130 SENSORY RECEPTORS 



Exercise XXVI 



varying greatly from one area of skin to another. 

 Our sense of spatial continuity — of the smooth- 

 ness of a surface — as well as of pattern is con- 

 veyed by such discontinuous mosaics of recep- 

 tors. 



The capacity of a sensory surface for evaluat- 

 ing pattern is measured by determining the 

 "two-point threshold," which is the smallest 

 separation at which two point stimuli are per- 

 ceived as two. This measures the density of 

 receptors, since for two stimuli to be appreciated 

 as two, they must excite two touch spots having 

 at least one unexcited touch spot between them. 



Perform the following experiments in pairs, 

 one student, with eyes closed during each test, 

 serving as subject, the other as experimenter 

 and recorder. 



Distribution of touch spots 



With a pen outline a square hairless or shaved 

 area about 3 cm on a side on the inner forearm. 

 Explore this area by touching it lightly with the 

 tip of a bristle, noting the points from which a 

 distinct sensation of touch is felt. Mark each of 

 them with a spot of ink. Draw a diagram of the 

 area, showing the locations of the touch spots. 

 Estimate the number per square centimeter. 



Two-point thresholds 



To test for these, the experimenter touches 

 various points in a region of skin very lightly 

 with one or both of the blunted points of a pair 

 of dividers, in haphazard order. At each touch 

 the subject reports the sensation as either "one" 

 or "two." 



At the start of each test adjust the separation 

 of the dividers so that all double stimuli are 

 reported as "two" and all single stimuli as 

 "one." Then gradually lessen the separation 

 until only about 8 in 10 reports are correct. 

 The separation of the points in centimeters is 

 then the approximate minimum perceptible sepa- 

 ration, or two-point threshold. In some areas 

 of skin this is much the same in all orientations 

 of the dividers; in others it differs greatly. 



Determine and record in a table the two-point 

 thresholds for the upper arm (longitudinal), 

 upper arm (transverse), forearm (longitudinal), 

 forearm (transverse), back of hand, palm of 

 hand, fingertip, and lips. 



Calculate the number of receptors per square 

 centimeter in each area tested, and enter this in 

 the table. In those areas in which the two-point 

 threshold is about the same in all orientations 

 of the dividers, use the formula : N/cm~ = 4/L^, 

 in which A'^ is the number of touch spots and L 

 is the two-point threshold in centimeters. This 

 formula is based on the assumption that the 

 two-point threshold represents twice the dis- 

 tance between neighboring touch spots. Why 

 twice the distance? 



For areas of skin in which the longitudinal 

 two-point threshold (Li) differs from the trans- 

 verse (Lo), the density is TV/cm^ = 4/(Li X /.2)- 



How does the density of touch spots on the 

 forearm, calculated from the two-point thresh- 

 old, compare with the density you found by 

 direct mapping? 



TASTE 



The special senses (sight, hearing, smell, taste) 

 are associated with dense aggregates of recep- 

 tors, concentrated in limited areas, some of them 

 supplied with highly adapted accessory struc- 

 tures, such as in the eye and ear. The sense of 

 taste is limited to the mucosa of the tongue and 

 mouth. The receptors are clustered in "taste 

 buds" and are of several types, each type mediat- 

 ing a primary taste. Since the receptors for the 

 several primary tastes are not uniformly dis- 

 tributed over the sensory surface, their nature 

 and distribution can be determined by applying 

 various solutions to different regions of the 

 tongue. 



A solution may stimulate more than one kind 

 of taste cell, resulting in a wide variety of taste 

 sensations. Other senses frequently enter: a 

 solution that is both bitter and hot may give 

 rise simultaneously to sensations of bitterness, 

 warmth, and perhaps pain. Very often also, the 



