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When humans respond to sensory stimulation, their reaction times tend to be long and variable relative to neural transduction and transmission times. The neural processes responsible for the duration and variability of reaction times are not understood. Single-cell recordings in a motor area of the cerebral cortex in behaving rhesus monkeys (Macaca mulatta) were used to evaluate two alternative mathematical models of the processes that underlie reaction times. Movements were initiated if and only if the neural activity reached a specific and constant threshold activation level. Stochastic variability in the rate at which neural activity grew toward that threshold resulted in the distribution of reaction times. This finding elucidates a specific link between motor behavior and activation of neurons in the cerebral cortex.
Table 4 provides a summary of the evidence that specific medications adversely affect the safety of the older driver. The preponderance of evidence suggests that benzodiazepines adversely affect the safety of the older driver, particularly for high doses and long half-life compounds. This conclusion is based upon the very consistent psychomotor function data showing pronounced dose-related impairment, the more limited epidemiologic data on crash involvement, epidemiologic data associating benzodiazepines with other types of injuries, and the fact that the reasons for most benzodiazepine use are not plausible confounders. This conclusion thus reinforces the need to prescribe benzodiazepines cautiously, including assessment of nonpharmacologic alternatives, use of the lowest possible dose for the shortest possible time, and avoidance of the very long half-life compounds. As more new nonbenzodiazepine anxiolytics and hypnotics become available, their effects on the safety of the elderly driver need to be determined. There is some evidence that cyclic antidepressants, currently the mainstay for treatment of depression in the elderly population, adversely affect driving safety; however, because of the paucity of experimental and epidemiologic data concerning the effects of depression per se on driving, further research is needed. Nevertheless, the existing data reinforce the need for careful prescribing of antidepressants, particularly avoidance of agents with high side-effect profiles (such as amitriptyline and imipramine) in the older driver. For hypoglycemics, although there is sufficient evidence of driving impairment to create a basis for concern, there are many unresolved questions. Currently, diabetic patients should be advised concerning the risk and management of hypoglycemia. For other sedating drugs, it always is prudent to advise patients concerning potential effects on driving. Unlike younger drivers, the typical older driver is a medication-taker. There now is a substantial body of evidence that commonly used medications can interfere with driving safety. Because many questions remain unanswered, there is a pressing need for further research that more fully elucidates how patient characteristics, disease, and drugs interact to affect driving safety; however, sufficient data are available to reinforce an underlying theme in geriatric medicine that is not yet fully implemented in practice: the need for caution in pharmacotherapy, with selection of a drug, dose, and regimen suitable for the unique characteristics of this population.
OBJECTIVE - To compare biomechanics force platform measurements of postural sway with clinical measures of balance and mobility, in frail elderly residents of community nursing homes, in terms of feasibility, correlation with other known risk factors for falls, and intercorrelation with each other.
DESIGN - Cross-sectional study.
SETTING - Twelve Tennessee community nursing homes.
SUBJECTS - Of 1315 residents 360 (> or = 65) could stand independently (> or = 10 seconds). Of these eligible subjects, 303 (84%) provided informed consent and were assessed.
MEASUREMENTS - The biomechanics force platform measurements were postural sway during quiet standing characterized as elliptical area and mean velocity. The clinical measures were functional reach, mobility maneuvers (adapted from Tinetti's Mobility Index), timed chair stands, and 10-foot walk. Resident characteristics and function were also obtained.
RESULTS - Balance measurements were obtained on most (100% for postural sway to 67% for chair stand) consenting residents and were reliable on test-retest (intraclass correlation from .56 to .98). Performance in both groups of balance measures deteriorated with increasing musculoskeletal disability. Functional reach and mobility maneuvers correlated with height, and mobility maneuvers with depressive symptoms. Elliptical area correlated with mean velocity of postural sway (Pearson's r = 0.72; P < 0.001), and the clinical measures of balance (functional reach, mobility maneuvers, timed chair stands and walk) were modestly intercorrelated (r from 0.35 to 0.65; all P values < or = 0.05). However, the biomechanical measures were not correlated with the clinical measures.
CONCLUSIONS - Standard measures of balance were obtained reliably from nursing home residents who could stand independently for > or = 10 seconds. However, in this group, further research is needed to determine which measures best predict falls. Further research is also needed to identify predictors of falls in the majority of residents who were too frail to undergo these standard assessments.
O'Boyle and Benbow (1990) have suggested that enhanced involvement of the right hemisphere (RH) during basic information processing is a neuropsychologic characteristic of the gifted brain. To provide converging evidence for this hypothesis, the present study was conducted using a concurrent finger-tapping paradigm. Specifically, 24 mathematically precocious and 16 average ability adolescent males were required to tap a key as quickly as possible while sitting silently (baseline condition), concurrently reading a paragraph aloud (verbal load), or encoding a random form into memory (spatial load). For average ability subjects, the concurrent verbal load reduced tapping rate for the right but not the left hand, reflecting a division of LH resources between linguistic processing of the paragraph and motor control of the contralateral hand. In contrast, for gifted subjects, both their left- and right-hand tapping rates were significantly reduced, suggesting that both hemispheres were engaged during verbal processing. The concurrent spatial task produced a small but reliable reduction in finger-tapping rate for both the left and right hand in each group. These findings provide additional support for the notion that enhanced reliance on RH functioning is a physiological correlate of mathematical precocity in gifted males.
BACKGROUND - Studies in nonhuman primates provide evidence that intact spatial working memory depends on the integrity of specific areas in the prefrontal cortex. Patients with schizophrenia have been shown to be impaired on spatial working memory tasks. Relatives of schizophrenic patients show a range of cognitive deficits in the absence of clinical symptoms (eg, thought disorder, eye tracking dysfunctions). We predicted that a significant proportion of relatives of schizophrenic patients would show deficits in working memory as measured by a delayed response task.
METHODS - In experiment 1, we tested 18 schizophrenic patients, 15 first-degree relatives of schizophrenic patients, and 18 normal control subjects on an oculomotor delayed response task. In experiment 2, we assessed the performance of another group of 12 first-degree relatives of schizophrenic patients and 16 different normal control subjects on a visual-manual delayed response task.
RESULTS - Relatives of schizophrenic patients showed significant deficits in working memory on both the oculomotor and visual-manual delayed response tasks.
CONCLUSIONS - Some relatives of schizophrenic patients are impaired on tasks that tap spatial working memory and that implicate the prefrontal system. The delayed response paradigm may be useful in elucidating the multidimensionality of the schizophrenic phenotype.
To determine if the risk of hip fracture difference between persons receiving benzodiazepines with long (greater than or equal to 24 hours) or short (less than 24 hours) elimination half-lives, we conducted a nested case-control study among residents of the Canadian province of Saskatchewan who were 65 years of age and older. We identified 4501 cases occurring between 1977 and 1985 from computerized hospital records and 24,041 population controls. Current benzodiazepine use, defined as having filled a prescription in the past 30 days, was ascertained from computerized pharmacy records. The relative risk of hip fracture was 1.7 (95% confidence interval, 1.5 to 2.0) for current users of long half-life benzodiazepines, in contrast to that of 1.1 (95% confidence interval, 0.9 to 1.3) for current users of short half-life drugs. This finding was not altered by sex, age, calendar year, nursing home residence, or history of hospitalization. Medical record review for a sample of 189 cases suggested that this finding was not due to confounding by dementia, ambulatory status, functional status, or body mass.
The present study demonstrates that schizophrenics are impaired on spatial delayed-response tasks, analogous to those that have been used to assess the working memory function of the dorsolateral prefrontal cortex in rhesus monkeys. Schizophrenic patients and two control groups, normal subjects and bipolar psychiatric patients, were tested on the oculomotor version of the memory task, a haptic version of the same task, and two control tasks: a sensory task that did not require working memory and a digit span test. The schizophrenic patients showed marked deficits relative to the two control groups in both the oculomotor and haptic delayed-response tasks. They were not, however, impaired on the digit span test, which taps verbal working memory as well as voluntary attention, and on the sensory control task, in which their responses were guided by external cues rather than by spatial working memory. These findings provide direct evidence that schizophrenics suffer a loss in representational processing and that this deficit is modality independent. These data on spatial working memory add to the growing evidence for involvement of the dorsolateral prefrontal cortex in schizophrenic disease.
To determine whether commonly used psychoactive drugs increase the risk of involvement in motor vehicle crashes for drivers > or = 65 years of age, the authors conducted a retrospective cohort study. Data were obtained from computerized files from the Tennessee Medicaid program, driver's license files, and police reports of injurious crashes. Cohort members were Medicaid enrollees 65-84 years of age who had a valid driver's license during the study period 1984-1988 and who met other criteria designed to exclude persons unlikely to be drivers and to ensure availability of necessary study data. There were 16,262 persons in the study cohort with 38,701 person-years of follow-up and involvement in 495 injurious crashes. For four groups of psychoactive drugs (benzodiazepines, cyclic antidepressants, oral opioid analgesics, and antihistamines), the risk of crash involvement was calculated with Poisson regression models that controlled for demographic characteristics and use of medical care as an indicator of health status. The relative risk of injurious crash involvement for current users of any psychoactive drug was 1.5 (95% confidence interval (CI) 1.2-1.9). This increased risk was confined to benzodiazepines (relative risk = 1.5; 95% CI 1.2-1.9) and cyclic antidepressants (relative risk = 2.2; 95% CI 1.3-3.5). For these drugs, the relative risk increased with dose and was substantial for high doses: 2.4 (95% CI 1.3-4.4) for > or = 20 mg of diazepam and 5.5 (95% CI 2.6-11.6) for > or = 125 mg of amitriptyline. Analysis of data for the crash-involved drivers suggested that these findings were not due to confounding by alcohol use or driving frequency.