Synesthetes show normal sound-induced flash fission and fusion illusions

•Synesthetes and controls are susceptible to the sound-induced flash illusion.•Synesthetes and controls are susceptible to the sound-induced fission illusion.•Synesthetes susceptibility to the illusions declines with age.

Idiopathic synesthesia, a neurological condition in which a stimulus in one sense generates a concurrent experience in a different sense, is often considered an example of multisensory integration. Consequently it has been suggested that synesthetes should experience multisensory illusions more consistently and compellingly than typical participants. To test this we measured the sound induced flash fission and fusion illusions in 22 coloured hearing synesthetes and 31 control participants. Analysis of the data using signal detection analysis, however, indicated no difference between the groups, either in perception or response bias, but a secondary analysis of the data did show evidence of a decline in the illusions for synesthetes with increasing age.