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Yoko Hirohara, Toshifumi Mihashi, Hiroyuki Kanda, Takeshi Morimoto, Tomomitsu Miyoshi, Takashi Fujikado; Functional imaging of retina in response to defocused or low-contrast grating stimuli in cats. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2104.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the effect of defocus on the retina, we recorded the retinal intrinsic signals in response to defocused gratings of different spatial frequencies (SFs) and compared them to those recorded in response to low-contrast focused gratings.
The intrinsic signals of the retina of the left eyes of six cats under general anesthesia were recorded with a fundus camera (TRC-50LX, Topcon). The retinal images were photographed (observation light 730-780 nm) at 40 Hz for 2 seconds before, 4 seconds during, and 20 seconds after white light stimulation. Vertical gratings of seven SFs from 0.05 to 3.46 cycles/deg (cpd) were flickered at 4 Hz. In the defocused grating conditions, the degree of defocus was either 6.0 D or 12.3 D in the near direction or 12.0 D and 23.0 D in the far direction. In the low contrast grating conditions, grating stimuli of six different Michelson contrasts between 0.2 to 0.9 for 0.22, 0.43 and 0.86 cpd were studied. The images recorded after stimulation were subtracted from those before stimulation, and were analyzed to obtain the peak value (PV) by two dimensional fast Fourier transform (FFT) (Hirohara Exp eye res 2013). For the defocused grating conditions, the stimulus contrast values (SCVs) were experimentally obtained from the grabbed defocused images. The PVs were compared with the corresponding SCVs under both conditions.
The PV was highest at 0 D (in focus) compared with other defocused conditions for SF equal or lower than 0.86 cpd (P<0.001, one way ANOVA). In the low contrast grating conditions, the PVs increased almost linearly as the SCVs increased (R2 >0.9). The PVs were larger for the lower SFs than for higher SFs. In the defocused grating conditions, the PVs did not increase with the SCVs in the range up to 0.85. With the SCV equal or larger than 0.85, the PVs increased steeply and did not depend on the SFs of the stimuli.
The response pattern of retinal intrinsic signals to the luminance contrast was different between low contrast grating and defocused grating conditions. These results suggest that the retinal mechanism to detect the defocused objects may be different from that to detect low-contrast objects.
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