Figure 1.
Trigeminal and olfactory chemosensory ERPs.
Trigeminal and olfactory chemosensory ERPs recorded at the scalp vertex (Cz vs. A1A2) in 11 healthy normosmic volunteers. Gaseous CO2 (50% v/v) was used to selectively activate trigeminal afferents. 2-Phenylethanol (50%v/v) was used to selectively activate olfactory afferents. 60 stimuli were presented, lasting 200 ms (20-ms rise-time), separated by a 30 s inter-stimulus interval. Individual ERP waveforms are shown in light grey, while the group-level average waveform is shown in black. Note that trigeminal chemosensory stimulation elicited a clear negative-positive complex (TRI-N1/TRI-P2), contrasting with the low signal-to-noise ratio of the response elicited by olfactory chemosensory stimulation, which was clearly identifiable in only a few subjects.
Figure 2.
Scalp distribution of the EEG responses elicited by trigeminal and olfactory chemosensory stimulation.
The scalp distribution of the EEG responses identified using across-trial averaging in the time domain (TRI-N1, TRI-P2, OLF-N1, OLF-P2) are expressed in microvolts, whereas the scalp distribution of the EEG responses identified using across-trial averaging in the time-frequency domain (TRI-TF1, TRI-TF2, TRI-TF3, OLF-TF1, OLF-TF2) are expressed as relative percentage increase or decrease relative to baseline.
Figure 3.
Receiver Operating Characteristic (ROC) curves.
ROC curves were computed to estimate the discrimination performance (ability to discriminate between presence vs. absence of stimulation) of each of the different EEG responses identified using across-trial averaging in the time domain (TRI-N1, TRI-P2, OLF-N1, OLF-P2) and across-trial averaging in the time-frequency domain (TRI-TF1, TRI-TF2, TRI-TF3, OLF-TF1, OLF-TF2). The shaded areas represent the 95% confidence interval of the obtained curves.
Table 1.
Discrimination performance of the phase-locked and non phase-locked EEG responses to trigeminal and olfactory chemosensory stimulation.
Figure 4.
Time-frequency representation of the phase-locked EEG responses to trigeminal and olfactory chemosensory stimulation (CWT-AVERAGE).
The time-frequency transform of the waveforms obtained by performing conventional across-trial averaging in the time domain was used to identify EEG responses that were phase-locked across trials, as these are preserved by the averaging procedure. Signal amplitude (group-level average, electrode Cz vs. A1A2) is expressed as percentage increase or decrease relative to baseline (−0.4 to −0.1 s) (ER%). Note that the trigeminal chemosensory ERP is mainly represented by an increase of low-frequency activities (1–5 Hz). Also note the lack of a clear EEG response following olfactory stimulation.
Figure 5.
Time-frequency representation of the non-phase locked EEG responses to trigeminal and olfactory chemosensory stimulation (CWT-SINGLE).
Non-phase locked EEG responses were identified by performing across-trial averaging in the time-frequency domain, a procedure which enhances time-locked EEG responses regardless of whether they are phase-locked to the onset of the stimulus. The upper panels show the group-level average time-frequency maps of oscillation amplitude (group-level average; electrode Cz vs. A1A2), expressed as percentage increase or decrease relative to baseline (−0.4 to −0.1 s) (ER%). Note that, in addition to the phase-locked EEG response (TRI-TF1), trigeminal stimulation also elicits event-related desynchronization in the alpha-band (TRI-TF2), as well as a transient and early increase of signal amplitude centered around 10–15 Hz (TRI-TF3). Also note that olfactory stimulation elicits a long-lasting non phase-locked increase of signal amplitude centered around 5 Hz (OLF-TF1), possibly followed by event-related desynchronization in the alpha-band (OLF-TF2). The lower panels highlight areas of the time-frequency matrix where signal amplitude deviated significantly from baseline (one-sample t-test).
Figure 6.
Correlation between psychophysical olfactory performance and magnitude of the significant EEG responses to chemosensory stimulation.
The left panel (A) shows the scatter diagram illustrating the correlation between TDI scores and the olfactory EEG response OLF-TF1. Note the significant positive correlation between the TDI score and OLF-TF1 magnitude (r = 0.70, p = 0.017). The right panel (B) illustrates the correlation between TDI scores and trigeminal EEG responses (TRI-TF1 (blue), TRI-TF2 (red) and TRI-TF3 (green). Note the absence of significant correlation between the TDI score and these different responses to trigeminal stimulation.
Figure 7.
Clinical usefulness of time-frequency analysis of chemosensory ERPs.
A. Time-frequency representation of the non-phase locked EEG responses to trigeminal and olfactory chemosensory stimulation (CWT-SINGLE; see Methods) in one hyposmic patient (TDI = 23) and one anosmic patient (TDI = 14). Signal amplitude is expressed as percentage increase or decrease relative to baseline (−0.4 to −0.1 s) (trigeminal stimulation: electrode Cz; olfactory stimulation: electrode Fz). B. Magnitude of TRI-TF1 and OLF-TF1 measured in the hyposmic patient (grey dot), the anosmic patient (white dot) and the 11 healthy controls (black dot). The horizontal red line indicates the cutoff amplitude value associated with the greatest Youden index. Note that the magnitude of OLF-TF1 measured in the hyposmic and anosmic patients are both below the cutoff value. In contrast, note that the magnitude of TRI-TF1 measured in the two patients was similar to those measured in the healthy controls.