Stimulating desires

A common problem with the study of volition is that the instructions can prime the participant with the experimenter’s expectations. ‘Tell me when you felt the urge to move’ is a common instruction (Libet et al., 1983) and implies that there was such an urge in the first place. The participants, trying to help the experimenter, will adapt accordingly and report the timing of an urge that they might never have had. There is one type of experiment where such a biasing does not occur though: direct cortical stimulation studies with patients undergoing awake brain surgery (Pacherie & Haggard, 2011).

Direct cortical stimulation is used predominately in patients with brain tumours. Because tumour tissue and healthy tissue are hard to distinguish in the brain, the surgeon wakes the patient out of anaesthesia[1] and stimulates various parts of the cortical surface while the patient does some simple motor or speech tasks. If the patient is impaired in his task during the stimulation, the stimulated tissue is thought to be healthy and will usually not be removed; if stimulation of a certain area does not affect the function, the underlying tissue is thought to be tumorous and will be removed subsequently by the surgeon[2]. Since the opportunity to stimulate the human brain with such temporal and spatial precision is near impossible in any other context, clinicians sometimes also perform short scientific experiments, given the patient’s prior consent.

One such experiment was performed by Desmurget et al and published in Science in 2009. They stimulated the posterior parietal and premotor cortices while the patients were doing nothing in particular and just lying there; they were asked to report anything that came to mind. Because the experimenters never mentioned anything about intentions, volition, urges or any of the like[3], these experiments are about as unbiased as they get.

What did Desmurget et al actually find then[4]? They say that they found motor intention when stimulating the posterior parietal cortex. This intention to move was specific to individual body parts (hand, arm, foot, lips, chest), although not necessarily to a specific action, just a general intention to move said body part. But did they actually find intentions?

With their title ‘Movement intention after parietal cortex stimulation in humans’ Desmurget et al clearly refer to an intention after stimulation and nothing else; at one point in their abstract they also mention a desire to move, but apart from that they always refer to an ‘intent’ or ‘intention’. They never define or outline what they mean by ‘intention’ or how this relates to other similar yet different concepts (desire, will, urge, wish, hope, plan, etc.), which is part of the problem, but even despite this lack of terminological precision, it surprises me that they can give a paper this title despite no patient ever using the word intention, or any of its cognates. What the patients do say, however, over and over again, is ‘desire’, and ‘want’. Patient 1: ‘I wanted to move my foot’, ‘I wanted to ‘(move my arm), ‘ I wanted to close’ (my left hand), ‘a desire to move my hand’; P2: ‘I had a desire to do something’, ‘A will to move’, ‘a desire to move my right hand’, ‘a will to move’; P3: ‘I had a desire to roll my tongue’, ‘I felt a desire to lick my lips’. Call me pedantic, but that doesn’t sound like an intention to me, that sounds like a desire, a wish[5].

This leads to the question of whether intention and desire refer to the same thing, or if they’re different. To paraphrase Wittgenstein (1958): What is left if I subtract the fact that I wanted my arm to go up from the fact that I intended to raise my arm[6]? Because Desmurget et al never define any of their terms (such as intention), I have to rely on my own definitions.

While a full terminological analysis is outside of the scope of this essay, I’d like to at least hypothesise a bit about the difference between an intention and a desire. It seems quite clear to me that a desire to do something is categorically different from an intention to do something. An intention is more closely related to a plan, something one genuinely thinks one will do, while a desire is more of a wish, something one wants to do or to happen, but that one might not actually ever do. In a sense, an intention is built on top of a desire: first we want something to happen, and then we plan to make it happen.

Now this leads to an even bigger question: does this matter, or am I just being pedantic? Surprisingly given that I’ve spent time writing this essay, I’d say that this does indeed matter. As mentioned, an intention is more advanced than a desire. I’d say that a desire is more of a precursor to an intention that occurs later in the process. This is not to say that Desmurget et al didn’t find something interesting, quite the opposite, but rather that the phenomenon they describe is not as high-level and abstract as they claim it is. ‘Motor desire’ might sound a bit silly, but I think this is what they found. So I wouldn’t quite say that PPC stimulation leads to an intention, but that it leads to a desire, something related, and yet categorically different.



Desmurget, Michel, et al. "Movement intention after parietal cortex stimulation in humans." Science 324.5928 (2009): 811-813.

Libet, Benjamin, et al. "Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential)." Brain 106.3 (1983): 623-642.

Pacherie, Elisabeth, and Patrick Haggard. "What are intentions." Conscious will and responsibility. A tribute to Benjamin Libet (2010): 70-84.

Wittgenstein, Ludwig. Philosophical investigations. John Wiley & Sons, 2010.


[1] Once the skull is open and the brain is exposed

[2] For those worried that this is all that’s involved in removing brain tumours: It’s more complicated than that, but for simplicity’s sake I’ve excluded these factors here

[3] Nor, presumably did they even know the topic of the study they were taking part in, although for ethical reasons they might have been informed that they were to ‘take part in a study on volition’ or something like that. Since I don’t know their consent sheet and all the other undocumented instructions, this is only a guess though

[4] In this essay, I’ll focus only on the findings of intention from parietal cortex, and ignore all other findings

[5] Two caveats. 1: This study was done in France, so I’m assuming that everyone spoke French and that these are the experimenters’ translations into English. God knows what they actually said, but we just have to assume that 1.1) the translations are correct and that 1.2) good English/French equivalents even exist for these terms. 2: Desmurget et al only provide samples of the patients’ reports in their supplementary material, so I don’t know what the patients said on other trials. Although it is possible that they referred to an intent in other trials, it would be rather odd that Desmurget et al still chose to report only trials that do not mention ‘intention’

[6] The actual quote goes: What is left over if I subtract the fact that my arm goes up from the fact that I raised my arm?

Thesaurus searches for volition

The simple beginnings of volition