EEG frequency tagging reveals higher order intermodulation components as neural markers of learned holistic shape representations

Abstract

Shape perception is intrinsically holistic: combinations of features give rise to configurations with emergent properties that are different from the sum of the parts. The current study investigated neural markers of holistic shape representations learned by means of categorization training. We used the EEG frequency tagging technique, where two parts of a shape stimulus were 'tagged' by modifying their contrast at different temporal frequencies. Signals from both parts are integrated and, as a result, emergent frequency components (so-called, intermodulation responses, IMs), caused by nonlinear interaction of two frequency signals, are observed in the EEG spectrum. First, participants were trained in 4 sessions to discriminate highly similar, unfamiliar shapes into two categories, defined based on the combination of features. After training, EEG was recorded while frequency-tagged shapes from either the trained or the untrained shape family were presented. For all IMs combined, no learning effects were detected, but post hoc analyses of higher-order IMs revealed stronger occipital and occipito-temporal IMs for both trained and untrained exemplars of the trained shape family as compared to the untrained shape family. In line with recent findings, we suggest that the higher-order IMs may reflect high-level visual computations, like holistic shape categorization, resulting from a cascade of non-linear operations. Higher order frequency responses are relatively low in power, hence results should be interpreted cautiously and future research is needed to confirm these effects. In general, these findings are, to our knowledge, the first to show IMs as a neural correlate of perceptual learning.

Holistic Integration in the Processing of Chinese Characters as Revealed by Electroencephalography Frequency Tagging

“…As the fifth harmonic of f 2 overlapped with the sixth harmonic of f 1, both at 36 Hz, we included only four harmonics for further analysis. For intermodulation responses, we analyzed various frequencies ( nf 1 ± mf 2, where n and m are integers) following previous studies (e.g., Vergeer et al., 2018). At f 2 −f 1 = 1.2 Hz, there was no clear response in the spectrum (Figure 2).…”

“…Such intermodulation responses cannot be explained solely by the inputs at f1 and f2 but can only be a result of nonlinear interactions between the input frequencies (Regan & Regan, 1988; see Gordon et al, 2019 for a recent review). By employing the frequency-tagged SSVEP technique, previous studies showed that the intermodulation response was observed when parts were bound into holistic representations such as faces (Boremanse et al, 2013(Boremanse et al, , 2014, Gestalt formation (Aissani et al, 2011;Alp et al, 2016;Gundlach & Mu¨ller, 2013), and newly learned shapes (Vergeer et al, 2018). For example, Boremanse et al (2013) frequency-tagged the left and the right half of face stimuli at different frequencies.…”

Interaction between steady-state visually evoked potentials at nearby flicker frequencies

“…3and 4for TF conditions f TARGET ≠ f MASK (three f MASK frequencies each above and below f TARGET ), using values of τ 0 , τ 1 and n estimated from the fits to data from Experiment 1 and using L amp (f TARGET ) estimated from the mask-only grating condition from Experiment 2. Values of f MASK were in [10,12,14,18,20,22] for f TARGET = 16 Hz, and in [5,6,7,9,10,11] for f TARGET = 8 Hz. For each electrode and response condition, we estimated tuned suppression parameters α and β using ordinary least-squares fits, allowing α and β to vary over the entire range.…”

“…Many studies have used IM components to study stimulus interactions 10,11,15,16,38 . When we analyzed the IM components at 2|F TARGET − F MASK | using a similar approach as above, we did not observe more suppression at low masking frequencies, but instead observed inconsistent suppression for mask frequencies very close to the target ( Supplementary Fig.…”

“…When two or more flickering stimuli are simultaneously presented, in addition to the SSVEP and their harmonics, intermodulation (IM) components are also produced due to nonlinear interactions between the neural substrates producing the SSVEP 4 . The SSVEP and IM responses are known to be sensitive to top-down cognitive modulatory effects, and numerous studies have used such paradigms to investigate a wide variety of cognitive and perceptual phenomena [5][6][7][8][9][10][11][12] .…”