The early general view of VPL — that conscious effort such as attention is necessary2,3
— has been challenged by the results of several studies, as described in this article. In addition, a growing body of evidence indicates that consolidation processed implicitly during wakefulness69
and during sleep65,79,114
may be important in VPL, as originally suggested by Sagi and colleagues65
. As we have outlined, it has also become clear that different types of VPL are associated with changes in the visual cortex and in areas responsible for decision making or with changes in the connectivity between the visual cortex and the decision-making areas4,5,111,113
However, the newer results that we have discussed are not necessarily inconsistent with the older views. For example, conscious and implicit processing may not be mutually exclusive. The usual training procedure used to generate VPL may include both conscious processing, such as focused attention to a task-relevant feature, and reinforcement processing that includes implicit components (). Furthermore, as outlined above, whether changes associated with the completion of VPL occur mainly in the visual cortex or beyond may depend on which changes lead to performance improvement in the most efficient manner.
What should future research focus on? It remains unclear how conscious processing involving attention and the more implicit processing of reinforcement signals interact to produce VPL. One model indicates that diffusive reinforcement signals are regulated by attention49
. Another possibility is that attentional and reinforcement signals are processed independently and that the degree to which each influences VPL depends on the situation under which VPL occurs. It is also unclear how consolidation of VPL is similar to or different from consolidation of other forms of learning and memory. As discussed above, certain aspects of VPL are distinct from other types of learning and memory. It will be necessary to systematically compare consolidation of VPL with that of other forms of learning and memory. To date, there has been a tendency for different researchers to use different parameters, such as stimuli or tasks, in their VPL studies. In some cases findings based on a particular set of parameters have been overgeneralized. However, it remains unclear whether different types of VPL have the same general underlying mechanism(s). For example, a condition that requires conscious effort during training may or may not result in a different type of consolidation and/or a change in a different cortical region to a condition that does not require conscious effort. We feel that the time has come to move research in a new direction in which the interactions and relations between different types of processing and the conditions leading to different types of VPL should be systematically investigated.