The lateral geniculate nucleus (LGN) is organized to set the stage for the cortex in many ways, including retinotopic organization, laminar segregation, and the relative representation of parallel afferent pathways. Another important function that occurs at the LGN is the control of the flow of visual information to the cortex. The recognition that the gamma-aminobutyric acid–releasing (GABAergic) inhibitory pathways can control the transfer ratio at the LGN provides a relatively simple explanation of a number of diverse changes that occur in the LGN and other thalamic relay structures. The control of the transfer ratio not only explains the way changes in signal detectability and contrast sensitivity may occur but also helps to explain the creation of a strengthened inhibitory surround at the LGN. Because the GABAergic circuitry at the LGN may itself be globally or focally controlled, it can serve as an early stage in the neural mechanisms that underlie attention. © 1992, Elsevier B.V. All rights reserved.