How animals discriminate between stimulus magnitudes: a meta-analysis

To maximize their fitness, animals must often discriminate between stimuli
differing in magnitude (such as size, intensity, or number). Weber’s Law
of proportional processing states that stimuli are compared based on the
proportional difference in magnitude, rather than the absolute difference.
Weber’s Law implies that when stimulus magnitudes are higher, it becomes
harder to discriminate small differences between stimuli, leading to more
discrimination errors. More generally, we can refer to a correlation
between stimulus magnitude and discrimination error frequency as a
magnitude effect, with Weber’s law being a special case of the magnitude
effect. If more discrimination errors are made when stimulus magnitudes
are higher, this could affect how signals evolve. However, the strength
and prevalence of the magnitude effect across species has never previously
been tested. Here, we conducted a meta-analysis to quantify the strength
of the magnitude effect across studies, finding that, on average,
perception followed Weber’s Law. However, the strength of the magnitude
effect varied widely, and this variation was not explained by any
biological or methodological differences between studies that we examined.
Our findings suggest that the magnitude effect is commonplace, and that
this sensory bias is therefore likely to affect signal evolution across a
diverse range of biological systems. Better discrimination at lower
magnitudes might result in signalers evolving lower magnitude signals when
being discriminated is beneficial, and higher magnitude signals when being
discriminated is costly.