How do our right and left eyes take in two separate streams of visual information and end up with a single view of the world? This question has come under intense scrutiny from neuroscientists for decades, and Mark Bear brings us up to date in his lecture. Single neurons in the visual cortex respond to particular stimuli (such as direction or color) and then the brain does some fancy filtering to process only the stimuli that match up in both eyes. Bear describes breakthrough experiments where researchers closed the eye of a kitten for just a day or so, and found that it was effectively “blind” after it opened. Correlating visual information to produce binocular images depends on neural connections that are forged during a “critical period” of visual cortex development. Bear’s work with visual system neurotransmitters has turned up intriguing connections to conditions like Fragile X syndrome. This form of mental retardation may result from a similar loss of neural connections during a parallel critical period after birth.