▁change ▁at ▁x ▁mainly ▁follows ▁the ▁change ▁in ▁the ▁band ▁at ▁y , ▁the ▁asynchronous ▁ 2 D ▁cross ▁peak ▁at ▁( x , y ) ▁is ▁negative ▁if ▁the ▁synchron ous ▁ 2 D ▁cross ▁peak ▁at ▁( x , y ) ▁is ▁negative , ▁the ▁interpretation ▁of ▁rule ▁ 3 ▁and ▁ 4 ▁for ▁the ▁asynchronous ▁ 2 D ▁peak ▁at ▁( x , y ) ▁has ▁to ▁be ▁revers ed ▁ ▁where ▁x ▁and ▁y ▁are ▁the ▁positions ▁on ▁the ▁x - x axis ▁of ▁two ▁bands ▁in ▁the ▁original ▁data ▁that ▁are ▁subject ▁to ▁intensity ▁changes . ▁ ▁Following ▁the ▁rules ▁above . ▁It ▁can ▁be ▁derived ▁that ▁the ▁changes ▁at ▁ 1 0 ▁and ▁ 3 0 ▁occur ▁simultaneously ▁and ▁the ▁changes ▁in ▁intensity ▁at ▁ 2 0 ▁and ▁ 4 0 ▁occur ▁simultaneously ▁as ▁well . ▁Because ▁of ▁the ▁positive ▁asynchronous ▁cross - pe ak ▁at ▁( 1 0 , ▁ 2 0 ), ▁the ▁changes ▁at ▁ 1 0 ▁and ▁ 3 0 ▁( pred omin antly ) ▁occur ▁before ▁the ▁intensity ▁changes ▁at ▁ 2 0 ▁and ▁ 4 0 . ▁ ▁In ▁some ▁cases ▁the ▁N oda ▁rules ▁cannot ▁be ▁so ▁readily ▁imp lied , ▁pre domin ately ▁when ▁spectral ▁features ▁are ▁not ▁caused ▁by ▁simple ▁intensity ▁variations . ▁ ▁This ▁may ▁occur ▁when ▁band ▁sh ifts ▁occur , ▁or ▁when ▁a ▁very ▁err atic ▁intensity ▁variation ▁is ▁present ▁in ▁a ▁given ▁frequency ▁range . ▁ ▁See ▁also ▁Cor relation ▁spect ro sc opy