Optical imaging of the intrinsic signal as a measure of cortical plasticity in the mouse

Abstract

The responses of cells in the visual cortex to stimulation of the two eyes changes dramatically following a period of monocular visual deprivation (MD) during a critical period in early life. This phenomenon, referred to as ocular dominance (OD) plasticity, is a widespread model for understanding cortical plasticity. In this study, we designed stimulus patterns and quantification methods to analyze OD in the mouse visual cortex using optical imaging of intrinsic signals. Using periodically drifting bars restricted to the binocular portion of the visual field, we obtained cortical maps for both contralateral (C) and ipsilateral (I) eyes and computed OD maps as (C − I)/(C + I). We defined the OD index (ODI) for individual animals as the mean of the OD map. The ODI obtained from an imaging session of less than 30 min gives reliable measures of OD for both normal and monocularly deprived mice under Nembutal anesthesia. Surprisingly, urethane anesthesia, which yields excellent topographic maps, did not produce consistent OD findings. Normal Nembutal-anesthetized mice have positive ODI (0.22 ± 0.01), confirming a contralateral bias in the binocular zone. For mice monocularly deprived during the critical period, the ODI of the cortex contralateral to the deprived eye shifted negatively towards the nondeprived, ipsilateral eye (ODI after 2-day MD: 0.12 ± 0.02, 4-day: 0.03 ± 0.03, and 6- to 7-day MD: −0.01 ± 0.04). The ODI shift induced by 4-day MD appeared to be near maximal, consistent with previous findings using single-unit recordings. We have thus established optical imaging of intrinsic signals as a fast and reliable screening method to study OD plasticity in the mouse.