Inject 2-deoxyglucose (2-DG) to an animal. Then, expose the animal to a ton of a particular frequency. 2-DG is taken up into the neurons, predominantly the most active neurons. Postmortem processing of the 2-DG distripution shows which area of the inferior colliculus was activated in responce to the specific tone. The most active areas will hve the most 2-DG.