Abstract
Purpose: :
Dark–adapted rods in the area surrounding a luminance–modulated field can suppress cone–mediated flicker detection. The receptoral or postreceptoral site of interaction is unclear. We investigated this issue by measuring the critical fusion frequencies (CFF) of isolated receptoral and postreceptoral systems in the presence of different levels of surrounding rod activity.
Methods: :
A 2–channel photostimulator, with 4–primaries for a central field and 4 for a surround, allowed independent control of rod and cone excitation (2 degree center, 13 degree surround; 7.5 degree eccentricity). Receptoral or postreceptoral luminance, or chromatic stimuli were sinusoidally modulated within the 80 photopic Td central field, using a 1–sec raised cosine envelope alternated with a 1–sec steady field. The L–, M– and Luminance (L+M+S or L+M+S+Rod) Michelson contrasts were 15%, the S–cone contrast was 30% and L/(L+M) contrast was 5%. Observers adjusted the modulation frequency of the 80 Td center to determine the CFF for each stimulus type in the presence of different surround illuminances (0, 0.05, 0.5, 5, 20 or 80 photopic Tds). Measurements followed either 30–min of dark adaptation or 2–min of light adaptation (10,000 Td broadband light).
Results: :
For L– or M–cone modulation and luminance modulation, the CFF was constant for the dark and dim surrounds but increased abruptly when the surround retinal illuminance was above cone threshold. Following light adaptation, dim surrounds did not result in lowered CFFs. The CFF for S–cones was unaffected by surround light level or light or dark adaptation and L/(L+M) postreceptoral signals showed a weak effect.
Conclusions: :
From physiological studies (Yeh et al, 1995, JOSA A, 12, 456), the expectation is that magnocellular pathway units would respond vigorously to all stimuli except S– or L/(L+M) modulations; parvocellular pathway units would not respond to the S– or luminance signals; and koniocellular pathway units would respond vigorously to L–, M– and S–cone modulations. Considering the expectation, our results indicate that rod–cone interactions in cone mediated flicker detection occur predominantly, if not exclusively, in the magnocellular pathway.
Keywords: temporal vision • photoreceptors: visual performance • chromatic mechanisms