Abstract
Purpose: :
The primate midget pathway has long been thought to serve a dual role in both 'red-green' color processing and peak achromatic spatial acuity. A difficulty with this 'double duty' hypothesis is that midget cells reportedly show poor achromatic contrast sensitivity (eg., Kaplan and Shapley, PNAS, 83:2755, 1986) and primarily signal color (eg., Martin et al., Nature, 410:933, 2001). In our recent study (Crook et al., in press) we found midget ganglion cells responsive to both fine achromatic stimuli restricted to the center and coarse chromatic stimuli that engaged center-surround interaction without recourse to a specialized color-coding circuitry. Our purpose therefore was to determine how both chromatic and achromatic responses arise from the strength of L and M cone inputs to the receptive field and to reexamine the achromatic contrast sensitivity of the midget pathway.
Methods: :
Spikes were recorded in vitro from midget ganglion cells from central retina to near peripheral retina driven by input from 1-5 L and/or M cones to the receptive field center. L and M cone selective drifting gratings were used to measure spatial and contrast sensitivity. Data were fit with a difference-of-Gaussian model and cone input weights to the center vs surround were derived. Contrast gain as a function of chromatic (L vs M), achromatic (L+M), and L or M cone contrast was determined.
Results: :
Midget cells showed either ‘chromatic-achromatic’ or ‘pure achromatic’ responses. For both groups L and M cones contributed with about equal weight to the surround (n=110). However, in chromatic-achromatic cells L vs M cone opponency arose via center-surround antagonism where either L or M cone input to the center was weak or absent (n=70). The weaker cone input to the center was cancelled by the surround giving rise to a distinctive dip or ‘notch’ in the spatial frequency response associated with a 180 deg reversal in response phase. By contrast, in pure achromatic cells both center and surround received combined L+M inputs of similar strength (n=40). Both cell groups showed band-pass tuning and high contrast sensitivity in response to L+M achromatic stimuli around the peak of the spatial frequency response (contrast gain mean±std=3±2, n=37).
Conclusions: :
In support of the ‘double-duty’ hypothesis, midget cells show high achromatic contrast sensitivity. The red-green chromatic signal is dependent on the locally variable weighting of L vs M cone inputs to the small receptive field center and on cancellation by the surround of the weaker cone input to the center.
Keywords: ganglion cells • chromatic mechanisms • electrophysiology: non-clinical