Visually evoked potentials

4.1.1. Early brain potentials (3 Hz and 1 Hz condition)

A. Effects of stimulus valence on early visual event-related potentials

Early selective processing of emotional adjectives elicited 200 - 280 ms after word presentation was indicated by significant main effects of ‘Valence’ (F(2,30) = 16.0, p<.001). Post hoc tests revealed significantly increased amplitudes for unpleasant and pleasant adjectives compared to neutral adjectives (F(1,15) = 23.5, p<.001 for unpleasant > neutral and F(1,15) = 23.9, p<.001 for pleasant > neutral).

B. Effects of stimulus repetition on early visual event-related potentials

Across ten repetitions (five per stimulation rate) repeated stimuli did not attenuate the amplitude of the early posterior negativity potential significantly (‘Repetition’ F(4,60) = 1.76, p >.15).

There was no evidence for habituation of the emotion effect (‘Repetition x Valence’, F(8,120) = 1.3, pHuynh-Feldt >.25 (Huynh-Feldt epsilon = 0.83)).

Figure 3a: Effects of stimulus repetition on the affective valence pattern of the early posterior negativity. Left panel: 3 Hz condition. Right panel: 1 Hz condition. The x-axis represents the blocks of stimulus repetition (five per stimulation rate).

Across ten stimulation blocks (333 ms and 1000 ms word duration) the amplitude of the early negative potential reliably distinguished repeatedly presented unpleasant and pleasant adjectives from repeatedly presented, neutral adjectives irrespective of whether words were presented for 333 ms or 1000 ms (‘Repetition x Duration x Valence’: F(8,120) = 0.8, pHuynh-Feldt >.54 (Huynh-Feldt epsilon = 1.0)). Effects are displayed in Figure 3a.

C. Effects of stimulus duration on early visual event-related potentials

No significant amplitude differences were found between adjectives presented at SOAs of 333 ms and 1000 ms (‘Duration’: F(1,15) = .23, p>.64).

However, affective processing tended to be more pronounced during the faster (333 ms) word presentations than during the slower (1000 ms) word presentations (‘Duration x Valence’:

F(2,30) = 3.2, pHuynh-Feldt =.059 (Huynh-Feldt epsilon = 1.0)). Particularly, unpleasant adjectives benefited from faster stimulus presentations (3 Hz > 1 Hz: F(1,15) = 5.2, p <.05; for unpleasant

> neutral adjectives and 3 Hz: unpleasant > pleasant F(1,15) = 3.6, p =.07). Effects are displayed in Figure 3b.

Figure 3b: Effects of stimulus duration on the affective valence pattern of the early posterior negativity. Left: 3 Hz condition. Right: 1 Hz condition.

Grand average waveforms of the visual event-related potentials during viewing unpleasant, pleasant, and neutral adjectives are presented in Figure 4 for two representative occipital electrodes. Affective modulation of the averaged brain potential during 333 ms word presentation is presented in Figure 4a and during 1000 ms word presentations in Figure 4b.

Topographic voltage maps of the difference potentials between emotional and neutral word categories (subtracting neutral from unpleasant, and neutral from pleasant adjectives) are also shown in Figure 4a (333 ms word duration) and Figure 4b (1000 ms word duration). Minimum norm plots giving an estimate of the brain electrical sources of the early affective modulation pattern are shown in Figure 4c.

Figure 4a: 3 Hz condition

Top: Topographic maps of the difference potentials of the early posterior negativity potential, subtracting neutral from unpleasant and neutral from pleasant word categories. Bottom:

Grand-averaged ERP waveforms during viewing of unpleasant, pleasant and neutral adjectives.

Effects are shown for left and right occipital electrodes (O9 and O10).

Figure 4b: 1 Hz condition

Top: Topographic maps of the difference potentials of the early posterior negativity potential, subtracting neutral from unpleasant and neutral from pleasant word categories. Bottom:

Grand-averaged ERP waveforms during viewing of unpleasant, pleasant and neutral adjectives.

Effects are shown for left and right occipital electrodes (O9 and O10).

Figure 4c: Figure 4c shows the cortical sources for the difference waves of the early posterior negativity potential, subtracting neutral from unpleasant and neutral from pleasant word categories. Top: 3 Hz condition. Bottom: 1 Hz condition. Minimum Norm Maps display the distribution for the respective maxima in global field power at 252 ms. Minimum Norm Maps were calculated according to the minimum norm algorithm of the BESA software package (MEGIS Software GmbH).

D. Laterality Effects on early visual event-related potentials

Analysis of the topographic distribution of the early posterior negativity did not yield any significant effects for the main factor ‘Hemisphere’ (F(1,15) = 1.16, p>.29). Laterality effects varied as a function of the processing speed with respect to affective modulation (‘Hemisphere x Duration x Valence’: F(2,30) = 4.6, pHuynh-Feldt <.05 (Huynh-Feldt epsilon = 1.0)). Post hoc tests revealed that the interaction effect was due to a stronger left-hemisphere dominant processing advantage for unpleasant compared to neutral adjectives at the shorter SOAs of 333 ms relative to the longer SOAs of 1000 ms word exposure (333 ms > 1000 ms: F(1,15) = 5.9, p <.05, for unpleasant > neutral adjectives). In contrast, no hemisphere dominant difference in affective processing across different processing speeds was obtained for pleasant adjectives compared to neutral adjectives (333 ms > 1000 ms: F(1,15) = .55, p >.47, for pleasant > neutral adjectives).

4.1.2. Late brain potentials (1 Hz condition only)

A. Effects of stimulus valence on the N400 and the LPP

When adjectives were presented for 1000 ms, they elicited a negative N400 brain potential starting at about 360 - 470 ms after word onset as well as a late positive brain potential (LPP) starting at about 470 - 600 ms after word onset. Both potentials had a definite regional focus over centro-parietal leads. Word content had a significant impact on both the N400 and the LPP amplitudes indicating better semantic processing (N400) and deeper encoding (LPP) of pleasant adjectives (N400: ‘Valence’ F(2,30) = 3.2, p=.05; LPP: ‘Valence’ F(2,30) = 5.5, p<.01). Post hoc test revealed smaller N400 amplitudes for pleasant compared to unpleasant adjectives (pleasant > neutral: F(1,15) = 8.3, p <.01) and larger LPP amplitudes for pleasant adjectives as compared to unpleasant and neutral adjectives (pleasant > neutral: F(1,15) = 7.04, p <.01;

pleasant > unpleasant: F(1,15) = 8.4, p <.01). For unpleasant adjectives no valence effect was obtained (unpleasant > neutral: N400 and LPP all p >.6).

B. Effects of stimulus repetition on the N400 and the LPP

Word repetition had a significant impact on the N400 potential (‘Repetition’ F(4,60) = 3.4, p

<.05). Across repeated word presentations the amplitudes of the N400 potential decreased significantly showing that word meaning was semantically integrated more easily at repeated occasion. By contrast, word repetition did not attenuate the amplitudes of the late positivity potential (LPP) significantly (‘Repetition’ F(4,60) = .71, p>. 58). For both brain potentials the interaction between word repetition and word category (unpleasant, pleasant, and neutral) yielded no significant valence effects (N400: ‘Repetition x Valence’ F(8,120) = .85, pHuynh-Feldt

>.56 (Huynh-Feldt epsilon = 1.0); LPP: ‘Repetition x Valence’ F(8,120) = .41, pHuynh-Feldt >.9 (Huynh-Feldt epsilon = 0.98)). Across word repetition valence effects were strongest for pleasant adjectives supporting the view that viewing pleasant words lead to better semantic and deeper encoding in contrast to viewing of any other word category during repeated presentations.

N400 and LPP affective modulation effects are presented in Figure 5. Grand average waveforms as well as topographic difference potential maps of the N400 component effects are shown in Figure 5a and in Figure 5b for the LPP component.

Figure 5: N400 and LPP emotion effects (1 Hz condition only)

Figure 5a: Right panel: Topographic maps of the difference potential of the N400 potential, subtracting unpleasant from pleasant adjectives. Bottom: Grand-averaged ERP waveforms of the N400 potential during viewing of unpleasant, pleasant and neutral adjectives. Effects are shown for a representative centro-parietal electrode (CPz).

Figure 5b: Right panel: Topographic maps of the difference potential of the LPP, subtracting pleasant from neutral adjectives. Bottom: Grand-averaged ERP waveforms of the LPP during viewing of unpleasant, pleasant and neutral adjectives. Effects are shown for a representative centro-parietal electrode (Pz).

4.2. Memory performance