Conditioned after-images. II.
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Acoustic Stimulation↗
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Thresholds for negative after-images.
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Afterimage↗
Bleached rhodopson and visual adaptation.
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Adaptation, Ocular↗
Bleached rhodopsin and retinal interaction.
1. The human rod retina exhibits lateral inhibition which is demonstrated by the sensitization, i.e. improvement in incremental threshold sensitivity, of a steadily illuminated patch of retina when its surround is also illuminated.2. Experiments were carried out to test whether the previously reported equivalence between steady illumination (real light) and signals from bleached rhodopsin (dark light) also carries over into lateral interaction.3. Symmetrical experiments involving real light patches surrounded by dark-light annuli, dark-light patches surrounded by real-light annuli, and dark-light patches surrounded by dark-light annuli showed that dark light is neither susceptible to sensitization by lateral interaction nor capable of exerting it.
Afterimage↗
The perceived spatial frequency shift: evidence for frequency-selective neurones in the human brain.
1. Prolonged observation of a high-contrast grating pattern causes an apparent shift in the spatial frequency of gratings subsequently viewed with the same retinal region. Gratings of higher and lower frequency than the adapting pattern seem, respectively, higher and lower than in fact they are.2. There is no significant after-effect at the adapting frequency itself nor at frequencies more than two octaves away.3. For very low adapting frequencies, the after-effect remains centred at about 3.0 c/deg and declines in strength as the adapting frequency is successively lowered.4. The magnitude of the after-effect increases with the contrast of the adapting grating and the length of time spent in adaptation. It takes several hours to recover completely from 30 min adaptation.5. The phenomenon is orientation-specific: a horizontal adapting grating has no effect on vertical test gratings. There is partial interocular transfer of the after-effect.6. These findings provide further evidence that the visual system of man, like those of the cat and the monkey, contains neurones selectively sensitive to the orientation and dimensions of retinal images, and that these adaptable cells are actually involved in the encoding and perception of the size of simple patterns.
Adaptation, Ocular↗
The orientation specificity of two visual after-effects.
1. Inspection of a high-contrast adapting grating produces two visual after-effects: (a) the contrast threshold is raised for test gratings of similar spatial frequency to that of the adapting pattern and (b) the apparent spatial frequency of test gratings shifts away from that of the adapting grating-higher frequencies seem higher and lower ones lower than they really are.2. Both after-effects are orientation-specific. A horizontal adapting grating influences neither the threshold nor the apparent spatial frequency of vertical test gratings.3. The magnitude of the two after-effects was measured with vertical test gratings as a function of (a) tilt of a high-contrast adapting grating and (b) contrast of a vertical adapting grating.4. At all frequencies of the test grating, the decline of both after-effects produced by an increase in tilt of approximately 6(3/4) degrees could be matched by a reduction in contrast by a factor of 2.5. We take this as evidence for a common neural origin for these two visual phenomena.
Afterimage↗
Disparity processing of spatial frequencies in man.
1. Adaptation to a high-contrast sine-wave grating has been shown previously by Blakemore & Campbell (1969) to raise the modulation required to detect a low-contrast grating that has the same or similar spatial frequency as the adapting grating.2. A similar adaptation effect occurs when the adaptation and test gratings are seen binocularly and are presented off the plane of fixation. When the gratings are not located on the plane of fixation, however, the greatest rise in threshold following adaptation occurs for test gratings presented in the same plane as the adapting grating. Thus, the neural mechanisms adapted to the high contrast patterns must be processing disparity information.3. The spatial frequency response of the disparity adaptation effect has been measured by adapting to gratings of different spatial frequencies presented at a given disparity, and comparing threshold elevations for identical test gratings presented in the same (disparate) plane as the adapting grating or in the plane of fixation.4. The unbiased adaptation effect specific to disparity is greatest for gratings whose periods are twice the disparity.5. There is no adaptation effect specific to disparity for individuals possessing only convergent or only divergent disparity mechanisms.6. The results suggest that disparity mechanisms make bar by bar correlations as opposed to edge by edge correlations and that narrow bar detectors feed small disparity mechanisms whereas wide bar detectors feed large disparity mechanisms.
Adaptation, Physiological↗
Evidence for disparity detecting neurones in the human visual system.
1. It is known that adaptation to a grating pattern causes a rise in the contrast threshold for test gratings of similar spatial frequency and orientation.2. We find this after-effect also to be disparity-specific. Adaptation to a grating at zero horizontal disparity (at the same distance as the fixation point) causes a greater elevation of threshold for patterns at the same disparity than for those at nearby disparities, closer or more distant than the fixation point.3. Adaptation to a grating at some disparity other than zero causes a disparity-specific elevation of threshold centred on the adapting disparity.4. This finding also applies if the observer adapts to a grating but single bright bars are used as the test stimuli.5. The disparity-specific ;tuning curves' revealed by these techniques are quite broad, having a half-width at half-amplitude of several min of disparity.6. Adaptation to a grating at one disparity causes an apparent change in the distance of test gratings at nearby disparities.7. We compare these psychophysical experiments with the properties of disparity-selective binocular neurones in the visual cortex of cats and monkeys.
Adaptation, Ocular↗
After-images on backgrounds of different luminance: a new phenomenon and a hypothesis.
1. After-images may be caused to vanish suddenly and reversibly by viewing them against a background of suitable luminance I(e).2. The value of I(e) at any moment is not related to any parameter (intensity, colour, duration, time) describing the stimulus that caused the after-image.3. On the other hand, I(e) is very closely related to the immediate adaptational history of the eye, so that viewing a dark background for a few seconds lowers, and a light one raises, the value of I(e).4. It is suggested that the signal conveying sensations of relative brightness in this situation is related to the rate of change of concentration of an intermediate photo-product decaying spontaneously with a time constant of some 14 sec.
Adaptation, Ocular↗
The dark adaptation curve of rods measured by their after-image.
1. The common dark adaptation curve exhibits two branches; the course of the rod branch cannot normally be measured at early times since it lies above the observed cone thresholds. In this paper we measure it. 2. this is done by observing the negative after-image against a uniform background critically adjusted in luminance. 3. adjacent to the bleached area to be studied is a second area more strongly bleached. If the background intensity is below threshold for the less bleached area it will not be seen there; but if the background is above that threshold, this area will be seen brighter than the other. 4. the dark adapted threshold on the less bleached area is therefore the background luminance which just permits the two areas to be distinguished in the after-image. 5. after 5 min cones have quite recovered, and thus have no after-image to contaminate the rod image. 6. the rod curve measured by after-image is traced over 5 units of log threshold: it is an exponential with half life of 4.5 min, and coincides with the time course of regeneration of rhodopsin in man.
Afterimage↗
Visual spatial aftereffect from prolonged head-tilt.
Subjects with head upright were required to adjust a lighted bar in a dark room until the bar appeared vertical; the task was performed before and after 2 and 3 minutes of lateral head-tilt with their eyes closed. A visual spatial aftereffect was observed which varied as a function of the angle of head-tilt and which was opposite in direction to head-tilt.
Afterimage↗
Monocular and binocular aftereffects of chromatic adaptation.
Supersaturated greens seen after long-wavelength adaptation depend upon contrast from the continuing afterdischarge of bleached "red" receptors in the surround, rather than merely upon inactivation from bleaching of "red" receptors in the test spot area. When test spot and bleach field coincide spatially, supersaturated greens are not seen. Since color mixing but not contrast was found binocularly, color contrast must be a retinal phenomenon.
Adaptation, Ocular↗
Adaptation and negative aftereffect to lateral optical displacement in newly hatched chicks.
Chicks wearing hoods containing 8.5-degree wedge prisms from the day of hatching showed both significant reduction in the average lateral displacement of pecking (adaptation) and significant pecking overcompensation in the direction opposite to the original displacement (negative aftereffect) when matched 0-degree plates were substituted for the prisms on the 8th day.
Adaptation, Ocular↗
Color: a motion-contingent aftereffect.
After human observers alternately view green stripes moving up and red stripes moving down for periods of 1/2 to 4 hours, they see a pink aftereffect when white stripes move up and a green aftereffect when white stripes move down. Longer exposures produce aftereffects which are visible 20 hours after stimulation. Thus, experience which pairs simple attributes (color and motion) of visual stimulation can result in a lasting modification of perception.
Afterimage↗
Interocular transfer of orientational effects.
Prolonged exposure of one eye to a diagonal line grating produces masking or decreased sensitivity for similar test gratings presented to the contralateral eye. These aftereffects are orientationally selective and suggest that narrow orientationally tuned channels found by electrophysiological methods in the visual cortex of the cat and the monkey may have neural correlates in the human brain.
Afterimage↗
Visual motion perception: experimental modification.
If a human observer fixates a moving spiral pattern for 15 minutes, a negative aftereffect of motion is perceived when he inspects a stationary spiral 20 hours later. The illusory motion is seen only when the stationary test stimulus falls upon the portion of the retina which had been stimulated by real motion. Thus previous stimulation can cause a relatively long-term modification of vision.
Adult↗
Motion perception: a color-contingent aftereffect.
When observers who watched repeated alternations of a red contracting spiral and green expanding spiral were later shown stationary spirals, red and a green the red stationary spiral appeared to be expanding and the green stationary spiral appeared to be contracting. These color-contingent motion after effects complement reports of motion-contingent color aftereffects and suggest that both may reflect adaptation of detectors specific to color and motion.
Adolescent↗
Iconic storage: the role of rods.
The hypothesis that rods mediate iconic storage was tested by presenting letters of one color against a field of another. The colors were chosen to be discriminable only by the cones, only by the rods, or both. Under dark adaptation, the rods had little if any effect on partial-report advantage; however, they were important in determining the phenomenal persistence of the stimulus. Under light adaptation, the rods played no apparent role in either type of persistence.
Afterimage↗