How to update a field where the field name is equal to a row

UPDATE [ECS_AccountAllocationSettings] 

SET    PeriodDate = (SELECT DISTINCT PeriodDate 

                     FROM   ECS_Period_Check 

                     WHERE  PeriodDate = variable), 

       Period = (SELECT DISTINCT Period 

                 FROM   ECS_Period_Check 

                 WHERE  PeriodDate = variable), 

       Debit = (SELECT DISTINCT Debit 

                FROM   ECS_Period_Check 

                WHERE  PeriodDate = variable), 

       Credit = (SELECT DISTINCT Credit 

                 FROM   ECS_Period_Check 

                 WHERE  PeriodDate = variable) 

FROM   [ECS_AccountAllocationSettings] 

WHERE  Substring(MainAccount, 1, 4) = variable