The basal ganglia are one of the fundamental processing units of the 
brain and have been associated with processes of reinforcement 
learning.  A strong line of supporting evidence comes from the 
recording of dopamine (DA) neurones in behaving monkeys. Unpredicted, 
biologically salient events, including reward cause a stereotypic 
short latency, short duration burst of DA activity the phasic 
response. A currently dominant hypothesis is that such responses 
represent reward prediction errors which are used as teaching signals 
in appetitive learning.   However, the comparative response latencies 
of DA neurones (~100ms) and orienting gaze-shifts (150-200ms) 
indicates that phasic DA responses are triggered by pre-attentive, pre-
saccadic sensory processing.  The discovery that subcortical sensory 
structures like the superior colliculus provide the main, if not 
exclusive short-latency visual inputs to DA neurones is consistent 
with this view. Therefore, in natural circumstances it is unlikely 
that the identity of an unexpected event is known at the time of DA 
signalling.  The limited quality of afferent sensory processing and 
the precise timing of phasic DA signals, suggests an alternative 
hypothesis that phasic DA signals may instead reinforce the discovery 
of sensory events for which the organism is responsible.  In cases 
where the event is caused by the organism, it is proposed that DA-
dependent neural plasticity acts to promote the discovery of novel 
actions.