The aim of this work is to understand the impact of lowercase letters and uppercase letters in terms of reading. Four sessions were held in which subjects are aged from 15 to 59 years old. Of the 19 participants with a mean age of 26.52 years (SD=13.14), to understand which letters (lowercase versus uppercase) presents a shorter reading time and higher levels of calmness, considering two different complexity texts (children and scientific) in two different forms of interaction (paper reading and screen reading). Several tests were carried out to ensure the intended result in order to comprehend the influence of various visual variables because of a more precise reading process. Four variables were examined using various sensors, including the Brain Computer Interaction (BCI) device, to measure heart rate activity (HRA) and levels of brain activity (active, neutral, and calm). The number of errors, the reading time, the heart rate variability and the calmness, active and neutral levels were considered. Our findings demonstrate that depending on the type of letters (lowercase versus uppercase), and the type of text (scientific versus children's text), and the reading text presentation (paper or screen), the visual variables have a different effect on reading performance.