Phase dynamics in the 40-Hz auditory steady state response

The phase of the 40-Hz auditory steady-state response (SSR) evoked by amplitude-modulated tones and recorded by MEG decreases monotonically with increasing carrier frequency, shifting about 6.25 ms or 90° over the range 250–4000 Hz. This phase shift has been attributed to propagation delays along the basilar membrane. Here we report that the slope of the SSR phase/carrier frequency characteristic is not invariant but depends on contextual factors relating to stimulus presentation. When steady-state stimuli were presented continuously with a random change to one of four carriers occurring every second, the phase difference between 500 and 1000 Hz was amplified by a factor of 2.5 compared to when the same frequencies were presented without shifting (baseline). Changing carrier frequencies in a predictable fixed order once per second resulted in a phase separation 1.6 times larger than baseline. We also tested whether a carrier frequency change was required to generate this wider phase separation. We presented continuous 15 min blocks at constant carrier frequencies of 500 or 1000 Hz, interrupted regularly once per second by increasing the amplitude of one modulation cycle. Once about every three seconds the enhancement occurred in only one ear, yielding a distinctive percept (target). The phase difference between 500 and 1000 Hz increased by 1.54 compared to baseline, regardless of whether the subject was attending to the target. Taken together, these results suggest that any disturbance in an on-going steady stimulus is sufficient to change the phase of the SSR for a period of time lasting on the order of several seconds or longer. The mechanisms responsible are unknown although a contribution of cortical dynamics to SSR phase is implied.