Assessments of Cognitive Function Following Traumatic Brain Injury - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 221

2.4.2. Novel Object
Recognition Procedure

The NOR test consists of two phases: a sample phase followed by
a test phase, with a variable delay. On the day before the fi rst trial,
animals are placed in the open fi eld for 30 min and allowed to
habituate to the empty open fi eld. On the following day, two iden-
tical objects are placed in the open fi eld, equidistant from the side
of the open fi eld. Animals are placed in the center of the open fi eld
and allowed to explore the objects over a 5-min period (sample
phase). Because injured rats may be less likely to spend time exploring,
criteria can be set to require the animals to spend a total of 20 s
exploring the objects during the sample phase of the task. The
cumulative time spent exploring each of the objects is recorded
by an observer blinded to the different groups. Variable delays can
be used between the sample phase and the test phase (e.g., 3 min,
1 h, 4 h, or 24 h) to test different aspects of memory. During the
test phase, a third object identical to the object used in the sample
phase and a novel object are placed in the open fi eld. Animals
are placed in the center of the open fi eld and allowed to explore.
The total time for exploring each object and the number of times
an animal's snout came within 2 cm or less of the object was
recorded by an observer blinded to group.
A third copy of the sample object is used in the test phase to
avoid possible scent marking by the animals. Additionally, the entire
open fi eld is cleaned with 30% ethanol before each experiment
begins and before each animal is placed in the open fi eld during
habituation and on each day of training. To avoid preference for
location within the open fi eld, the location of the novel object
was randomly assigned for each trial. Pairs of objects should be
randomly assigned as either sample or test objects in matched pairs
of shams and injured animals to control for any innate preference
for the objects.
The primary dependent measure for recognition memory is
the percent of time spent exploring the novel object during the test
phase. A memory discrimination index (DI) can also be used as
described by Sonmez et al. ( 23 ): DI = ( N − F )/( N + F ), where N is
the time spent exploring the novel object and F is the time spent
exploring the familiar object from the sample phase. Higher DI
refl ects greater memory ability.

3. Conclusions

Experimental brain injury models have been successful at replicating
many of the same cognitive impairments observed in TBI patients.
Still, the exact nature of cognitive impairments following TBI in
animals is unknown and it is uncertain if the cognitive assessments
most commonly used in animals fully address what is seen in the
human population. It is also unclear which anatomical structures