Early studies suggested that what determines
whether a task depends on declarative memory (and
the HF) or on habit memory (and the striatum) is how
rapidly the task is ordinarily learned. In nonhuman
primates, tasks that were acquired rapidly were more
sensitive to the effects of medial temporal lobe damage
than tasks acquired slowly (Squire 2009). For example,
intact monkeys learned pattern discrimination and
concurrent discrimination tasks gradually over hundreds
of trials but could learn object discriminations
in 10 or 20 trials (Resende et al. 2003). Correspondingly,
monkeys with HF damage acquired pattern discrimination
and concurrent discriminations normally
but were impaired at learning the easier object discriminations.
On the contrary, monkeys with lesion that
included the tail of the caudate showed impairment at
acquiring pattern discriminations as well as concurrent
discrimination tasks. Moreover, when rats were trained
in a T-maze, performance during the early trials relied
on a hippocampus-dependent form of place learning.
With extended training, the rats shifted to a dorsal
striatum–dependent strategy based on response learning.
These earlier findings supported the hypothesis