Purpose:
Frequency-doubling (FD) stimuli are used in perimetry to preferentially stimulate magnocellular-projecting (M-) retinal ganglion cells relative to parvocellular-projecting (P-) cells. However, no studies of primate retinal ganglion cells have compared responsiveness of M- and P- cells to FD stimuli versus the conventional size III perimetric stimuli. Therefore we measured contrast responses of primate M- and P- cells to FD and size III stimuli.
Methods:
Data were recorded from macaque retinal ganglion cells, 42 M-cells and 28 P-cells, via an in vivo preparation, at eccentricities of 5°-15°. Frequency-doubling stimuli were 0.5 cycle/degree sinusoids flickered in counterphase at 18 Hertz. Size III stimuli were circular luminance increment pulses 200 msec in duration, 26 minutes of arc (minarc) in diameter. Responses to FD stimuli were measured from 22 M-cells and 7 P-cells. Responses to size III stimuli were measured from 39 M-cells and 28 P-cells. Contrasts for both stimuli were expressed as Weber contrast, (peak- mean)/mean, ranging from 6% to 500% contrast for size III and 1.5% to 100% contrast for FD. Stimuli were centered on the ganglion cell’s receptive field, and a Michaelis-Menten function was fit to a cell’s contrast responses to compute contrast gain as impulses per second per change in % contrast.
Results:
For FD stimuli, mean (± 1 SD) for log contrast gain was 0.6 ± 0.3 for M-cells and -0.4 ± 0.2 for P-cells. For size III stimuli, contrast gain averaged 0.4 ± 0.2 for M-cells and -0.9 ± 0.3 for P-cells. For those cells tested with both stimuli, the decline in contrast gain from FD to III was larger for P-cells than for M-cells (t =2.75, p < .02)
Conclusions:
For both frequency-doubling stimuli and size III stimuli, contrast gain for M-cells averaged about one log unit higher than for P-cells. Conventional size III perimetry can be considered an M-cell test, to the same extent as is the case for frequency-doubling perimetry.
Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • ganglion cells • perimetry