Despite the common negligence of Müller and Pilzecker's (1900)
original RI research and definition (i.e. diversion RI) in Psychology today, it is highly evident from the above review and research that such factor does play a considerable role in human memory. While such ‘rediscovery’ of RI does not signify the demise of similarity RI, it does call for a revision of the current RI theory.
We propose such a potential revision that integrates both diversion as well as similarity RI in neurologically intact people as well as anterograde amnesiacs. We suggest that both diversion and similarity RI exist (as proposed by Skaggs in 1925
) but that they differ (a) in terms of the cognitive processes they affect (as proposed by Skaggs in 1933
) as well as (b) potentially in terms of the neural substrates they have an effect on (see ).
Figure 15 Long-term memory processes affected by interference. The above diagram is an illustration of the potential effects of diversion RI and similarity RI on the normal memory process. The horizontal thick arrow depicts the cognitive processing of a to-be-retained (more ...)
We propose, on the basis of the old and new research reviewed above, that in neurologically intact people diversion RI affects the consolidation stage of memory. Given the frequent association of consolidation and temporal lobe structures (i.e. the hippocampus) we further suggest that such interference occurs within the temporal lobe.
Similarity RI, on the other hand is likely to affect retrieval processes. Due to its close resemblance to PI and the association between PI and frontal lobe activity, we tentatively propose that similarity RI affects frontal lobe processes, though this warrants further research. Furthermore, while diversion RI can occur independently of similarity RI, the reverse does not seem plausible. Hence, any interpolated material/task, including similar material, causes diversion RI and will therefore affect consolidation. The memory decrement induced by interpolation of similar material is thus likely to be the cause of an additive effect of diversion RI and similarity RI, and hence the cause of interference at the consolidation and retrieval stage. Therefore, while minimising similarity RI should only allow for uninterrupted retrieval, minimising all RI should allow for both uninterrupted consolidation and retrieval. Further similarity RI studies based on Müller and Pilzecker's (1900)
delayed RI paradigm may allow for a teasing apart of the similarity and diversion RI effects caused by similar interpolated material (i.e. due to a reduced susceptibility to diversion RI when interpolation of subsequent material is delayed) and hence an elucidation of the true magnitude of the actual similarity RI effect.
We further propose that the memory deficits in at least some amnesiacs may be the result of a severely heightened susceptibility to the diversion and/or similarity RI (and PI) experienced to a mild extent by all neurologically intact people. Patients with specific damage to the retrieval system (as may be the case in patients with lesions to the frontal lobes) may thus present with a severely heightened susceptibility to similarity RI and PI and ‘normal’ susceptibility to diversion RI. On the other hand, patients with specific damage to the storage processes or to the mechanisms that feed into such processes (as appears to be the case for the patients described by Cowan et al., 2004
and Della Sala et al., 2005
) may present with severely heightened susceptibility to diversion RI.
It appears from Cowan et al.'s (2004)
research that an intact temporal lobe is required for amnesic patients to benefit from minimal RI. With respect to the above discussion on potential cognitive processes underlying the benefit in the amnesic group, such notion would imply that parts of such undamaged temporal lobe could serve as (a) a capacity limited retention buffer (which would still be intact in patients who would be no longer able to consolidate new memories) or (b) a consolidation system (as it is assumed to be in neurologically intact people). If the benefit in the amnesiacs could be explained by the former, patients would not in actual fact have an ‘increased’ susceptibility to diversion RI. Hence they would ‘simply’ present with impaired consolidation and spared capacity limited STM; i.e. the effects of RI would underlie the ‘normal’ limits of a functioning capacity limited STM mechanism.
If the latter could explain the benefit, a main characteristic of this type of amnesia could be a highly raised susceptibility to the diversion RI. The requirement of an intact temporal lobe would thus most likely indicate the requirement of a consolidation system for emergence of a benefit in this patient group. The data by Cowan et al. (2004)
suggests that in such case the actual cause of the susceptibility to diversion RI would most probably underlie the patients' non-temporal lesions. It is for example possible that such non-temporal structures are required for the filtering and structured input of to-be-retained information into the consolidation mechanism within the temporal lobe. Damage to such structures may thus lead to an overload of information into such system leading to no consolidation taking place. In the absence of RI however the damage of such filtering mechanisms would not matter, as only the to-be-retained material would be entered into the temporal lobe system.
The finding of a benefit in MCI patients with some hippocampal atrophy further indicates that less extensive damage to the consolidation system itself may also lead to heightened susceptibility to diversion RI (possibly because such damage renders the system unable to process more than a few stimuli at a time).
Future research will elucidate whether or not the patients who benefit from minimal RI do so due to preserved consolidation or due to the use of a capacity limited STM buffer. It is even plausible that both types of ‘benefit’ exist.