 The primary challenge is to ensure that any candidate formulation uses only local information, rather than relying on global (error) signals , as in orthodox backprop.
<clarity>  The primary challenge is to ensure that any candidate formulation uses only local information, rather than relying on global (error) signals , as in orthodox backprop.
 The primary challenge is to ensure that any candidate formulation uses only local information, rather than relying on global signals as in standard backprop.