Convergence insufficiency (CI) is one of the most common binocular vision disorders. The prevalence of CI ranges from 3% to 6% in school-aged children. CI symptoms include visual fatigue, headache, blurred vision, and diplopia and could be caused while using near-distance viewing. These symptoms might become more severe with increasing need to perform near-distance tasks. Long-term visual symptoms could result in a negative impact on learning behaviors and work performance in patients. Nowadays, visual training is the main type of management for CI patients clinically; however, the existing training protocol required patients to make several visits to the clinic for visual training, which might increase the burden for patients and result in higher failures in training. Therefore, this study will use a simple training method by utilizing prisms for CI patients to train at home, then compare visual symptoms and binocular vision after the training. In stage I of this study, investigators will recruit 60 symptomatic CI participants aged 9 to 30 years old to do a 6-week visual training with the prisms (15 min/time, 3 times/week). The post-training outcomes will be collected at week 4 and week 6. In stage II, all of the participants will be randomly divided into the "stop training group" and the "continue training group." The participants in the "continue training group" will have the same 6-week prism training and in the "stop training group" will stop all the prism training during this period. The final post-training outcomes of all the participants will be collected again at week 12. In this study, investigators will investigate the effectiveness of the prisms training for 6 weeks and for 12 weeks on visual symptoms and binocular vision in CI patients, and evaluate whether the training effect will be affected by stopping training after 6 weeks.

Inclusion Criteria:

1. The overall score of CI symptom survey ≥ 16 (9-18 years old) or ≥ 21 (19-30 years old)
2. The break point of near point of convergence (NPC) ≥ 6 cm
3. Exophoria at near distances be at least 4∆ higher than at far distances
4. Near positive fusional vergence (PFV) ≤ 15∆ or failing Sheard's criterion at near.

Exclusion Criteria:

1. The best corrected visual acuity (BCVA) of < 20/25 in each eye at far and near distances
2. Amblyopia patients or the difference of BCVA between the two eyes ≥ 2 lines
3. Constant strabismus patients
4. History of strabismus surgery or refractive surgery
5. Systemic diseases that would affect binocular vision
6. Acquired brain injury or neurological disorder.