Halliday & Matthiessen (2014: 246-7):
There is one further type of ‘mental’ clause: I can feel something on my foot. This is a clause of perception, the can feel as the Process, I as the Senser and something on my foot as the Phenomenon being perceived. Such clauses are similar to emotive and cognitive ‘mental’ clauses in that the Senser is construed as a conscious being. But they also have properties that set them apart from the other subtypes of ‘mental’ clause. For example, while Pat could have said I feel something on my foot with the Process in the simple present, she has used a modulation of readiness instead – can feel. This is quite common with ‘mental’ clauses of perception, as with Can you see those pelicans flying across the lagoon alongside Do you see .... This example also illustrates another feature specific to clauses of perception: what is construed as the phenomenon being perceived can be a thing (such as cockroach); but it can also be an act, realised by a non-finite clause, as in I can feel [[something crawling up my foot]].