We read the findings by Cecato et al. (Reference Cecato, Martinelli, Izbicki, Yassuda and Aprahamian2016) with great interest. In their study, naming the rhinoceros discriminated between patients with amnestic mild cognitive impairment (aMCI) and Alzheimer's disease (AD) but not healthy controls (HC). Of note, HC participants were significantly younger than aMCI and AD patients. All participants were administered the original version of the Montreal Cognitive Assessment (MoCA) instrument.

We recently recruited a group of Parkinson's disease (PD) patients and matched HC as part of an eye-movement study. A total of 45 participants were included: 15 PD with normal cognition (PD-N), 14 PD with mild cognitive impairment (PD-MCI) and 16 age-, education-, and sex-matched HC. All PD-MCI participants at our institution are diagnosed as such according to the Movement Disorders Society level II criteria, which incorporate the MoCA as one of the utilized instruments (Litvan et al., Reference Litvan2012). All participants were asked to name the animals of the original MoCA as well as those in two alternative MoCA versions (i.e. nine animals in total; Nasreddine, Reference Nasreddine2017). Animals were presented three-at-a-time, as on the paper form.

The percent correct animal-naming was equivalent among the study groups: PD-N 96%, PD-MCI 94%, and HC 98% (F _2,44 = 1.2, p = 0.3). Naming errors included the misidentification of the rhinoceros (MoCA 1), hippopotamus (MoCA 2), giraffe (MoCA 2) and donkey (MoCA 3), but did not differ among the participants (see Table S1, available as supplementary material attached to the electronic version of this paper at https://doi.org/10.1017/S1041610217000345).

Several factors may explain the superior performance of our PD patients compared with aMCI and AD patients in the study by Cecato and colleagues (Reference Cecato, Martinelli, Izbicki, Yassuda and Aprahamian2016). Unlike patients with aMCI and AD, PD patients do not normally exhibit naming deficits until later in the disease process when PD-dementia supervenes (Frank et al., Reference Frank, McDade and Scott1996). Our sample did not include patients with PD-dementia.

Our PD participants were generally younger than aMCI and AD patients reported by Cecato et al. (Reference Cecato, Martinelli, Izbicki, Yassuda and Aprahamian2016), were predominantly male and had many years of formal education – all factors that have been found to significantly influence animal-naming on the MoCA (Del Brutto and Wright, Reference Del Brutto and Wright2015). In addition, the sample for our study is from New Zealand, compared with a Brazilian sample reported by Cecato et al. (Reference Cecato, Martinelli, Izbicki, Yassuda and Aprahamian2016). The influence of sociocultural factors on performance has not been extensively evaluated, but Del Brutto and colleagues found that a common mistake made by elderly participants in rural Ecuador was mistaking the rhinoceros for a cow – a much more familiar animal to the studied farming community (2015).

In conclusion, we did not find MoCA animal-naming to discriminate among PD-N, PD-MCI, and HC participants. It is possible that the small number of our sample led to an underpowered study. Future larger studies – especially those that include PD-dementia patients – ought to provide a fuller picture of the discriminatory value of MoCA animal-naming in this disorder.