Biomedical subjects

R A Tyrrell

Publications and source records attributed to R A Tyrrell.

8 recordsLinked to original sources

PubMed1992

Age differences in dynamic contrast thresholds.

Contrast thresholds for sinusoidal gratings of 1.5-18 cpd were measured in young (M = 24 years) and older adults (M = 69 years). Thresholds were determined for stationary gratings, and for gratings that traveled along a circular path at 5, 10, and 15 deg/sec. For static gratings, age differences in thresholds were greater for intermediate and higher spatial frequencies. Grating motion increased thresholds for all observers, but older adults exhibited threshold increases at lower target velocities than the young. Age differences in velocity effects were generally greater at intermediate and higher spatial frequencies. Results are discussed in relation to age changes in temporal resolution and smooth pursuit gain.

Adolescent↗

PubMed1992

The effects of visual factors and head orientation on postural steadiness in women 55 to 70 years of age.

This study was designed to establish reference data for the effects of head position and visual conditions on the standing posture of healthy women ages 55 to 70. Center of pressure (CoP) data were analyzed using ANOVA models. Total CoP displacement was 45% greater (significant, p less than .05) when the subjects stood with their heads back and their eyes closed, as compared to standing looking straight ahead. Visual surrounds consisting of vertical visual cues resulted in less postural sway (PS) than surrounds made of horizontal visual cues (significant, p less than .05 for anteroposterior CoP range). Experimentally reduced visual acuity resulted in greater PS, while changes in ambient lighting and spatial frequency of visual cues affected PS minimally. These results suggest that a reduction in the height of shelving, and the provision of high-contrast visual cues and vertical geometric patterns are potential intervention strategies to optimize vestibular sensory input and visual orientational cues for the maintenance of stability during stance.

Aged↗

PubMed1992

Lateral phoria at distance: contributions of accommodation.

Lateral heterophoria at distance often is cited as an index of tonic vergence, yet recent research has shown that vergence in darkness is more convergent than most measures of phoria. Two experiments were conducted to investigate the possible role of accommodation in this discrepancy. Experiment I compared measures of distance phoria of 19 young adults with measures of vergence and accommodation of the same subjects. Dark vergence was found to be correlated with but significantly more convergent than distance phoria. Hierarchical regression analysis showed that individual measures of phoria were related to subjects' negative accommodation (from the dark focus to the distant target) as well as to dark vergence. Experiment II tested 13 young adults to evaluate three simple models of the influence of accommodation on distance phoria. Distance phoria was predicted best by a model that included the subjects' negative accommodation, accommodative vergence, and dark vergence. These findings support the hypothesis that distance phoria is influenced by accommodation for the fixation target. Dark vergence is a simpler index of tonic vergence.

Accommodation, Ocular↗

PubMed1991

Age differences in estimating vehicle velocity.

Automobile accidents among older adults may be related to difficulties in judging the speed of other vehicles. To examine this possibility, 3 groups of observers in the young adult, middle-aged, and older adult age ranges were asked to estimate the velocity of an isolated automobile traveling at 15-50 mph (24-80 kph). Across all age groups, perceived and actual velocity were related by a power function with an exponent of 1.36. Age was significantly and positively correlated with intercepts, but negatively correlated with exponents; that is, older observers showed less sensitivity to changes in actual velocity. Results bear on the issues of ontogenetic changes in accident involvement and sensitivity to motion.

Acceleration↗

PubMed1991

The influence of visual factors on fall-related kinematic variables during stair descent by older women.

Despite the documented health hazards associated with stair descent, the mechanisms of falling on stairs remain relatively unexamined. The objectives of this study were to define kinematic variables that could be used to describe foot-stair spatial relationships during the mid-stair phase of stair descent, and to investigate the effects of various visual and environmental conditions on those variables in a group of 36 healthy women between the ages of 55 and 70. Foot clearance and foot placement were measured through high-speed film analysis. Clearance between the foot and the stair during swing phase was small under all visual conditions. Degraded visual acuity had a significant effect on cadence, foot placement, and foot clearance, but visual surround conditions did not. The kinematic variables used in this experiment may be helpful in future studies to assess the results of interventions aimed at reducing the frequency of falls on stairs.

Accidental Falls↗

PubMed1990

Bandwidth of the contrast sensitivity function as an index of spatial vision with application to refraction.

The contrast sensitivity function (CSF), although containing more information than traditional measures of acuity, has found difficulty gaining clinical acceptance. The hesitancy of clinicians to adopt the CSF stems, in part, from the fact that it is not as readily interpreted as is acuity. In order to facilitate such interpretation, five indices of spatial vision which are derivable from the CSF were examined in a sample of 287 persons aged 5 to 85 years. All indices were found to be both age-sensitive and strongly related to each other, but bandwidth of the CSF was chosen as a practical index for clinical settings. In a second study, acuity and CSF bandwidth were measured under 0 to +/- 1 D optical blur. It was found that the correction providing best acuity also maximized CSF bandwidth, and that bandwidth was more sensitive to optical blur than was acuity. Results support the assertion that CSF bandwidth is a readily interpreted index of spatial vision that can be measured efficiently within the context of clinical refraction.

Adolescent↗

PubMed1990

The relation of vergence effort to reports of visual fatigue following prolonged near work.

Two experiments examined the link between vergence effort and subjective reports of visual fatigue following prolonged near work. In Experiment 1, dark vergence and reports of visual fatigue symptoms associated with computer use were measured in 104 persons. Dark vergence was significantly correlated with four of six symptoms of visual fatigue. In Experiment 2, 16 subjects who exhibited clear "Heuer effects" (systematic gaze-related changes in dark vergence) read from a video display terminal (VDT) at a distance of 20 cm for 60 min under two gaze elevation conditions: 20 deg above and 20 deg below a baseline gaze elevation. This manipulation was designed to vary the amount of vergence effort required to fixate the VDT. Results indicate a positive relation between vergence effort and visual fatigue both within and between subjects, and that prolonged near work leads to decreased vergence accuracy at far distances. Implications for VDT workstation design are discussed.

Adolescent↗

PubMed1988

Age differences in Vistech near contrast sensitivity.

Contrast sensitivity was measured at near using the Vistech 6000 Contrast Test System (VCTS) in 287 persons aged 4 to 87 years. Obtained data compared well with those previously reported for the Vistech charts. Results indicate an age-related loss in sensitivity to intermediate and higher spatial frequencies. This is consistent with the age trends observed using other methods for measuring contrast sensitivity functions (CSFs). However, the Vistech charts yield contrast sensitivity values which are lower than those obtained previously with the Nicolet system. These between-method differences are greatest at lower spatial frequencies, and are attributed to the small number of grating cycles in the Vistech charts. The data are used to develop provisional age norms for the Vistech charts which allow the assessment of an individual's performance relative to their age cohort. Recommendations for use and future development of the Vistech charts are also given.

Adolescent↗