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Philip B. Kruger, Frances J. Rucker, Lawrence R. Stark; Defocus cues and accommodation. Journal of Vision 2003;3(12):20. doi: 10.1167/3.12.20.
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Previous research shows that the eye can identify the sign of defocus (myopic or hyperopic) for accommodation in humans and for emmetropization in animals. However the optical signals (cues) that specify the sign of defocus remain obscure. Accommodation experiments suggest that luminance contrast is accompanied by an unknown directional signal, perhaps from wavefront aberration and/or the Stiles-Crawford (S-C) effect. In some eyes, defocussed point spread-functions can be skewed differently for myopic and hyperopic defocus, and this might indicate the sign of defocus. In addition, longitudinal chromatic aberration (LCA) provides the sign of defocus. Previous experiments suggest that the ratio of L-cone to M-cone contrast specifies the defocus sign. The present aim was to identify the cone types and pathways that mediate these luminance and chromatic signals. Accommodation was monitored continuously while sine-wave grating targets (2.2 c/d; 0.27 contrast) moved sinusoidally (0.195 Hz) towards and away from the eye (+/−1 D) from a mean position of 2 D. Twelve illumination conditions (Cambridge Research Systems VSG2/5; Sony GDM-F500R display) were used to test specified locations in cone-contrast space. Mean accommodation level and dynamic gain and phase were calculated. Mean accommodation level varied significantly when the L/M-cone contrast ratio was altered in both luminance and chromatic quadrants of cone-contrast space. High M-cone-contrast with low L-cone-contrast increased the mean response to the near target, while high L-cone-contrast with low M-cone-contrast reduced the mean response. Dynamic gain was higher for L-cones alone than for M-cones alone, but gain was highest for a combination of L- and M-cone-contrasts. We conclude that L- and M-cones contribute to luminance (L+M) and chromatic (L−M) signals that drive accommodation via magnocellular and parvocellular pathways respectively. Although the chromatic signals are fairly well understood, the directional signal that accompanies luminance contrast is unknown.
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