Knut Nordby

Dark-adaptation of the human rod system

Following substantial bleaching, dark-adaptation thresholds of a complete rod monochromat and of a subject with normal colour vision were measured using a Wright colorimeter. When precautions were taken to ensure that the fixation point fell on the same retinal area during the threshold measurements as during the bleaching period, the dark-adaptation threshold curves of the rod monochromat followed exactly the same course as those of the normal subject subsequent to the cone-rod break of the long-term, normal dark-adaptation curve; irrespective of the intensity and the duration of the bleaching and the wavelength of the test stimulation. In contrast to the normal subject, however, the dark-adaptation curves of the rod monochromat showed no evidence of any cone function at photopic intensities. Furthermore, as opposed to previous measurements which show a simple linear relationship between fraction of bleached rhodopsin and log threshold, the present results show that there is a close linearity between log fraction of bleached rhodopsin and log threshold. This linear relationship is obtained despite varying extents of bleaching and subsequent dark-adaptation periods.

Time course of contrast adaptation to VDU-displayed text

Abstract VDU text-editing induces contrast adaptation at the predominant spatial frequencies (periodicity) of the text page. Visual contrast sensitivity was tested after 10 and 60 min reading of VDU-displayed text of positive and negative contrast polarity. Contrast sensitivity impairments in-the order of 0-4 to 0-7 log unit change in contrast thresholds were observed. This contrast threshold elevation after-effect decays as a power function of time, with time required to recover from adaptation approximately corresponding to the reading times. At low spatial frequencies (horizontal periodicity of rows), displays of negative polarity induce stronger contrast adaptation than displays of positive polarity, at medium spatial frequencies (vertical periodicity of characters) no effect of contrast polarity was observed. The results are discussed in relation to VDU-induced visual fatique.

The expanding telephone number Part 2: Age variations in immediate memory for multiple-digit numbers

Immediate ordered recall of multiple-digit numbers was investigated in a practical read-and-key or listenand-key task for three age groups whose mean ages were 25.2 years (range 23-27), 44.1 years (range 42-44) and 63.6 years (range 61-68), all recruited from students and faculty staff at the University of Oslo. The two younger groups performed at comparable levels on the immediate memory task, surpassing the performance of the older group for both visual and auditory presentation of the digit-strings. Increasing the presentation time of the numbers affected the young and older age groups similarly by improving the memory performance. Analyses of the serial position curves revealed an enhancement of the well-known modality effect in the older subjects. The overall decline in memory performance observed at the age of 65 should be taken into account in design of communication technology for the general public.

Temporal summation in a rod monochromat

Below a certain critical stimulus duration (t,), the visual system shows complete temporal summation (Bloch, 1885). The value of Q depends on the level of adaptation, so that the brighter the background the shorter tc (Barlow, 1958; Biersdorf, 1955; Graham and Kemp, 1938; Herrick, 1956; Keller, 1941) and increases in the course of dark adaptation (Montellese et al., 1979; Stewart, 1972). Neither of these changes appears to be related to the transition from cones to rods (Brown and Black, 1976; Montellese et al., 1979), suggesting essentially similar summation characteristics in the two systems. Summation in the rods has not been determined at stimulus levels well above the scotopic range, since the rods at these levels are overshadowed by the more sensitive cones. By having access to a rod monochromat, i.e. an individual whose retinae are devoid of cones*, we have had the unique opportunity to compare the summation in the rods to that in the cones at a level of stimulation where the rods are substantially less sensitive than the cones.

Binaural time/intensity trading

Time/intensity trading functions (Δt/ΔI) were established psychophysically by three subjects for three interaural intensity differences (ΔI) at ten mean intensity levels (?). The sound image of repeating, dichotic, all‐pass clicks was centered, using the objective nullity method combined with the method of adjustment. It was found that trading ratios increased with increasing values of ΔI and tended to decrease with increasing ?. It is argued that for intensity induced neural latency (intensity and latency being inversely related) to account for time/intensity trading, a single intensity/latency curve is required. Since, for any particular mean stimulus intensity (?), the trading ratio increases with increasing ΔI, this requirement is not met; indicating that a single peripheral intensity‐to‐latency mechanism is not sufficient to account for time/intensity trading.

MICRO-POSITION EFFECTS IN VISUAL SHORT-TERM MEMORY

Short-term memory for visually presented eight-digit numbers was tested in a forced recall, read-and-dial task. The digits were presented non-grouped or grouped in 2+2+2+2 or 2+3+3 digit formats. Spatial grouping enhanced memory performance and produced local discontinuities in the serial position curves related to grouping format. These micro-position effects are explained in terms of the relative spatial distinctiveness of individual digits.

Spatial vision of the achromat: the tilt aftereffect.

Tilt aftereffects measured in an achromat and a trichromat control subject exhibit similar strength and angular extent, indicating normal orientation coding in the rod-based achromat vision.