Psychophysical assessment of the L:M weighting of inputs to the ON and OFF S-cone pathways

New data on S-OFF ganglion cells (Dacey et al., 2001) reveal anatomical and physiological differences between ON and OFF S-cone pathways. From transient tritanopia data, McLellen and Eskew (2000) reasoned that ON and OFF S-cone pathways may have different L:M cone weights. Here, in Experiment One, we measured temporal 2AFC thresholds for detecting increment and decrement pulses (abrupt onset, ramped offset; 400 ms) that were defined only by a change in S-cone stimulation. We measured threshold versus radiance (tvr) curves for two adapting chromaticities: high L:M ratio or low L:M ratio. Stimuli originated from a colorimeter with four primaries in centre and in surround that allowed independent modulation of the rods and each of the 3 cone types. In Experiment Two we measured detection of increment and decrement pulses (rectangular; 200 ms) on a neutral reference field, 400 ms after the offset of a long-wavelength adapting field. We measured thresholds as a function of the retinal illuminance of the adapting light, and used two different adapting chromaticities. We used two classes of test stimuli. One offered pure S-cone modulation, and the other included a rod modulation in order to simulate McLellen andEskew's nominally S-isolating stimuli generated on a CRT. Results from Experiment One were inconsistent with different ON/OFF L:M cone weightings: increment and decrement tvr curves superposed for both adapting fields. Data from Experiment Two replicated McLellen andEskew's result: the relation between increment and decrement thresholds changed as a function of the L:M ratio in the adapting field. We suggest it is unlikely that the inferred ON and OFF S-cone pathways differ in L:M weighting. McLellen andEskew's result may in part be ascribed to the rod signal associated with their test stimuli