Effects of Long-Term Object Familiarity on Event-Related Potentials in the Monkey

Jessie J. Peissig, Jedediah Singer, Keisuke Kawasaki, David L. Sheinberg
Cerebral Cortex June 2007;17:1323--1334 doi:10.1093/cercor/bhl043

Although some change in the neural representation of an object
must occur as it becomes familiar, the nature of this change is not
fully understood. In humans, it has been shown that the N170—an
evoked visual potential—is enhanced for classes of objects for
which people have visual expertise. In this study, we explored
whether monkeys show a similar modulation in event-related
potential (ERP) amplitude as a result of long-term familiarity by
recording ERPs with chronically implanted electrodes over extended
training periods spanning many sessions. In each of 3
experiments, we found larger amplitude visual evoked responses to
highly familiar images for the time period of 120--250 ms after
stimulus onset. This difference was found when the monkeys were
trained in an individual-level discrimination task, in a task that
required only color discrimination, and even following a viewingonly
task. We thus observed this familiarity effect across several
tasks and different object categories and further found that the
difference between ‘‘familiar’’ and ‘‘novel’’ became smaller as the
animals gained experience with the previously unfamiliar objects
across multiple test sessions. These data suggest that changes in
visual responses associated with familiarity are evident early in the
evoked visual response, are robust, and may be automatic, driven at
least in part by repeated object exposure.