Abstract
Objective
Previous studies of handedness and cognitive function rely on self-classification and yield inconsistent results. This study examines the associations of self-reported versus grip-strength-based handedness with cognitive function in healthy older men and women.
Design
Cross-sectional study.
Setting
1988–91 follow-up clinic visit and 1991 mailed survey.
Participants
684 men and 985 women aged 55–95 who were community dwelling.
Measurements
Cognitive function was assessed with 12 tests and grip strength was measured by hand-held dynamometer. Self-reported handedness was obtained with a mailed survey.
Results
By self-report, 92.1% of men and women were right-handed; 2.0% were left handed. By grip strength, in men, 64.3% were right-handed, 22.5% left-handed, and 13.2% ambidextrous. In women, 61.3% were right-handed, 17.3% left-handed, and 21.4% ambidextrous. No cognitive function differences were found by self-reported handedness in either sex (p’s>0.10). However, based on grip strength, left-handed women scored poorer than right-handed or ambidextrous women in immediate and delayed memory, attention, and verbal fluency (p’s<0.05). Using categorical definitions, left-handed or ambidextrous individuals based on grip strength were more likely to show poor cognitive function on 4 of 5 tests.
Conclusion
Grip strength is a useful alternative to self-reports for classifying handedness. Left-handedness by grip-strength, may be related to poorer cognitive function; this association may vary by gender.
Keywords: cognitive function, grip strength, handedness, laterality, memory
Approximately 10% of all individuals are left-handed, with men comprising the majority (1–4). However, variable criteria exist to assess handedness and reported rates of left-handedness range from 3.6% to 39.6% (5). Additionally, defining the spectrum of handedness to classify individuals as “consistently left-handed,” “consistently right-handed,” or “ambidextrous” has proved difficult (6–8). The term handedness encompasses a wide range of definitions including one or more of the following: a) hand “preference” (6, 7, 9); b) hand “skillfulness” (6, 7, 9); and/or c) hand “strength” (6, 10, 11).
Previous studies examining the association of handedness with cognitive function showed inconsistent results. These studies generally rely on self-classification of handedness, and often exclude or reclassify ambidextrous individuals. Furthermore, few studies examine the association of handedness with cognitive function in older adults (3, 12, 13, 14), who are at the age when memory loss and cognitive impairment become more apparent.
The ability to show an association of handedness with cognitive performance depends on the ability to accurately classify handedness. However, forced hand switching in early childhood coupled with an inability to objectively classify ambidextrous individuals into a separate group makes self-reports less than optimal for classifying handedness and may account for some of the inconsistency. A more objective measure, such as grip strength, may be a viable alternative for the classification of handedness.
According to a classic study by Woo and Pearson (15), measurement with a grip dynamometer is more effective than direct questioning for determining native handedness. It has been reported that grip strength between hands may differ by as much as 40% (6, 16). While it was suggested (17) that the nondominant or “holding” hand has more strength than the dominant or “operating” hand, the majority found stronger grip strength in the dominant hand (6, 16, 18–22). This was especially evident for right-handed individuals, whereas left-handed individuals exhibited a greater equality of strength although differences were still apparent (13, 19, 23).
The purpose of the present study is to compare the associations of self-reported handedness versus handedness objectively measured by grip strength with standard tests of cognitive function in older community-dwelling adults.
METHODS
Participants
Participants were community-dwelling men and women aged 55–95 years from the Rancho Bernardo Study. In 1972–1974, 6,339 individuals, representing 82% of all adults aged ≥30 living in Rancho Bernardo (a community in Southern California), were enrolled in a study of heart disease risk factors. All were Caucasian, relatively well-educated, and middle-class. A total of 1,727 men and women aged ≥50 participated in a 1988–91 clinic visit when a battery of cognitive function tests was administered and grip strength was measured. After excluding 54 individuals with a history of stroke, TIA, or other impairment that could affect grip strength, and 4 participants missing grip strength measurements, there remained 684 men and 985 women aged 55–95 for whom cognitive function and grip strength data were available. In 1991, a mailed questionnaire including self-reported handedness was completed by 1,415 of the participants who had attended the 1988–91 clinic visit. Cognitive function data were available for 1,363 (n=556 men and 807 women) of these respondents. All participants were ambulatory and gave written consent; this study was approved by the Human Subjects Protection Program of the University of California, San Diego.
Procedures
At the 1988–1991 clinic visit, age, education, and current medication use were assessed by questionnaire. Medication use was validated by pills and containers brought to the clinic for that purpose.
A battery of standardized tests, chosen to assess multiple, diverse aspects of cognitive function, was administered by a trained interviewer. All cognitive tests had demonstrated adequate reliability and validity (24) and included:
The Buschke-Fuld Selective Reminding Test (25) assesses short and long-term storage, retention, and retrieval of spoken words. Ten unrelated words are read to the subjects at the rate of 1 every 2 seconds. Immediately after, the subject is asked to recall the entire list. This procedure is followed for six trials. Measures of long- and short-term storage and total recall were used. Higher scores on the short-term memory test indicate poorer performance.
The Heaton Visual Reproduction Tests (26) adapted from the Wechsler Memory Scale (27) assesses memory for geometric forms. Three stimuli of increasing complexity are presented, one at a time, for 10 seconds each. The subject is asked to reproduce the figures immediately and after 30 minutes of unrelated testing. After both memory trials have been administered, the subject is asked to copy the stimulus figures to assess visual-spatial impairments. Three scores are obtained: immediate recall, delayed recall, and copying.
The Mini-Mental State Examination (28, 29) assesses orientation, registration, attention, calculation, language, and recall. Total Mini-Mental State Examination scores range from 0 to 30; subjects with dementia usually score below 24. Two items from the Mini-Mental State Examination were analyzed separately: counting backward from 100 by sevens (Serial 7’s), which assesses calculation, and spelling the word “world” backward (“World” Backwards), which assesses attention. For both items, the maximum possible score is 5.
Two items from the Blessed Information-Memory-Concentration Test (Blessed) (24) assess mental control and concentration by having the subject name the months of the year backward, and assess memory by asking the subjects to recall a five-part name and address following a 10-minute delay. The maximum possible score across the two items is 7.
Trails B (from the Halstead-Reitan Neuropsychological Test Battery) (30) tests visuomotor tracking and attention. Subjects scan a page continuously to identify numbers and letters in a specified sequence while shifting from number to letter sets. A maximum of 300 seconds is allowed; performance is rated by the time required to finish the test; the greater the score, the poorer the test performance.
In Category Fluency, (31), the subject names as many animals as possible in 1 minute. The score is the number of animals named correctly. Repetitions, variants (e.g., dogs after producing dog), and intrusions (e.g., apple) are not counted.
Bilateral grip strength was measured using a hand-held dynamometer (Therapeutic Instruments Hand Dynometer, Serial No. 0687083). At the start position, the subject’s elbow was flexed at a 90-degree angle with the forearm parallel to the floor. The dynamometer was maximally squeezed for a 3-second count while simultaneously lowering the arm to full extension. Subjects practiced this procedure once per hand, after which measurements were recorded to the nearest 0.1 pound. The instrumental precision error was 1.0%.
The 1991 mailed questionnaire included the following question about hand preference: “If you were originally left-handed, were you ever strongly encouraged or forced to switch to your right hand?” and allowed the following response choices: 1) Never left-handed, always have been right-handed; 2) Naturally and still left-handed; 3) Yes, forced to change to right hand; 4) Naturally left-handed, now ambidextrous; and 5) Naturally ambidextrous.
Statistical analysis
Educational level was dichotomized into no college versus some college or more (32). Based on self-reported handedness on the 1991 mailed questionnaire participants were classified as “right-handed” versus “all-others” which included those naturally left-handed, switched from left-handed to right-handed, naturally left-handed and now ambidextrous, or naturally ambidextrous. Comparisons of mean cognitive function scores by self-reported handedness after adjustment for age, education, and current estrogen use in women were performed with analysis of covariance.
Classification of handedness by grip strength was as follows: Individuals whose grip strength was >.45 kg (>1 pound) greater in the right than the left hand were categorized as “Right-Handed.” Individuals whose grip strength was >.45 kg (>1 pound) greater in the left than the right hand were classified as “Left-Handed.” Those who had a ≤.45 (≤1 pound) difference in grip strength between their left and right hands were classified as “Ambidextrous.” Comparisons of age by handedness based on grip strength were performed using analysis of variance. Comparisons of educational level and, in women, current estrogen use by handedness based on grip strength were performed with chi-square analysis. Comparisons of cognitive function scores by handedness based on grip strength after adjustment for age, educational level, and current estrogen use in women were performed with analysis of covariance; the right-handed group served as the reference group.
Cognitive function scores were also analyzed as categorical outcome variables. Cutoff values indicative of poor performance were obtained from the Alzheimer’s Disease Research Center of the University of California, San Diego and were available for five tests: the long-term recall task of the Buschke Selective Reminding Test, the immediate recall task of the Heaton Visual Reproduction Test, the Mini-Mental Status Exam, Trails B, and Category Fluency. Logistic regression was used to examine the association of handedness by grip strength with risk of poor performance on cognitive function tests after adjustment for age, education and in women, estrogen use.
RESULTS
At the 1988–91 clinic visit, age ranged from 55 to 95 years (means=73.4±9.0 for men and 73.0±9.2 for women). Overall, 78.6% of men and 62.5% of women had completed at least some college education. All women were postmenopause; 36.8% reported current use of estrogen therapy.
On the 1991 questionnaire, 92.1% of respondents reported they were “always right-handed,” 2.0% were “naturally and still left-handed,” 3.5% were “forced to change to their right hand,” and the remainder were “naturally left-handed, now ambidextrous” or “naturally ambidextrous” (each accounting for 1.2%). Comparisons of grip strength by self-reported handedness showed that among those self-classified as “always right-handed,” grip strength was concordant in 68.4% of cases; among those “naturally and still left handed,” grip strength was concordant in 67.9% of cases. The majority (57.1%) “forced to change to the right hand” had a stronger right hand. Only 5.9% of the “naturally ambidextrous” group was classified as ambidextrous based on similarity of bilateral grip strength. Comparisons of cognitive function scores by self-reported handedness are shown in Table 1. There were no significant differences (p’s>0.10) between those who were and were not right-handed on any of the cognitive function tests in men or women after adjustment for age, education, and use of postmenopausal estrogen.
Table 1.
Adjusted* mean cognitive function scores by self-reported handedness (always right-handed versus all other hand preference types) for men and women
| MEN Right Handed | Women Right Handed | |||||
|---|---|---|---|---|---|---|
| n | Yes | No† | n | Yes | No† | |
| Buschke Selective Reminding | ||||||
| Total recall- Age adjusted | 537 | 36.1 | 34.7 | 796 | 40.4 | 40.1 |
| Age & college adjusted | 527 | 36.1 | 34.7 | 793 | 40.4 | 40.0 |
| Age, college & estrogen adjusted | 793 | 40.4 | 39.9 | |||
| Long-term recall- Age adjusted | 537 | 28.5 | 27.5 | 796 | 34.7 | 33.8 |
| Age & college adjusted | 527 | 28.5 | 27.4 | 793 | 34.8 | 33.7 |
| Age, college & estrogen adjusted | 793 | 34.8 | 33.5 | |||
| Short-term recall- Age adjusted | 537 | 7.6 | 7.2 | 796 | 5.6 | 6.5 |
| Age & college adjusted | 527 | 7.6 | 7.2 | 793 | 5.6 | 6.5 |
| Age, college & estrogen adjusted | 793 | 5.6 | 6.6 | |||
| Heaton Visual Reproduction | ||||||
| Immediate recall- Age adjusted | 550 | 10.6 | 10.2 | 802 | 9.7 | 9.1 |
| Age & college adjusted | 540 | 10.6 | 10.2 | 799 | 9.7 | 9.0 |
| Age, college & estrogen adjusted | 799 | 9.7 | 9.0 | |||
| Delayed recall- Age adjusted | 546 | 8.3 | 7.8 | 797 | 7.0 | 6.7 |
| Age & college adjusted | 536 | 8.3 | 7.8 | 794 | 7.0 | 6.7 |
| Age, college & estrogen adjusted | 794 | 7.0 | 6.7 | |||
| Copying- Age adjusted | 546 | 15.4 | 15.3 | 798 | 15.3 | 15.1 |
| Age & college adjusted | 536 | 15.3 | 15.1 | 795 | 15.3 | 15.1 |
| Age, college & estrogen adjusted | 795 | 15.3 | 15.1 | |||
| Minimental-Age adjusted | 556 | 27.1 | 27.1 | 807 | 27.4 | 27.3 |
| Age & college adjusted | 546 | 27.1 | 27.1 | 804 | 27.4 | 27.3 |
| Age, college & estrogen adjusted | 804 | 27.4 | 27.3 | |||
| Serial 7’s- Age adjusted | 552 | 4.3 | 4.4 | 779 | 4.0 | 4.1 |
| Age & college adjusted | 542 | 4.4 | 4.5 | 776 | 4.0 | 4.1 |
| Age, college & estrogen adjusted | 776 | 4.0 | 4.0 | |||
| “World” backward- Age adjusted | 553 | 4.7 | 4.7 | 803 | 4.9 | 4.9 |
| Age & college adjusted | 543 | 4.7 | 4.7 | 800 | 4.9 | 4.9 |
| Age, college & estrogen adjusted | 800 | 4.9 | 4.9 | |||
| Blessed Items- Age adjusted | 554 | 6.1 | 6.1 | 801 | 6.2 | 6.1 |
| Age & college adjusted | 544 | 6.1 | 6.1 | 798 | 6.2 | 6.1 |
| Age, college & estrogen adjusted | 798 | 6.2 | 6.0 | |||
| Trails B (seconds)- Age adjusted | 546 | 119.1 | 125.8 | 803 | 132.7 | 139.0 |
| Age & college adjusted | 536 | 119.3 | 126.0 | 800 | 132.7 | 139.5 |
| Age, college & estrogen adjusted | 800 | 132.8 | 139.3 | |||
| Category fluency- Age adjusted | 554 | 19.2 | 18.3 | 802 | 17.7 | 17.2 |
| Age & college adjusted | 544 | 19.2 | 18.3 | 799 | 17.7 | 17.1 |
| Age, college & estrogen adjusted | 799 | 17.7 | 17.1 | |||
Adjusted comparisons performed with analysis of covariance, all p’s >0.10
No= Always left handed, forced to change to right hand, naturally left and now ambidextrous, and naturally ambidextrous;
Among men, grip strength in the right hand ranged from 1.5 to 26.6 kg (mean=16.3±3.9) and in the left hand ranged from 1.5 to 26.1 kg (mean=15.4±3.7). Among women, grip strength in the right hand ranged from 1.4 to 20.0 kg (mean=9.1±2.4) and in the left hand ranged from 1.4 to 18.2 kg (mean=8.4±2.3). Based on grip strength, 22.5% of men and 17.3% of women were classified as left-handed, 64.3% of men and 61.3% of women were right-handed, and the remaining men and women were ambidextrous. Mean grip strength in each hand for men and women classified as right, left, or ambidextrous is shown in Figure 1. For both men and women, the largest differences in grip strength between the left and right hands was observed among those classified as right-handed (means=15.1 kg vs. 17.1 kg in men and 8.0 kg vs. 9.6 kg in women for comparisons of left vs. right hand grip strength, respectively).
Figure 1.
Grip strength measured in each hand for men and women classified left-handed, right-handed or ambidextrous
As shown in Table 2, left-handed men were significantly older than right-handed and ambidextrous men (p’s<0.05). However, there were no significant differences by handedness in the proportion completing some college or more in men and there were no significant differences by handedness in age, education or estrogen use in women (p’s>0.10).
Table 2.
| Left-handed† | Ambidextrous† | Right-handed† | p | |
|---|---|---|---|---|
| Men (N=684) | ||||
| n (%) | 154 (22.5%) | 90 (13.2%) | 440 (64.3%) | |
| Age (mean) | 75.5§ | 72.8 | 72.8 | .006 |
| Collegea (%) | 72.4 | 80.7 | 80.3 | .106 |
| Women (N=985) | ||||
| n (%) | 211 (21.4%) | 170 (17.3%) | 604 (61.3%) | |
| Age (mean) | 73.5 | 73.4 | 72.7 | .402 |
| Collegea (%) | 59.2 | 64.9 | 62.9 | .495 |
| Current estrogen use (%) | 37.0 | 41.2 | 35.4 | .389 |
Analysis of variance was used to compare age. Chi-square tests were used to compare the proportions who completed some college or more, and in women, the proportions currently using estrogen.
Individuals whose grip strength was >.45 kg greater in the right than the left hand were labeled “right-handed”; individuals whose grip strength was >.45 kg greater in the left than the right hand were labeled “left-handed”; those with ≤.45 kg difference between hands were labeled “ambidextrous”.
College=some college or more
p<0.05 for comparisons of left-handed vs. right handed and left-handed vs. ambidextrous
Table 3 shows comparisons of cognitive function test scores by handedness based on grip strength after adjustment for age, education, and, in women, current estrogen use. For men, there were no significant differences in cognitive function scores by handedness based on grip strength (p’s>0.10). In contrast, left-handed women had significantly lower scores than right-handed or ambidextrous women on the Heaton Visual Reproduction immediate recall (p’s<0.05) and delayed recall (p’s<0.01) tests. Additionally, differences by handedness were also found for women in scores on the Blessed and category fluency tests with left-handed women having significantly lower scores than right-handed women (p’s<0.05). There were no other differences by handedness based on grip strength for any of the other cognitive function tests.
Table 3.
Adjusted comparisons of cognitive function scores by handedness based on grip strength in men and women
| MEN |
Women |
|||||||
|---|---|---|---|---|---|---|---|---|
| n | LH | AMB | RH | n | LH | AMB | RH | |
| Buschke Selective Reminding | ||||||||
| Total recall- Age adjusted | 630 | 35.1 | 35.5 | 35.0 | 914 | 39.3 | 39.2 | 40.2 |
| Age & college adjusted | 618 | 35.0 | 35.5 | 34.9 | 905 | 39.4 | 39.2 | 40.2 |
| Age, college & estrogen adj | 905 | 39.4 | 39.2 | 40.2 | ||||
| Long-term recall- Age adjusted | 630 | 27.0 | 27.7 | 27.4 | 914 | 33.5 | 33.2 | 34.3 |
| Age & college adjusted | 618 | 27.0 | 27.7 | 27.3 | 914 | 33.6 | 33.2 | 34.4 |
| Age, college & estrogen adj | 914 | 33.6 | 33.2 | 34.4 | ||||
| Short-term recall- Age adjusted | 630 | 8.0 | 7.9 | 7.6 | 914 | 5.9 | 6.1 | 5.9 |
| Age & college adjusted | 618 | 8.1 | 7.9 | 7.6 | 905 | 5.8 | 6.1 | 5.8 |
| Age, college & estrogen adj | 905 | 5.8 | 6.1 | 5.8 | ||||
| Heaton Visual Reproduction | ||||||||
| Immediate recall- Age adjusted | 649 | 10.1 | 10.0 | 10.1 | 927 | 9.0 | 9.6* | 9.6‡ |
| Age & college adjusted | 637 | 10.2 | 9.9 | 10.1 | 918 | 9.0 | 9.6* | 9.6‡ |
| Age, college & estrogen adj | 918 | 9.0 | 9.6† | 9.6‡ | ||||
| Delayed recall- Age adjusted | 642 | 7.5 | 7.9 | 7.7 | 920 | 6.1 | 7.0† | 6.9§ |
| Age & college adjusted | 630 | 7.5 | 7.8 | 7.7 | 911 | 6.2 | 7.0* | 6.9§ |
| Age, college & estrogen adj | 911 | 6.2 | 7.0* | 6.9§ | ||||
| Copying- Age adjusted | 642 | 15.1 | 15.2 | 15.2 | 922 | 15.1 | 15.3 | 15.2 |
| Age & college adjusted | 630 | 15.1 | 15.2 | 15.2 | 913 | 15.1 | 15.3 | 15.2 |
| Age, college & estrogen adj | 913 | 15.1 | 15.3 | 15.2 | ||||
| Minimental-Age adjusted | 657 | 26.5 | 27.0 | 26.8 | 934 | 27.1 | 27.3 | 27.4 |
| Age & college adjusted | 645 | 26.5 | 27.0 | 26.7 | 925 | 27.1 | 27.2 | 27.4 |
| Age, college & estrogen adj | 925 | 27.1 | 27.2 | 27.4 | ||||
| Serial 7’s- Age adjusted | 652 | 4.3 | 4.4 | 4.4 | 902 | 3.9 | 3.9 | 4.0 |
| Age & college adjusted | 640 | 4.3 | 4.3 | 4.4 | 893 | 3.9 | 3.9 | 4.0 |
| Age, college & estrogen adj | 893 | 3.9 | 3.9 | 4.0 | ||||
| “World” backward- Age adjusted | 650 | 4.6 | 4.7 | 4.7 | 930 | 4.8 | 4.8 | 4.9 |
| Age & college adjusted | 638 | 4.6 | 4.7 | 4.7 | 921 | 4.8 | 4.8 | 4.9 |
| Age, college & estrogen adj | 921 | 4.8 | 4.8 | 4.9 | ||||
| Blessed Items- Age adjusted | 654 | 6.0 | 5.7 | 6.0 | 927 | 6.0 | 6.0 | 6.2‡ |
| Age & college adjusted | 642 | 6.0 | 5.7 | 6.0 | 918 | 6.0 | 6.0 | 6.2‡ |
| Age, college & estrogen adj | 918 | 6.0 | 6.0 | 6.2‡ | ||||
| Trails B (seconds)- Age adjusted | 638 | 134.5 | 127.1 | 124.4 | 925 | 138.2 | 130.8 | 140.8 |
| Age & college adjusted | 626 | 134.5 | 128.4 | 124.5 | 916 | 137.8 | 130.9 | 141.3 |
| Age, college & estrogen adj | 916 | 137.8 | 130.8 | 141.4 | ||||
| Category fluency- Age adjusted | 654 | 18.3 | 18.4 | 18.6 | 929 | 16.8 | 17.2 | 17.7‡ |
| Age & college adjusted | 642 | 18.3 | 18.5 | 18.6 | 920 | 16.8 | 17.2 | 17.7‡ |
| Age, college & estrogen adj | 920 | 16.8 | 17.2 | 17.7‡ | ||||
Comparisons adjusted for age, some college education, and estrogen use in women with analysis of covariance;
p <.05,
p<0.01, for comparisons of left-handed with ambidextrous;
p<0.05,
p<0.01 for comparisons of left-handed with right-handed.
Risk of poor performance on specific cognitive function tests based on categorically defined criteria was examined by handedness based on grip strength using logistic regression (see Table 4). Compared to those who were right-handed, left-handed men were significantly more likely to perform poorly on the mini-mental status exam (odds ratio [OR]=1.80, 95% confidence interval [CI]=1.03, 3.16) and Trails B (OR=1.61, CI=1.03, 2.51), and left-handed women were significantly more likely to perform poorly on category fluency (OR=1.63; CI=1.04, 2.56). Additionally, as compared to those who were ambidextrous, left-handed men had almost twice the risk of poor performance on the Heaton immediate recall task (OR=1.91; CI=1.07, 3.40). No other differences by handedness in risk of poor performance were observed on any of the other cognitive function tests.
Table 4.
Comparisons of risk of poor performance† on cognitive function tests in left-handed and ambidextrous versus right-handed as classified by grip strength
| Sex | Test | Left vs. Right | Ambidextrous vs. Right | ||
|---|---|---|---|---|---|
| OR | CI | OR | CI | ||
| Men | |||||
| Buschke- Long Term Recall ≤13 | 1.02 | 0.59 – 1.74 | 1.04 | 0.52 – 2.08 | |
| HVR Immediate recall ≤7 | 1.09 | 0.68 – 1.74 | 1.91 | 1.07 – 3.40 | |
| MMSE ≤24 | 1.80 | 1.03 – 3.16 | 0.61 | 0.24 – 1.58 | |
| Trails B ≥132 seconds | 1.61 | 1.03 – 2.51 | 1.15 | 0.64 – 2.06 | |
| Category fluency ≤12 | 0.72 | 0.40 – 1.30 | 0.59 | 0.26 – 1.37 | |
| Women | |||||
| Buschke Long term recall ≤13 | 1.01 | 0.51 – 2.01 | 1.75 | 0.92 – 3.32 | |
| HVR Immediate recall ≤7 | 1.23 | 0.86 – 1.78 | 0.94 | 0.63 – 1.41 | |
| MMSE ≤24 | 1.48 | 0.73 – 3.01 | 2.01 | 0.98 – 4.10 | |
| Trails B ≥132 | 1.01 | 0.70 – 1.46 | 0.73 | 0.49 – 1.09 | |
| Category fluency ≤12 | 1.63 | 1.04 – 2.56 | 1.39 | 0.85 – 2.29 | |
Categorical cutoffs indicative of poor performance were: Buschke Long-term Recall ≤13; Heaton Visual Reproduction (HVR) ≤7; Mini-mental Status Exam (MMSE) ≤24; Trails B ≥132; Category fluency ≤12. Odds ratios for risk of poor performance were calculated with logistic regression and adjusted for age, education and in women, estrogen use; odds ratios with confidence intervals including 1.0 are not considered statistically significant.
OR=odds ratio, CI=95% confidence interval.
DISCUSSION
No differences in cognitive function test scores by handedness based on self-report were observed in men or women, but only 2% reported natural left-handedness. Women, but not men, classified by grip strength as left-handed or ambidextrous showed statistically significant deficits in performance on tests of immediate and delayed memory, attention, and verbal fluency relative to those classified as right-handed. Left-handed and ambidextrous men and left-handed women were more likely to score below established cutoffs in 4 tests.
In one study that classified individuals by self-reported handedness, 96% of right-handers demonstrated left hemisphere lateralization of language on functional MRI, while only 4% had bilateral recruitment of cerebral hemispheres (33). In contrast, 76% of left-handers had left hemisphere lateralization, 14% had bilateral involvement, and 10% had right hemisphere lateralization (33). Thus, although left-handedness is an obvious behavioral asymmetry possibly corresponding to an alternate cerebral lateralization pattern and subsequent differences in cognitive performance, the majority of left-handers have left-hemisphere language dominance. In another study, “converted left-handers,” those forced to write with their right hand in elementary school and who continued to write right-handed (mean age=47), continued to display neuroanatomical differences when compared with age-matched adult right-handers (34). These studies support the concept of neuroanatomical heterogeneity among individuals with either right- or left-hand writing preference, and may account for the inconsistency of results when switched individuals are misclassified by self-reports.
Most studies investigating sex-specific cognitive performance report that woman perform better on verbal and memory recall tasks while men perform better on visuospatial tasks (35). In this study, women classified as left-handed by grip strength had poorer performance on visual memory and category (verbal) fluency than right-handed women. While others suggest that left-handed men have better visuospatial performance than right-handed men (36), this study found no handedness differences in the copying task assessing visuospatial skill or Trails B in men, and supports other studies reporting that handedness is unrelated to spatial ability (37, 38). As grip strength measurements eliminate misclassification of handedness occurring by self-report, these findings may have application in understanding the physiology of cognitive performance. It is possible that laterality and dominance of hemisphere have a role in cognitive performance that varies between men and women.
Porac and Searleman (14) investigated the associations of handedness and early switching of handedness with well-being and cognitive performance in a 1277 older adults (mean age=75.1). Although decreased levels of “life satisfaction” and “general health satisfaction” were reported for left-handers who had unsuccessfully tried to change handedness, cognitive function was unrelated to hand preference or history of successful or unsuccessful early hand switching. Porac and Searleman (14) did not report sex-specific results, but their observations are similar to the absent association of handedness with cognitive performance observed here in men classified by grip strength and in men and women classified by self-report.
In the present study there was a high level of discordance between self-reported handedness and classification by grip strength (31.6% and 32.1% discordant for reported right-handers and left-handers, respectively). Lewandowski et al. (39) also noted discordance between self-reported handedness and grip strength in 173 young adults; 36% of self-identified left-handers had a stronger right hand and 19% of right-handers had stronger left hand grip strength.
While questionnaires requesting hand preference on a multitude of tasks (i.e., “Which hand do you brush your teeth with?”) permit estimating degrees of handedness, they may not reflect actual patterns (2). Parental or societal influences may shape an individual’s hand preference or attitudes about handedness. For example, some cultures or religions encourage right-hand use predominantly or exclusively (2, 4). Intolerance of left-handedness or ambidextrous patterns at a young age can prevent the use of the left hand for future tasks (4). Thus, hand-switching at an early age and remembering if such an event occurred can affect the accuracy of self-classification. In this cohort, only 3.5% recalled switching, making it unlikely that forced switching is an explanation of the discordance between reports and hand strength.
Measuring handedness by grip strength relies on participant effort, but it may be more reliable than self-reports (39). The absence of differences in cognitive performance among women when categorized by self-report, but the presence of differences when categorized by grip strength, suggests that grip strength is a more global measure of handedness. Hand strength differences are less prominent in left-handed than right-handed writers (19, 40). In accord with this, the present study found more prominent differences between the left and right grip strengths of men and women who were classified as right-handed. Gilbert and Wysocki (41) estimated that 30% of individuals who write left-handed throw right-handed; such exercise may asymmetrically train hand strength and affect grip performance. This would reduce the accuracy of classification of ambidextrous versus left-handed groups in grip strength studies and may account for the discordances between self-reported handedness and handedness by grip strength. Furthermore, if true ambidextrousness encompasses equivalent handedness (42), then 100% concordance would be expected between self-report and grip strength. However, only 5.9% of individuals identifying themselves as ambidextrous had a difference of <.45 kg in grip strength between hands. Nevertheless, studies using grip strength may be preferable over questionnaires because such studies eliminate some of the potential misclassification inherent in self-reports.
Educational attainment is related to cognitive function later in life (32). The Rancho Bernardo cohort is relatively well-educated; 78% of the men and 62% of the women had at least some college education versus an average of 15% of the college age population in the 1940s (43). Significant differences in cognitive function by handedness were primarily observed for women in this study, possibly because women had more variation in attained education level. If higher educational levels protect against cognitive decline (44), differences would be less likely to be observed in men.
The present study is limited by the relatively small number of left-handers and the possibility that combining the self-reported subgroups into a non-right-handed (n=111) and right-handed (n=1293) may have limited the ability to observe differences in cognitive function. Thus, we cannot exclude heterogeneity in test performance among non-right-handers. We also cannot exclude the possibility that handedness defined by other criteria would yield different associations with cognitive function. For example, in a study of 11-year old children where handedness was measured by motor skill, lower cognitive performance was found in those who were mixed-handed relative to those who were wither left- or right-handed (45).
Another potential limitation of this study is survival bias. Left-handedness has been associated with increased risk of injuries (46), which could have led to differential nonparticipation. Several, though not all studies (e.g. 14, 47–50) suggest left-handers have shorter lifespans (1, 2). However, survival bias seems unlikely here because men and women classified as left-handed by grip strength were either older or of equivalent age to right-handed men and women. de Leon et al. (3) studied Alzheimer’s Disease (AD) in 60–80 year olds and found lower rates among left-handers. Although another study found left-handers with AD had earlier onset of symptoms, cognitive function by handedness was not significantly different (13). Individuals with AD or dementia did not attend this clinic visit. It is also unlikely that differential prevalence of other brain pathology such as stroke or TIA influenced these results; Rancho Bernardo participants were ambulatory, relatively healthy, and data from those reporting a history of stroke or TIA were excluded from the present analysis. Results of the present study support grip strength as a useful alternative to self-reports for classifying handedness. Grip strength, as an objective measure of handedness, may alleviate misclassification of individuals whose hand preference was switched at an early age.
Results of previous studies relying on self-reports may be less accurate than thought, which would add to the inconsistency in the literature. In the present study, handedness makes a relatively small but statistically significant, contribution to cognitive function, with left-handed individuals, especially women, exhibiting poorer cognitive performance. Grip strength decreases with age. Whether serial grip strength measurements will predict decline in various tests of cognitive function remains to be studied.
Acknowledgments
This study was supported by grant AG07181 from the National Institute of Aging.
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