The soundscapes that we experience vary widely from a quiet office or library to a busy pub or high street. Auditory neurons in the brain cope with these constantly changing inputs by adapting their responses to match the statistics of the sounds that are heard. Professor Andrew King’s group has previously shown that neurons in the auditory cortex adjust their gain – their sensitivity to changes in sound level – in response to the contrast or range of sound levels that are heard. When the contrast is low, perhaps due to the presence of background noise, neurons become more sensitive to changes in level. On the other hand, they reduce their gain when the contrast is high.
Professor Andrew King observes: "This is important because contrast adaptation helps to match the range of stimulus values that neurons can encode through a change in their firing rate to the stimulus range that is currently being experienced. This is therefore an efficient way of representing the changing auditory scene." The group’s research has also shown that adaptation to contrast helps the brain to construct stable representations of complex sounds, such as speech, despite the presence of noise in the stimulus.
Until now, it has been assumed that contrast gain control is primarily a property of cortical neurons and several research groups around the world are exploring the role of cortical inhibitory interneurons in adaptation to stimulus statistics.
Now, a new study published in Nature Communications by the King Group has shown in mice that neurons in the thalamus and the midbrain show a comparable degree of contrast gain control to that seen in the auditory cortex. Furthermore, by using optogenetic silencing, they found that descending projections from the cortex are not responsible for the adaptation observed subcortically.
The perceptual consequences of adaptation have so far received little attention. The team, which includes former DPhil student Dr Michael Lohse, Associate Professor Victoria Bajo Lorenzana and Dr Ben Willmore, went on to show that the ability of human listeners to distinguish small changes in sound level varies with stimulus contrast in a similar way. In confirmation, they found that contrast adaptation measured physiologically predicts these behavioural results.
According to Professor King, “Our findings show that these adaptive properties, which are emerging as an important feature of sensory processing, arise at an earlier stage in the auditory processing pathway than previously thought and are sufficient to account for contrast-dependent effects on human perception. Demonstrating how neurons adjust to the statistics of sounds will both improve our understanding of the computational principles underlying the representation of sensory information in the brain and provide insight into its capacity to process the abnormal inputs that result from sensory disorders."
The publication, Neural circuits underlying auditory contrast gain control and their perceptual implications, is available to read here.