Unspoken testimony: why neural speech decoding does not produce testimonial evidence

This article warns against the idea that information from neural speech decoding (NSD) can be treated as testimonial evidence. We contend that NSD and testimony differ in their epistemic status. While the evidential validity of testimony depends essentially on someone’s sincerity, by contrast NSD depends upon the accuracy of decoding the person’s inner speech. To show this, we first explore how NSD poses a problem for rights against self-incrimination by testimony in, for instance, the Fifth Amendment to the US Constitution. Building on Pardo’s epistemic account of the distinction between physical and testimonial evidence, we argue that since NSD produces ambiguous information, it does not qualify as testimony. Conversely, denying our claim makes one liable to what we call the informational symmetry fallacy (ISF), viz. the conflation of decoded output from someone’s brain with that person’s speech act. We then set out the consequence of falling into the ISF by criticizing Farahany’s proprietary conception of thought. Her conception is shaped by goal of providing a new taxonomy of evidence better suited for relating constitutional rights to brain data and privacy. Here, we argue that Farahany must presume that we already own and are thereby accountable for all of our intercepted thoughts. Hence, she precludes the possibility of our disclaiming some thoughts and thereby justifies others in treating those thoughts as our beliefs, without considering the dependence of belief our epistemic authority. Lastly, we discuss a practical problem entailed by our account, namely that users of neural speech prostheses will not qualify as witnesses in a legal setting. In response, we argue that although NSD prosthetics enable users to communicate, there are insuperable limits in assessing someone’s testimonial sincerity solely on the basis of an NSD readout. For this reason, we conclude that NSD is better analogized to hearsay evidence.