Abstract
Purpose::
We are trying to identify ultra-violet (UV) and middle wavelength (M) cone inputs to cells in murine visual cortex in order to find physiological evidence for color vision.
Methods::
The mice are anesthetized with either ketamine and xylazine, or pentobarbital introduced intraperitonealy. We find the former leads to spontaneous rhythmic oscillations, which though interesting, are troublesome for our purposes. Tungsten microelectrodes record cell responses in penetrations from the surface to a cortical depth of 1 mm. The contralateral eye is stimulated diffusely by 370 or 505 nm LEDs. The 370 nm stimulus affects UV-cone opsin strongly, 505 nm affects only M-cone opsin and rhodopsin. Rods are saturated with a strong orange background. If the cell responds strongly to 370 and at all to 505 nm, we assume it receives inputs from both UV- and M-cones. We also combine this with intensity/ response patterns in order to see if a response can be made quasi-identical to the two wavelengths by varying energy, i.e. univariant. If it cannot, we also assume it receives both cone inputs.
Results::
Of the cells encountered, about 80% are driven by this form of stimulation at on, off or on-off. All of 139 cells studied so far appear to receive inputs from both cone types. No cell has been found that is conclusively cone specific or cone opponent. There are differences in the effectiveness of these two wavelengths at different cortical areas. There is a suggestion that the difference in effectiveness reflects the retinotopic distribution of UV- and M-cone opsins, but this point has not been easy to establish. The effectiveness of the two chromatic stimuli is usually similar at different cortical depths in the same penetration. Cells in the upper layers receive a strong, machine-like input of impulses, which have a latency of 35-50 msec; those in the lower layers have a longer latency and a lower frequency of responding.
Conclusions::
So far there is no conclusive evidence that any cortical cell can detect color contrast. All cells sampled appear to receive inputs from both UV- and M-cones in contrast to what we have found in the retina, where some cells receive inputs driven by cones responding only to UV- or only M-stimuli. There are differences in the strengths of the UV- and M-cone inputs at different areas of the cortex, but less so in the same penetration.
Keywords: visual cortex • electrophysiology: non-clinical