2 Figure 6.144 Distribution of the JOL effects without memory confounds

6.5. Discussion

The second of the experiments reported in this thesis examined the relationship between JOLs and memory encoding in a similar way to Experiment 1, however rather than sorting the trials included in the memory encoding contrast based on subsequent cued recall, the trials were sorted based on subsequent recognition.

Altering the test requirements did not produce noticeably different ERP correlates of subsequent memory, but the experimental manipulation did, however ensure an increase in the trials that contributed to the SM contrast and the ERPs are considerably cleaner in comparison to those from Experiment 1. In addition, the JOL effect could be analysed within trials that only included subsequent hits allowing the examination of JOL effects without memory confounds. These analyses revealed a similar pattern of effect to the original effect that included both

subsequently missed and remembered items suggesting that the observed JOL effects are genuine. The ERP data in Experiment 2 were analysed using the same two time windows as the data from Experiment 1: early (550-1000 ms) and late (1300-1900 ms). Findings for each time window are discussed in turn.

6.5.1. Early Time Window (550-1000 ms)

Similar to the findings from Experiment 1, the SM effect in Experiment 2 is characterised by an increase in positivity for subsequently recognised relative to subsequently missed items. In experiment 2, however, the effect did not have as clear a posterior focus, but rather seemed to exhibit two peaks – one at midline frontal electrode sites and one at midline parietal electrode sites. Although it is possible that this pattern reflects the existence of two separate effects, statistical analyses did not, however, verify the presence of two peaks. Backsorting study trials based on subsequent recognition rather than cued recall therefore seems not to have produced qualitatively different SM effects across Experiment 1 and 2.

The early JOL effect found in Experiment 2 also strongly resembles the pattern of activity observed in Experiment 1; items assigned high JOLs showed a clear increase in positivity relative to items assigned low JOLs. Unlike the SM effect, the maximum amplitude was recorded at posterior electrode sites, but statistical analyses did not establish a reliable interaction between condition and location (as in Experiment 1).

In the discussion section of Chapter 5 three possible interpretations of the early effects were outlined: i) the effects reflect a ‘pure’ measure of successful memory encoding, ii) the effects reflect a ‘pure’ measure of JOL, and finally iii) the effect is a product of a interaction between JOL and encoding (i.e. JOL-specific encoding strategies). Given the similar pattern of results, Experiment 2 does not provide much additional evidence in favour of any one of the theories. The present data do, however, provide a more fine-grained analysis, demonstrating that the ERP for Medium JOL responses lies between the High and Low JOL ERPs, revealing a clear correlation between the JOL rating and the magnitude of the early JOL effect.

In both Experiments 1 and 2 there are clear increases in memory performance as JOL ratings get higher. For that reason it remains impossible to determine whether the modulation of the JOL effect is a direct consequence of the JOL ratings or simply reflect an increase in the proportion of recognised trials. Again, however, if the JOL effect is primarily driven by successful encoding operations, it is reasonable to expect it to be smaller in comparison to the SM effect. As in Experiment 1, however, a visual inspection of the SM and JOL effects from Experiment 2 gives the impression that the JOL effect is larger in magnitude than the SM effect. Although visual inspection does not provide strong evidence that the early effect is at last partly related to JOL processes, the consistency of this observation across two experiments makes it hard to argue that successful encoding processes exclusively give rise to the early positivity.

One way to further investigate the functional significance of the early effect is to replicate the present experiment without JOL instructions; if successful encoding in the absence of the explicit requirement to make JOLs produces effects that are comparable to those observed in Experiments 1 and 2, it would imply that JOLs themselves are not necessarily causing the early effects seen in Experiments 1 and 2. On the other hand, if the SM effect turns out to be qualitatively different, this would imply that JOLs were at least partly responsible for the effects.

6.5.2. Late Time Window (1300-1900 ms)

Experiment 2 replicated the findings of Experiment 1; there were no significant SM effects in the late time window, but there was clear evidence of a negative-going JOL effect with a centro-parietal maximum. Unlike the late JOL effect in Experiment 1, however, this effect was not left-sided, but focussed instead on midline electrodes. Follow-up analyses of the data suggest that this difference in topography is the result of counterbalancing the rating scale. Since the distribution of the late JOL effect seems to be dependent on the choice of response hand, this raised the concern that the effect reflects response preparation (i.e. CNV) rather than JOL-related processes per se. When ERPs elicited by Medium JOL items plotted against Low and High JOL items, it was clear that High JOL and Medium JOL ERPs overlapped and differed significantly in amplitude from Low JOL items. By contrast, the largest difference in reaction time was between Low and Medium JOL items compared to High JOL items. It is therefore very unlikely that the late JOL effect is caused solely by response preparation processes.

The fact that the waveforms associated with Medium JOLs overlap with High JOLs in the late time windows is itself an interesting observation. The early JOL effect showed a graded increase; the higher the JOL rating the larger the amplitude of the effect. Why then does the late JOL effect not display a similar pattern? One possibility is that the early effect reflects processes involved in determining JOL responses, whereas the late JOL effect reflects processes that work on the product of the JOL decision. These components of processing correspond to what Nelson &

Narens (1990) refer to as monitoring and control in their theoretical framework for metacognition (see Chapter 1). Although this interpretation does not provide a simple answer to why low JOL items should be processed differently from High JOL and Medium JOL items, one speculation is that when memorability is judged as low (as opposed to high or ‘uncertain’) participants engage in specific control strategies to compensate for poor learning. Although examinations of Medium JOL activity do not provide comprehensive insights into the functional significance of the early and late JOL effects, their outcomes are consistent with the view that these effects are functionally distinct (because only the early effect is clearly graded).

This claim could not have been supported with the same degree of confidence based on the data from Experiment 1 alone.

6.6. Summary and Conclusion

Experiment 2 further investigated the correspondence between the neural correlates of successful memory encoding and JOLs by altering task instructions at test. As in Experiment 1, a positive-going effect shared by SM and JOL was evident in an early time window. This effect seemed to be modulated by the JOL ratings; higher

JOL resulted in an increase in positivity. The primary purpose of altering test instructions at test was to provide an alternative basis upon which encoding trials could be sorted. Successful encoding defined as successful recognition did not, however, produce effects that differed noticeably from successful encoding defined as successful cued recall. In a later time window there was only evidence for a negative-going JOL effect distinguishing High and Medium JOL items from Low JOL items. Overall, the findings from Experiment 2 replicate and extend the findings from Experiment 1, which suggested that there are associations as well as dissociations between the neural systems that mediate successful memory encoding and JOLs.

Chapter 7.