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INTRODUCTION
The Symbol Digit Modalities Test, also known for its acronym SDMT, is an experimental task for the detection of possible brain dysfunctions in children and adults. The activity performed during the test to be evaluated consists on converting a series of symbols, with different shapes, into numbers using the key provided on the test(1).
SDMT is part of typical neuropsychological pen and paper type-specific tests, used to evaluate a particular neuropsychological function(2). Its content is based on 120 stimuli distributed into 8 rows per sheet for its application. It takes 5 minutes to be completed and it can be applied individually or collectively. There the possibility to use oral or visual versions.
The visual version of the SDMT (format of interest for the current study) allows evaluating quickly and as an initial exploration, the presence of brain problems with manual, motor, visual or learning implications. On the other hand, the oral version permits to analyze reading capabilities or difficulties, or speech impairments, and the state of connections in neurophysiological areas of visual recognition, comprehension and oral articulation can be appreciated(1).
SDMT has been identified as an important tool for the evaluation and neuropsychological diagnosis of humans. For example, it has been stated that the test contributes to the evaluate focal or selective attention(3), execution intelligence, discrimination and visual precision facing abstract stimuli, short term memory, cognition flexibility(4) (5), cognitive deterioration for multiple sclerosis(6) (7). It is also used as a tool to diagnose ADHD(8), cognitive functioning on patients suffering a major depression disorder(9), information processing speed of patients with brain problems(10), among other areas of interest in neuropsychology.
Based on this explanation, it is of vital importance to have SDMT parameters in Ecuador so we can use the test in our context, and specially, on our line of research, to apply SDMT on university students.
Reviewing the mail research databases such as Scopus, Thomson Reuters, Scielo and Latindex, it was evidenced that there is a lack of studies where the performance of students of all educational levels in Ecuador has been analyzed; even less where clinical population results have been studied. In this sense, and based on the lack of prior research, proposing a research study where normative and descriptive performance values can be analyzed for this test was eminent. The resulting data can be useful to determine reference values of great utility for the clinical and scientific Ecuadorian context interested in the analysis of neuropsychological functions involved during the execution of SDMT.
METHOD
Participants
The sample included 250 young university students, 142 (56,8%) men and 108 (43,2%) women. The age range was between 18 and 34 years old (M=21,53, DE=2,25). All the students belonged to private universities of Quito, Ecuador. As far as the socioeconomic level of the participants, they all belong to middle, upper-middle and high class. All college students who participated showed a neuropsychological development within normal parameters during their evaluation.
RESULTS
In terms of percentiles (P), the number of right answers show that P5 had 31,55 correct answers; P25 had 46; P75 had 60; and p95 had 74,45 right answers. Table 1 illustrates descriptive statistics of the resulting scores.
Table 1 Measurement Results of SDMT
| N | Minimum | Maximum | Media | Standard Deviation | |
| Total errors | 250 | 0 | 78 | 1,70 | 5,39 |
| Total hits | 250 | 10 | 108 | 52,83 | 13,60 |
| Total omissions | 250 | 0 | 82 | 6,03 | 16,22 |
The analysis of means between men and women found that for total errors t(248)=0,68, p=0,49 and total omissions t(248)=1,54, p=0,12 the result does not show statistical differences of significance. Nevertheless, for total hits, there were significant differences, where women had a higher performance with respect to men t(248)=2,07, p=0,04 (see Figure 1).
The correlation analysis between the different variables evaluated during the SDMT found that total hits is inversely correlated to total errors (r=-025, p<0,001) and total omissions (r=-0,17, p=0,007).
Comparing means of total hits f(3, 246)=0,14, p=0,94; total errors f(3, 246)=0,67, p=0,57 and total omissions f(3, 246)=1,98, p=0,12 when considering four factors of age rages, showed there are no significant statistical differences (see Figure 2). Figure 3 illustrate the means of hits of the applied SDMT according to the age of the subjects.
Discussion
The current paper has made a normative and descriptive analysis for the interpretation of the SDMT test when applied to a sample of college students of Ecuador. Findings showed the average hits or correct answers for young participants, 18 years old or older, will be 52,83; which is concordant with prior research results, such as one of Spain where the hits mean was 49,10 for a sample of 1036 subjects over 20 years old(7). Another normative study of SDMT performed in Spain as well that counted with 1307 young participants between 18 and 24 years of age, with normal development, of similar characteristics as the ones of our study, shows an average of 54,40 hits, while for participants 25 to 34 years old the mean was 53,30 for hits on the test.(11) Finally, a study performed in Italy found a mean for errors of 56,3 in a sample of 273 young adults without any pathology(12).
As far as percentile values, prior research in Spain reported the following findings: 25 (P5), 42 (P25), 50 (P75), 57 (P75) and 59 (P95), while in our research results were 31 (P5),55 (P25), 46 (P50), 60 (P75), 74,45 (P95). These are similar results which enlighten us, at least basically, since there is no prior research, and will lead us to further research to understand normal and pathological performance during the SDMT test of university students.
As far as the difference between men and women, our research shows a similar performance for both of them. On the other hand, in terms of number of hits, women reach a higher score in the SDMT. This result is not concordant with prior research(1),(7),(13) where findings state that gender has no inference in the performance of attention evaluation instruments. Nevertheless, our study shows a 3,5 error difference and la minimum level of significance; which can´t lead to a significance difference with certainty specially at a clinical level.
With respect to the relationship between SMDT performance and the age of the participants, as seen in prior research papers, findings show that this variable has no significant correlation for subjects 18 to 30 years old. This result makes complete sense when compared to nervous system theoretic background which states that for humans 18 years old or older, brain processes involved with visual perception are completely mature; hence there will be no change during this age range(11).
A constraint that needs to be highlighted for this study lays in the fact that the sample used for the research was taken only from one city of Ecuador, which means that results will apply for the described context. Although there is no prior research data, this paper is an important contribution and can be used as reference in order to interpret SDMT resulting data. On this line, as further research, we seek to analyze the oral version of the SDMT and relate its performance results with the findings obtained from the current study. Furthermore, it will be interesting to analyze the correlation of the test with other psychometric instruments used to evaluate similar cognitive functions.
