In Lewy body diseases (LBD), complex visual hallucinations (CVH), visuo-perception (VP) and attention (ATTN) deficits are common and distressing symptoms. Despite a number of proposed models, our understanding of CVH is limited as we currently do not have good methods to objectively quantify the phenomenon whilst in state, nor do we know what aspects of VP and ATTN function are important in for their manifestation. This study aimed to explore what aspects of VP and ATTN correlate best with CVH in a LBD population, and how performance on these tasks correlated with performance on two illusion tasks which could be considered as surrogates for CVH. Methodology In this study, seven patients with Lewy body dementias (LBD) and twelve healthy aged-controls completed a series of neuropsychological measures, in addition to a battery of computerised ATTN and VP tasks. We also utilised two novel illusion tasks considered to be potential surrogates for CVH: the pareidolia task and the mirror gazing task. Results: We found significant VP and ATTN impairment in LBD patients, although these deficits did not correlate with typical CVH measures. In the pareidolia task, it was found that LBD patients see more illusory images in noise and this was associated with (a) greater VP impairment and (b) greater CVH frequency and intensity. Surprisingly, we found that a greater percentage of healthy controls than LBD patients reported perceptual changes within the mirror-gazing task. Conclusions: We believe that the pareidolia task may be an effective clinical tool in objectively measuring CVH in LBD, and findings using this task are suggestive implicate impairment in top-down processing in the manifestation of CVH. Furthermore, findings from the mirror-gazing task could be suggestive of impairment in bottom-up processing in LBD hallucinators.