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Elizabeth N. Johnson, Thomas R. Tucker, David Fitzpatrick; Mapping cone specific activity in primary visual cortex. Journal of Vision 2005;5(8):1018. doi: 10.1167/5.8.1018.
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© ARVO (1962-2015); The Authors (2016-present)
We are investigating how cone inputs are mapped across the cortical surface using intrinsic signal optical imaging in the dichromatic tree shrew. We determined an empirical short wavelength (S-) cone isolating stimulus using color-exchange during extracellular recordings of multiunit sites. In these experiments, the blue CRT gun contrast was held constant at 0.8 (80%), while the green gun contrast varied from 0 to −0.8, with the green and blue modulation 180° out of phase. This produced heterochromatic blue/green gratings with a spatial frequency of 0.4 cycles/deg drifting at 8 Hz. A prediction of color-exchange is that a particular value of blue:green modulation will produce no net change (a null) in response from the dominant middle-long wavelength (ML-) cones. At this modulation ratio, only S-cones produce a differential response, so this stimulus isolates the contribution from the S-cones. The color-exchange experiments yielded a mean response minimum at a green gain of −0.19±0.03 (s.d.), with most sites giving a response null at or below the spontaneous firing rate. An ML-cone-isolating stimulus of approximately the same cone contrast as the S-cone-isolating stimulus was calculated from the known tree shrew S-cone spectral sensitivity. We then performed optical imaging experiments while presenting full-screen drifting sinusoidal S- and ML-cone isolating gratings of varying orientation and spatial frequency. The spatial tuning for S-cone activation peaks at lower spatial frequency with more high frequency attenuation than the ML-cone response. In addition, S-cone isolating stimuli produce activity that is roughly half the magnitude of ML-cone isolating stimuli of equivalent cone contrast. The map of orientation tuning is comparable in orientation preference, but has broader tuning with S-cone isolating stimuli. These results suggest that although the S-cone mediated signals are lower in spatial frequency resolution, they are able to drive orientation specific responses in V1.
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