Using their large mystacial vibrissas, rats perform a variety of tasks, including localization and
identification of objects. We report on the discriminatory thresholds and behavior of rats trained in
a horizontal object localization task. Using an adaptive training procedure, rats learned to discriminate
offsets in horizontal (anteroposterior) location with all, one row, or one arc of whiskers intact,
but not when only a single whisker (C2) was intact on each cheek. However, rats initially trained with
multiple whiskers typically improved when retested later with a single whisker intact.
Individual rats reached localization thresholds as low as 0.24 mm (approximately 1 degree). Among the
tested groups, localization acuity was finest (<1.5 mm) with rats that were initially trained with
all whiskers and then trimmed to one arc of whiskers intact. Horizontal acuity was finer than
the typical inter-vibrissal spacing (approximately 4.8 mm at contact points).
Performance correlated with the net whisking spectral power in the range of 5-25 Hz but not in
nonwhisking range of 30-50 Hz. Lesioning the facial motor nerves reduced performance to chance level.
We conclude that horizontal object localization in the rat vibrissal system can reach hyperacuity level
and is an active sensing process: whisker movements are both required and beneficiary, in a graded manner,
for making accurate positional judgments.