1. A total of 3 12 cutaneous afferent units identified in the rat foot
as belonging to one of nine major types of sensory receptors were inc
luded in the present study. A natural stimulus set was defined to diff
erentiate optimally among those receptor types according to the distin
guishing response patterns that it produced. It included air puffs, 30
- and 300-Hz sinusoids, 200-mN force indentation of the skin, 1.2- and
6-N compressions of a skin fold, cooling the skin by 5 and 20-degrees
-C, warming by 5-degrees-C, and heating by 15-degrees-C.2. The respons
es to predefined stimuli of 188 units were subjected to multivariate s
tatistical analyses. The responses of an individual unit were measured
as the number of impulses evoked by 10 stimuli, each lasting 10 s. Ad
ditionally, the number of impulses occurring for 5 s after withdrawal
of a 200-mN indentation (I of the 10 stimuli) was counted. 3. In discr
iminant analysis, the 11 stimulus variables predicted fairly correctly
the grouping of afferent units into nine predetermined receptor categ
ories (175 of 188, 93.1%), indicating a powerful ability to discrimina
te among different receptor types. Using hierarchical cluster analysis
, afferent unit data described by 11 variables were divided into clust
ers that well represented prior receptor categories (170 of 188, 90.4%
), suggesting the reliable application of this procedure to the classi
fication of newly recorded cutaneous sensory receptors. 4. Eleven vari
ables were then reduced to 7 on the basis of the results of factor ana
lysis (95% of variance accounted for). The seven variables corresponde
d to 1.2-N compression, heating the skin by 15-degrees-C, cooling the
skin by 20-degrees-C, 30- and 300-Hz sinusoids, withdrawal of a 200-mN
indentation, and air puffs. 5. The seven selected variables correctly
assigned afferent units into five modality-based categories in the di
scriminant solution (177 of 188, 94.1%). In the cluster solution, affe
rent units described by the seven selected variables were divided into
clusters, most of whose members were modality specific (I 76 of 18 8,
93.6%). 6. The results indicate that cutaneous receptors can be divid
ed into modality-specific groups according to similarities in their re
sponses to seven stimulus variables. It is proposed that the stimulus
set developed here and multivariate statistical methods can be used as
powerful tools for the functional classification of central somatosen
sory neurons.