  Periodic modulations are seen in normal period pulsars ($P >$ 0.1 sec) over
timescales ranging from a few seconds to several minutes. Such modulations have
usually been associated with the phenomenon of subpulse drifting. A number of
recent studies have shown subpulse drifting to exhibit very specific physical
characteristics : i) drifting is seen only in conal components of the pulse
profile and is absent in central core emission; ii) drifting pulsars are
distributed over a narrow range of spin-down energy loss ($\dot{E}$), where
pulsars with $\dot{E} < 2\times$10$^{32}$ erg s$^{-1}$ show this behaviour,
iii) drifting periodicity ($P_3$) is anti-correlated with $\dot{E}$, such that
pulsars with lower values of $\dot{E}$ tend to have longer $P_3$. These
detailed characterisations of drifting behaviour on the other hand also
revealed the presence of other distinct periodic modulations, which can be
broadly categorised into two types, periodic nulling and periodic amplitude
modulation. In contrast to drifting these periodic phenomena are seen across
the entire profile in both the core and conal components simultaneously and are
not restricted to any specific $\dot{E}$ range. In this work we have assembled
an exhaustive list of around 70 pulsars which show such periodic modulations,
22 of which were newly detected using observations from the Giant Meterwave
Radio Telescope and the remaining compiled from past publications. The presence
of such a significant group in the pulsar population suggests that periodic
nulling and periodic amplitude modulations are newly emergent phenomena in
pulsars with their physical origin distinct from subpulse drifting.
