Table 1.

Criteria used to develop the MBT.

Criteria	MBT
Restricted to upper limb function	The MBT is performed seated and requires bilateral function to grasp, transfer and accurately release tokens into a container.
Ecologically valid	The MBT is a dual task assessment that consists of five items, from which three are transfer tasks with incremental difficulty; Baseline Transfer, Complex Transfer, and Dual Transfer tasks. The remaining two items are baseline tasks to ensure the subject can count backwards from values presented and recite the alphabet in preparation for the Complex Transfer and Dual tasks respectively. The MBT was designed so it was sensitive for individuals with different levels of functional ability, such as people with HD. Reciting the alphabet was used for the Dual Transfer task to increase task complexity. This specific task was selected as it is less likely to be confounded by education or job type compared to other commonly used secondary tasks, such as addition, subtraction, or verbal fluency tasks.
The assessment is applicable to people with all stages of HD	The MBT consists of a hierarchy of items with increasing levels of difficulty. Participants had to meet set criteria before proceeding to the more complex MBT items to minimize the chances of floor and ceiling effects. The pass/fail criteria is presented in the Supplementary Material.
The assessment is sensitive to functions that involve the degenerating neuroanatomy in HD	MBT items were developed to target behaviors that involve the cortico-basal ganglia-thalamo circuitry. This included: Dexterity: The lateral striatum is required for fine motor tasks (Döbrössy and Dunnett, 2003). To account for this, participants were required to pick up different sized tokens and accurately release these into a defined target on a container Repeated motor transitions: Rhythmic, repeated motor transitions leads to a change in neuronal firing patterns in the dorsolateral striatum (Ashby et al., 2010), and may relate to new skill learning (Turner and Desmurget, 2010). The MBT was designed to take advantage of these functions, as the participant is required to repeatedly transfer eight tokens as quickly as possible into a container Oculomotor function (Harting and Updyke, 2005): It was hypothesized that optimal MBT performance required occulo-motor function to rapidly saccade the eyes to the next token target Attention (De Diego-Balaguer et al., 2008): The increasing levels of difficulty in the MBT intended to demand increasing levels of attention. Throughout the MBT, participants are required to transfer tokens between hands and in a given order. In the Dual Transfer task, attentional capacity is challenged again as participants are required to transfer tokens in a set order whilst simultaneously reciting the alphabet Alphabet recitation: Previous studies have shown that less cognitively demanding tasks can be more sensitive in people with HD than those with high cognitive demands (Snowden et al., 2001; Thompson et al., 2010). In addition, pre-clinical research suggests that the dorsolateral striatum is involved in performing fixed, automatic behaviors (Yin et al., 2004). Reciting the alphabet is a fairly simplistic task that is regularly recited from a young age. For many, by early adulthood, this recitation would pose little attentional demand as the memory is retrieved and automatically recited (Ashby et al., 2010; Turner and Desmurget, 2010). It was hypothesized that reciting the alphabet would load extra stress on the fronto-striatal circuitry making the Dual Transfer task more challenging for people with striatal dysfunction.
Minimal burden for the administrator and the participant	The MBT is uncomplicated to set up and takes between 5 and 10 min for the participant to perform. Due to the criteria developed for each MBT item, the length of the MBT assessment is dependent on the participant's functional ability. In addition, as the MBT is used to measure bilateral function, unlike pegboard tests, it only need to be performed once, which reduces the time of the assessment.
Compact	As clinic space is often limited and equipment needs to be stored and transported to different clinic locations, the MBT was designed so it was compact, lightweight and so construction involved few and small test components.
Quantitatively scored	The MBT is quantitatively evaluated, using time as a primary measure, which can be combined with accuracy to calculate an MBT total score. This method was used to improve inter-rater reliability and to sensitively measure change over time (Hobart et al., 2000).