Why are the blind and severely visually impaired children with mental retardation much more retarded than the sighted children?

Five per cent of mentally retarded children in Denmark have a visual acuity below 6/60 as compared to 0.02% in the normal child population. Profound mental retardation was more prevalent among the visually impaired than among the sighted patients. An analysis was performed concerning the correlation between the degree of retardation and the severity of visual impairment, chronological age, number of complicating signs, and degree of motor impairment. The degree of mental retardation was also compared with the aetiology of visual impairment. Severe motor impairments was closely correlated with profound mental retardation.