Patients that meet the inclusion criteria, have no exclusion criteria and who give written informed consent, will be assigned a consecutive number during their outpatient visit according to their order of inclusion in the trial. These participants will be randomized to either HBE or control group in the moment that a member of the research team verifies, among the stored prisms, the existence of a prism pair marked with the same number as the consecutive number assigned to the patient. In the case that this pair does exist, then the patient will take home that pair of wooden prisms and use them during the first period of the trial according to spoken and written instructions to be given at that moment. On the contrary, if a pair of wooden prisms marked with the same consecutive number as the patient, does not exist, then it will be understood that the study participant has been allocated to control group during the first period of the trial. The member of the research team who verifies the storehouse of wooden prisms will not be in charge of confirming inclusion and exclusion criteria and will not assign consecutive numbers to patients during outpatient visits.