Participant     face    observation
          1     sad1             x1
          1     sad2             x2
                 ...
          1     sad5             x5
          1     joy1             x6
          1     joy2             x7
                 ...
          1     joy5            x10
          2     sad1            x11
          2     sad2            x12
                 ...
          2     joy1            x16
          2     joy2            x17
                 ...
        100     sad4           x999
        100     sad5          x1000

aov(observation ~ face + Error(Participant/face), Data)

Participant     face               mean_values
          1      sad          mean(x1, ... x5)
          1      joy         mean(x6, ... x10)
          2      sad        mean(x11, ... x15)
          2      joy        mean(x16, ... x20)
                 ...
        100      sad      mean(x991, ... x995)
        100      joy     mean(x996, ... x1000)

glm(mean_values ~ face, Data, family="Gaussian")