Effects of conflicting stimuli on neurons in posterior parietal cortex
(PPC)

We define stimuli which require different responses in different
behavioral contexts as conflicting. Humans and animals respond more
slowly and less accurately to conflicting stimuli (e.g., Stoet &
Snyder 2003).  We report the effect of conflicting stimuli on neural
activity in the PPC of rhesus monkeys.

Two discrimination tasks were randomly interleaved.  Trials started
with a 250 ms visual task instruction. After a 450 ms delay, a
colored, oriented stimulus appeared.  Depending on the task
instruction, animals judged either stimulus color (red or green) or
orientation (horizontal or vertical). To indicate their judgment
animals moved the left paw from a central resting position to one of
two buttons, positioned 16 degrees left and right from screen center.
In half of the trials the stimuli were conflicting; the required
response was different in the two tasks.  In the other half, stimuli
were not conflicting; they required identical responses in the two
tasks.  Monkeys responded, on average, 32 ms slower to the conflicting
stimuli.

We recorded from 392 isolated neurons in the PPC. We selected neurons
with a significantly higher spike rate during movements contralateral
to the recording site.  For these 154 neurons, we compared the onset
of spatially tuned responses for conflicting versus non-conflicting
stimuli.  The onset of a tuned response was defined as the earliest
time at which activity was significantly different for responses in
the two directions.  Conflicting stimuli delayed the onset of spatial
tuning by 47 ms, but had no other effect on the pattern of the
response.

Given that neuronal activity is delayed, but otherwise similar
following conflicting and non-conflicting stimuli, we conclude that
PPC reflects but is not the site of increased processing due to
conflict.