Optimization of large field tritan stimuli using concentric isoluminant annuli.

Large, nominally isoluminant chromatic gratings containing a short-wavelength component are prone to luminance contrast intrusions due to retinal inhomogeneity, especially as a result of the uneven distribution of macular pigment. Isoluminance is usually determined for a relatively small, central area, but a significantly larger stimulus cannot be isoluminant across the whole field, largely due to macular pigment absorption of short-wavelength light. This confounds attempts to maintain high selectivity, particularly in suprathreshold electrophysiological and brain-imaging studies that require large stimulus fields. Here we introduce the concept of a panisoluminant grating (PIG), which comprises a series of concentric annular regions, each adjusted to location-specific isoluminance for the observer. Gratings were modulated along subject-specific tritanopic confusion lines and the selectivity of responses to the PIG was tested according to both psychophysical and electrophysiological criteria. The psychophysically-determined temporal tuning function obtained using the PIG showed lower sensitivity and lower resolution than with a conventional tritan grating of equal diameter (18°). Chromatic onset visual evoked potentials (VEPs) to the PIG were dominated by a chromatic-specific negative wave and reduced achromatic response components that were prominent in VEPs to the conventional grating. These data demonstrate that a large tritan PIG is capable of eliciting selective responses of the S-cone-driven pathway at threshold and at suprathreshold levels. The PIG stimulus may prove beneficial in investigations that require large fields such as electrophysiological and brain imaging studies of chromatic processing.