Making sense of effort and persistence are an important part of knowing how, why, and when we choose to act or to give up. Insights can help improve teaching and training methods, as well as understand situations where motivation is chronically impaired – in anxiety or depression. Developing tasks to look at these ideas in rats and mice has benefits of testing in humans. One is that rodents can be trained and tested over an extended period of time to do a fairly mundane task, while a human participant may object to the tedious request. Another is that the neuronal basis of the behaviour can be studied in more detail.
It is important to be able to separate effort from persistence, as the motivation to continue to perform a task that requires little effort may be distinct from the motivation to perform a single momentous task. Or at least, there may be a shared pool of motivation, but the way in which we draw from this pool to summon the will to perform the task may differ.
Several standardised tasks to measure effort in rodents already exist. Two common tasks are a lever pressing task and barrier tests. As their name suggests, lever pressing tasks involve pressing a small lever until a reward is granted. This task is not ideal because it does not separate the effort of performing the task from the persistence of continuing to push the lever. The task is also very simple, and so the act of pushing the lever may quickly become an automated habit. Performing an habitual behaviour is likely to require less effort than performing non-habitual behaviours, so it is not easy to generalise the task to more complex and conscious behaviour. Barrier tests involve the subject either climbing or jumping over a barrier to reach a reward. Barrier tests allow persistence and instantaneous effort to be separated to some extent by varying the height of the barrier vs how many times the animal is required to cross it. However, one problem with the task is that the as the height of the barrier increases, the task becomes more dangerous as well as more effortful. Also, the task still only relies on fairly instant effort in a simple behaviour, rather than more prolonged engagement.
The paper by Porter & Hillman describes a new task for studying effort and persistence in rats the task involves pulling a cord via a pulley to lift weights. The cord must be pulled a distance that requires body movement, and the cord can be manipulated with paws or mouth, giving a more complex and engaging multi-action task. The task can vary both required effort (the weight applied to the cord) and the persistence (how many times the cord must be pulled).
A range of measures of effort and persistence can be taken by measuring e.g. the number of times an animal will attempt to lift a given weight to obtain a reward, the number of times an animal will persist in attempting to lift a challenging weight, or the point at which an animal does not attempt to lift a weight as the weight increases.
Porter and Hillman describe the experimental equipment and the training needed to shape the behaviour to perform the task. They also provide some benchmark values for success in the training process, as well as for performance in progressive and fixed weight tasks.
Why it is important
The task provides a way of examining effort and persistence through neurological and pharmacological means. The rats who performed this task were also implanted with intracranial electrodes. The intention was not to analyse any related data, but simply to show that it would be possible. The electrode implant also contains an LED, which allows the position of the rat to be tracked automatically.
The equipment is made from off the shelf customisable equipment such as an Arduino microcontroller. The code for the Arduino is freely available on GitHub (https://github.com/blakeporterneuro/weightLiftingTask). This open approach to creating research equipment allows it to be easily and transparently reused and re-purposed.
Finally, the researchers describe some incidents during training where the behavioural shaping did not go as intended. They describe changes to the training or equipment that they made to solve these problems. This open approach to describing potential pitfalls that a researcher might encounter is admirable and very helpful.
A question I might have liked to see answered is how measures on this task relate to scores on other effort tasks.
Beyond this, many of the questions that arise from this work will be answered through its use in research – what are the neural bases of effort and persistence? How is the threshold for persistence set and how does it change? How does nutrition, sleep, and more serious conditions effect effort and persistence by what mechanism? How does performance on this task vary between individuals or from day to day?
Posted on: 16th September 2019Read preprint
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