The Relationship between Intelligence and Executive Functions in Hispanic Patients with Mild Traumatic Brain Injury

Executive functions have been described as the “brain of the brain” and to include a variety of processes, including, but not limited to, abstraction, formulation of intentions, reasoning, formulation of strategies, monitoring their success, working memory, and the self-regulation of behavior. Executive functions have been determined to be associated with the integrity of the frontal lobes. On the other hand, intelligence has also been described in similar terms, and the concept of general intelligence, or the g factor has also been associated with the frontal lobes. As constructs, intelligence and executive functions have been also described as “overlapping” in many features, while maintaining a certain degree of conceptual independence. The purpose of this study was to compare the intellectual functioning of a sample of patients with mild traumatic brain injury, obtained by means of the Reynolds Intellectual Assessment Scales (RIAS), with their performance in a number of neuropsychological measures of executive functions, such as the Halstead Category Test (HCT), the Wisconsin Card Sorting Test (WCST), the Stroop Word and Color Test (SWCT), and the Trail Making Test (TMT). Pearson Product Moment Correlations were obtained between the various measures of the RIAS and the measures of executive functions (HCT, WCST, SWCT, TMT). Whereas the magnitudes of the correlations were within the moderate range, intelligence, particularly non-verbal intelligence and working memory were noted to be significantly correlated with the measures of executive functions. The conclusion was reached that intelligence and executive functions are related to each other as constructs, but yet maintain a certain degree of inde-pendence.


Introduction and Literature Review
According to Tirapu-Ustárroz et al. (2017), the term executive functions was proposed for the first time by neuropsychologist Muriel D. Lezak (1982) who defined it as mental capacities essential to carry out effective, creative, and socially accepted behavior with four components: formulation of goals (ability to generate and select the desirable states in the future), planning (selection of actions, means and sequences necessary to reach goals), development (ability to initiate, stop, maintain, and change among planned behaviors), and implementation (ability to monitor and correct behaviors).
For Lopera-Restrepo (2008), executive function refers to the directing, managerial, and guiding function of the brain. It is the "brain of the brain". In reality, the guiding or managerial function of the brain is a series of directing functions that include a variety of programming and implementing cerebral activities.
Friedman & Miyake (2017) described executive functions as high-level cognitive processes that, through their influence on lower-level processes, allow individuals to regulate their thoughts and actions during behavior aimed at the attainment of goals.
For E. Goldberg, one of the disciples of the great teacher of neuropsychology, Alexander R. Luria, executive functions are intimately related with the integrity of the frontal lobes, to which he refers to as the "lobes of human civilization" (Goldberg, 2001). Within the model of executive functions developed by Goldberg (2009), executive functions include a series of high-level cognitive processes designed to facilitate the independent attainment of behavioral goals.
Executive functions include, in the model designed by Goldberg, processes such as abstraction, formulation of intentions, reasoning, formulation of strategies, monitoring their success, working memory, and the self-regulation of behavior (Goldberg, 2001(Goldberg, , 2009. The relationship between the constructs of intelligence and executive functions has raised the interest of neurocognitive researchers. The most classical and widely used definition of intelligence was proposed by David Wechsler, the author of the most frequently used instruments for the measurement of intellectual ability. According to this author, intelligence is "the aggregate or global ability to behave purposely, think rationally, and deal effectively with the environment" (Wechsler, 1958). This definition of intelligence has many elements in common with the concept of executive functions, as it is currently understood. García-Molina et al. (2010) carried out a revision of the concepts of intelligence and executive functions with the aim of answering the question if these are the same or not. They reviewed, in the first place, the concept of the g factor, within different theories of intelligence. As with the concept of executive functions, the g factor has been closely related to the functioning of the pre-frontal cortex and is viewed as a mechanism responsible for the coordination and control of cognitive processes involved in problem-solving. The intellectual processes that were estimated to have a closer relationship to executive functions are those Psychology related to fluid intelligence.
On the other hand, these authors concluded that the diversity of theoretical views of both intelligence and executive functions, as well as the absence of instruments to assess both processes in a comprehensive manner, makes it difficult to answer the question if both constructs represent or not the same reality. According to these authors, in regards to this question: "The studies described in this article offer different answers, being the hypothesis that has greater acceptance the one that considers intelligence and exectutive functions as overlapping in certain aspects, but not in others (p. 744)".

Purpose of This Study
The purpose of this study is to determine the relationship that exists between intelligence, measured by means of the Reynolds Intellectual Assessment Scales (RIAS; Reynolds & Kamphaus, 2009) and executive functions assessed by means of the Halstead Category Test (HCT; Halstead, 1947;Reitan & Wolfson, 1993), the Wisconsin Card Sorting Test (WCST; Berg, 1948;Grant & Berg, 1948;Heaton et al., 1993Heaton et al., , 2009, the Stroop Word and Color Test (SWCT; Golden, 2001) and the Trail Making Test (TMT; Reitan, 1958;Reitan & Wolfson, 1993), as well as to determine which component or components of intellectual functioning, that is, crystallized intelligence, fluid intelligence, or working memory are more closely related with the construct of executive functions.

Ethical Considerations
This study was authorized by the Institutional Review Board of the Neurobehavioral Institute of Miami as an archival research.

Participants
The participants of this study were 39 Hispanic patients referred for neuropsychological evaluation by their treating neurologists to the Neurobehavioral Institute of Miami for neuropsychological evaluation with a history of having sustained a closed head injury resulting in a mild traumatic brain injury (mTBI), with alterations in consciousness for less than one hour, as well as having retrograde amnesia also for less than one hour. The mean age of this sample was 41 years, with a standard deviation (sd) of 17.34, with values between 13 and 77 years and 56% being women. The major cause of mTBI in this sample was motor vehicle accidents (20). Patients with mTBI resulting from falls and physical assaults were also included. The distribution of the educational level of the sample was as follows: 38% had a college level education, 26% had a technical level education, and 23% had a high school level education. The remaining 10% had an elementary level or middle school level education. All the patients had negative neurological examinations, as well as negative electroencephalograms (EEG), computerized axial to-Psychology mographies (CAT Scan), and magnetic resonances (MRI).

Procedures
All patients were administered a battery of neuropsychological instruments, which included a measure of intelligence, the Reynolds Intellectual Assessment In order to determine if the participants had made an appropriate effort in the performance of the instruments included in the neuropsychological battery administered to them, they were administered two instruments designed for that purpose, the Test of Memory Malingering (Tombaugh, 2011) and the Rey 15-Item Memory Test (Strauss, Sherman, & Spreen, 2006). Only participants with scores in these two instruments that reflected the application of appropriate effort were included in this study.
The instruments selected for the assessment of executive functions in this study have a solid trajectory of being sensitive to lesions in the frontal lobes, particularly in patients with mild traumatic brain injury. Relative to the Halstead Category Test the reader is referred to the following publications: Choca et al. (1997) andJubiz Bassi (2008). In order to determine the ability of the Wisconsin Card Sorting Test to assess executive functions mediated by the frontal lobes, the reader is referred to the following works:

Statistical Analysis
The SPSS.20 program was used to analyze the data in terms of frequencies, descriptive statistics, and correlations.

Results
The means and standard deviations of the four indices of the RIAS can be found in  Table 2).
Following, the Pearson Product Moment Coefficients of Correlation between the variables of the Reynolds Intellectual Assessment Scales and the measures of executive functions selected for this study are presented (See Table 3). The

Conclusion
The findings of this research study showed that the constructs of intelligence and executive functions have a statistically significant relationship, although moderate in magnitude. In particular, fluid intelligence and working memory showed the highest correlations between these two constructs. The moderate magnitude of the correlations obtained supports the notion that, although intelligence and executive functions are related, they nonetheless maintain a certain independence.
On the other hand, crystallized intelligence, which represents the level of knowledge acquired by means of experience and education, did not show statistically significant correlations with executive functions. It is quite interesting to note that crystallized intelligence is closely related to another important construct, cognitive reserve. It would be interesting that other research studies explore the relationship between cognitive reserve and executive functions.