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Peripheral nerve conduction velocity, reaction time, and intelligence: An attempt to replicate Vernon and Mori (1992)
INTELLIGENCE
18, 127- 13 I ( 1994)
Peripheral Nerve Conduction Velocity, Reaction Time, and Intelligence: An Attempt to Replicate Vernon and Mori (1992) JOHN PHILIP
C. WICKETT A.
VERNON
University of Western Ontario
In an attempt to examine the relationship between neural speed, as indicated by nerve conduction velocity (NCV) along the median nerve of the arm, and intelligence, four relevant studies have been carried out: two finding the relationship and two not finding evidence of such a relationship. In an attempt to replicate the two studies (Vernon & Mori, 1992) finding this relationship, 38 healthy, right-handed females, aged 20 to 30 years, completed the Multidimensional Aptitude Battery (Jackson, 1984), a series of reactiontime tasks and were submitted to the same NCV procedures as in Vernon and Mori (1992). Contrary to prediction, NCV did not correlate with intelligence or reaction time. A reanalysis of Vernon and Mori (1992), however, showed evidence for a possible sex difference in relation to NCV and intelligence, with a pronounced correlation between these variables being found in males but a much smaller correlation being found in females.
One of the predominant views within contemporary intelligence research is that speed of information processing is a key component of what is meant by “intelligence.” That is, the intelligent individual is one who can quickly and effectively process incoming stimuli, with this efficient system allowing for increased information handling capacity. Evidence for this speed factor is provided by studies employing the reaction time (RT) paradigm, and it is well established that RT on elementary cognitive tasks is negatively correlated with IQ (e.g., Vernon, 1987). It is logical, given the relationship between RT and IQ, to ponder what the neural substrate of this speed factor might be. The simplest, and clearest, explanation is that the speed at which the neuron fires is the key neural underpinning. In an attempt to assess the viability of neural speed as a factor underlying intelligence, four studies have been conducted assessing the relationship between peripheral nerve conduction velocity (NCV) and IQ. Peripheral NCV (specifically, in the arm) involves no cognitive component and is thus purely physiologi-
Funding for this project was provided by a grant to the second author from The Pioneer Fund, Inc. Correspondence and requests for reprints should be sent to P.A. Vernon, Department of Psychology, University of Western Ontario, London, Ontario, Canada N6A 5C2.
127
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cal. Interest in the relationship is primarily due to this noncognitive nature of NCV. The discovery of a positive correlation between NCV and IQ would show evidence that IQ can be measured and explained, at least in part, by purely neural mechanisms. Vernon and Mori (1992) present two studies that provide such evidence. In the first study, 85 male and female undergraduates (M age = 24.5 years; M IQ = 114.9) completed the Multidimensional Aptitude Battery (MAB; Jackson, 1984) as the intelligence measure, a set of RT tasks, and had eight NCV measurements taken along three different segments of the median nerve of the arm (wrist-tofinger, wrist-to-elbow, and wrist-to-axilla). First unrotated factor scores for each of the three sets of measures (IQ, RT, and NCV) were obtained, and intercorrelations determined. As expected, general RT correlated negatively with general IQ (r = -.44); additionally, general RT correlated negatively with general NCV (Y = -.28), and general NCV correlated positively with general IQ (Y = .42). Correcting for sex or age had no effect on the correlations derived. This indicates that the speed at which a neural impulse travels is related to intelligence, with faster speeds being related to higher intelligence. Study 2 of Vernon and Mori (1992) was essentially identical to Study I, except that fewer RT tasks were employed, and only wrist-to-finger NCV was measured. In this second sample of 88 male and female undergraduates (M age = 23.1 years; M IQ = 115) essentially the same findings were discovered: RT correlated negatively with IQ (r = - .4S), and also negatively with NCV (r = - .18), and NCV correlated positively with IQ (r = .48). As with Study 1, age and sex had no appreciable effect on the correlations derived. Thus, given the results of both studies, fairly substantial support is provided for a moderate positive correlation between NCV and IQ. Two other studies, however, do not provide such support, although procedural differences may account for at least some of the discrepancy. Barrett, Daum, and Eysenck (1990), in a sample of 44 males and females, did not find a significant correlation between NCV and intelligence as measured by the Advanced Raven Matrices (Raven, 1983a). This study, however, did not use supramaximal stimulation of the median nerve, and as such all nerve fibers would not have been activated. This is a substantial departure from the Vernon and Mori ( 1992) procedure, and thus the findings are not comparable. Reed and Jensen (1991), in a sample of 200 male university and community college students, also did not find evidence for a relationship between NCV and IQ as measured by both the Standard and Advanced Raven Matrices (Raven, 1983a, 1983b). One difference between this study and those of Vernon and Mori (1992) is that Reed and Jensen controlled statistically for arm temperature, rather than directly using a heating pad as per Vernon and Mori. Arm temperature is a key factor in nerve conduction speed, and as such the appropriate regulation of temperature is imperative. The contradictory findings, coupled with the methodological differences between the Vernon and Mori ( 1992) studies and those of Barrett et al. ( 1990) and Reed and Jensen (1991), make it difficult to draw any firm conclusions regarding
NCV, RT, AND IQ: A REPLICATION
129
the relationship between NCV and IQ. There is clearly a need to adopt similar methodology in order to shed some light on this problem. To this end, and in an attempt to provide confirmation of a positive relationship between NCV and IQ, this study was carried out using methodology identical to that of Vernon and Mori (1992). Specifically, it is predicted that NCV along the median nerve will be positively correlated with IQ as assessed by the MAB. Additionally, RT, as measured by a set of four RT tasks, will be negatively correlated with both IQ and NCV.
METHOD Subjects Subjects for this study were 38 healthy, right-handed adult females, aged 20 to 30 years (M = 24.59). Subjects volunteered by answering an advertisement in a local newspaper, and were paid for their participation. Forty-one subjects originally took part in the study, but three subjects had to be discarded when it was not possible to get a “clean” reading of their NCV. All subjects signed a consent form and were debriefed. Measures IQ was assessed using the MAB which provides a Full-Scale IQ (FSIQ), a Verbal IQ (VIQ), and a Performance IQ (PIQ) score. It also provides subscale scores similar to those provided by the WAIS-R (Wechsler, 1981) on which the MAB is based. The MAB is a timed, multiple-choice test. General intelligence, or GIQ, was estimated by submitting the 10 subtests of the MAB to a factor analysis and then computing factor scores on the first unrotated factor, which accounted for 50.5% of the variance. Four RT measures were obtained using standard RT tasks. Detailed description of these tasks can be found in Vernon, Nador, and Kantor (t98.5), with the particular tasks used in the present study including measures of short-term memory, synonyms and antonyms, and simple arithmetic. Scores for each task for each subject were transformed into z scores and then summed together within each subject to obtain a composite RT score. NCV was measured using techniques identical to that of Vernon and Mori (1992) with the following exception: Only one wrist-to-finger and one wrist-toelbow velocity were obtained. Supramaximal stimulation of the median nerve of the right arm was employed, and a heating pad maintained arm temperature between 32 and 35°C. NCV measures are reported in meters per second. Procedure IQ testing was completed during one session lasting approximately 1.5 hr, with groups of I to 5 subjects completing the test together. Additionally, subjects were
130
WICKETI’ AND VERNON
tested individually for handedness (Kimura, 1986) during this session to insure that all were right-handed. The RT and NCV measures were obtained in a second individual session with the RT tasks being completed first. Subjects were paid and debriefed upon completion of the NCV procedure.
RESULTS Mean FSIQ for the group of 38 subjects was 107.82 (SD = 11.6X), indicating that they were within the average range of intelligence. Mean wrist-to-elbow NCV was 60.52 m/s (SD = 3.74), and mean wrist-to-finger NCV was 59.03 m/s (SD = 4.48). These NCVs are comparable to those in Vernon and Mori (1992). Mean RTs (in ms) on the four RT measures ranged from 661.73 to 1156.40. These are also comparable to the RTs of Vernon and Mori’s (1992) subjects. As predicted, GIQ correlated negatively with the composite RT score (r = -.24, ns.). Although this correlation is not significant, it is within the normal range of RT-IQ correlations. Contrary to prediction, neither wrist-to-finger NCV nor wrist-to-elbow NCV correlated significantly with GIQ (rs = -. 12 and .02, respectively, both n.s.). Neither NCV measure correlated significantly with any of the MAB subscales, and they also did not correlate significantly with VIQ or PIQ. Also contrary to prediction, the NCV measures were not correlated with composite RT (TS = .02 and .Ol, both n.s.).
DISCUSSION In direct contrast to the findings of Vernon and Mori (1992), the present study failed to find a significant relationship between NCV and IQ. Additionally, and also contradictorily, NCV was not related to RT, although RT did show the traditional negative relationship with IQ. These results would seem to indicate that nerve conduction velocity is not an impo~ant factor in explaining individual differences in intellectual ability. This confirms the null.findings of Reed and Jensen (1991) and Barrett et al. (1990) although, as mentioned earlier, these two studies used methodologies different from Vernon and Mori (1992). The present findings prompted a reanalysis of the Vernon and Mori (1992) data. Because only females were used in the present study, separate analyses for males and females were performed on the data from both of the Vernon and Mori (1992) samples. Surprisingly, the correlations obtained for males and for females were substantially different. In Study 1 of Vernon and Mori (with no sex difference for mean RT, NCV, or IQ), the obtained correlations between RT, NCV, and IQ for males were in each case higher than those found for females. Specifically, for males (n = 40) and females (n = 45), respectively: general IQ and general NCV correlated Y = .62 (p < .Ol) and r = .28 (n.s.); general IQ and general RT
NCV, RT, AND IQ: A REPLICATION
131
correlated r = - .66 (p < .Ol) and r = - .43 (p < .Ol); and general NW and general RT correlated r = - .58 (p < .Ol) and r = - .05 (n.s.). A similar pattern is observed in Study 2. Again for males (n = 38) and females (n = 50), respectively: IQ and NCV correlated r = .54 (P < .Ol) and r = ‘37 (p < .Ol); IQ and RT correlated r = -.68 (p < .Ol) and r = -.37 (p < .Ol); and NCV and RT correlated r = -.24 (n.s.) and r = -.03 (n.s.). The only comparisons that reached significance were the male-female difference in Study I for general NCV and genera1 RT (Z = 2.71, p < .Ol), and in Study 2 for IQ and RT (Z = 1.98, p < .05); all other Zs ranged from 0.96 to 1.94. Although only two of these six comparisons show a significant sex difference, the fact that the same pattern exists in both sets of data suggests what may be a true sex difference. Indeed, use of the technique outlined by Fisher (1958) for combining p values from independent studies reveals that the male-female difference for the IQ-RT correlation is significant [x2(4) = 9.999,~ < .OS], as is the difference observed in the NCVRT correlations [x2(4) = 14.443, p < .Ol]. For NCV and IQ the male-female difference is marginally significant [x*(4) = 8.087, p < . lo]. In conclusion, the disparity between the correlations obtained for males and females in the Vernon and Mori (1992) studies, along with the null findings for females in the present study, raise interesting questions concerning sex differences in intelligence. Although it is clear that further study is required, the present results suggest males rely quite heavily on neuronal speed to perform cognitive tasks, whereas for females neural speed is only minimally implant, and it is rather some other, and as yet unknown, neural processes that play the predominant role.
REFERENCES P.T., Daum, I., & Eysenck, H.J. (1990). Sensory nerve conduction and intelligence: A meth~ological study. Journal of Psychophysiology, 4, I- 13. Fisher, R.A. (1958). Sratisticul methods for research workers (13th ed.). New York: Hafner. Jackson, D.N. (1984). Multidimensional Aptitude Battery. Port Huron, MI: Research Psychologists Press. Kimura, D. (1986). Neuropsychology tesr procedures. London, Ontario: DK Consultants. Raven, J.C. (1983a). Advu~ced Raven Progressive Matrices. London: H.K. Lewis. Raven, J.C. (1983b). Standard Progressive Matrices. London: H.K. Lewis. Reed, T.E., & Jensen, A.R. (1991). Arm nerve conduction velocity (NCV), brain NCV, reaction time, and intelligence. Intelligence, IS, 33-47. Vernon, P.A. (Ed.). (1987). Speed of jnformurionprocessing and imeiligence. Norwood, NJ: AbIex. Vernon, P.A., & Mori, M. (1992). Intelligence, reaction times, and peripheral nerve conduction velocity. Intelligence, 16, 273-288. Vernon, P.A., Nador, S., & Kantor, L. (1985). Reaction times and speed of information processing: Their relationship to timed and untimed measures of intelligence. fntef&gence, 9, 357-374. Wechsler, D. f 198 1). Manual for the Wechsler Adult intelligence Scale-Revised. New York: Psychological Corporation. Barrett,
18, 127- 13 I ( 1994)
Peripheral Nerve Conduction Velocity, Reaction Time, and Intelligence: An Attempt to Replicate Vernon and Mori (1992) JOHN PHILIP
C. WICKETT A.
VERNON
University of Western Ontario
In an attempt to examine the relationship between neural speed, as indicated by nerve conduction velocity (NCV) along the median nerve of the arm, and intelligence, four relevant studies have been carried out: two finding the relationship and two not finding evidence of such a relationship. In an attempt to replicate the two studies (Vernon & Mori, 1992) finding this relationship, 38 healthy, right-handed females, aged 20 to 30 years, completed the Multidimensional Aptitude Battery (Jackson, 1984), a series of reactiontime tasks and were submitted to the same NCV procedures as in Vernon and Mori (1992). Contrary to prediction, NCV did not correlate with intelligence or reaction time. A reanalysis of Vernon and Mori (1992), however, showed evidence for a possible sex difference in relation to NCV and intelligence, with a pronounced correlation between these variables being found in males but a much smaller correlation being found in females.
One of the predominant views within contemporary intelligence research is that speed of information processing is a key component of what is meant by “intelligence.” That is, the intelligent individual is one who can quickly and effectively process incoming stimuli, with this efficient system allowing for increased information handling capacity. Evidence for this speed factor is provided by studies employing the reaction time (RT) paradigm, and it is well established that RT on elementary cognitive tasks is negatively correlated with IQ (e.g., Vernon, 1987). It is logical, given the relationship between RT and IQ, to ponder what the neural substrate of this speed factor might be. The simplest, and clearest, explanation is that the speed at which the neuron fires is the key neural underpinning. In an attempt to assess the viability of neural speed as a factor underlying intelligence, four studies have been conducted assessing the relationship between peripheral nerve conduction velocity (NCV) and IQ. Peripheral NCV (specifically, in the arm) involves no cognitive component and is thus purely physiologi-
Funding for this project was provided by a grant to the second author from The Pioneer Fund, Inc. Correspondence and requests for reprints should be sent to P.A. Vernon, Department of Psychology, University of Western Ontario, London, Ontario, Canada N6A 5C2.
127
128
WICKETT AND VERNON
cal. Interest in the relationship is primarily due to this noncognitive nature of NCV. The discovery of a positive correlation between NCV and IQ would show evidence that IQ can be measured and explained, at least in part, by purely neural mechanisms. Vernon and Mori (1992) present two studies that provide such evidence. In the first study, 85 male and female undergraduates (M age = 24.5 years; M IQ = 114.9) completed the Multidimensional Aptitude Battery (MAB; Jackson, 1984) as the intelligence measure, a set of RT tasks, and had eight NCV measurements taken along three different segments of the median nerve of the arm (wrist-tofinger, wrist-to-elbow, and wrist-to-axilla). First unrotated factor scores for each of the three sets of measures (IQ, RT, and NCV) were obtained, and intercorrelations determined. As expected, general RT correlated negatively with general IQ (r = -.44); additionally, general RT correlated negatively with general NCV (Y = -.28), and general NCV correlated positively with general IQ (Y = .42). Correcting for sex or age had no effect on the correlations derived. This indicates that the speed at which a neural impulse travels is related to intelligence, with faster speeds being related to higher intelligence. Study 2 of Vernon and Mori (1992) was essentially identical to Study I, except that fewer RT tasks were employed, and only wrist-to-finger NCV was measured. In this second sample of 88 male and female undergraduates (M age = 23.1 years; M IQ = 115) essentially the same findings were discovered: RT correlated negatively with IQ (r = - .4S), and also negatively with NCV (r = - .18), and NCV correlated positively with IQ (r = .48). As with Study 1, age and sex had no appreciable effect on the correlations derived. Thus, given the results of both studies, fairly substantial support is provided for a moderate positive correlation between NCV and IQ. Two other studies, however, do not provide such support, although procedural differences may account for at least some of the discrepancy. Barrett, Daum, and Eysenck (1990), in a sample of 44 males and females, did not find a significant correlation between NCV and intelligence as measured by the Advanced Raven Matrices (Raven, 1983a). This study, however, did not use supramaximal stimulation of the median nerve, and as such all nerve fibers would not have been activated. This is a substantial departure from the Vernon and Mori ( 1992) procedure, and thus the findings are not comparable. Reed and Jensen (1991), in a sample of 200 male university and community college students, also did not find evidence for a relationship between NCV and IQ as measured by both the Standard and Advanced Raven Matrices (Raven, 1983a, 1983b). One difference between this study and those of Vernon and Mori (1992) is that Reed and Jensen controlled statistically for arm temperature, rather than directly using a heating pad as per Vernon and Mori. Arm temperature is a key factor in nerve conduction speed, and as such the appropriate regulation of temperature is imperative. The contradictory findings, coupled with the methodological differences between the Vernon and Mori ( 1992) studies and those of Barrett et al. ( 1990) and Reed and Jensen (1991), make it difficult to draw any firm conclusions regarding
NCV, RT, AND IQ: A REPLICATION
129
the relationship between NCV and IQ. There is clearly a need to adopt similar methodology in order to shed some light on this problem. To this end, and in an attempt to provide confirmation of a positive relationship between NCV and IQ, this study was carried out using methodology identical to that of Vernon and Mori (1992). Specifically, it is predicted that NCV along the median nerve will be positively correlated with IQ as assessed by the MAB. Additionally, RT, as measured by a set of four RT tasks, will be negatively correlated with both IQ and NCV.
METHOD Subjects Subjects for this study were 38 healthy, right-handed adult females, aged 20 to 30 years (M = 24.59). Subjects volunteered by answering an advertisement in a local newspaper, and were paid for their participation. Forty-one subjects originally took part in the study, but three subjects had to be discarded when it was not possible to get a “clean” reading of their NCV. All subjects signed a consent form and were debriefed. Measures IQ was assessed using the MAB which provides a Full-Scale IQ (FSIQ), a Verbal IQ (VIQ), and a Performance IQ (PIQ) score. It also provides subscale scores similar to those provided by the WAIS-R (Wechsler, 1981) on which the MAB is based. The MAB is a timed, multiple-choice test. General intelligence, or GIQ, was estimated by submitting the 10 subtests of the MAB to a factor analysis and then computing factor scores on the first unrotated factor, which accounted for 50.5% of the variance. Four RT measures were obtained using standard RT tasks. Detailed description of these tasks can be found in Vernon, Nador, and Kantor (t98.5), with the particular tasks used in the present study including measures of short-term memory, synonyms and antonyms, and simple arithmetic. Scores for each task for each subject were transformed into z scores and then summed together within each subject to obtain a composite RT score. NCV was measured using techniques identical to that of Vernon and Mori (1992) with the following exception: Only one wrist-to-finger and one wrist-toelbow velocity were obtained. Supramaximal stimulation of the median nerve of the right arm was employed, and a heating pad maintained arm temperature between 32 and 35°C. NCV measures are reported in meters per second. Procedure IQ testing was completed during one session lasting approximately 1.5 hr, with groups of I to 5 subjects completing the test together. Additionally, subjects were
130
WICKETI’ AND VERNON
tested individually for handedness (Kimura, 1986) during this session to insure that all were right-handed. The RT and NCV measures were obtained in a second individual session with the RT tasks being completed first. Subjects were paid and debriefed upon completion of the NCV procedure.
RESULTS Mean FSIQ for the group of 38 subjects was 107.82 (SD = 11.6X), indicating that they were within the average range of intelligence. Mean wrist-to-elbow NCV was 60.52 m/s (SD = 3.74), and mean wrist-to-finger NCV was 59.03 m/s (SD = 4.48). These NCVs are comparable to those in Vernon and Mori (1992). Mean RTs (in ms) on the four RT measures ranged from 661.73 to 1156.40. These are also comparable to the RTs of Vernon and Mori’s (1992) subjects. As predicted, GIQ correlated negatively with the composite RT score (r = -.24, ns.). Although this correlation is not significant, it is within the normal range of RT-IQ correlations. Contrary to prediction, neither wrist-to-finger NCV nor wrist-to-elbow NCV correlated significantly with GIQ (rs = -. 12 and .02, respectively, both n.s.). Neither NCV measure correlated significantly with any of the MAB subscales, and they also did not correlate significantly with VIQ or PIQ. Also contrary to prediction, the NCV measures were not correlated with composite RT (TS = .02 and .Ol, both n.s.).
DISCUSSION In direct contrast to the findings of Vernon and Mori (1992), the present study failed to find a significant relationship between NCV and IQ. Additionally, and also contradictorily, NCV was not related to RT, although RT did show the traditional negative relationship with IQ. These results would seem to indicate that nerve conduction velocity is not an impo~ant factor in explaining individual differences in intellectual ability. This confirms the null.findings of Reed and Jensen (1991) and Barrett et al. (1990) although, as mentioned earlier, these two studies used methodologies different from Vernon and Mori (1992). The present findings prompted a reanalysis of the Vernon and Mori (1992) data. Because only females were used in the present study, separate analyses for males and females were performed on the data from both of the Vernon and Mori (1992) samples. Surprisingly, the correlations obtained for males and for females were substantially different. In Study 1 of Vernon and Mori (with no sex difference for mean RT, NCV, or IQ), the obtained correlations between RT, NCV, and IQ for males were in each case higher than those found for females. Specifically, for males (n = 40) and females (n = 45), respectively: general IQ and general NCV correlated Y = .62 (p < .Ol) and r = .28 (n.s.); general IQ and general RT
NCV, RT, AND IQ: A REPLICATION
131
correlated r = - .66 (p < .Ol) and r = - .43 (p < .Ol); and general NW and general RT correlated r = - .58 (p < .Ol) and r = - .05 (n.s.). A similar pattern is observed in Study 2. Again for males (n = 38) and females (n = 50), respectively: IQ and NCV correlated r = .54 (P < .Ol) and r = ‘37 (p < .Ol); IQ and RT correlated r = -.68 (p < .Ol) and r = -.37 (p < .Ol); and NCV and RT correlated r = -.24 (n.s.) and r = -.03 (n.s.). The only comparisons that reached significance were the male-female difference in Study I for general NCV and genera1 RT (Z = 2.71, p < .Ol), and in Study 2 for IQ and RT (Z = 1.98, p < .05); all other Zs ranged from 0.96 to 1.94. Although only two of these six comparisons show a significant sex difference, the fact that the same pattern exists in both sets of data suggests what may be a true sex difference. Indeed, use of the technique outlined by Fisher (1958) for combining p values from independent studies reveals that the male-female difference for the IQ-RT correlation is significant [x2(4) = 9.999,~ < .OS], as is the difference observed in the NCVRT correlations [x2(4) = 14.443, p < .Ol]. For NCV and IQ the male-female difference is marginally significant [x*(4) = 8.087, p < . lo]. In conclusion, the disparity between the correlations obtained for males and females in the Vernon and Mori (1992) studies, along with the null findings for females in the present study, raise interesting questions concerning sex differences in intelligence. Although it is clear that further study is required, the present results suggest males rely quite heavily on neuronal speed to perform cognitive tasks, whereas for females neural speed is only minimally implant, and it is rather some other, and as yet unknown, neural processes that play the predominant role.
REFERENCES P.T., Daum, I., & Eysenck, H.J. (1990). Sensory nerve conduction and intelligence: A meth~ological study. Journal of Psychophysiology, 4, I- 13. Fisher, R.A. (1958). Sratisticul methods for research workers (13th ed.). New York: Hafner. Jackson, D.N. (1984). Multidimensional Aptitude Battery. Port Huron, MI: Research Psychologists Press. Kimura, D. (1986). Neuropsychology tesr procedures. London, Ontario: DK Consultants. Raven, J.C. (1983a). Advu~ced Raven Progressive Matrices. London: H.K. Lewis. Raven, J.C. (1983b). Standard Progressive Matrices. London: H.K. Lewis. Reed, T.E., & Jensen, A.R. (1991). Arm nerve conduction velocity (NCV), brain NCV, reaction time, and intelligence. Intelligence, IS, 33-47. Vernon, P.A. (Ed.). (1987). Speed of jnformurionprocessing and imeiligence. Norwood, NJ: AbIex. Vernon, P.A., & Mori, M. (1992). Intelligence, reaction times, and peripheral nerve conduction velocity. Intelligence, 16, 273-288. Vernon, P.A., Nador, S., & Kantor, L. (1985). Reaction times and speed of information processing: Their relationship to timed and untimed measures of intelligence. fntef&gence, 9, 357-374. Wechsler, D. f 198 1). Manual for the Wechsler Adult intelligence Scale-Revised. New York: Psychological Corporation. Barrett,