Rats use their large facial hairs (whiskers) to detect, localize and identify
objects in their proximal three-dimensional (3D) space. Here, we focus on recent
evidence of how object location is encoded in the neural sensory pathways of the
rat whisker system. Behavioral and neuronal observations have recently converged
to the point where object location in 3D appears to be encoded by an efficient
orthogonal scheme supported by primary sensory-afferents: each primary-afferent
can signal object location by a spatial (labeled-line) code for the vertical axis
(along whisker arcs), a temporal code for the horizontal axis (along whisker rows),
and an intensity code for the radial axis (from the face out). Neuronal evidence
shows that (i) the identities of activated sensory neurons convey information about
the vertical coordinate of an object, (ii) the timing of their firing, in relation
to other reference signals, conveys information about the horizontal object
coordinate, and (iii) the intensity of firing conveys information about the radial
object coordinate. Such a triple-coding scheme allows for efficient multiplexing of
3D object location information in the activity of single neurons. Also, this scheme
provides redundancy since the same information may be represented in the activity
of many neurons. These features of orthogonal coding increase accuracy and
reliability. We propose that the multiplexed information is conveyed in parallel to
different readout circuits, each decoding a specific spatial variable.
Such decoding reduces ambiguity, and simplifies the required decoding algorithms,
since different readout circuits can be optimized for a particular variable.