Cross-modal performance: behavioural processes, phylogenetic considerations and neural mechanisms

Issues relating to cross-modal performance (CMP) are examined from various points of view, with major emphasis on phylogenetic comparisons and neurological mechanisms. Although it now seems likely that certain distinctions that were made based on training procedures (i.e., among transfer, matching, and recognition) have no functional significance, research on this topic has demonstrated how the level of performance is affected by certain task variables (such as number of trials in the first modality). It has not yet been shown that these relationships differ from ones that would be seen in comparable within-modal studies. Overall phylogenetic differences specific to CMP cannot be sustained from the data for humans, apes, monkeys, and non-primates. However, two possible differences--one phylogenetic and one ontogenetic--require further study. Metaphorical matching has not been demonstrated in nonhumans, and it may be the case that 'categorical' CMP appears earlier in development than 'specific' CMP. Efforts to establish that CMP is mediated by representations localized in regions of polysensory neural convergence have not provided convincing evidence, so that 'leakage' between perceptual/memory systems previously considered to be modality-specific is proposed as the mechanism for CMP. Based primarily upon findings from a study using 2-DG, the suggestion is made that one pathway for such leakage is through the ventral claustrum. Polysensory areas of cortex may play a special role during the initial formation of a multisensory engram.