Experimental studies,
designed to index symbolic numerical magnitude processing, have indeed been very successful in
identifying cognitive mechanisms that may underlie the processing of symbolic numerical magnitude.
For instance, the numerical distance effect (NDE), an inverse relationship between reaction time and
the numerical distance (the numerical difference between two numbers) of two compared numerals,
has been argued to index an internal analog representation in which numerical magnitudes are represented
as approximate quantities along a hypothetical mental number line (Moyer & Landauer,
1967).