LOCALITY, Perception of. Our sensa tions are associated with positions in space. That this is the case with vision and joint sensations is obvious, while in all the other senses a stimulus is referred to the approxi mate spatial position of the organ that perceives it, and is possibly further referred to some place in space from which it is believed to arise. The senses with the most direct spatial reference appear to be vision, the sensations of the skin, and those of the joints. The spatial relations of two objects presented to the eye are as immediately given as their relations of color-intensity or color-quality. In the same way, the location of an object on the skin or the position of a joint are primitive attributes of the experience of the object. However, the visual, tactile and articular spatial worlds are per se of a bi-dimensional character. The relations of space in the third dimension are not directly given in experience, but are the result of the synthesis of a large number of different factors. In the case of vision, these factors are the parallax of the visual objects with reference to the interocular distance, the experiences of the muscles of the eyeballs as they focus the eyes and regulate their degree of convergence and such phenomena of geo metrical and atmospheric perspective as have been given by habit a three-dimensional inter pretation. The cutaneous senses acquire their three-dimensional interpretation through asso ciation with visual experiences and experiences of joints, and through the complex of cutaneous sensations resulting when two parts of the in tegument are brought into contact with one another. The sensations of a single joint are necessarily uni-dimensional or bi-dimensional, as the joint cannot have more than two degrees of freedom, but the combined sensations of two adjacent joints furnish a three-dimensional manifold, corresponding to the three degrees of freedom of a point at the end of a doubly jointed rod. The three-dimensional worlds of sight, touch and joint-position formed in the manner thus indicated are amalgamated into a single space by that intimate association of their phenomena which is impressed on us from the very start of our experience, and which is possibly even prior to our consciousness of this association. An important theory in this con nection is that due to Lotze, and known as the theory of local signs. According to this view, each cutaneous sensation carries with it a peculiar qualitative distinction that is always associated with the point of its origin. As it appears impossible, however, to discover any qualitative characteristic peculiar to each place on the skin, the local sign of a cutaneous sensa tion is probably just that visual localization that accompanies it.
It is altogether probable that hearing and smell, at any rate, have no distinctly spatial attribute, and that the localization of sounds and scents is due to the association of certain non-spatial characters on their part with the sight-touch spatial system. In the case of sound
the chief factor is the relative intensity of the sensations in the two ears, as is shown by the poorness of localization in cases where hearing of one ear is lost, and by the practical impos sibility of determining the source of a sound in the median plane. The quality and absolute intensity of the sound are also indices of its source. The quality is somewhat dependent on the direction from which the sound reaches the pinna, and so enables a certain degree of local ization in hearing with a single ear. In the case of smell, localization depends on the relation between the position of the had and the in tensity of the sensation.
The accuracy of localization can be measured in two ways. The first is to determine the lower limen (see LIMINA) of the interval be tween two stimuli; the second is to indicate the absolute position of a stimuli, and measure the average error of the localization. While the former method often indicates a greater sen sitivity than the latter, the relation between the acuity of absolute and relative discrimination of locality varies in the different senses. In sight relative discrimination preponderates, and two points subtending an angle of one minute or a retinal distance of .004 millimetre can be dis tinguished. A binocular parallax of five seconds suffices to give an impression of depth. In the case of the pressure sense, the average absolute error of localization is 5 to 10 milli metres on the wrist. The discrimination linen for the tactile senses is not dearly defined, and the sensation from pressure at two points goes through several stages of elongation before it is distinctly double. If the pressure is applied at pressure spots, the two point limen assumes a much smaller value than otherwise, and is about 0.3 millimetre on the back of the hand, 0.5 millimetre on the forehead, .08 millimetre on the chest, and 4 millimetres an the back. The corresponding values for the cold sense are 2, 0.8, 2 and 1.5 millimetres; for the sense of warmth, 3, 4, 4, and 4 millimetres respectively. The least noticeable movement of the hip
0.5 degree; for the ankle, 1 degree. The localization of organic sensations is highly irregular, and exhibits many anomalies. (See LIMINA; SPACE; VISION). Consult Kiilpe, 0., (Outlines of Psychology) (tr. New York 1909) ; Ladd, G. T., and Woodworth, R. S.,