Auditory Stimuli
| Stimulus Waveform | Stimulus Spectrum | |||
| 0%AM 0%FM |  |  | ||
| 100%AM 0%FM |  |  | ||
| 50%AM 0%FM |  |  | ||
| 0%AM 50%FM |  |  | ||
| 0%AM 20%FM |  |  | ||
| 50%AM 20%FM |  |  |  |  |
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The figure consists of 6 stimuli (left column) and their corresponding power spectra. The first stimulus is an unmodulated carrier frequency of 1000 Hz which produces a single peak in the power spectrum. While the frequency specificity of this stimulus is very good, it will not produce an evoked response because the stimulus is always on.
The second stimulus is a 1000 Hz carrier frequency which has been amplitude modulated (AM) at 100% using a modulation frequency (MF) of 80 Hz. Each "energy packet" will evoke a response from the participant. Because the stimuli occur very rapidly, the brain’s response to each stimulus is evoked before the response to the prior stimulus has terminated: rather than being allowed to return to a baseline state, a steady-state response is elicited. Because the envelope of energy determines the evoked response, the steady-state response is sometimes called the envelope following response. AM stimuli contain energy both at the carrier frequency (CF) and at the CF+MF and CF-MF.
In the next row the carrier frequency has been modulated at only 50%. This type of stimulus might be used to test the sensitivity of the auditory system to amplitude modulation. Amplitude modulation of as little as 15% can still evoke a response in many participants. Notice that more energy remains at the carrier frequency, while the sidebands are smaller than in the case of the 100% AM stimulus.
The next stimulus is a 50% FM stimulus. Here you can visually see the carrier frequency sweeping from the low to the higher frequency range. A 50% FM stimulus would not usually be used in a multiple stimulus test, however, for demonstration purposes it clearly shows the frequency transitions. The next stimulus is carrier frequency that has been frequency modulated at 20% (for a 1000 Hz stimulus this roves between 900 Hz (10% below) and 1100 Hz (10% above). Notice that the power spectra contains additional sidebands than are seen in the power spectra of the AM stimuli.
The last stimulus is a carrier frequency that has been modulated in both the amplitude and frequency or "mixed modulation". Because the AM and FM response are generated relatively independently, combining the two types of stimulation causes the evoked response to be considerably larger than the response to either AM or FM alone. The phase of the FM has been adjusted so that the highest frequency in the FM sweep occurs simultaneously with the maximum of the AM envelope.
