You may have heard the statistic that, on average, the brain comprises 2% of the human body mass, but uses up 20% of the total oxygen supply, and 25% of the glucose production. This makes the brain one of the most energetically demanding organs for its size. In particular, the retina is incredibly energetically demanding. The retina is a layer of neurons in back of each eyeball, and serves as the step where light is converted into neural signals. The rods and cones of the retina are photoreceptors responding to light, which then activate or deactivate the subsequent layers of cells.