Homeostatic control of interocular balance revealed with contrast mismatch

Abstract

Short-term monocular deprivation (STMD), wherein one eye is deprived of input for 1–3 hours, induces a temporary (30–60mins) shift in interocular balance. Surprisingly the deprived eye (DE) gets relatively stronger post-deprivation in comparison to the non-deprived eye. STMD may be viewed as presenting an interocular contrast mismatch where one eye (DE) receives a lower contrast than the other. What if the contrast imbalance is in the opposite direction, a contrast increment relative to baseline? Is the STMD effect symmetrical? Primate V1 was optically imaged for ocular dominance columns (ODCs) starting with a baseline of 40% contrast sine gratings of four orientation to each eye. Then the left (manipulated) eye (ME) was switched to 80% contrast, back to 40%, then to 20%, then back to 40% while the right eye was constantly stimulated with 40% (each period lasted for 1–2 hours). Line profiles across the ODCs show DC offset shifts indicative of the relative strength of each eye’s response. The result is that the response to the ME stimuli at first is commensurate with the contrast change, but later begins to shift to match the fellow eye. Whenever the ME returns to baseline (40%), the response “overshoots” or “undershoots” DC zero implying a relative gain change that opposes the previous contrast imbalance. This behavior indicates a binocularly-regulated interocular balancing mechanism, with a time constant of ~30mins. An additional analysis that examines the peak-peak amplitude of the ODC signal shows a companion result, supporting the notion of a binocular homeostatic mechanism regulating interocular balance. Although this STMD gain shift effect was approximately symmetrical, there was an asymmetry such that the 40–80 increment yielded a larger gain shift than the 40-20 decrement. A variant of the DSKL model for binocular combination replicates key features of these interocular contrast imbalance responses.