ERP Acquisition, Analysis and Data Reduction

The EEG (electroencephalogram) signals were recorded from 64 electrodes (FP1, FPz, FP2, AF3, AF4, F7, F5, F3, F1, Fz, F2, F4, F6, F8, FT7, FC5, FC3, FC1, FCz, FC2, FC4, FC6, FT8, T7, C5, C3, C1, Cz, C2, C4, C6, T8, TP7, CP5, CP3, CP1, CPz, CP2, CP4, CP6, TP8, P7, P5, P3, P1, Pz, P2, P4, P6, P8, PO7, PO5, PO3, POz, PO4, PO6, PO8, CB1, O1, Oz, O2, CB2) which were arranged on the scalp according to the 10-20 system. Cz formed the average reference during ERP recording. To improve electrical conduction, a conductive gel was used (Compumedics Neuromedical Supplies, Quik Gel).

To measure vertical eye movements (VEOG), two electrodes were placed just above and below the left eye (about 1 cm). To measure horizontal eye movements (HEOG), two electrodes were placed on the outer canthi of the right as well as the left eye. Electrical impedance was kept below 10KΩ

.

To analyse the signal, Scan 4.4 acquisition and analysis software (Compumedics Neuroscan Ltd.) were used toamplify (x 1000) and band-pass filter (0.1 – 100Hz) the EEG waves. ‘Bad’ electrodes were identified by visually observing the EEG data. This only applied to one participant, where the electrode F8 had to be excluded as it was not recording correctly.

Offline, the signal was band-pass filtered

(

0.1hz-30hz, 12 db/octave)

and

electrodes were re- referenced to the average of both linked mastoids. A spatial filter paradigm (Scan 4.2.) was applied to remove eye blink artefacts by Principal Component analysis. The EEG signal was baseline corrected (entire sweep) and peaks exceeding ±150μV were automatically excluded in the subsequent analysis. Event-codes identified stimulus-locked responses which were then averaged across all sweeps.

176 Epochs were created from the cleaned data by including 100ms of pre-stimulus interval and 900ms of stimulus processing which was locked to stimulus onset at 0s. For each participant, ERPs were averaged across 8 conditions (i.e. angry/happy face or voice, emotional/neutral face or voice). Averaged ERPs for each individual and each condition were then baseline corrected to pre-stimulus interval of -100ms. For each average ERP per participant and condition, amplitudes and latencies were detected via a peak detection algorithm in Scan 4.3. This was conducted across all electrodes and for three specific time-windows: 70-130ms for early sensory processing, 130-200ms for specific face or voice processing, and 300-600ms for late effects during emotion-specific processing.

For the present analysis of ERPs, separate ANOVAs were conducted for auditory and for facial stimuli due to their perceptual differences in basic perception. Further, separate ANOVAs were conducted for latency and amplitude measures. Although stimuli were presented in four blocks (2 modality x 2 task), for analysis, the data was subsequently split into 8 conditions. For each of the two modalities, there were separate analyses for (1) emotional versus neutral stimuli derived from the explicit emotion-task and secondly for (2) angry versus happy stimuli which were collapsed across gender and emotion-tasks to increase the number of trials. This way, number of trials were kept constant across tasks (N = 60) as is illustrated in Figure 35. Running separate analyses for emotional versus neutral and happy versus angry stimuli allowed observing hierarchical processing stages within the human mind from more general to narrower emotion concepts. The current study did not analyse ERP responses in relation to gender discrimination nor did it specifically investigate the contrast of direct versus indirect emotion ratings because at this point, the present investigation is of preliminary nature only.

Regions of interest (ROI): Regional averaging by clustering several electrodes in a smaller amount of regions reduces the levels of comparisons and degrees of freedom (Dien & Santuzzi, 2005). Visual inspection of topographical maps for all conditions and each time-window revealed ROIs for the subsequent analyses (see Table 5.1.). After initial screening for hemispheric lateralisation, no significant effects were found (p > .05). Therefore, lateralisation was not included as additional variable. Hence, in the present analysis, several electrodes were averaged together in order to observe activity in three main regions of the brain: Frontal (FP1, FP2, AF3, AF4, F3, F1, F2, F4), Central (FC3, FC1, FC2, FC4, C3, C1, C2, C4, CP3, CP1, CP2, CP4) and Posterior (P3, P1, P2, P4, PO3, PO4, O1, O2) ROIs within the 10-20 system. There was one exception: for faces during the second time-window, four individual posterior electrodes P7, P8, PO7 and PO8 - rather than one averaged region - were selected. Those electrodes are commonly used to investigate the face-typical N170 response in posterior regions (e.g. Batty & Taylor, 2003; Smith, 2012). This is illustrated in Figure 5.3.

177 Table 5.1

Identification of regions of interest (ROI) for each modality and time window

Faces Voices

70 – 130 ms

Frontal region: negativity Posterior region: positivity

Frontal region: positivity Posterior region: negativity

130 – 200 ms

P7, P8, PO7, PO8: negativity

Frontal Region: positivity Posterior region: negativity

300 – 600 ms

Posterior region: positivity Frontal Region: positivity Central Region: positivity

Note.Topographic maps show difference waves between emotional and neutral stimuli.

00:00:00+25 ms 25/50 ms 50/75 ms 75/100 ms 100/125 ms 125/150 ms 150/175 ms 175/200 ms 200/225 ms 225/250 ms 250/275 ms 275/300 ms 300/325 ms 325/350 ms 350/375 ms 375/400 ms 400/425 ms 425/450 ms 450/475 ms 475/500 ms 500/525 ms 525/550 ms 550/575 ms 575/600 ms +3.0 +2.6 +2.3 +1.9 +1.5 +1.1 +0.8 +0.4 0 -0.4 -0.8 -1.1 -1.5 -1.9 -2.3 -2.6 -3.0 Subject:

EEG file: GROUP_~1.AVG Recorded : 12:10:57 24-Jul-2014 Rate - 1000 Hz, HPF - 0 Hz, LPF - 200 Hz, Notch - off

Neuroscan SCAN 4.4 Printed : 15:03:23 23-Oct-2014 00:00:00+25 ms 25/50 ms 50/75 ms 75/100 ms 100/125 ms 125/150 ms 150/175 ms 175/200 ms 200/225 ms 225/250 ms 250/275 ms 275/300 ms 300/325 ms 325/350 ms 350/375 ms 375/400 ms 400/425 ms 425/450 ms 450/475 ms 475/500 ms 500/525 ms 525/550 ms 550/575 ms 575/600 ms +3.0 +2.6 +2.3 +1.9 +1.5 +1.1 +0.8 +0.4 0 -0.4 -0.8 -1.1 -1.5 -1.9 -2.3 -2.6 -3.0 Subject:

EEG file: GROUP_~1.AVG Recorded : 12:10:57 24-Jul-2014 Rate - 1000 Hz, HPF - 0 Hz, LPF - 200 Hz, Notch - off

Neuroscan SCAN 4.4 Printed : 15:03:23 23-Oct-2014 00:00:00+25 ms 25/50 ms 50/75 ms 75/100 ms 100/125 ms 125/150 ms 150/175 ms 175/200 ms 200/225 ms 225/250 ms 250/275 ms 275/300 ms 300/325 ms 325/350 ms 350/375 ms 375/400 ms 400/425 ms 425/450 ms 450/475 ms 475/500 ms 500/525 ms 525/550 ms 550/575 ms 575/600 ms +3.0 +2.6 +2.3 +1.9 +1.5 +1.1 +0.8 +0.4 0 -0.4 -0.8 -1.1 -1.5 -1.9 -2.3 -2.6 -3.0 Subject:

EEG file: GROUP_~1.AVG Recorded : 12:10:57 24-Jul-2014 Rate - 1000 Hz, HPF - 0 Hz, LPF - 200 Hz, Notch - off

Neuroscan SCAN 4.4 Printed : 15:03:23 23-Oct-2014 00:00:00+25 ms 25/50 ms 50/75 ms 75/100 ms 100/125 ms 125/150 ms 150/175 ms 175/200 ms 200/225 ms 225/250 ms 250/275 ms 275/300 ms 300/325 ms 325/350 ms 350/375 ms 375/400 ms 400/425 ms 425/450 ms 450/475 ms 475/500 ms 500/525 ms 525/550 ms 550/575 ms 575/600 ms +10.0 +8.8 +7.5 +6.3 +5.0 +3.8 +2.5 +1.3 0 -1.3 -2.5 -3.8 -5.0 -6.3 -7.5 -8.8 -10.0 Subject:

EEG file: GROUP_~1.AVG Recorded : 12:33:58 24-Jul-2014 Rate - 1000 Hz, HPF - 0 Hz, LPF - 200 Hz, Notch - off

Neuroscan SCAN 4.4 Printed : 14:56:27 23-Oct-2014 00:00:00+25 ms 25/50 ms 50/75 ms 75/100 ms 100/125 ms 125/150 ms 150/175 ms 175/200 ms 200/225 ms 225/250 ms 250/275 ms 275/300 ms 300/325 ms 325/350 ms 350/375 ms 375/400 ms 400/425 ms 425/450 ms 450/475 ms 475/500 ms 500/525 ms 525/550 ms 550/575 ms 575/600 ms +10.0 +8.8 +7.5 +6.3 +5.0 +3.8 +2.5 +1.3 0 -1.3 -2.5 -3.8 -5.0 -6.3 -7.5 -8.8 -10.0 Subject:

EEG file: GROUP_~1.AVG Recorded : 12:33:58 24-Jul-2014 Rate - 1000 Hz, HPF - 0 Hz, LPF - 200 Hz, Notch - off

Neuroscan SCAN 4.4 Printed : 14:56:27 23-Oct-2014 00:00:00+25 ms 25/50 ms 50/75 ms 75/100 ms 100/125 ms 125/150 ms 150/175 ms 175/200 ms 200/225 ms 225/250 ms 250/275 ms 275/300 ms 300/325 ms 325/350 ms 350/375 ms 375/400 ms 400/425 ms 425/450 ms 450/475 ms 475/500 ms 500/525 ms 525/550 ms 550/575 ms 575/600 ms +10.0 +8.8 +7.5 +6.3 +5.0 +3.8 +2.5 +1.3 0 -1.3 -2.5 -3.8 -5.0 -6.3 -7.5 -8.8 -10.0 Subject:

EEG file: GROUP_~1.AVG Recorded : 12:33:58 24-Jul-2014 Rate - 1000 Hz, HPF - 0 Hz, LPF - 200 Hz, Notch - off

Neuroscan SCAN 4.4

178

Figure 5.3. Electrodes used to create average for frontal (blue), central (yellow) and posterior (red) regions of interest (ROI). Green electrodes used as individual electrodes for the second time-window in the face analysis.

Posterior ROI Face-specific Electrodes

Frontal ROI

179

5.3. Results

The present section will report findings from the ERP study on emotion-effects across two separate modalities across three individual time-windows. It will firstly summarise behavioural findings before moving on to describe ERP patterns which are split according to task: emotional versus neutral and angry versus happy. Within each subsection, data will be reported for all three time-windows and for both modalities. Mean amplitudes and latencies for each time-window and condition are presented in Table 5.2.

In document Emotion recognition in the human face and voice (Page 175-179)