Event:

Almost 200 years ago Weber noticed that the accuracy of discriminations between the intensity of two sensory stimuli depends only on their ratio. Although this regularity has been replicated in hundreds of studies involving all sensory modalities, no principled way has been identified to choose between many proposed alternative explanations. We trained rats to report the lateralization of sounds -- which relies on a comparison of intensity across the two ears -- varying the overall sound level at various fixed intensity-ratios. Although, consistent with Weber’s law, accuracy depends only on intensity-ratio, we found that reaction times decrease with sound level when the intensity-ratio is fixed. Surprisingly, changes in reaction time at fixed ratio take place through a rigid stretching of the reaction time distribution, implying that variations in overall level are equivalent to a uniform rescaling of time. We demonstrate mathematically that these results allow a detailed specification of the mechanism underlying Weber’s law, placing strict requirements on how stimulus intensity is encoded in the stochastic activity of sensory neurons, and revealing that discriminative choices must be based on perfect temporal accumulation of this sensory activity up to a constant bound. In support of these claims, we show that Weber’s law breaks down if sound duration is limited --preventing sensory evidence from reaching the bound -- and that fits of a minimal diffusion model with the required characteristics describe the rats’ performance and reaction time distributions with virtually no error. Manipulations of motivation were unable to increase discrimination accuracy, demonstrating that the hard limit on the precision of a sensory system is set by the value of the evidence bound. Our results reveal the mechanistic basis of a fundamental law of sensation.