Transient brain responses predict the temporal dynamics of sound detection in humans

The neural events leading up to the conscious experience of stimulus events have remained elusive. Here we describe stimulation conditions under which activation in human auditory cortex can be used to predict the temporal dynamics of behavioral sound detection. Subjects were presented with auditory stimuli whose energy smoothly increased from a silent to a clearly audible level over either 1, 1.5, or 2 s. Magnetoencephalographic (MEG) recordings were carried out in the passive and active recording conditions. In the active condition, the subjects were instructed to attend to the auditory stimuli and to press a response key when these became audible. In both conditions, the stimuli elicited a prominent transient response whose emergence is unexplainable by changes in stimulus intensity alone. This transient response was larger in amplitude over the right hemisphere and in the active condition. Importantly, behavioral sound detection followed this brain activation with a constant delay of 180 ms, and further the latency variations of the brain response were directly carried over to behavioral reaction times. Thus, noninvasively measured transient events in the human auditory cortex can be used to predict accurately the temporal course of sound detection and may therefore turn out to be useful in clinical settings.