  Figure 5. Note that the pair of sounds circled in blue differ in F2  starting frequency less than those circled in red, yet the former are both reliably counted as instances of the sounds /b/ whereas the  latter are reliably classified as different sounds, /d/ and /g/. This pattern, together with the abrupt switch from one classification to  another (e.g. /b/ to /g/), is characteristic of categorical  perception.