Na sua edição especial, o Colóquio do Departamento de Física desta semana vai trazer o professor Alfonso Renart, do Champalimaud Neuroscience Programme (CNP), para falar sobre “O fundamento mecanicista da lei de Weber”. O encontro é aberto ao público e ocorrerá amanhã (6), às 9h, no auditório do Centro de Ciências Exatas e da Natureza da UFPE (CCEN).

Resumo

The accuracy of a sensory system can be probed by measuring the smallest intensity difference that can be reliably discriminated. Weber noticed, almost 200 years ago, that this just-noticeable-difference (JND) is not a difference, but a certain ratio between the two stimulus intensities which is constant across a wide range of absolute stimulus magnitudes. Although Weber’s law is the most firmly established regularity in sensation, no principled way has been identified to choose between its many proposed explanations. We studied how accurately rats could discriminate the lateralization of sounds, which relies on a comparison of intensity across the two ears, at various absolute levels. These experiments revealed the existence of a novel psychophysical regularity – which we term time-intensity equivalence in discrimination (TIED) – describing how reaction times change as a function of absolute intensity while intensity-ratios, and thus discrimination-accuracy, are kept fixed. The relationship between absolute intensity and reaction time is so stringent that it allows us to mathematically specify the computational basis of the sensory discrimination process, 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 bounded exact temporal accumulation of sensory evidence. This mechanism is not only necessary for the TIED to hold, it is also sufficient to provide a virtually complete quantitative description of the rats’ behavior. Furthermore, we provide an empirical, model-independent demonstration of the necessity of bounded evidence accumulation, verifying that Weber’s law breaks down for short fixed-duration sounds. The JND was robust against manipulations of motivation, which suggests that in the present conditions, against common belief, the bound for evidence accumulation is not under subject control and sets the hard-limit for the accuracy of discrimination. Our results reveal the mechanistic underpinnings of a fundamental psychophysical law and clarify the factors that set the accuracy of perception.

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Departamento de Física - UFPE
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