Additional Analyses of the Late JOL Effect

For the late time window, the initial ANOVA revealed a significant main effect of condition [F(1.8,41.2) = 6.8, p < 0.01], along with interactions between condition and site [F(1.9,43.5) = 5.5, p < 0.01]. Two additional ANOVAs were then carried out to investigate in more detail how the three conditions differ from each other; High JOL versus Medium JOL and Medium JOL versus Low JOL (waveforms are provided in Figure 6.10 and scalp maps in Figure 6.11). High JOL versus Low JOL were reported above in the original analyses.

The first of the comparisons (High JOL versus Medium JOL) revealed no significant main effect or interactions (all Fs < 1.5). The second comparison (Medium JOL versus Low JOL) revealed only a significant main effect of condition [F(1,23) = 6.7, p < 0.05]. As reported previously, the original comparison between High JOL and Low JOL revealed a significant main effect of condition and significant interactions between condition and location and between condition and site. Altogether, the additional analyses suggest that i) there are no differences between ERPs elicited by Medium and High JOLs, and ii) the ERP elicited by Low JOLs is significantly more positive-going compared to both Medium and High JOLs. The statistical outcomes are consistent with the impression provided in Figures 6.10 and 6.11.

Medium JOL/ Low JOL High JOL/ Low JOL High JOL/ Medium JOL 2 0 µV -2 2 0 µV -2

Figure 6.11 Distributions of late JOL effects.

The topographic map illustrates the scalp distributions of the High JOL versus Medium JOL, Medium JOL versus Low JOL and High JOL versus Low JOL effects during the late time window.

The late negative-going JOL effect observed in Experiment 1 showed a clear left- hemispheric distribution, however the effect in Experiment 2 showed no such hemispheric differences. Except from this disparity, the effects were remarkably similar with comparable morphologies and time courses. One inconsistency across the experimental procedures might possibly explain this difference; while the rating scale used in Experiment 2 was counterbalanced, the scale used in the preceding experiment was not (see Chapter 4). To investigate if the late JOL effect is sensitive to the choice of response hand, the late JOL effect from Experiment 2 was plotted separately for the participants who used a standard scale (as in Experiment 1) and for the participants who used a reversed version of the scale (see Figure 6.12).

Experiment 1

Experiment 2

Figure 6.12 The late JOL effect for standard and reversed scales.

The illustration shows the effect from Experiment 1 (upper left) and Experiment 2 (upper right). The topographical maps below show the effect from Experiment 2 separately for the group of participants (N = 12) who used standard scale (left) and the group of participants (N = 12) who used a reversed scale (left).

Subtraction data (High JOL minus Low JOL) from the two groups were analysed using ANOVA with a between-participant factor of group (standard scale versus reversed scale) and within-participant factors of location (frontal, fronto-central, central, centro-parietal, parietal), hemisphere (left, right) and electrode site (superior, mid, inferior). The ANOVA revealed significant interactions between group and hemisphere [F(1,22.0) = 18.1, p < 0.001] confirming the impression in Figure 6.12 that the two groups produce significantly different late JOL effect; whereas the group using the standard scale produce effects that are slightly skewed to the left (and maximal towards the midline), the group using a reversed scale

produce effects that are slightly skewed to the right (and maximal at more lateral electrodes).

To confirm that the differences in distribution are real, scalp distribution analyses were carried out after rescaling the data, revealing significant interactions between group and hemisphere [F(1,22.0) = 18.1, p < 0.001] and between group, location and hemisphere [F(1.6,36.3) = 5.8, p < 0.01]. These analyses confirm that the distribution of the late JOL effect is dependent on the choice of response hand for making the JOL ratings; when a standard scale is used, the effect is most prominent over the right hemisphere, whereas when a reversed scale is used, the effect is most prominent over the left hemisphere.

The fact that the late JOL effect is sensitive to choice of response hand could be indicating that the effect is reflecting differential activity associated with the motoric preparation of making JOL ratings. Motoric activity is associated with one of the first observed ERP deflections, the Contingent Negative Variation (CNV). This effect was first demonstrated by Walter, Cooper, Aldridge, McCallum & Winter (1964; see Luck, 2005), who presented participants with a warning signal followed by a target stimulus and instructed them to press a button when they detected the target. In the time period between the presentation of the warning signal and the target, Walter et al. (1964) observed a negative voltage at frontal recording sites that appeared to reflect participants preparing to respond to the upcoming stimulus. The time course of the negative-going late JOL effect observed in Experiments 1 and 2 could be interpreted as reflecting a CNV potential, however

this interpretation is dependent on the pattern of reaction times at study matching the pattern of the effect; the largest differences in the CNV potential should be present between conditions which show the greatest differences in reaction times. Looking at the reaction times across Low, Medium and High JOL (Figure 6.13), however, it is clear that the difference is largest between Medium JOL and High JOL. An ANOVA comparing the reaction times for all three conditions revealed a significant main effect of JOL [F(2,46) = 4.2, p < 0.05]. Post hoc comparisons revealed a marginally significant difference between High and Medium JOL (p = 0.05). By contrast, looking at the ERPs for the same three conditions (see Figure 6.10 above), the biggest difference in this case is between Low JOL and the two remaining conditions. This observation makes a CNV interpretation of the late JOL effect very unlikely.

0 200 400 600 800 1000 1200

Low JOL JOL3 High JOL

R T ( m s )

Figure 6.13 Reaction times across JOL.

Mean (and S.E.) reaction times for making Low, Medium and High JOLs at study.