Field additivity of the middle-wavelength cone pathway under various test and field configurations.

The field additivity of the M-cone pathway was measured with psychometric functions at 10 times absolute threshold on monochromatic fields and their mixtures. Observers detected a 500 nm test on 530 or 610 nm fields, and a 530 nm test on 481 or 622 nm fields. For both sets of wavelengths, field additivity held with the 1 deg test, 10 deg field condition which defines II-4 and with the 3.6 min are test on a 8.6 min arc field used to isolate the M fundamental by Stockman [(1983) Ph. D. thesis, Trinity College, Cambridge University, Cambridge]. Sub-additivity occurred for a 1 deg test on a 1 deg field, a condition for Foster's "spectral sharpening" which may evince opponency.