Temporal induction of blackness--II. Spectral efficiency and tests of additivity

The perception of blackness was investigated by measuring spectral efficiency and field additivity under conditions of temporal induction. For both purposes, observers foveally viewed a 0.75 deg, inducing stimulus for 5 sec followed immediately (or 200 msec later) by a broadband (5500 K) reference stimulus of the same size and spatial location. For spectral efficiency measurements, the inducing field was a monochromatic light between 400 and 700 nm (10 nm steps), while the additivity studies involved various wavelength mixtures. The psychophysical task was to increase the radiance of the inducing field until the reference stimulus just turned completely black. For two observers the spectral efficiency function of temporally-induced blackness more closely resembled their heterochromatic flicker photometry (HFP) function than their direct brightness-matching function. Their brightness functions were characterized by an inflection at about 580 nm which is generally ascribed to subtractive cone interactions, but their blackness-induction and HFP functions did not show this inflection. The brightness function of the third observer did not show an inflection at 580 nm, thereby making it difficult to differentiate between her three spectral efficiency functions. Overall, the subtle differences between the various spectral efficiency functions made it difficult to determine whether blackness induction was more similar to HFP or brightness matching. The results from the additivity tests of blackness induction, HFP and direct brightness matching removed this ambiguity from the spectral efficiency findings. Blackness induction and HFP were shown to be additive, whereas the results from brightness matching showed clear additivity failures of the cancellation type. These data support the view that the perception of blackness is mediated by neural mechanisms that additively combine the inputs of middle- and long-wave photoreceptors.