title: Electrical coupling between mammalian cones.
abstract: BACKGROUND
  Cone photoreceptors are noisy because of random fluctuations of photon absorption, signaling molecules, and ion channels. However, each cone's noise is independent of the others, whereas their signals are partially shared. Therefore, electrically coupling the synaptic terminals prior to forward transmission and subsequent nonlinear processing can appreciably reduce noise relative to the signal. This signal-processing strategy has been demonstrated in lower vertebrates with rather coarse vision, but its occurrence in mammals with fine acuity has been doubted (even though gap junctions are present) because coupling would blur the neural image.
  
  
  RESULTS
  In ground squirrel retina, whose triangular cone lattice resembles the human fovea, paired electrical recordings from adjacent cones demonstrated electrical coupling with an average conductance of approximately 320 pS. Blur caused by this degree of coupling had a space constant of approximately 0.5 cone diameters. Psychophysical measurements employing laser interferometry to bypass the eye's optics suggest that human foveal cones experience a similar degree of neural blur and that it is invariant with light intensity. This neural blur is narrower than the eye's optical blur, and we calculate that it should improve the signal-to-noise ratio at the cone terminal by about 77%.
  
  
  CONCLUSIONS
  We conclude that the gap junctions observed between mammalian cones, including those in the human fovea, represent genuine electrical coupling. Because the space constant of the resulting neural blur is less than that of the optical blur, the signal-to-noise ratio can be markedly improved before the nonlinear stages with little compromise to visual acuity.
authors.affiliation: Department of Ophthalmology, Northwestern University, 303 East Chicago Avenue, Chicago, IL 60611, USA.
authors.forename: Steven H
authors.initials: SH
authors.lastname: DeVries
authors.forename: Xiaofeng
authors.initials: X
authors.lastname: Qi
authors.forename: Robert
authors.initials: R
authors.lastname: Smith
authors.forename: Walter
authors.initials: W
authors.lastname: Makous
authors.forename: Peter
authors.initials: P
authors.lastname: Sterling
mesh_terms: D000818:Animals; D002450:Cell Communication / Q000502:physiology*; D004553:Electric Conductivity; D004594:Electrophysiology; D007365:Intercellular Junctions / Q000502:physiology*; D008322:Mammals; D018408:Patch-Clamp Techniques; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology*; D012589:Sciuridae