1. 3. Arnold Bentley. Monotones—A Comparison with Normal Singers in Terms of Incidence and Musical Abilities. London: Novello, 1968. 74 pp.
2. 4. J. Mc Leish and G. Higgs. “An Inquiry into the Musical Capacities of Educationally Sub-Normal Children”, Occasional Research Papers No. 1. Cambridge Institute of Education, 1967. 12 pp.
3. The Effect of Scale Direction on Pitch Acuity in Solo Vocal Performance