The spectral tilt has inspired significant debate [52,53]. Is the tilt pattern (1) a unitary broad-band effect with increases in high-frequency power and decreases in low-frequency power inherently yoked? Or, (2) do decreases in narrow-band low-frequency oscillations often -- but not exclusively -- co-occur with high-frequency increases due to overlapping but independent neural processes? If hypothesis (1) is true, underlying theta rhythms may exist on top of a tilt of the power spectrum, potentially resolving some of the discrepancy between invasive and non-invasive studies of human theta (see Box 4 and Figure 3). If hypothesis (2) is true, theta oscillations are reduced during successful memory formation, requiring us to rethink theta’s role in memory formation and retrieval.
