a curvilinear form by age, and asymptotically with intelligence and education; with openness to experience, creative self-concept and to a lesser degree with other personality traits; with cognitive/attitudinal variables (DT and pro-risk), intrinsic motivation and self-efficacy, low extrinsic motivation, emotional intelligence (EI), and positive affect. It is also expected that the association with negative affect is weaker or less clear. Finally, we expect that there will be a stronger association between creativity and motivation than between creativity, cognition, and attitude.

We now turn to an explanation of the method used to explore the hypothesis, the results obtained and the discussion of objectives. The limitations of the study will also be outlined before conclusions for future research are drawn.

A search was made for all articles which d the terms meta-analysis, state of the art, review, creativity, and innovation (PsycINFO & Google Scholar), without searching exhaustively for primary articles and without imposing time limits. Meta-analyses which used creativity or innovation as a criterion variable and individual characteristics as a predictive variable were d (highlighted with an asterisk in the references). Also d were meta-analyses which had as an axis creativity and its relationship with given and acquired individual characteristics, personality traits, attitude, intelligence, and cognitive styles, motivational processes, affectivity, and emotional processes (see Table 1).

Table 3 displays individual factors of creativity which are present in two or more meta-analyses. Their effects are described by providing z and k for each. To provide the effects of all the factors, correlations r were run. When there were Cohen's ds (e.g., Ma, 2009, Davis, 2009), they were ed into r s. When there was only one meta-analysis, the r effect size is reported, and when there were two or more, a mean r¯ effect size is shown. We calculated r¯ on the basis of the r s not corrected for reliability to group the results together, given that it was the common indicator for all. N , or sample size, was used to weight the effects. Given that Ma's meta-analysis only reported k , or the number of studies, and not N on which these were based, the sample size was estimated on the basis of the average N in other studies. Given the large values of N , all confidence intervals exclude 0, e.g., with N = 15,279 the 95% confidence interval of emotional instability was between -.05 and -.01 ( DeCoster & Iselin, 2005).

To the relevance of factors facilitating creativity, Rosenthal's Binomial Effect Size Display (BESD) was used. Assuming the variable creativity or innovation is normally distributed, a high level (above average) of the facilitating factor implies a probability of .50 + r /2 greater creativity (e.g., the association of neuroticism with creativity is r = -.03). This means that 51.5% (.50 + .03/2 = .515) of people with emotional stability will display a more creative than average response, compared to 48,5% of people with high neuroticism.

The hypotheses will be examined in order of effect size (greater to smaller), taking into account whether we are dealing with an integration of two or more meta-analyses or commentary on a single one. In the first case, the estimated weighted r¯ is given.

The results of the meta-analyses reviewed clearly (see Table 3 ) that more creative people are characterized by greater DT (r¯ = .27). This ( r = .20) is associated with creativity, as is convergent thinking ( r = .10) in the meta-analysis of Ma (2009). The meta-analysis of Hülsheger et al. (2009a) found that DT is associated (r = .30) with innovation and that people with a systematic thinking style displayed lower workplace innovation ( r = -.26). Having the knowledge relevant to the task is associated with innovation ( r = .33) in the same meta-analysis. DT is the most important effect of those reviewed based on more than one meta-analysis and its BESD can be seen in Table 3.

Results indicate that creativity is associated with personality traits such as CSC, r¯ = .21, OE, r¯ = .22, and to a lesser extent with being extrovert (r¯ = .11). OE is also associated with creativity ( r = .33, Ma, 2009), and innovation at work (r = .19 Hammond et al., 2011; Hülsheger et al., 2009a ). CSC is also associated with creativity in ( r = .20, Ma, 2009) and workplace innovation (r = .24 Hammond et al., 2011 ). Creativity is also associated with greater extraversion ( r = .12) and in some meta-analyses like Ma's (2009) with emotional stability (r = .13), with conscientiousness ( r = .11) and agreeableness (r = .08), although in others creativity is negatively related to conscientiousness and agreeableness (see Feist, 1998; Hülsheger et al., 2009a ). Low neuroticism (r¯ = ?.03) and agreeableness (r¯ = ?.05) and being less conscientious (r¯ = ?.06) are globally associated with creativity. The effects of CSC and OE are second and third in order of importance of those reviewed on the basis of two or more meta-analyses (see BESD in Table 3).

Creativity is associated with low alexithymia and high empathy, considered to be indicators of EI ( Ma, 2009, r = .31). Two meta-analyses found that positive affect is associated with creativity (r¯ = .19) ( Baas et al., 2008, r = .15; Davis, 2009, r = .25). However, this effect was greater in experimental rather than correlational studies, and the type of task and the context influenced the effect (see in Table 1 Baas et al., 2008; Davis, 2009 ). Comparing positive with negative affect groups, it was found that positive affect reinforces originality and fluidity of creative response ( r = .11 and r = .09), although it does not influence flexibility or a global indicator of creativity ( Baas et al., 2008 ). Finally, these same authors suggest that high activation reinforces creativity, while Davis (2009) offers evidence that if positive affect is too high it inhibits it. This is the fifth most important effect of those reviewed in more than one meta-analysis (see BESD in Table 3).

Intrinsic motivation is associated with creativity, r¯ = .20 ( Ma, 2009, r = .15; Hammond et al., 2011, r = .24). This last meta-analysis found a positive association between rewards and pressures, or extrinsic motivation, and creativity ( r = .11). The results that the relation between creativity and intrinsic motivation is greater than with extrinsic motivation, although it is positive in both cases. The effect on intrinsic motivation is the fourth most important of those reviewed based on more than one meta-analysis (see BESD in Table 3 ). Androgynous gender identity, or possessing expressive alongside instrumental characteristics is associated with creativity ( Ma, 2009, r = .19)

Regarding age, a linear relation was revealed between this and creativity ( Ma, 2009, r = .17). Intelligence was associated with creativity (r¯ = .17). Kim (2005) found that the relation between IQ and creativity is linear ( r = .17), while Hülsheger et al. (2009a) discovered a correlation between general mental ability and innovation of r = .04.

In terms of self-efficacy and creativity, it was found that these motivation indicators were associated with creativity r¯ = .13. The first is linked to performance in workplace innovation, r = .22 for workplace self-efficacy and r = .28 for creative self-efficacy (Hülsheger et al., 2009a ). This association was also found in Ma's (2009) meta-analysis (r = .11). In that of Hülsheger et al. (2009a) , the associations with self-efficacy were stronger than that shown by creativity with the OE ( r = .19) and CSC (r = .20), signaling that motivation would be a better facilitator for innovation than personality. Conversely, in the meta-analysis of Ma (2009) , self-efficacy was associated less strongly with creativity ( r = .11) than was CSC and OE, which were linked more intensely ( r = .30 & r = .35 respectively).

Ma's (2009) meta-analysis confirms a significant association between an attitude favorable to taking risks and creativity ( r = .08). A further significant relation was found between being female and creativity ( r = .07, Ma, 2009 ). The level of academic achievement was not significantly associated with innovation ( Hammond et al., 2011, r = .14). The meta-analyses of Baas et al. (2008) and Davis (2009) revealed that negative affectivity had a non-significant relation with creativity.

The analyses carried out provide evidence about the factors which reinforce creativity and innovation. Sixty percent of the effects found were smaller than .19, which, despite being a low correlation, are coherent with those found in social psychology, where 50% of the same effects are below this value ( Richard, Bond, & Stokes-Zoota, 2003 ). The r¯ between individual variables and creativity was .14, below the average in social psychology which is .21. Individual factors reviewed explain 2% of the variance in creativity, while variance explained by role, group, and organizational factors was 7.2%, with r¯ at .27, higher than that explained by individual variables ( da Costa, Páez, Sánchez, Gondim, & Rodriguez, 2014 ). This reinforces the importance of a model of person-context interaction to understand creativity.

Results confirm most of the hypotheses, with the exception of the negative association between convergent thinking and extrinsic motivation with creativity, of the curvilinear or asymptotical relation with age, intelligence, and education, and the hypothesis of a stronger association between creativity and motivation than between creativity, cognition, and attitude. These negative findings are important, because they disconfirm some assumptions shared in the domain of creativity and innovation by relevant authors (e.g., Amabile, 1996).

Results that low alexithymia and high empathy are strongly linked to creativity, confirming that knowing and using emotions is al, both for thinking and information processing ( Mayer & Salovey, 2007 ). To explain this, we can suppose that EI reinforces the al use of emotions in thinking and problem solving. In terms of cognition, DT is also strongly linked to creativity and its higher scores are associated with workplace innovation, just as having knowledge relevant to the task is associated with innovation ( Hülsheger et al., 2009a; Ma, 2009 ). This may be explained by positing that the thinking style most strongly associated with creativity is thinking that diverges, disagrees, and looks for alternatives or original possibilities for solving a given problem ( Ma, 2009 ). In one meta-analysis, convergent thinking is associated with creativity, but in another, systematic thinking was negatively linked to innovation. This discrepancy could be due to how both the dependent and independent variables are measured ( Hülsheger et al., 2009a; Ma, 2009 ). In the creative process, both styles of thinking are necessary, with the role of convergent thinking varying, depending on whether the focus is on the generation of ideas or their application. The model of person-context interaction is important for understanding this role ( Csikszentmihalyi, 2011; Hammond et al., 2011; Ma, 2009; Simonton, 2012 )

The strong association between CSC, OE, and creativity is due to people possessing these traits to a high degree being more flexible and being strongly attracted to new perspectives and feelings ( Feist, 1998; Hülsheger et al., 2009a; Ma, 2009 ), to the extent that they would be more likely to try changes and innovate. It has also been suggested that this association could be due to the overlap between descriptors of OE, CSC, and self-reports of creativity. However, the link between CSC, OE, and creativity and innovation is found when indicators of achievement and external judges are used, which questions whether this only arises because of the overlap of method and semantic content. The moderately strong association between extraversion and creativity can be explained by the fact that people with this character trait are more energetic, more likely to look for new stimuli (new ways of doing tasks and solving problems), resulting in greater creative achievement. Being friendly, conscientious, and emotionally unstable slightly hinders creativity ( Feist, 1998; Hülsheger et al., 2009a; Ma, 2009 ).

The relation between motivation and creativity ( Ma, 2009 ) was confirmed both in studies of creativity, and creativity applied to organizations ( Hülsheger et al., 2009a ). In order to explain the effect of intrinsic motivation, it is suggested that this refers to a natural tendency towards control, interest, and discovery, representing an important source of pleasure and vitality ( Hammond et al., 2011 ). Creative performance involves high levels of energy, concentration, and willpower, which means that having high levels of intrinsic, and to a lesser extent extrinsic motivation, will drive the creative effort. A further motivational factor, self-efficacy, is associated with creativity, albeit to a lower degree than intrinsic motivation. General and creative self-efficacy act as motivational forces to raise creative performance via the perception that one possesses the capacity necessary to attain goals. Questioning the hypothesis of the general superiority of motivation, however, it has to be said that its effect is less than that of DT and similar to that of some personality traits. Regarding the relationship of these individual factors in the different stages of the creative process, we recall that Hammond et al. (2011) found that personality and motivation were associated with the generation of ideas, while the characteristics of work and the organization do so more strongly with their implementation in the workplace.

The meta-analyses confirm that, compared to a neutral state, positive affect reinforces creativity with an effect size similar to intrinsic motivation ( Baas et al., 2008; Davis, 2009 ). To explain this, it is suggested that positive affect reinforces creativity primarily via memory and thought. Positive affect would engender a greater flow or number of ideas because the associative network of emotional states and positive materials promotes memory and accessibility of information. It would facilitate a more flexible treatment of categories because people with a positive state of mind are more inclusive when categorizing stimuli. Furthermore, positive affect would reinforce the generation of more uncommon ideas, i.e., originality ( Baas et al., 2008; Davis, 2009 ). Secondly, with positive feelings acting as a signal suggesting that a state of wellbeing prevails, they evoke a playful, relaxed approach to tasks (e.g. DT), whereas negative feelings would indicate danger, thereby provoking systematic thought in problem solving. A positive outlook would reinforce DT, boost attention and the repertoire of actions and ideas ( Fredrickson, 2009 ). Finally, positive affect and associated emotions will influence creativity through the tendency towards action that they prime. Positive emotions of high levels of activation, such as happiness and interest, which involve action tendencies of an active approach, motivate creative performance ( Baas et al., 2008 ). Mere experiencing of such states of positive valence does not reinforce creativity ( da Costa, Páez, Oriol, & Unzueta, 2014 ); it is the high level of activation which reinforces it when it is positive and blocks it when it is negative. Furthermore, a congruency hypothesis is partially confirmed: positive affect reinforces creativity in tasks which are enjoyable and are intrinsically motivating, while undermining it in serious tasks of extrinsic reinforcement ( Baas et al., 2008).

Being older is associated with creativity, demonstrating that experience provides resources for it and that it is far from being typical of young people. Rather than a curvilinear relation between the two, a linear one was found ( Ma, 2009 ). It has been suggested that intelligence measured by IQ is associated with creativity, although it is not decisive beyond a certain level; this is known as an asymptotic relationship. Empirically, in terms of cognitive capacity and creativity, it was found that the relation between IQ and creativity is linear ( Kim, 2005 ). These results suggest that greater intelligence means greater creativity, but permit a large amount of variability in the latter (see Kim, 2005 in Table 1 ), i.e., one can be creative without being very intelligent, but the greater one's cognitive resources, in general the greater one's creativity.

Confirming that creativity has to do with attitude ( Amabile, 1996; Hennessey & Amabile, 2010 ), Ma's meta-analysis (Ma, 2009 ) supports a slight association between pro-risk attitude and creativity. Taking risks in trying out ideas and new processes is an important part of the creative process. From the gender perspective, women displayed slightly more creativity ( Ma, 2009 ). In terms of gender identity, it was found that having expressive traits alongside instrumental ones, i.e., androgyny, is associated with creativity more strongly than being male or female, which suggests that the psychological is more relevant that the biological sex ( Ma, 2009; Stoltzfus et al., 2011 ). That is to say that internalization of instrumental (e.g., being assertive) and expressive attributes in one's self-concept may aid innovation.

Regarding the relation between education and creativity, it was seen that academic achievement, an education level indicator, was not associated significantly with innovation. However, this conclusion is diminished by the idea that relevant knowledge for the task in hand does associate with it ( Hülsheger et al., 2009a ), and it is understood that to be an expert in a given field s ten years of systematic learning ( Csikszentmihalyi, 2011; Gardner, 2010 ).

Negative affect is in general not associated with creativity ( Davis, 2009 ). Compared to its positive counterpart, negative affect slightly reduces the flow and originality of the creative response, although it does not affect other facets of creativity ( Baas et al., 2008 ). Some studies have discovered positive relations between indicators of psychopathological imbalance and creativity, as in the case of artists, but not in scientists ( Feist, 1998; Ma, 2009 ). Nevertheless, the relation between affective disorders and creativity is essentially found in the experience of intensely affective or moderately manic states which heighten consciousness and flexibility of thought ( Ivcevic et al., 2007).

Regarding limitations of the study, we acknowledge in our conclusions that they are based on correlations uncorrected for measurement error, and are thus likely to be underestimates of the real relations; for example the size effect for positive affect was larger than r = .20 when corrected for reliability of measurement ( Davis, 2009 ). Furthermore, we do not take into account the overlap between the studies of meta-analyses, although we believe that since they derive from traditions of experimental psychology on the one hand and organizational psychology on the other, any overlap will be minimal. In addition, in some cases there is only one meta-analysis, which limits generalization. In this regard, we note that results have been unified for tasks of cognitive creativity with evaluations of the application of ideas and products in organizations, although this also reinforces the generality of the conclusions.

The results confirm that creativity is associated with emotional intelligence (e.g., high empathy, emotional expressiveness, and good capacities of affect regulation), divergent thinking, creative personality, openness to experience, positive affect, intrinsic motivation, and androgyny. To a lesser degree it is associated with age, intelligence, extrinsic motivation, self-efficacy, and somewhat less with pro-risk attitude and the female sex. On the basis of the evidence, we can conclude that when recruiting staff with creativity in mind, these should be emotionally intelligent, have a high degree of divergent thinking, be motivated primarily for the interest and challenge involved in the work, and be simultaneously expressive and assertive. As a practical conclusion, we emphasize that for innovation in the workplace emotional aspects such as EI and cognitive facets like DT are more important than motivational factors such as self-efficacy and intrinsic motivation. An organizational culture that boosts the EI of its workforce increases employees’ capacity for understanding, using, expressing, and regulating emotions, while also reinforcing instrumental and cognitive capacities will promote innovation considerably.

In recruitment it should also be remembered that employees with openness to experience traits and a creative self-concept could strengthen organizational innovation. Furthermore, an organizational culture which s in its occupational roles the goals of generating ideas and applying original ideas in problem solving, as well as a commitment to innovative behavior and practices may help to turn this into the norm, and, through organizational socialization, it may become internalized in the self-concept. To a lesser extent, ing employees with high positive affect would benefit innovation. From an organizational point of view, practices and norms which reinforce a positive climate and positive emotions among the staff could have the same effect. In both emotional climate and positive emotions of the individual, it should be specifically emotions of high activation that are boosted, not those of calm, since only the former favor innovation. Recruiting people with high intrinsic motivation and high self-efficacy, or reinforcing these in existing employees by raising their decision-making and autonomous work planning capacities, as well as increasing their self-control and cognitive competences, will promote innovation moderately. Prioritizing the ion of women above men, older people and those with a pro-risk disposition would (moderately) benefit innovation. Finally, it should be remembered that organizational factors (e.g., complexity and autonomy in the work role, and structural resources of the company) are more important than those of personality in terms of increasing creativity and organizational innovation. With regard to future research, we believe it necessary to use more sophisticated statistical methods to verify theoretical models using moderation and mediation analyses. We also consider it important to broaden socially and culturally the samples of the studies, incorporating workers for different organizational areas.

The authors of this article hereby declare no conflict of interest.

This study has been made possible thanks to research PSI2011-26315 and to UFI 11/04 of UPV/EHU.