Haptic Object Localization in the Vibrissal System: Behavior and Performance

Abstract

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 (ϳ1°). Among the tested groups, localization acuity was finest (Ͻ1.5 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 (ϳ4.8 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.

Biomechanics of the Vibrissa Motor Plant in Rat: Rhythmic Whisking Consists of Triphasic Neuromuscular Activity

“…Our head-fixed animals did not whisk in the foveal range (15-20 Hz), so additional data points were obtained from two separate studies. The first dataset involved unrestrained rats whisking in air (n ϭ 20) , and the second used unrestrained rats whisking in air during an object localization task (n ϭ 150) (P. M. Knutsen, M. Pietr, and E. Ahissar, unpublished observations) (for methods, see Knutsen et al 2005Knutsen et al , 2006. The behavioral data have comparable boundaries with the estimated behavioral limits at all frequencies and exhibit the same trend toward smaller, protracted whisks at higher frequencies.…”

“…Motivated by evidence that the range of whisking amplitudes is diminished for large offset angles and high values of whisking frequency (Carvell and Simons, 1995;Berg and Kleinfeld, 2003;Knutsen et al, 2006), we analyzed the available range of whisk amplitudes and set points, defined here as the point of maximal retraction, within the context of our model. Our strategy was to use all possible realistic patterns of muscle activity as input for the model to determine the bounds on its range of motion.…”

Layer-Specific Touch-Dependent Facilitation and Depression in the Somatosensory Cortex during Active Whisking

“…Active exploration involves head movements; thus, even if a whisker repeats its scanning pattern from cycle to cycle, it may not sample the same location in different cycles (Knutsen et al, 2005(Knutsen et al, , 2006. Therefore, even if input signals are identical from cycle to cycle, they convey new information in world coordinates (Ahissar and Zacksenhouse, 2001).…”

“…To obtain sensory information, rat whiskers move forth and back with a rhythmic motion (whisking) at 4 -25 Hz (Welker, 1964;Carvell and Simons, 1990;Fanselow and Nicolelis, 1999;Kleinfeld et al, 1999;Berg and Kleinfeld, 2003;Knutsen et al, 2006). Experimentally, deflections of passive whiskers can be useful for studying responses that occur during behavioral epochs in which the whiskers are not actively moved, such as when running along walls without whisking.…”

“…The force of the muscle acts on the base of the whisker, whereas the force of the object acts on a point along the external shaft of the whisker. These two forces cause whiskers to bend considerably while contacting objects (Derdikman et al, 2003;Szwed et al, 2003;Knutsen et al, 2006). In contrast, during passive stimulation, force is applied only at a single point, and whiskers hardly bend.…”