Cosyne 2007
Saturday evening, Poster III-51
Dissociating Rule-based Categorization and Set-shifting from
Action Selection in Monkeys
Alexander Lerchner1 , Takafumi Minamimoto1 , David Soucy1 , and Barry
Richmond1
1
Lab. of Neuropsychology, NIMH, NIH, DHHS, Bethesda, MD, USA
Dynamic, rule-based categorization is usually tested by requiring subjects to choose between actions based
on current category membership. This confounds categorization and action selection. Such confounding
makes it difficult to investigate neural correlates of cognitive functions such as rule-based categorization if
the brain areas under investigation are also considered important for dynamic selection between actions (the
prefrontal cortex being a typical example). Here, we show how categorization and attentional set-shifting
can be tested in monkeys with a task that does not require action selection. We find that eliminating the
need for action selection substantially reduces the training time for acquiring the task when compared to
conventional approaches. Thus the cognitive and the action-selection requirements are two separate steps.
In our task, there is a single instrumental response, identical for all trials. To obtain a reward, the monkey
has to release a bar when a red target changes to green. Error trials have to be repeated until performed correctly. At the beginning of each trial, we also present a visual cue that, although not relevant for performing
the instrumental task, indicates whether the reward after a correct response will be delivered immediately
(”pleasant”) or delayed (”unpleasant”) – based on the current category assignment of the cue. The cues were
created by producing morphs between two shapes and two colors, which allows us to use either the color
or the shape dimension for dividing the stimulus set into two subsets. Assigning immediate and delayed
rewards to the two subsets results in four different categorization rules.
For all four monkeys tested, both the error rates and the reaction times differed significantly between the
immediate and the delayed conditions. Thus, even though the identities of the cues are irrelevant for carrying
out the task, the monkeys learn their meanings. By switching the categorization rule without announcement
within sessions, we found that monkeys follow both extra-dimensional (color to shape, or vice versa) and
intra-dimensional (reversal within color or shape) set-shifting in this task.
Our results show that monkeys can form rule-based categories and switch between them in a task that
requires no choice between actions. This offers the opportunity to probe the contribution of different brain
areas, e.g., individual parts of prefrontal cortex, to rule-based categorization and set-shifting when there is
no requirement for action selection.
Acknowledgments
Support Contributed By: IRP/NIMH, JSPS/Japan
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