Abstract
Purpose :
Functional tests that isolate retinal ganglion cell populations and their retinogeniculate pathways have proven to be useful for monitoring glaucoma. We hypothesized that signatures of the parvo- and magnocellular pathways can be identified with combinations of L- and M-cone isolating stimuli and that a test based on this principle can be used to demonstrate functional loss in glaucoma patients.
Methods :
We measured psychophysical thresholds in the perifovea for combinations of temporal modulation of L- and M-cones in 59 subjects at temporal frequencies of 2 and 20 Hz (fifteen normal subjects, 13 with ocular hypertension, 11 with preperimetric and 20 with perimetric open angle glaucoma). Stimuli were created using the triple-silent-substitution paradigm with a four-primary LED stimulator. Eight measurements were performed per subject and frequency with L:M ratios of 1:1, 0:1, 1:0, 1:2, 2:1, -(1:1), -(1:2), and -(2:1), where negative ratios correspond to stimulation in counterphase. A mathematical model based on vector summation was used to describe these measurements. The parameters of this model included L-cone gain, M-cone gain and phase angle.
Results :
In normal subjects, thresholds can be explained by vector summation of L- and M-cone signals. At 2 Hz, thresholds were highest when L- and M-cones were stimulated in phase, suggesting vector subtraction (median phase angle of 179.9°). Furthermore, contrast gains were similar for L- and M-cones (L-cone- / M-cone gain 1.06). This is characteristic for the parvocellular system. At 20 Hz, thresholds were higher when L- and M-cones were stimulated in counter-phase, resulting in smaller phase angles (median of 127.8°). L-cone gain was higher than M-cone gain (L-cone- / M-cone gain 3.21). This is typical for the magnocellular system. In glaucoma patients, thresholds were generally higher. At 2Hz, phase angles were similar to those in normal subjects (median of 179.9°). At higher temporal frequencies, modeling of the data was more difficult as thresholds were frequently close to instrument gamut.
Conclusions :
Combinations of L- and M-cone-isolating stimuli can be used to create stimuli favoring the parvo- and magnocellular system. We demonstrated the feasibility of measuring temporal contrast sensitivity based on silent substitution in glaucoma patients.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.