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Biomedical subjects

G M Long

Publications and source records attributed to G M Long.

At least 19 recordsLinked to original sources
PubMed1992

Contrast sensitivity during horizontal visual pursuit: dynamic sensitivity functions.

The contrast sensitivity functions of college students for grating targets presented at angular velocities of 0, 30, 60, and 90 deg s-1 were determined for target durations of 200 and 600 ms. The most pronounced effects of target movement were evident at the mid to high spatial frequencies in which sensitivity was markedly reduced as velocity increased. These adverse effects were greatest in the 200 ms condition, in which performance was largely limited to the saccadic eye movement system. In the 600 ms condition, in which both saccadic and smooth pursuit eye movements were possible, contrast sensitivity for the low-frequency target actually improved significantly for the 30 and 60 deg s-1 targets, whereas only adverse effects of target motion were found for targets of mid and high spatial frequencies. The results are discussed in terms of the limitations of traditional visual assessment procedures and the practical and theoretical benefits of conceptualizing the joint effects of target composition and target movement.

Adult↗
PubMed1992

Dynamic visual acuity and contrast sensitivity for static and flickered gratings in a college sample.

For a sample of college observers, visual performance was assessed with three types of measures: (1) contrast sensitivity for a range of spatial frequency gratings with 4-Hz flicker, (2) contrast sensitivity for a range of stationary wallchart gratings, and (3) dynamic visual acuity (DVA) for a range of target velocities. Within each of these three performance measures, mean results were generally consistent with previous reports. However, across-test comparisons revealed little commonality of processes between either of the contrast sensitivity scores and the dynamic acuity scores. These findings are discussed in terms of the separate contributions to overall visual performance made by the increasingly popular contrast sensitivity function (CSF) and DVA.

Adolescent↗
PubMed1992

Prime time: fatigue and set effects in the perception of reversible figures.

Subjects viewed unambiguous versions of both stationary and rotating Necker cube illusions for varying durations prior to the presentation of the standard ambiguous figure. In each case, the subjects were more likely to report the ambiguous figure to be (1) in the same configuration as that of the preceding prime following brief preexposure periods and (2) in the opposite configuration from that of the preceding prime following long preexposure periods. In addition, the number of reversals of the figure during the test period was also strongly related to the duration of the preexposure period, with progressively fewer reversals reported following longer preexposure periods. The results are interpreted as revealing the concurrent roles of "set" effects in the brief preexposure conditions and neural fatigue effects in the long preexposure conditions. Furthermore, the ability of the proposed two-process model to integrate the myriad of empirical effects in the reversible-figure literature is emphasized.

Adult↗
PubMed1991

Training effects on dynamic visual acuity with free-head viewing.

Training effects on dynamic visual acuity (DVA) were determined over a range of target durations and target velocities under the viewing condition of free-head movement. Dynamic acuity levels were lower than static acuity levels, and training was found to be very effective in improving performance, particularly under the most difficult conditions in which performance was poorest. Improvement was also most evident among those observers who initially exhibited the poorest DVA scores. Preliminary comparisons between DVA scores of a nonselect sample of college students and those of a small group of college athletes indicated that the athletes were not very different from the nonselect group and would also benefit from training on the task.

Attention↗
PubMed1991

Comparison of contrast sensitivity functions across three orientations: implications for theory and testing.

The contrast sensitivity functions of a large group of observers (N = 71) were determined for three orientations of test gratings: vertical, oblique, and horizontal. Comparison of group means indicated that, consistent with previous findings for the 'oblique effect', sensitivity was poorer for the oblique orientation-but only for the mid-high spatial frequencies. Correlation analyses indicated that contrast sensitivity for a particular spatial frequency at a given orientation was highly correlated with contrast sensitivity for that same frequency at the other two orientations. Factor analysis of the intercorrelations revealed two strong factors, a low frequency factor, and a mid-high frequency factor. Results are discussed in terms of: (a) the implications for contrast-sensitivity testing across orientations, (b) the basis for the oblique effect, and (c) a different type of evidence for a dichotomy among spatial-frequency channels that may reflect the distinction between X cells and Y cells or between the parvocellular and magnocellular systems.

Adult↗
PubMed1990

The nature and basis of age-related changes in dynamic visual acuity.

The ability of observers to resolve moving targets, or dynamic visual acuity (DVA), was determined for a group of young adults (M age = 19.6 years) and a group of older adults (M age = 67.6 years). Targets were presented at two luminance levels over a range of velocities (30, 60, 90, and 120 deg/s) and at 2 durations (200 and 600 ms). The younger subjects exhibited superior DVA under nearly all conditions, but this effect was essentially eliminated by the luminance adjustment. These results are interpreted in terms of decreased retinal illumination in the older subjects rather than in terms of age-related changes in the underlying eye movement systems. Practical and theoretical implications are discussed.

Adult↗
PubMed1990

The role of small-field tritanopia in two measures of colour vision.

The present work extends the findings of previous efforts examining the comparability of current colour-screening tests. Several popular tests are shown to differ greatly in the performance exhibited by colour-normal observers as well as in their differential sensitivity to experimental manipulations of viewing duration and viewing distance. Those tests designed to identify yellow-blue dichromacy are especially sensitive to the manipulation of viewing distance, which is interpreted as reflecting 'small-field tritanopia' and the asymmetry in retinal density of the three cone types. These findings are discussed in terms of factors that influence the comparability of current colour-screening devices and the particular need for close adherence to standardized conditions with such instruments.

Color Perception↗
PubMed1989

Effects of viewing conditions on standard measures of acquired and congenital color defects.

We examined the effect of variations in viewing distance and viewing duration on the performance of color-normal observers with four standard tests of color vision. Significant effects of the experimental manipulations were obtained: both increasing viewing distance and decreasing viewing duration significantly increased the number of errors made by observers. Moreover, the four tests differed widely in their sensitivity to the variations in viewing conditions. Practical implications of the findings for the administration and selection of plate tests are discussed, and possible mechanisms underlying the results are suggested.

Color Perception↗
PubMed1989

Training effects on the resolution of moving targets--dynamic visual acuity.

In initial 60-min sessions, the dynamic visual acuity (DVA) of 54 male college observers was determined over a range of target velocities (60, 90, 120, and 150 deg/s) at each of three durations (200, 400, and 600 ms). Following four 30-min practice sessions with the task, a final test session identical to the first was then conducted. Highly significant training effects on DVA were obtained; contrary to previous work, these effects were most marked for observers with initially poorer performance.

Humans↗
PubMed1989

Paradoxical punishment as it relates to the battered woman syndrome.

The main goal of this article is to suggest some processes by which the behavioral interactions of an abusive couple can be developed and maintained. A summary of these processes as they have been adapted from the animal analog and human research follows. First, the pairing of abuse (punishment) with love and affection (reinforcement) could indicate that the battered woman responds to the reinforcement that follows an abusive incident. Second, the battered woman may seek to escape the fear of the tension-building phase but during the escape behavior she is punished (physical abuse), which fortifies and strengthens her initial fear. However, after the abusive incident, there is an absence of tension and fear (negative reinforcement) compounded by the positive reinforcement of the contrition phase. Thus, a cyclical pattern of behavior evolves and sustains itself. Possibly augmenting the vicious cycle is habituation or negative adaptation to the physical abuse, as well as the lack of opportunities to learn adaptive behavioral responses that may break the cycle of violence. Finally, the need for primary reinforcement (i.e., love and affection) may be so powerful that it somewhat mitigates the abuse endured to obtain it.

Animals↗
PubMed1989

The changing face of visual persistence.

In three experiments, the effect of target luminance on estimates of visual persistence was determined on a popular successive-field task. Even though the experiments differed from each other only in terms of minor changes in stimulus conditions, very different effects were obtained across the three experiments. The different findings were attributed to the often overlooked roles of masking and target degradation on tasks of persistence. Implications for the numerous contradictory findings in the persistence literature are discussed, and the current popularity of the successive-field task is questioned.

Afterimage↗
PubMed1988

The effects of target borders on dynamic visual acuity: practical and theoretical implications.

The effects of target borders on the ability of observers to resolve moving targets (Landolt Cs) under a range of conditions were examined. Contrary to reported findings with stationary targets, it was predicted that the presence of borders would improve acuity for slow-moving targets because (i) overall stimulus energy is kept relatively constant as target detail varies, and (ii) a low-spatial-frequency component is held constant as target detail varies. In an experiment in which a two-sided border (above and below the target) was used, the predicted beneficial effect of the border at slow speeds was obtained. The results are discussed in terms of practical implications for the assessment of dynamic visual acuity as well as the potential neural mechanisms underlying performance.

Humans↗
PubMed1988

Reliabilities of alternate measures of contrast sensitivity functions.

In four studies, the comparability of several measures of contrast sensitivity functions (CSF's) as well as their individual test-retest reliabilities were determined with observers drawn from a college population. The relation between CSF scores obtained by different tests or by different psychophysical procedures with the same test was consistently low and frequently nonexistent. These low intertest reliabilities were found to be due to the lack of reliability of some of the individual tests. In particular, with both stationary and flickering gratings, the von Bekesy tracking procedure produced unacceptable variations in contrast estimates. Fortunately, the reliabilities of other procedures, particularly the simple adjustment method, were much higher (greater than 0.70). CSF scores obtained from the wallchart method exhibited insufficient variability among observers in the college sample to permit similar reliability estimates to be determined. Implications of the test characteristics are discussed for users of the CSF.

Adult↗
PubMed1987

Foveal and parafoveal processing of asynchronous Stroop stimuli.

Two experiments are presented which used a form of the Stroop colour-word task to study the processing of target and distractor information under changing conditions of temporal and spatial proximity. Experiment 1 focused on the marked empirical discrepancies in previous research reported in the recent literature. A coloured target and a distractor (neutral, facilitative or interfering) that were separated by variable onset asynchronies were presented foveally to observers. The findings of this study indicate strong distractor effects on response latency at the intermediate stimulus onset asynchronies (SOAs) (around 0 ms) and a clear monotonic decrease in reaction time for colour naming at extreme positive and negative SOAs. Experiment 2 used this same task but presented targets parafoveally and distractors foveally. This manipulation resulted in the same general pattern of processing as found in Expt 1. In addition, a laterality effect was found when the target and distractor were incongruent; the left visual field/right hemisphere showed significantly greater interference as indicated by longer reaction times. These results are discussed in terms of current models of information processing of compound stimuli.

Adult↗
PubMed1987

Normative contrast sensitivity functions: the problem of comparison.

Sixty college age observers who were prescreened for 6/6 (20/20) acuity were examined twice on an automated contrast sensitivity system. A significant practice effect was evident in the obtained contrast sensitivity functions (CSF's). In addition, marked differences were found between the contrast sensitivity scores in the present work and the normative CSF reported by other investigators using similar methods. Implications for the rapid assessment of CSF in applied settings and for the use of published CSF norms are discussed.

Adult↗