Simulations of nanocrystalline materials reveal that the pressure gradient present within the structure can play a key role in the movement of self-interstitial atoms (SIA's) to surrounding grain boundaries and therefore in the resulting defect structure formed during displacement cascades. Initially SIA's sense the grain boundary region as a "defect collector plate," a two-dimensional (2D) indistinguishable region under tension to which they are attracted. The SIA's approach the "defect collector plate" and at a certain distance are able to distinguish the local variations in pressure specific to the particular grain boundary misorientation, changing their direction in response to the local pressure environment. Consequently even large SIA clusters undergo a change in direction, moving 1D/3D in order to reach and follow a lower compressive and where possible a tensile pressure path to the grain boundary.