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Biomedical subjects

G M Long

Publications and source records attributed to G M Long.

At least 37 records · Page 2Linked to original sources
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↗
PubMed1985

Visual persistence from brief letters and pictures.

The visual persistence from briefly presented letters and pictures was assessed by the popular probe-matching procedure over a range of background and target luminance levels and for several color conditions. It was determined that the fading visible persistence measured in this way increased with increasing target luminance and with decreasing background luminance. For small foveal presentations, photopically-matched targets of differing wavelength produced equivalent persistences; but for larger, parafoveal presentations, scotopically-matched targets of differing wavelength produced equivalent persistences. This was true for both letter and picture targets. Results were discussed in terms of an early sensory locus to such persistence effects. The strong consistency of these findings to some previous work and the apparent inconsistency with other work were treated in terms of different kinds of visual persistence effects assessed by different experimental methods.

Afterimage↗
PubMed1985

Perceptual style and dual-task performance as a function of task difficulty and task emphasis.

Differences in dual-task performance by field-dependent and field-independent college students were investigated. The perceptual styles of the 102 subjects were determined by their performance on the Rod-and-Frame Test. Dual-task performance required the subjects to perform concurrently a central tracking task and a peripheral light-detection task. The effect of stress on dual-task performance was examined by employing three widely differing levels of tracking task difficulty. Three sets of instructions varied the relative importance of the two tasks within the dual-task situation. Both manipulation of task difficulty and instructional set had powerful effects on performance. However, performance differences among subjects with differing perceptual styles were found only on the central tracking task, with field-independent subjects consistently outperforming field-dependent ones. The results are discussed in terms of the theoretical basis for the perceptual style of field-dependence/field-independence as well as the potential value of this dimension in dual-task situations.

Attention↗
PubMed1985

Distance, duration and blur effects on the perception of pseudoisochromatic stimuli.

Two experiments are reported that examine the effects of viewing distance, viewing duration, and stimulus clarity on performance with three popular versions of the pseudoisochromatic tests of colour deficiency. With a large sample of colour-normal observers, highly significant effects of these viewing conditions are found. In addition, large differences in sensitivity to these conditions are obtained among the three tests employed (The American Optical Corporation test, the Dvorine test, and the Ishihara test). Results are discussed in terms of practical implications for test administration and in terms of likely processes, in addition to colour discrimination, assessed by these tests.

Adult↗
PubMed1984

The pretreatment weighting technique.

A common procedure that is frequently used in perceptual studies interested in group differences (e.g., young vs. aged) involves initially equating all subjects in a pretreatment condition by differentially weighting the subjects on a particular stimulus dimension. The experimental treatment of interest is then introduced to these equal-baseline groups. The present article questions the general validity of this procedure by focusing on the critical assumption that the pretreatment weighting and the experimental treatment are independent. A research example is included demonstrating that differentially weighting individuals in order to produce equal-baseline performance may, in fact, provide the basis for later performance differences under the experimental treatment.

Aged↗
PubMed1984

Task and size effects in the Oppel-Kundt and Irradiation illusions.

The present study examined the effects of type of task and stimulus size on two well-known illusions of perceived size: the Oppel-Kundt illusion and the Irradiation illusion. In Experiment 1, a forced-choice task, in which 87 male and female college observers chose the "longer" of two side-by-side stimuli, was used to determine the relative perceived size of stimuli of different length that were all white, all black, or white with various numbers of black stripes. In Experiment 2, an absolute judgment task, in which 73 male and female college observers reported their direct estimate of a single target's length, was used for the same determinations. For both kinds of perceptual judgment, the Oppel-Kundt illusion is strongly in evidence (p less than .001), but the magnitude of the illusion does not increase proportionally with the size of the stimuli. The Irradiation illusion is only found with the forced-choice judgments in Experiment 1. Implications for the generalizability of these illusions to nonlaboratory settings are noted.

Female↗
PubMed1984

Complexity effects on reaction-time measures of visual persistence: evidence for peripheral and central contributions.

Two experiments are presented that attempt to determine the effect of operationally defined stimulus complexity on visual persistence. Two common measures of complexity are employed in both experiments. In Experiment 1, visual persistence for high-contrast, solid black figures increases with increasing target duration and with decreasing complexity. In Experiment 2, which uses outline figures of the same sizes and shapes as in the first experiment, visual persistence increases both with increasing target duration and with increasing complexity. It is proposed that the very different results from the two experiments may reflect the separate functioning of a visible, sensory persistence and a higher order, schematic persistence. The reaction-time (RT) measure of persistence that was employed may be sensitive to either of these persistences, depending upon stimulus and task conditions. Implications are suggested for (a) the importance of sensory processes in some manipulations of target complexity and (b) the possible role of peripheral and central factors in visual persistence.

Adult↗
PubMed1983

Positive and negative afterimages from brief target gratings.

Threshold luminance levels for the production of negative afterimages from brief target gratings were determined as a function of background luminance and grating frequency. The obtained thresholds were extremely low--typically below the values that would maintain constant space-average luminance between target and background. The implications of these results for other studies that may have inadvertently produced negative afterimages with their stimulus conditions were noted. As a demonstration, visual persistence estimates from these gratings were determined under conditions that carefully excluded negative afterimages, and clear differences from previously published work were obtained.

Afterimage↗
PubMed1982

Receptor interactions and visual persistence.

Two experiments are described that provide evidence for an interaction between the cone and rod systems following the offset of a stimulus. Experiment 1 employs a visual persistence task in which the observer is required to integrate two brief flashes separated by a variable ISI. Threshold luminance as a function of ISI is determined for several conditions of target wavelength. Striking differences between short and long wavelength targets are found which indicate the possible exclusion of rod participation at the brief ISIs. Experiment 2 employs a forward-masking task to demonstrate that the stimulus conditions of the first experiment result in a transient reduction in rod system activity following stimulus offset while the cone system is still active. Results support this proposal. Implications for the locus of persistence within the visual system are discussed.

Color Perception↗