A new study has found that errors in perceptual decisions occurred only when there was confused sensory input, not because of any ‘noise’ or randomness in the cognitive processing. The finding, if replicated across broader contexts, will change some of our fundamental assumptions about how the brain works.
The study unusually involved both humans and rats — four young adults and 19 rats — who listened to streams of randomly timed clicks coming into both the left ear and the right ear. After listening to a stream, the subjects had to choose the side from which more clicks originated.
The errors made, by both humans and rats, were invariably when two clicks overlapped. In other words, and against previous assumptions, the errors did not occur because of any ‘noise’ in the brain processing, but only when noise occurred in the sensory input.
The researchers supposedly ruled out alternative sources of confusion, such as “noise associated with holding the stimulus in mind, or memory noise, and noise associated with a bias toward one alternative or the other.”
However, before concluding that the noise which is the major source of variability and errors in more conceptual decision-making likewise stems only from noise in the incoming input (in this case external information), I would like to see the research replicated in a broader range of scenarios. Nevertheless, it’s an intriguing finding, and if indeed, as the researchers say, “the internal mental process was perfectly noiseless. All of the imperfections came from noise in the sensory processes”, then the ramifications are quite extensive.
The findings do add weight to recent evidence that a significant cause of age-related cognitive decline is sensory loss.
 . Rats and Humans Can Optimally Accumulate Evidence for Decision-Making. Science [Internet]. 2013 ;340(6128):95 - 98. Available from: http://www.sciencemag.org/content/340/6128/95