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E Kaplan, E Dinnerstein, Y Chen, T Yokoo, V Brodsky, L Sirovich; The Contrast Dependence of the Intrinsic Optical Signal in the Cat Visual Cortex . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4749.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: In vision, the most important stimulus parameter is contrast. The dependence of neural responses on stimulus contrast is well known for single unit recordings (Geisler et al., Vis. Neurosc., 1997), and has recently been determined for fMRI signals (Heeger et al., Nature Neurosc., 2000). Imaging of the intrinsic optical signal (IOS) has become a widespread and powerful tool in brain research, and it is therefore of interest to know how the IOS amplitude depends on stimulus contrast. Methods: We imaged the exposed visual cortex (area 17, 18) of anesthetized and paralyzed cats while they were viewing drifting gratings of optimal spatial frequency at several orientations and contrasts. We estimated the amplitude of the IOS with a new analysis method (Yokoo et al., NeuroImage, 2002). The resulting functions of response amplitude vs. contrast were fitted with various analytical functions: linear, power law, logarithmic function and others. Results: We found that our results were best fit by the Michaelis-Menten function: R=Rmax×Cn/(Cn+Hn), where Rmax is the maximal response, C is stimulus contrast, H is the contrast that produced half the maximal response, and n is a constant. This function is also the one that best fits data from single units (Albrecht and Hamilton, 1982). Conclusion: This result provides an important quantitative link between the firing of cortical neurons and the IOS, which reflects indirectly the activity of large cell populations.
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