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Biomedical subjects

K Nordby

Publications and source records attributed to K Nordby.

At least 19 recordsLinked to original sources
PubMed1999

Effects of moderate doses of alcohol on immediate recall of numbers: some implications for information technology.

OBJECTIVE: Moderate doses of alcohol will have detrimental effects on memory functions used in various aspects of human interaction with information technology. The need to deal with multidigit numbers while under alcohol intoxication (e.g., making telephone calls) is a reality of life. To determine these relations, the effects of alcohol on immediate ordered recall were studied in a practical number-dialing task. METHOD: Immediate ordered recall of eight-digit numbers, presented visually or auditorily, was tested in read-and-dial or listen-and-dial forced ordered recall tasks with male subjects (N = 11) under two levels of blood alcohol concentration (BrAC = 0.05% and BrAC = 0.1%) and a no-alcohol placebo control condition (BrAC = 0.0%). RESULTS: With increasing alcohol level, immediate ordered recall of whole numbers (all eight digits correct and in the right order) fell by 9% (visual presentation) and 15% (auditory presentation). No significant interaction effect between alcohol level and digit position in the number was found for visually presented numbers, while a weak, but significant, effect was found for auditorily presented numbers (most prominent at digit positions 5, 6 and 7). This is probably due to the difference in rehearsal opportunities in the two presentation modes. No significant interaction effects between alcohol level and presentation modality were found. CONCLUSIONS: The results, tailored to a practical everyday task of dialing unfamiliar eight-digit telephone numbers, show that even moderate doses of alcohol will affect the performance of an already fragile short-term memory system engaged whenever reproduction of digit strings is required.

Acoustic Stimulation↗
PubMed1998

Is the rod visual field temporally homogeneous?

Cone vision has been shown to be temporally inhomogeneous across the visual field. In the periphery, contrast sensitivity is lower for low temporal frequencies and higher for high temporal frequencies. Here we ask a similar question for rod vision at mesopic luminances. Isolation is obtained by testing a well documented rod monochromat. We show that the rod visual field exhibits only a modest degree of temporal inhomogeneity.

Color Vision Defects↗
PubMed1996

Rod temporal channels.

Mechanisms underlying rod temporal contrast sensitivity have been considered in terms of a fast retinal signal predominating at mesopic levels and a slower retinal signal predominating at scotopic levels. Here we use a small signal masking method, which has previously been used to delineate the cone-mediated cortical temporal channels, to investigate their rod-mediated cortical counterparts. The results suggest that there are three different rod-mediated cortical temporal channels, one which is lowpass and two which are bandpass. These mechanisms co-exist at all light levels and their relative sensitivity depend on the stimulus spatio-temporal frequency.

Color Vision Defects↗
PubMed1993

[Access to family practitioners by telephone. A practical example of quality assurance].

A record of telephone calls in a Norwegian town (Molde) in 1987 showed that fewer inquiries were answered by general practitioners and their receptionists than by personnel in any other profession or trade. The general practitioners and the receptionists agreed on measures to improve accessibility, and more records of telephone calls were made. The measures and the process are described. At local level, a record of calls which differentiates between those answered by the providers of primary care and those answered by automatic machines seems to be a simple and useful means of quality assurance which we encourage others to apply.

Family Practice↗
PubMed1993

Spatial integration and sensitivity changes in the human rod visual system.

1. The factor by which increment threshold rises with increasing background intensity is less if the target is small than if it is large. The difference is usually attributed to a reduction in the area over which visual signals are integrated as the visual system light adapts. Recently, however, it has been argued that the difference in slope may instead be caused by an increase in the gain of the local response function with light adaptation. 2. To test this hypothesis in the rod-driven visual system, we compared monoptic, small and large target increment thresholds, and dichoptic, large target brightness matches, measured as a function of background intensity in a typical, complete achromat, who has no cone vision. 3. The dichoptic brightness matches were made using a large target of a similar intensity to the threshold intensity of the small target. If local intensity is important, the large target brightness matching curve should be more similar to the shallow, small target threshold curve. But, if changes in spatial integration are important, the brightness matching curve should be similar to the steeper, large target threshold curve. 4. The slope of the large (1.85 deg) target increment threshold functions measured with either 200 or 50 ms test flashes were steeper than those of the small (10 min of arc) target functions by 0.10 (on logarithmic co-ordinates) or about 15%. 5. The logarithmic slopes of the dichoptic brightness curves were also slightly steeper than the small target increment functions. This is contrary to the local response (only) hypothesis, which predicts that the brightness curve should have the same slope as the small target function because the luminance of the targets in the two cases is the same. 6. We conclude that there must be a change in spatial integration in the rod visual system during light adaptation, over and above that due to local gain changes.

Adaptation, Ocular↗
PubMed1992

Mutual rod-cone suppression within the central visual field.

Under mesopic conditions the contrast sensitivity of the central visual field is reduced as the result of a non-linear interaction between rod- and cone-mediated signals, each of which is capable of higher sensitivity in isolation. The interaction is produced only when the rod-mediated system is driven at flicker rates above 6 Hz. This finding bears upon how rod and cone signals are combined and therefore affects our interpretation of the significance of the relationship between retinal illuminance and both contrast sensitivity and temporal resolution.

Contrast Sensitivity↗
PubMed1991

Slow and fast pathways in the human rod visual system: electrophysiology and psychophysics.

Under most conditions, increasing the intensity of a flickering light makes the flicker more conspicuous. For a light flickering at 15 times per second, however, increasing the intensity can cause the flicker to disappear before reappearing again at higher intensities [Vision Res. 29, 1539 (1989)]. This flicker disappearance or null is also evident in human electrophysiological recordings at the same intensity levels. These results point to a duality within the rod visual pathway, in which flicker signals travel through a slow and a fast pathway and then recombine at a later stage. At 15 Hz the slow rod flicker signals are delayed by half a cycle relative to the fast signals. Thus, when the two signals are recombined, they destructively interfere and diminish the perception of flicker. The dual-pathway interpretation is supported by both electroretinographic and psychophysical evidence showing a phase difference of half a cycle between 15-Hz rod signals just below and just above the null region. These effects are apparent not only in the normal observer but also in an achromat observer who lacks functioning cone vision.

Electrophysiology↗
PubMed1989

The incremental threshold of the rod visual system and Weber's law.

The incremental threshold of the isolated rod visual system is believed, under certain conditions, to obey Weber's law (that is, to increase in direct proportion to the intensity of the background). This relation was tested at several background wavelengths, over an intensity range for which the target was seen only by the rods. Although the slope on long-wavelength background approximates unity (that is, Weber's law on log-log coordinates), it averages less than 0.8 on short- and middle-wavelength backgrounds. This is the same value as that found for the thresholds of a typical, complete achromat--who lacks cone vision--regardless of background wavelength. These results force the conclusion that Weber's law for incremental threshold detection is achieved not by the rods alone but only by the rods acting together with the cones.

Dark Adaptation↗
PubMed1988

Temporal summation in the achromat.

We investigated temporal summation of the rods in a complete achromat, who lacks cone vision. Critical duration (tc) was estimated both at the achromat's preferred area of fixation and at an area 12 deg laterally in the nasal visual field. Comparable tc determinations were made in a normal trichromat. At background luminances of 0.0 and 0.6 scot. td, where the rods mediate detection, the values of tc were similar for the achromat and the normal. At a luminance of 813 scot. tds, however, where the middle-wavelength sensitive cones mediate detection in the trichromat, the tc for the achromat was much longer than that for the trichromat.

Adaptation, Ocular↗
PubMed1988

Spatial vision of the achromat: spatial frequency and orientation-specific adaptation.

1. The psychophysical technique of selective adaptation to stationary sine-wave gratings of varying spatial frequency and orientation was used to investigate the central processing of spatial information in the visual system of the complete achromat. 2. For adapting spatial frequencies of 1 and 2 cycles/deg, the spatial frequency and orientation selectivity of contrast threshold elevation is similar for achromatic and trichromatic vision. 3. For adapting frequencies below 1 cycle/deg, the achromat shows threshold elevations of normal magnitude with symmetrical spatial frequency and orientation tuning for adapting frequencies as low as 0.09 cycles/deg with 'bandwidth' estimates similar to those found at high frequencies in the trichromat. Below 0.66 cycles/deg no after-effect could be obtained in the trichromat, and the frequency tuning at 0.66 cycles/deg was skewed towards higher frequencies. 4. The interocular transfer of low-frequency adaptation in the achromat was 50%, which is the same value obtained at higher frequencies. 5. The time course of the decay of low spatial frequency adaptation in the achromat was similar to that found at higher frequencies. 6. Control experiments show no low-frequency adaptation in peripheral vision or in central vision in the dark-adapted trichromat indicating that low spatial frequency adaptation cannot be elicited through the rod system of the trichromat. 7. It is proposed that the observed range shift of adaptable spatial frequency mechanisms in the achromat's visual cortex is the result of an arrest at an early stage of sensory development. The visual cortex of the achromat is comparable, with respect to spatial processing, to that of the young, visually normal human infant.

Adaptation, Ocular↗
PubMed1988

The directional sensitivity of the photoreceptors in the human achromat.

1. The anatomical nature of the retinal photoreceptors in a typical, complete achromat was investigated by measuring their directional sensitivity. 2. A small (0.5 deg), brief (94 ms) test flash was placed at threshold by varying either its intensity (indirect method) or the intensity of a large (5 deg) adapting field (direct method). The dependent variable was the intensity of light required for the detection threshold as a function of the position of entry in the pupil. An infra-red viewing system was used to monitor the achromat's pupil and eye position. 3. At both scotopic and mesopic adapting field luminances, the complete achromat's receptors displayed only a small directional sensitivity effect. The effect was not wavelength dependent and could be attributed solely to the rods. 4. The results are consistent with other psychophysical evidence indicating that the complete achromat's retinae lack the post-receptoral function of the cone photoreceptors. Therefore they do not confirm previous reports that complete achromats have a second, high-intensity type of photoreceptor.

Color Vision Defects↗