Measurements were carried out using the RETIport recording system (Roland Consult, Wiesbaden, Germany). The stimuli were sine waves presented using a six-primary Ganzfeld bowl (Q450 SC; Roland Consult). The primaries consisted of arrays of LEDs, of which only four were used in the following protocols: green (peak wavelength 523 nm, CIE1931 coordinates: x = 0.2016, y = 0.7371), orange (594 nm, CIE1931 coordinates: x = 0.5753, y = 0.4240), blue (469 nm, CIE1931 coordinates: x = 0.1255, y = 0.0926), and red (638 nm, CIE1931 coordinates: x = 0.6957, y = 0.2966). All responses were recorded under identical mean luminance (284 cd/m2; the luminance of each LED array: green 40 cd/m2, orange 160 cd/m2, blue 4 cd/m2, and red 80 cd/m2) and chromaticity (i.e., reddish appearance, CIE1931 coordinates: x = 0.5951, y = 0.3857). The luminance measurements were made using a Minolta LS-110 photometer. Spectral outputs and CIE coordinates were measured using a CAS 140 spectroradiometer (Instrument Systems, Munich, Germany).
Eight stimulus protocols were employed in total: L-cone isolating, M-cone isolating, isochromatic luminance, and isoluminant red–green chromatic stimuli, each at 12 and 36 Hz temporal frequencies. Stimulus specifications of each of these are presented in
Table 1.
Briefly, L- or M-cone isolation was obtained using the triple silent substitution method (Kremers,
2003; Shapiro, Pokorny, & Smith,
1996). L-cone responses were isolated with 19% cone contrast with all other photoreceptor types silenced (i.e., 0% modulation). Similarly, M-cone isolation was obtained with 18% cone contrast and with L-cone, S-cone, and rods silenced. These stimuli were presented at 12 and 36 Hz to assess separate contributions from the chromatic- and luminance-based pathways, respectively (Jacob et al.,
2015; Kremers & Link,
2008; Kremers & Pangeni,
2012; Kremers et al.,
2010; Martins et al.,
2016; Parry et al.,
2012). Additional measurements with isochromatic luminance and isoluminant red–green chromatic stimuli were performed at 12 and 36 Hz. For the luminance stimuli, the outputs of all four LEDs were modulated in phase with 75% luminance contrast. The red–green chromatic stimuli were obtained with counterphase modulation of the red and green LEDs (blue and orange LEDs not modulated). Modulation contrast was 100% for the green and 50% in the red LEDs to achieve isoluminance considering that the mean luminance of the red LED (80 cd/m
2) was twice that of the green LED (40 cd/m
2). Due to individual variation in isoluminance conditions, residual luminance stimulation may be present. However, previous data have shown that, with these conditions, the ERG response to the chromatic contents of the stimulus dominates at 12 Hz (Kremers et al.,
2010).