Lateral suppression of rhythmic evoked responses in the dolphin's auditory system.

In the auditory system of bottlenose dolphins (Tursiops truncatus), a brain-evoked response to rhythmic sound amplitude modulations (the envelope-following response) was markedly suppressed by addition of another sound with a frequency 5-20 kHz higher and an intensity down to 40 dB lower than that of the amplitude-modulated signal. This effect was called paradoxical lateral suppression. This phenomenon was primarily observed when the amplitude-modulated stimulus had a carrier frequency above 30 kHz and modulation rates above 500 Hz. Only the sustained rhythmic response was suppressed, while the transient on-response was not. This indicates that the suppression influenced the ability of evoked potentials to follow rapid amplitude modulations. This prevents weak sounds from being masked by stronger ones.