It might be easier to view both as opposite ends of a continuum. If a person is under responsive, their sensitivity to sensory input is low and may not even be aware of it. In short, because they don’t sense what others do, this affects nervous system arousal. Sensation seeking can be viewed as the other end of the continuum. They are not only aware of sensory input, there is a desire for more, intense stimulation.
Underresponsivity has two types, one is sensory seeking in which children seek more sensory input by running, jumping, spinning so that their brain release feel good neurochemicals to become organized and focused.
The other type children are 'passive under responsive children who needs more sensory input but doesn't know how to get it, hence they seem to be day dreaming all time and doesnt interact with the surroundings and they have learning difficulty as they donot process information quickly like other children