Intrinsic noise
Biological systems carry appreciable amounts of internal noise. The magnitude of this noise source can be estimated by presenting the human observer with two identical sequences of noisy stimuli, and measuring the "consistency" with which the observer responds to the two repeated presentations (Fig. 1). If the observer were noiseless, it would behave like a deterministic device and generate identical response sequences on the two presentations of the same stimulus sequence. Real observers, however, often respond differently to repeated identical stimuli (at threshold they do so about every 4 trials); this inconsistent behaviour reflects internally generated randomness, and can be used to estimate the magnitude of internal noise.
We have exploited this double-pass paradigm to estimate internal noise across a wide range of tasks, stimuli and observers (Neri 2010), including language processing (Diependaele et al 2012). Collectively, the magnitude of the internal noise source stands at roughly 1.4 times the magnitude of the variability induced by the external noise source (Fig. 2). This is huge: it essentially means that the response generated by the human observer depends more on stimulus-decoupled processes going on in her/his brain than it does on what the experimenter presents on the screen.
We have also characterized the distribution of the internal noise source, which had previously been assumed Gaussian. We have found that it is more kurtotic than Gaussian (Fig. 3), closer to a Laplacian distribution (Neri 2013).
In recent theoretical work (Neri 2020), we have used calculus of variations to demonstrate the counter-intuitive result that ideal templates for signal detection/discrimination depend on internal noise intensity for some plausible constraints (Fig. 4H-I). We find empirical evidence to support the theoretical expectation when internal noise is compared across participants (but not within each participant), indicating that implicit knowledge about internal variability in different individuals is reflected by their detection templates (Fig. 4K).
Relevant publications:
• Neri P Optimal templates for signal extraction by noisy ideal detectors and human observers 2020 Journal of Computational Neuroscience in press
• Spilioti M, Vargesson N, Neri P Quantitative assessment of intrinsic noise for visually guided behaviour in zebrafish 2016 Vision Research 127 104-114
• Neri P The statistical distribution of noisy transmission in human sensors 2013 Journal of Neural Engineering 10 016014
• Diependaele K, Brysbaert M, Neri P How noisy is lexical decision? 2012 Frontiers in Language Sciences 3 348
• Hasan BAS, Joosten ERM, Neri P Estimation of internal noise using double passes: does it matter how the second pass is delivered? 2012 Vision Research 69 1-9
• Neri P How inherently noisy is human sensory processing? 2010 Psychonomic Bulletin & Review 17 802-808