"Upon identification of the celebrity, individuals consciously attempted to 'partial out' the perceived influence of their attitude toward that celebrity when asked about their attitudes toward the brand in the advertisement," explain Mark Forehand (University of Washington) and Andrew Perkins (Rice University).
"However, when unable to identify the celebrity, individuals were unaware that their attitudes toward the celebrity were affecting their responses regarding attitude toward the brand in the advertisement."
In their experiment, the researchers found that when a consumer identified the celebrity from the voice, there was no tie between the consumer's feeling for the celebrity and the brand. However, this changed when a consumer could not identify the celebrity. In this instance there was a definite effect on brand attitude.
Forehand, Mark R. and Andrew Perkins. "Implicit Assimilation and Explicit Contrast: A Set/Reset Model of Response to Celebrity Voiceovers." Journal of Consumer Research, Dec 2005.