You are doing quality control in a factory that makes smartphone cases, and you have no equipment that is sensitive enough to detect scratches in the surface of the cases. If you could hire humans to detect these scratches, What would you look for because research has shown?
1) people with a large number of Meissner corpuscles; these receptors respond well to fine texture; only respond when the stimulus starts and stops
2) people with a large number of Meissner corpuscles; these receptors respond well to low frequency tactile gratings; are highly concentrated in the fingertips
3) people with a large number of Merkel receptors; these receptors respond well to fine surface texture; are highly concentrated in the fingertips
4) people with a large number of Merkel receptors; these receptors respond well to continuous stimulation; are used for haptic judgments