Abstract
Prior studies have shown that high-frequency activity (HFA) is modulated by the phase of low-frequency activity. This phenomenon of phase-amplitude coupling (PAC) is often interpreted as reflecting phase coding of neural representations, although evidence for this link is still lacking in humans. Here, we show that PAC indeed supports phase-dependent stimulus representations for categories. Six patients with medication-resistant epilepsy viewed images of faces, tools, houses, and scenes during simultaneous acquisition of intracranial recordings. Analyzing 167 electrodes, we observed PAC at 43% of electrodes. Further inspection of PAC revealed that category specific HFA modulations occurred at different phases and frequencies of the underlying low-frequency rhythm, permitting decoding of categorical information using the phase at which HFA events occurred. These results provide evidence for categorical phase-coded neural representations and are the first to show that PAC coincides with phase-dependent coding in the human brain.
Highlights
Perceptual representations of the environment are critical to an animal’s survival and are believed to occur through coactivated neuronal groups known as cell assemblies
phase-amplitude coupling (PAC) was visible in the raw trace (Figure 2C), the modulatory signal showed rhythmicity (Figure 2D), and there was a clear peak in the power spectrum of both the raw signal and the modulatory signal (Figure 2E, see Figure 2—figure supplement 1 for more examples)
High frequency activity could occur at particular phases of low frequency oscillations (LFO), as reflected by PAC, but these phases may not vary with stimulus category (Figure 1B, left)
Summary
Perceptual representations of the environment are critical to an animal’s survival and are believed to occur through coactivated neuronal groups known as cell assemblies. Several recent studies have shown that these two signals are positively correlated (Ray et al, 2008; Manning et al, 2009; Whittingstall and Logothetis, 2009; Miller et al, 2014; Rey et al, 2014; Burke et al, 2015) and are each modulated by the phase of low frequency oscillations (LFO) (O’Keefe and Recce, 1993; Bragin et al, 1995; Skaggs et al, 1996; Canolty et al, 2006; Jacobs et al, 2007; Tort et al, 2009; Axmacher et al, 2010; Rutishauser et al, 2010; McGinn and Valiante, 2014) This modulation is detectable as phase-amplitude coupling (PAC) of gamma amplitude to LFO phase (Buzsaki, 2010; Miller et al, 2014; Aru et al, 2015). The PAC observed in humans (Canolty et al, 2006; Axmacher et al, 2010) has been thought to reflect phase-coding, this assumption has yet to be validated because prior studies have not investigated the relation between PAC and decoding from LFO phases
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