Cats exhibit the Franssen Effect illusion

The Franssen Effect (FE) is a striking auditory illusion previously demonstrated only in humans. To elicit the FE, subjects are presented with two spatially-separated sounds; one a transient tone with an abrupt onset and immediate ramped offset and the other a sustained tone of the same frequency with a ramped onset which remains on for several hundred ms. The FE illusion occurs when listeners localize the tones at the location of the transient signal, even though that sound has ended and the sustained one is still present. The FE illusion occurs most readily in reverberant environments and with pure tones of ∼1-2.5 kHz in humans, conditions where sound localization is difficult in humans. Here, we demonstrate this illusion in domestic cats using, for the first time, localization procedures. Previous studies in humans employed discrimination procedures, making it difficult to link the FE to sound localization mechanisms. The frequencies for eliciting the FE in cats were higher than in humans, corresponding to frequencies where cats have difficulty localizing pure tones. These findings strengthen the hypothesis that difficulty in accurately localizing sounds is the basis for the FE.