  We present results of model atmosphere/line formation calculations which
quantitatively test how the 21 classical and four higher-order Balmer-line
Lick/IDS indices (Worthey et al. 1994; Worthey & Ottaviani 1997) depend on
individual elemental abundances (of carbon, nitrogen, oxygen, magnesium, iron,
calcium, natrium, silicon, chromium, titanium) and overall metallicity in
various stellar evolutionary stages and at various metallicities. At low
metallicities the effects of an overall enhancement of alpha-elements are also
investigated. The general results obtained by Tripicco & Bell (1995) at solar
metallicity are confirmed, while details do differ. Tables are given detailing
to which element every index reacts significantly, as a function of
evolutionary stage and composition.
  This work validates a number of assumptions implicitly made in the stellar
population models of Thomas, Maraston & Bender (2003), which utilized the
results of Tripicco & Bell (1995) to include the effects of element abundance
ratios variations. In particular, these computations confirm that fractional
changes to index strengths computed at solar metallicity (and solar age) can be
applied over a wide range of abundances and ages, also to model old stellar
populations with non-solar abundance ratios. The use of metallicity-dependent
response functions is, however, required for the proper modelling of the
Balmer-line indices. While H beta still responds only moderately to abundance
ratio variations, the higher-order Balmer indices H gamma and H delta display
very strong dependencies at high metallicities. As shown in Thomas, Maraston &
Korn (2004), this result allows to remove systematic effects in age
determinations based on different Balmer-line indices. (abridged)
