Encoding principles of active vibrissal touch

Encoding of Active Touch

Using this technique, we found that the signals sent to the brain by the whiskers during active touch differ from those transmitted during passive touch. Specifically, we found that active touch is reported to the brain by different classes of signals that either describe the movement of the whiskers, contact, or combinations of whisking and contact.

 

Encoding of Location

Using the same technique we studied the neural codes used by vibrissal receptors to encode the coordinates of object location in three dimensions.

We found that the most efficient neural code for each of the three spatial dimensions is different: temporal for the horizontal axis (along whisker rows), spatial for the vertical (along whisker arcs) and rate for the radial axis (from the face out).

 

Behavior

 We examined the ability of rats to resolve horizontal object location and its dependence on whisker configuration and whisking parameters. We found that rats could discriminate offsets in horizontal (posterior-anterior) location as small as 0.24 mm (~1 deg). Rats could learn the task with all the whisker configurations tested (all, one row, or one arc intact on each cheek), except when only a single intact whisker (C2) was left intact on each cheek.

However, rats initially trained with multiple whiskers typically improved their performance when re-tested later with a single whisker intact on each cheek. In general, lower thresholds were obtained with fewer intact whiskers, and the lowest thresholds were significantly less than the typical inter-vibrissal spacing. Rats typically whisked when performing this task, and performance dropped to chance level when whisking was prevented by cutting the facial motor nerve.

Furthermore, performance levels for all whisker-array configurations tested, except that of a single pair of whiskers from onset of training, correlated positively with the net whisking spectral power at 5-25 Hz. From these experiments, we conclude that object localization in the rat vibrissal system is an active process: whisker movements are both required and beneficiary, in a graded fashion, for making accurate positional judgments.

 

On-going computation of whisking phase by mechanoreceptors

In order to attribute spatial meaning to sensory information, the state of the sensory organ must be represented in the nervous system. In the rodent’s vibrissal system, the whisking-cycle phase has been identified as a key coordinate, and phase-based representation of touch was reported in the somatosensory cortex. Where and how phase is extracted in the ascending afferent pathways is still unknown.

Using a novel closed-loop interface in anesthetized rats, we found that whisking phase is encoded in a frequency- and amplitude- invariant manner already by primary vibrissal afferents. We show that for naturally-constrained whisking dynamics, such invariant phase coding can be obtained by tuning each receptor to a restricted kinematic sub-space.

We then show that invariant phase coding is preserved in the brainstem, where paralemniscal neurons filter out the slowly evolving offset while lemniscal neurons preserve it. These results demonstrate an accurate perceptually-relevant mechanically-based processing at the sensor level.