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D. Cao, B. B. Lee, H. Sun; Physiological Assessment of Rod and Cone Inputs to the Magnocellular Pathway in the Mesopic Luminance Range. Invest. Ophthalmol. Vis. Sci. 2009;50(13):6151.
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© ARVO (1962-2015); The Authors (2016-present)
To determine how rod and cone inputs are combined in the magnocellular (MC) pathway in the mesopic luminance range where both rods and cones are active.
The responses of parafoveal ganglion cells were recorded from macaque retina using an in vivo preparation, as a function of temporal frequency (0.62 - 20 Hz) and at mesopic light levels (2, 20 and 200 Td). Stimuli were of three modulation types: 1) isolated rod stimuli (only rod signals were modulated), 2) isolated cone stimuli (only cone luminance signals, L+M, were modulated), and 3) combined rod and cone stimuli (both rod and cone luminance signals were modulated in phase). The stimuli were generated using a four-primary photostimulator that allowed independent control of rod and cone stimulation.
With an increase in light levels, MC cell responses decreased to the isolated rod stimuli but increased to the isolated cone stimuli. For the isolated rod stimuli, the MC cell response amplitude was highest at 2 Td, with peak sensitivity between 4-6 Hz. For the isolated cone stimuli, the response amplitude increased monotonically with increases in light level; the temporal frequency of the peak sensitivity also increased with light level. MC cell responses to the combined stimuli were dominated by rod input at 2 Td but were dominated by cone input at 20 or 200 Td. Further, MC cell response to the combined stimuli could be predicted as a linear summation of responses to the isolated rod and cone stimuli. The strength of rod input relative to cone input at each light level estimated from MC-cell recordings was comparable to that estimated from a psychophysical experiment using comparable stimuli (Cao et al., Vision Research, 48, 2586-2592, 2008).
The rod and cone inputs to the MC pathway can be isolated. Their balance varies with light level, and they are combined linearly in the mesopic range.
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