Svein Magnussen

Temporal aspects of spatial adaptation. A study of the tilt aftereffect

Growth and decay characteristics of the tilt aftereffect were studied for aftereffects induced by normal or continuous adaptation routines, and for aftereffects induced by successive or spaced adaptation to the same or different orientations on an adapt-partial decay-readapt schedule. In the continuous adaptation condition, growth and decay of the aftereffect were logarithmic functions of time. There was no evidence for saturation after 30 min adaptation. Aftereffect decay following spaced adaptation progresses as by continuous adaptation, but an adapting stimulus introduced during recovery from previous adaptation is more effective on the time scale than when introduced to a fully recovered system, summing approximately linearly with the residual aftereffect and off-setting the recovery process to zero. A second adapting stimulus whose orientation is of opposite sign (ccw vs cw) induces a two-phased decay process consisting of an early cancellation and a later enhancement of the original aftereffect. A two-stage model of adaptation is proposed.

Interactions among spatial frequency and orientation channels adapted concurrently

Interactions between size and orientation-specific mechanisms in the human visual system were investigated using a sequential adaptation technique. Subjects adapted to a vertical, 4 c/deg high-contrast (0.7) sinewave grating that was interleaved at a rate of 0.5 Hz with another adapting grating differing either in (1) spatial frequency or (2) orientation. Before and after adaptation contrast thresholds were measured for a vertical 4 c/deg sinewave test grating. The resultant elevation in contrast threshold was plotted as a function of the (1) spatial frequency or (2) orientation differences between the first and second adapting gratings. Maximum threshold elevation was found when both adapting gratings shared the same spatial frequency and orientation. Minimum elevations were found when the second gratings spatial frequency or orientation differed by approx. 1.5 octaves or 45 deg, respectively. Beyond these values threshold elevations reapproached the baseline value measured in a control condition, where the 4.0 c/deg adapting grating was interleaved with a blank. The minimum threshold elevations were 0.2-0.3 log units below the baseline level. The results suggest the existence of inhibitory interactions between neural mechanisms tuned to the size and orientation of retinal images.

Marathon adaptation to spatial contrast: Saturation in sight

The contrast thresholds for detecting a 6.0 c/deg vertical sinusoidal test grating were tracked during and after 3 hr inspection of a high-contrast adapting grating of the same spatial frequency and orientation. Log contrast threshold increased linearly with log adaptation time, attaining a final stable value after approximately 30 and 60 min of adaptation for the two subjects tested. The recovery function was likewise linear on double logarithmic axes. The results further suggest that adaptation beyond the saturation point had no influence on the subsequent rate of recovery.

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 achromats 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.

A test for contrast-polarity selectivity in the tilt aftereffect.

The tilt aftereffect (TAE) was studied with adapting and test stimuli consisting of black or white bars (experiment 1), and of luminance edges (experiment 2). Both experiments failed to demonstrate selectivity of the TAE to the polarity of luminance contrast.

Contrast threshold elevation following continuous and interrupted adaptation.

Contrast thresholds for a 6 c/deg sinewave grating were measured following continuous and interrupted adaptation of 10 min duration to a high-contrast (0.6) sinewave grating of the same spatial frequency. Interrupted adaptation was administered as five 2-min segments, and the interadaptation interval (IAI) was varied from 10 to 180 sec. The results indicate that adaptation to spatial contrast can be described by a two-staged process, each stage having a different time constant of adaptation decay.

Different Neural Codes for Spatial Frequency and Contrast

S. S. Stevens (1957) argued that there are two different types of sensory dimensions, extensive and intensive, and that they have different types of neural codes. Stevens based his arguments on the results of psychophysical scaling experiments. However, if the two types of dimensions are coded in fundamentally different ways, the difference should be reflected in other tasks as well, including short-term memory tasks. In this paper, we present evidence that the short-term memory for spatial frequency, an extensive dimension, does not degrade over periods up to 10 sec, whereas the memory for contrast, an intensive dimension, does. In our experiments, the viewer must discriminate between two, successively presented stimuli. To respond correctly, the viewer must have an accurate memory of the first stimulus to compare with the second. If the memory degrades over time, discrimination performance must decline as the interval between the stimuli (ISI) lengthens. We find that spatial frequency discrimination, as measured by the difference threshold, is independent of ISI for intervals up to 10 sec, whereas the threshold for contrast discrimination progressively increases over this same period. This difference occurs at all contrast levels and has the same magnitude when the viewer judges both dimensions simultaneously as when the dimensions are judged separately.

Spatial waveform discrimination following higher-harmonic adaptation

Campbell and Robson [J. Physiol. (London) 197, 551 (1968)] proposed that a near-threshold square-wave grating can be distinguished from a sine-wave grating of the same spatial frequency and fundamental amplitude when the channel tuned to the third-harmonic component of the square wave reaches its own threshold. To test this hypothesis, we measured waveform discrimination thresholds with two-interval forced-choice methods before and after 4-min adaptation to a high-contrast sine-wave grating, the spatial frequency of which equaled that of the square waves third harmonic. The results indicate that 3f adaptation has only a negligible effect on discrimination thresholds. In a further experiment, we adapted observers to both 3f and 5f harmonic frequencies of the square-wave test grating presented sequentially over 4 min. Although substantial threshold elevations occurred at the 3f and 5f frequencies, the elevation in waveform discrimination threshold was small. These results suggest that the independent-channel hypothesis alone cannot account for the visibility of complex features (edges) following harmonic adaptation.