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Robert Barlow, Mukhtar Khan, Bart Farell; Time of day and glucose modulate visual sensitivity. Journal of Vision 2003;3(9):356. doi: 10.1167/3.9.356.
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Glucose and oxygen power our visual system. Our metabolic state is not constant: blood glucose levels and utilization decrease at night. We investigated how time of day and metabolic state influence visual sensitivity. We used a 2AFC psychophysical paradigm to measure contrast sensitivity and multifocal ERG (mERG) to measure retinal sensitivity. We tested subjects day and night and in various metabolic states set by a Glucose Clamp Technique (GCT) or Glucose Tolerance test (GTT). Testing subjects over a 48h period, we found up to 4-fold increases in contrast thresholds at night. The threshold increases correlated with mild nocturnal hypoglycemia controlled by endogenous circadian oscillators. Inducing hypoglycemia and hyperglycemia artificially with GCT and GTT respectively, we found up to 6-fold decreases in contrast sensitivity evoked by moderate hypoglycemia (∼50–60mg/dl) and up to 3-fold increases in sensitivity evoked by moderate hyperglycemia (>200mg/dl). Severe hypoglycemia (∼40mg/dl) can transiently block central vision (∼15deg). Preliminary mERG recordings show that moderate hypoglycemia can decrease the amplitude of the “b-wave” generated by central retina (5deg) but not that generated by peripheral retina (>10deg). Moderate hyperglycemia, on the other hand, appears to uniformly up regulate retinal sensitivity. We conclude that metabolic state can modulate visual sensitivity and suggest that central vision may be more sensitive to metabolic stress (hypoglycemia) than peripheral vision. mERG recordings indicate that the metabolic modulation of visual sensitivity begins in the retina.
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