If you are an Indian resident, and have a dependent with a disability, you can claim tax deductions under Section 80DD. The disability must be certified by a medical doctor.
In case of
Normal Disability ( i.e. at least 40%) the deduction allowed from gross total income is Rs. 75,000.
Severe Disability (i.e. 80% or more) the deduction allowed from gross total income is Rs.1,25,000.
Full deduction can be claimed under Section 80DD even if the actual expenses on the said disabled dependant are less than the mentioned amount.
Mental disability is among the listed disabilities covered under 80DD- for others, you must refer to clause (i) of Section 2 by the 'person with Disabilities Act, 1995.