The relationship between Fitts' Index of Difficulty (ID = log2 2A/W) and movement time was investigated for finger, wrist, and whole arm motions over a wide range of movement distances (0.25 to 30.5 cm). Results supported Fitts' original speculation that various limb segments may show different maximum information processing rates. Short-distance finger and wrist motions showed much higher rates (38 and 23 bits/sec, respectively) than longer-distance arm motions (10 bits/sec). Examination of motion trajectories qualitatively supported a descriptive model whereby a visually mediated discrete-correction control process is used, as proposed by Crossman and Goodeve (Note 1) and Keele (1968). However, evidence of severe nonlinearities in the measured human movement responses did not support the use of linear control models in explaining the empirical validity of Fitts' law in predicting human motor performance.

We examined 502 subjects, 247 of whom had occupational elemental mercury exposures 20 to 35 years previously, to identify potential exposure-related neurological abnormalities. Few significant (p less than 0.05) differences existed between exposed and unexposed subjects. However, multiple linear regression analysis demonstrated several significant correlations between declining neurological function and increasing exposure as determined by urine mercury measurements from the exposure interval. Subjects with urine mercury peak levels above 0.6 mg/L demonstrated significantly decreased strength, decreased coordination, increased tremor, decreased sensation, and increased prevalence of Babinski and snout reflexes when compared with the remaining subjects. Furthermore, subjects with clinical polyneuropathy had significantly higher peak levels than normal subjects (0.85 vs 0.61 mg/L; p = 0.04), but not increased exposure duration (20.1 vs 20.8 quarters; p = 0.34), and 28% of subjects with peak levels above 0.85 mg/L had clinical evidence of polyneuropathy, compared with 10% of remaining subjects (p = 0.005). Although exposure was not age dependent, several neurological measures showed significant age-mercury interaction, suggesting that natural neuronal attrition may unmask prior exposure-related subclinical abnormalities.

Workers exposed to metallic mercury vapor were subjects for tremor, EMG, and psychomotor tests. Regression analysis revealed statistically significant trends in these test results related to workers' urine mercury histories. Effects were subclinical, functionally insignificant and most associated with those workers whose urine mercury had exceeded 0.5 mg/L in the previous year. In agreement with previous reports, effects were reversible upon reduction of mercury exposure.

Neurologic and electrodiagnostic evaluations and urine mercury level determinations were performed on 138 chlor-alkali plant workers, some of whom were chronically exposed to inorganic mercury vapor. Eighteen subjects had a mild polyneuropathy on clinical examination. These subjects had significantly (p less than 0.05) elevated urine mercury indexes, reduced sensation on quantitative testing, prolonged distal latencies with reduced sensory evoked response amplitudes, and increased likelihood of abnormal needle electromyography compared with the remaining 120 subjects. Similar results were found for subgroups matched by sex and age. We conclude that elemental mercury exposure is associated with a sensorimotor polyneuropathy of the axonal type; the degree of neurologic impairment appears related to the magnitude of exposure.

In many industries workers perform manual assembly tasks with hands postured above the shoulders. Awkward shoulder and arm postures are often viewed as acceptable given costs of workplace modification, postural exertions which are in compliance with current design recommendations, ready availability of strong workers, and numbers of electromyographic studies which fail to detect significant signs of localized muscle fatigue (LMF). An experiment was conducted to: (a) study the onset and severity of (LMF) in the shoulder when performing a stylus-to-hole Fitts reciprocal movement task under a range of postures, hand loads, ratios of work-to-rest, and task durations, and (b) to evaluate the efficacy of three techniques (i.e., changes in EMG behaviour, postural tremor, and cross-modal matching estimates) in detecting and monitoring posturally-based LMF and discomfort in the shoulder complex. Experimental findings showed that posturing hands above shoulder level significantly increased the risk of LMF and postural discomfort even in light-weight manual assembly environs where postural exertions are small, and that cross-modal matching estimates and postural tremor were more sensitive metrics of LMF in the shoulder complex than EMG RMS voltage and mean spectral power frequency metrics. The basis for experimental findings, as well as potential application of LMF metrics in future postural stress investigations, are discussed. Recommendations for workplace posture are provided for job designers facing work height decisions in manual assembly environs.

the STM capacity should also be adversely affected by exposure to mercury. This study was conducted to test this prediction and to provide converging evidence that one locus of the neurotoxic effect of mercury is short term memory.Two alternative measures of STM capacity were evaluated under field study conditions. The first, the Wechsler digit span forward,6 was included because it is the test of STM span that has traditionally been used in behavioural toxicology studies.7A second, more extensive test of STM span was also evaluated because it has been suggested that the Wechsler digit span test taken alone "is not sufficiently discriminating to detect significant differences in intellectual (that is, retention) functioning."8 This second test entails estimating an individual's 50% threshold for serial recall-that is, the number of digits that can be correctly recalled 50% of the time, in serial order.The reliability and precision of these two tests were assessed within the context of evaluating the effects of chronic exposure to elemental mercury on STM. This provided an opportunity to study the performance of a heterogeneous population (mercury cell chlor-alkali workers) and to illustrate the usefulness of precise behavioural measures in assessing the neurotoxic effects of chronic exposure to elemental mercury. 413

Fitts' Law describes human movement time in the horizontal and frontal planes, but its application to movement in the sagittal plane has been questioned by Beggs and Howarth. T o evaluate the Law in this plane, eight subjects performed a forearm action, similar to dart-throwing without release, moving a stylus through the sagittal plane from a back contact plate forward to a target. Four movement distances, 8, 12, 16 and 20 in, were combined with four target widths, 4, 4, I and z in, in 16 experimental conditions. Following practice all subjects were tested in each condition, with visual knowledge of results. The plot of movement time against log zA/W was essentially linear, accounting for 92.6% of the variance.

A comparison of toluene and ethanol (EtOH) induced changes in central nervous system (CNS) function and symptoms were evaluated in two studies, and when possible the effects of toluene were expressed in EtOH equivalent units. The toluene concentrations were 0, 75, and 150 ppm, bracketing the American Conference of Governmental Industrial Hygienists threshold limit value (ACGIH TLV) of 100 ppm. The socially relevant EtOH doses were 0-00, 0 33, and 0-66g EtOH/kg body weight, equivalent to two and four 3 5% 12 ounce beers. Forty two paid college students were used in each study. In the first study, subjects were exposed to toluene and an odour masking agent menthol (0-078 ppm) for seven hours over three days. In the second study EtOH or a placebo was administered at 1530 across three days also in the presence of menthol. Verbal and visual short term memory (Sternberg, digit span, Benton, pattern memory), perception (pattern recognition), psychomotor skill (simple reaction time, continuous performance, symboldigit, hand-eye coordination, finger tapping, and critical tracking), manual dexterity (one hole), mood (profile on mood scales (POMS), fatigue (fatigue checklist), and verbal ability were evaluated at 0800, 1200, and 1600. Voluntary symptoms and observations of sleep were collected daily. A 3 x 3 latin square design evaluated solvent effects simultaneously controlling for learning and dose sequence. An analysis of variance and test for trend were performed on am-pm differences reflecting an eight hour workday and on pm scores for each solvent, in which subjects were their own control. Intersubject variation in absorbance was monitored in breath. A 5 to 10% decrement was considered meaningful if consistent with a linear trend at p < 0-05. At 150 ppm toluene, British Journal of Industrial Medicine 1991;48:750-761 losses in performance were 6-0% for digit span, 12 1% for pattern recognition (latency), 5% for pattern memory (number correct), 6-5% for one hole, and 3% for critical tracking. The number of headaches and eye irritation also increased in a dose-response manner. The greatest effect was found for an increasing number ofobservations ofsleep. A range of2 to 7% decrements suggest the ACGIH TLV of 100 ppm toluene may be a good estimate of the biological threshold supporting a re-evaluation ofthe TLV. At 0-66g EtOH/kg body weight symptoms and performance decrements were 6-6% for digit span, 9-2% for pattern recognition, 4 0% for continuous performance, 7-9% for symbol-digit, 16-5% for finger tapping, 6-2% for critical tracking, and 5-2% for the one hole test. The EtOH equivalents at 150 ppm toluene for digit span (0-56g EtOHlkg/body weight), the latency for pattern recognition (0-66 g EtOH kg body weight), and the one hole element "move" (0-37 g EtOH kg body weight) show that the first two xneasures would be affected at or above the 50 mg% blood alcohol concentration. This concentration is recognised as the lowest alcohol concentration associated with increased numbers of automobile accidents. The r...