The introduction of the signal detection theory has been a significant step in psychophysics, and d' score estimations have helped us understand cognitive processing to a significant degree. However, researchers in the field of speech perception have been using this sensitivity measure while assuming that the decision spaces are symmetrical, neglecting thus the internal structure of categories. In a two-interval 'same-different' paradigm, for example, the areas covered by the S1S1, S1S2, S2S1 and S2S2 distributions are not necessarily equivalent. In this paper, we will contest the assumption of symmetry by revisiting some papers which provide evidence in favour of asymmetrical vowel perception. Perceptual distance is dependent on the salience of the acoustic properties of the stimuli, with salient stimuli either decaying slower or interfering more with the processing of adjacent stimuli. We will then attempt to provide a hypothetical decision space for a 'same-different' task implementing two vowels, and and will discuss the usefulness of decision spaces as tools for understanding the perceptual robustness of stimuli.
