In this model, the incoming acoustic signal is believed to be first processed to determine the so-called landmarks which are special spectral events in the signal; for example, vowels are typically marked by higher frequency of the first formant, consonants can be specified as discontinuities in the signal and have lower amplitudes in lower and middle regions of the spectrum. These acoustic features result from articulation. In fact, secondary articulatory movements may be used when enhancement of the landmarks is needed due to external conditions such as noise. Stevens claims that coarticulation causes only limited and moreover systematic and thus predictable variation in the signal which the listener is able to deal with. Within this model therefore, what is called the lack of invariance is simply claimed not to exist.