Perceptual and Motor Skillr, 1976, 42,671-674. @ Perceptual and Motor Skills 1976
NUMEROSITY DISCRIMINATION OF TACTILE STIhWLI THOMAS B. POSEY' A N D MICHAEL R. JAMES Murray State University Summmy.-The ability of 40 college smdents to discriminate the number of tactile stimuli presented simultaneously was measured in two experiments. Stimulation was provided by 12 solenoids fixed to points on the arms and legs. Exp. I showed a mean correct discrimination level of between 1.55 and 2.10 solenoids. Exp. I1 compared discrimination of numerosity by a group who received immediate feedback with a no-feedback control group. Feedback produced a small increase in this ability. However, the most stimulation points correctly discriminated did not exceed three. The data are discussed in terms of the possible role of cutaneous masking.
The ability of people to discriminate the number of multiple, simultaneous stimuli presented for brief exposures has been investigated for several sensory systems (Miller, 1956). Visual numerosity ability under these conditions is about six and other sensory systems seem to possess a numerosity ability, or channel capacity, close to this value (Miller, 1956). However, while several studies have reported the use of the skin in coded communication, tactile numerosity has apparently not been directly measured. (See Kirman, 1973, for a review of this area which includes the work of Geldard, Alluisi, von Bekesy, and others.) The purpose of this research was to measure the channel capacity of the tactile sense and to do so by means of single pressure pulses. (This is different from most of the studies referred to above in that they have used vibrating stirnuli.) In Exp. I, subjects were asked to report the number of points on their skin which were touched simultaneously. In a second experiment, some subjects were given feedback as to the number of stimuli on each presentation, and the effect of feedback was measured.
EXPERIMENTI Method
Subjects were 8 male and 12 female vol~inteersfrom introductory psychology classes. The apparatus consisted of 12 Dormeyer solenoids placed on the subjects' arms and legs, a chair for the subject, and a control panel. The solenoids provided tactile stimulation of 11 oz. ( 312 gm) on a plunger 1/16 in. ( 1.5 mm) in diameter. The solenoids were fastened to arched, metal plates which in turn were fastened to the subjects' limbs with elastic straps. The arch in the plate provided no contact between the skin and the plunger when the solenoid was in an open position. When the experimenter operated a pushbutton, the plunger descended to touch the skin for .5 sec. and then returned to a resting position. The solenoids were wired through a control panel which allowed the experimenter to 'Requests for reprints may be sent to the first author, Department of Psychology, Murray State University, Murray, Kentucky 42071.
672
T.B. POSEY & M. R. JAMES
select any number of solenoids for multiple operation. The subjects were seated in a chair about 2 m from the control panel with their backs to the experimenter. The 12 skin points used for solenoid stimulation were as follows: the ventral surface of each wrist horizontal to the thumb, approximately 3 cm. from the hand; the top (or radial surface) of each forearm, approximately 5 cm. below the elbow; the lateral surface of the arm, approximately 10 cm. below the shoulders; the inside of each ankle; the outside of each calf; and the top of the thighs approximately 10 cm. above the knee. The 12 locations were selected SO that bilaterally each solenoid was on a different dermatome. In addition, these 12 areas were spaced no closer together than 20 to 25 cm. on the average subject and were well in excess of two-point threshold distances. The force of each solenoid was constant as the number of solenoids activated simultaneously varied and calibrations were made during the course of experimentation to verify constancy. The pressure each solenoid delivered (312 gm. on its 1.5 mm plunger) is well above detection threshold values and it was not considered important in this case to adjust for the differences in subjective intensity at different body loci. After being familiarized with the equipment and shown how the solenoids worked, the subjects were seated and the 12 solenoids were attached. In all cases the experimenter explained that the solenoids would only push on the skin and that no shock could occur. When all the solenoids were connected, the subject was presented with one solenoid at a time turned on for .5 sec., beginning with the right ankle and continuing up the right side, then proceeding down the left arm and leg to the left ankle. During this pretesting period, the subject was asked to tell when each solenoid was on in order to test the equipment and to familiarize the subject further with the stimulation he/she would receive. The subject then received 60 presentations of between 1 and 12 solenoids turned on for .5 sec. and was asked to report how many he/she felt immediately after each presentation. The presentations were grouped into five sets of 12 each, with each number of combinations of solenoids presented in a random order within each set. In addition, the particular solenoids activated for any given combination were determined at random. Prior to each presentation, the experimenter said, "ready." The stimulus was then presented and the subject's verbal report was recorded. Results The results of the first experiment indicated a mean channel capacity of 1.55 stimuli (SD = .60) or .63 bits. This value was based on each subject's maximum errorless discrimination, which was the highest number of solenoids accurately reported in all five sets of presentations. Only one of the 20 subjects was completely accurate in reporting threes, that is, the presentation of three solenoids. The other 19 had 100% accuracy at the level of ones and twos.
NUMEROSITY DISCRIMINATION OF TACTILESTIMULI
673
At an 80% accuracy level, that is four right out of five presentations, the mean number correct was 2.10 stimuli ( S D = .72) or 1.07 bits. At this level ( 8 0 % ) , two subjects were able to report only ones, while the other 18 subjects did either twos or threes. No subject reached four with either 100% or 80% accuracy, and only two subjects were successful with fours more than half the time (60%). This f~ndingof a channel capacity for tactile numerosity on the order of less than 1 bit was unexpected and a second experiment was run to determine if immediate feedback could increase this ability. EXPERIMENTI1 Method Subjects were 20 different volunteers from introductory psychology classes divided equally into an experimental group, which received feedback, and a control group, which received no feedback. There were five men and five women in each group. The apparatus and the basic procedure were the same as in Exp. I. The solenoids were placed on the same areas and the presentation procedures and rate of presentation were the same. A new collection of 6 3 presentation combinations was selected at random so that there were seven sets of nine presentations each. Each set contained all combinations of one through 12 solenoids except eights, tens, and elevens. As in Exp. I, order of presentation and solenoid combinations on presentations were determined randomly. The subjects were run as in Exp. I, except that the subjects in the feedback group were told by the experimenter on each presentation that their response was correct or were told the actual number of solenoids that were turned on if their response was incorrect. Subjects in the control group were given no feedback. The total number of correct responses was recorded for each subject in each of the seven presentation sets. Thus, any change in performance over sets could be observed. Results The data were analyzed by means of a 2-groups by 7-sets repeated-measures analysis of variance. The analysis showed both of the main effects and the interaction were significant (groups: F = 21.74, df = 1/18, p < .001; sets: F = 9.05, df = 6/108, p .001; groups X sets: F = 4.40, df = 6/108, p .01) and indicated improvement in performance occurred with feedback. These results are accounted for in part by the feedback subjects learning to discriminate cwelves, the condition with all the solenoids on. When twelves are disregarded, the mean number correct ( n o error) on the last three sets for the feedback group was 3.00 ( S D = .94) and for the control group, 1.8 ( S D = 1.47). This difference was significant ( t = 2.1, df = 18, p .05). The results of Exp. I1 indicate that feedback can improve performance on
NUMEROSITY DISCRIMINATION OF TACTILE STIhWLI THOMAS B. POSEY' A N D MICHAEL R. JAMES Murray State University Summmy.-The ability of 40 college smdents to discriminate the number of tactile stimuli presented simultaneously was measured in two experiments. Stimulation was provided by 12 solenoids fixed to points on the arms and legs. Exp. I showed a mean correct discrimination level of between 1.55 and 2.10 solenoids. Exp. I1 compared discrimination of numerosity by a group who received immediate feedback with a no-feedback control group. Feedback produced a small increase in this ability. However, the most stimulation points correctly discriminated did not exceed three. The data are discussed in terms of the possible role of cutaneous masking.
The ability of people to discriminate the number of multiple, simultaneous stimuli presented for brief exposures has been investigated for several sensory systems (Miller, 1956). Visual numerosity ability under these conditions is about six and other sensory systems seem to possess a numerosity ability, or channel capacity, close to this value (Miller, 1956). However, while several studies have reported the use of the skin in coded communication, tactile numerosity has apparently not been directly measured. (See Kirman, 1973, for a review of this area which includes the work of Geldard, Alluisi, von Bekesy, and others.) The purpose of this research was to measure the channel capacity of the tactile sense and to do so by means of single pressure pulses. (This is different from most of the studies referred to above in that they have used vibrating stirnuli.) In Exp. I, subjects were asked to report the number of points on their skin which were touched simultaneously. In a second experiment, some subjects were given feedback as to the number of stimuli on each presentation, and the effect of feedback was measured.
EXPERIMENTI Method
Subjects were 8 male and 12 female vol~inteersfrom introductory psychology classes. The apparatus consisted of 12 Dormeyer solenoids placed on the subjects' arms and legs, a chair for the subject, and a control panel. The solenoids provided tactile stimulation of 11 oz. ( 312 gm) on a plunger 1/16 in. ( 1.5 mm) in diameter. The solenoids were fastened to arched, metal plates which in turn were fastened to the subjects' limbs with elastic straps. The arch in the plate provided no contact between the skin and the plunger when the solenoid was in an open position. When the experimenter operated a pushbutton, the plunger descended to touch the skin for .5 sec. and then returned to a resting position. The solenoids were wired through a control panel which allowed the experimenter to 'Requests for reprints may be sent to the first author, Department of Psychology, Murray State University, Murray, Kentucky 42071.
672
T.B. POSEY & M. R. JAMES
select any number of solenoids for multiple operation. The subjects were seated in a chair about 2 m from the control panel with their backs to the experimenter. The 12 skin points used for solenoid stimulation were as follows: the ventral surface of each wrist horizontal to the thumb, approximately 3 cm. from the hand; the top (or radial surface) of each forearm, approximately 5 cm. below the elbow; the lateral surface of the arm, approximately 10 cm. below the shoulders; the inside of each ankle; the outside of each calf; and the top of the thighs approximately 10 cm. above the knee. The 12 locations were selected SO that bilaterally each solenoid was on a different dermatome. In addition, these 12 areas were spaced no closer together than 20 to 25 cm. on the average subject and were well in excess of two-point threshold distances. The force of each solenoid was constant as the number of solenoids activated simultaneously varied and calibrations were made during the course of experimentation to verify constancy. The pressure each solenoid delivered (312 gm. on its 1.5 mm plunger) is well above detection threshold values and it was not considered important in this case to adjust for the differences in subjective intensity at different body loci. After being familiarized with the equipment and shown how the solenoids worked, the subjects were seated and the 12 solenoids were attached. In all cases the experimenter explained that the solenoids would only push on the skin and that no shock could occur. When all the solenoids were connected, the subject was presented with one solenoid at a time turned on for .5 sec., beginning with the right ankle and continuing up the right side, then proceeding down the left arm and leg to the left ankle. During this pretesting period, the subject was asked to tell when each solenoid was on in order to test the equipment and to familiarize the subject further with the stimulation he/she would receive. The subject then received 60 presentations of between 1 and 12 solenoids turned on for .5 sec. and was asked to report how many he/she felt immediately after each presentation. The presentations were grouped into five sets of 12 each, with each number of combinations of solenoids presented in a random order within each set. In addition, the particular solenoids activated for any given combination were determined at random. Prior to each presentation, the experimenter said, "ready." The stimulus was then presented and the subject's verbal report was recorded. Results The results of the first experiment indicated a mean channel capacity of 1.55 stimuli (SD = .60) or .63 bits. This value was based on each subject's maximum errorless discrimination, which was the highest number of solenoids accurately reported in all five sets of presentations. Only one of the 20 subjects was completely accurate in reporting threes, that is, the presentation of three solenoids. The other 19 had 100% accuracy at the level of ones and twos.
NUMEROSITY DISCRIMINATION OF TACTILESTIMULI
673
At an 80% accuracy level, that is four right out of five presentations, the mean number correct was 2.10 stimuli ( S D = .72) or 1.07 bits. At this level ( 8 0 % ) , two subjects were able to report only ones, while the other 18 subjects did either twos or threes. No subject reached four with either 100% or 80% accuracy, and only two subjects were successful with fours more than half the time (60%). This f~ndingof a channel capacity for tactile numerosity on the order of less than 1 bit was unexpected and a second experiment was run to determine if immediate feedback could increase this ability. EXPERIMENTI1 Method Subjects were 20 different volunteers from introductory psychology classes divided equally into an experimental group, which received feedback, and a control group, which received no feedback. There were five men and five women in each group. The apparatus and the basic procedure were the same as in Exp. I. The solenoids were placed on the same areas and the presentation procedures and rate of presentation were the same. A new collection of 6 3 presentation combinations was selected at random so that there were seven sets of nine presentations each. Each set contained all combinations of one through 12 solenoids except eights, tens, and elevens. As in Exp. I, order of presentation and solenoid combinations on presentations were determined randomly. The subjects were run as in Exp. I, except that the subjects in the feedback group were told by the experimenter on each presentation that their response was correct or were told the actual number of solenoids that were turned on if their response was incorrect. Subjects in the control group were given no feedback. The total number of correct responses was recorded for each subject in each of the seven presentation sets. Thus, any change in performance over sets could be observed. Results The data were analyzed by means of a 2-groups by 7-sets repeated-measures analysis of variance. The analysis showed both of the main effects and the interaction were significant (groups: F = 21.74, df = 1/18, p < .001; sets: F = 9.05, df = 6/108, p .001; groups X sets: F = 4.40, df = 6/108, p .01) and indicated improvement in performance occurred with feedback. These results are accounted for in part by the feedback subjects learning to discriminate cwelves, the condition with all the solenoids on. When twelves are disregarded, the mean number correct ( n o error) on the last three sets for the feedback group was 3.00 ( S D = .94) and for the control group, 1.8 ( S D = 1.47). This difference was significant ( t = 2.1, df = 18, p .05). The results of Exp. I1 indicate that feedback can improve performance on
