The purpose of this investigation is to determine whether the mental load of a cognitive task prevents the processing of visual stimuli, that is, whether the mental load produces inattentional blindness, and at what point in the cognitive-task processing more interference is produced. An arithmetic task with two levels of mental load was used in a dual-task situation with a visual search and detection task. An experiment was performed with 35 participants. An eye tracker system (ASL model 5000) was used to verify which targets were looked at. The results show impairment in the detection task when carrying out the two tasks simultaneously; it was higher when the arithmetic task had a higher mental load. The impairment cannot be explained by alteration in the ocular pattern. The moment at which the process or sub-process of the arithmetic task produces the greatest interference in visual detection corresponds to the purely cognitive moment of calculus, versus sub-processes with perceptive or motor components, such as listening to the stimuli or emitting responses.