Intrinsic signals from human cone photoreceptors

To develop noninvasive means to relate structure to function in human eyes, the authors investigated intrinsic retinal signals at high resolution using an adaptive optics scanning laser ophthalmoscope (AOSLO).

The AOSLO was used in dual-wavelength mode to stimulate the retina with 658 nm visible light and simultaneously to image the retina with 840 nm infrared light. Modulation of each laser beam using acousto-optic modulators allowed the integration of complex, patterned stimuli into the projected raster, whose exact locations on the retina were recorded into the movie in real time. Stimulus luminance was 12,000 cd/m(2). Twenty- to 30-second movies were recorded, with stimulation occurring at 5 seconds. Intensity changes in the infrared image in response to the visible stimulus were monitored over time.

In five subjects, results showed a clear increase in infrared light scattering in the stimulated region with respect to its surroundings, reproduced in four subjects across multiple imaging sessions. Signal increase began immediately at the onset of the stimulus, reached a peak 2 to 3 seconds after stimulus onset, and decreased to baseline within 2 to 10 seconds. The magnitude of the increase over the stimulated area varied from 0% to 5% between subjects.

Results suggested that the signal originated in the cone photoreceptors, though not all cones contributed to the same extent. In individual cones, signal increases over 20% were measured. Excessive eye movements and dim images gave insufficient signal to noise. Eight subjects showed spurious results for these reasons and were eliminated from the study.