Gamma waves may participate in the formation of coherent, unified perception, also known as the problem of combination in the binding problem, due to their apparent synchronization of neural firing rates across distinct brain regions. 40-Hz gamma waves were first suggested to participate in visual consciousness in 1988, .e.g, two neurons oscillate synchronously (though they are not directly connected) when a single external object stimulates their respective receptive fields. Subsequent experiments by many others demonstrated this phenomenon in a wide range of visual cognition. In particular, Francis Crick and Christof Koch in 1990 argued that there is a significant relation between the binding problem and the problem of visual consciousness and, as a result, that synchronous 40 Hz oscillations may be causally implicated in visual awareness as well as in visual binding. Later the same authors expressed skepticism over the idea that 40-Hz oscillations are a sufficient condition for visual awareness.