We aimed to investigate how ordered mappings (e.g., left-to-right and right-to-left order representations) would interfere with each other.

Mental representations of numbers and letters are linked with spatial representation and can be changed intentionally.

The experiment consisted of three sessions. In the digit-alone session, two digits randomly selected from [1], [2], and [3] were shown. If the two digits were the same, participants pressed the button corresponding to the digit, and if the digits differed, they pressed the remaining button. The response buttons were ordered [1][2][3] from the left. In the letter-alone session, three different button configurations were prepared: sequential [A][B][C], reversed [C][B][A], or partially reversed [B][A][C]. The same-versus-different rules were basically identical to those in the digit task. In the mixed session, trials of the digit task and those of the letter task were randomly mixed.

We found that two ordinal representations did not interfere with each other when they shared the same direction of order ([1][2][3] vs. [A][B][C]), two ordinal mappings interfered with each other when they had different directions of order ([1][2][3] vs. [C][B][A]), and an ordinal mapping ([1][2][3]) was affected by a nonordinal mapping ([B][A][C]), but the nonordinal mapping was less affected by the ordinal mapping.

The mapping between ordinal information and space can be modulated by top-down processes, and it is prone to interference depending on the nature of another coexisting mapping.

Our findings may be used in designing response assignments for input devices for multiple functions.