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The brain uses neural ‘codes’ to represent the sensory world, movements, and internal variables. These codes have been critical for developing strategies for driving prosthetic limbs and understanding perception. A common view is that a given form of a code is fixed, and attention or learning optimize the responses within that coding framework. In our lab we have data that indicates that the form of a code can change with learning and attention and the nature of these changes depends on the nature of the task. Specifically, we contrasts two codes for the temporal modulation of sound. One is that aggregate increases of activity in a brain area provide evidence of modulation. The other code requires subtracting the responses from opponent populations of cells that represent modulation differently. We find that which code is present in a brain area changes depending on the task and its demands on attention . An possible explanation of this is that auditory cortex might be a site where task and cognitive parameters in addition to sensory parameters are processed, and that the degree to which a specific code is used in an area depends on the balance of sensory and cognitive demands.