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At least 775 records · Page 43Linked to original sources
PubMed2001

A diagnostic sign in migraine?

At the bedside it was noted that, after ocular fundoscopy, patients with migraine complained more often of an after-image than did non-migraineurs. This phenomenon was then investigated in consecutive patients attending a general neurology outpatient clinic. The relative risk for the diagnosis of migraine in patients reporting an after-image was 2.91 (95% confidence interval 1.96 to 4.34), and the sensitivity, specificity and positive predictive value of this observation for the diagnosis of migraine were 0.63, 0.75 and 0.55 respectively. After-images were equally likely to be reported by migraineurs with and without aura, and by patients with migraine equivalents. The after-image phenomenon probably reflects the heightened sensitivity to visual stimuli of patients with migraine. Although a diagnosis of migraine is primarily established by the patient's history, the presence of an after-image following ocular fundoscopy may support this diagnosis.

Adolescent↗
PubMed1980

Palinopsia: cerebral localization with computed tomography.

We report three patients whose vivid visual experiences were initially attributed to psychiatric or metabolic derangements, in the absence of obvious neurologic findings. Each depicted repetitive visual images of previously viewed objects or scenes, not necessarily confined to the region of homonymous visual deficit. Each patient had a large occipital lobe lesion, well demonstrated by computed tomography. An important diagnostic clue was documentation that the portrayed images had actually been viewed recently and therefore represented visual transpositions in time or space.

Afterimage↗
PubMed2000

Palinopsia and polyopia in the absence of drugs or cerebral disease.

OBJECTIVE: To report the occurrence of palinopsia and polyopia in patients who neither used drugs nor had diseases of the cerebral hemispheres, a group in which these visual symptoms have not been reported. METHOD: The patient records in the database of an academic neuro-ophthalmology unit were reviewed. RESULTS: Seventeen patients were identified in the database with the diagnosis of palinopsia or polyopia, of whom eight had diseases of the cerebral hemispheres, leaving nine patients for analysis. No patients with a history of drug toxicity were identified. In one patient the symptoms presented during an initial episode of demyelinative optic neuritis in the absence of clinical or laboratory evidence of cerebral lesions. In another patient they developed immediately after laser treatment of diabetic macular edema. A third patient developed the symptoms in association with visual loss from Leber's hereditary optic neuropathy. The other six patients were healthy individuals. CONCLUSION: Palinopsia and related visual symptoms can occur in otherwise healthy individuals and in patients with disease apparently confined to the eye or the optic nerve.

Adult↗
PubMed1979

Motion and vision. I. Stabilized images of stationary gratings.

To demonstrate that eye movements have profound effects on the sine-wave contrast threshold, the author uses a new method of stabilizing the retinal image, in which the Purkinje reflections from the eye move the stimulus pattern displayed on a CRT screen. Calibration of this compensatory motion is very critical; a gain error greater than 1% may produce significant destablization. Under optimum conditions, image stabilization elevates the subject's contrast threshold by a factor of about 20; it also produces after-images with resolution greater than 12 c/deg. These results compare favorably with those obtained by other methods.

Afterimage↗
PubMed1980

Phase selectivity of spatial frequency channels.

The phase selectivity of spatial frequency channels was measured, using an adaptation technique. Subjects first adapted to a grating of a given spatial frequency; subsequent threshold measurements were made at various spatial frequencies and phase shifts. Changes in the phase relationship between the test and adaptation gratings due to eye movements were circumvented by viewing the grating through an image stabilization apparatus. Local retinal adaptation was minimized by using an adaptation grating whose contrast flickered sinusoidally as a function of time. We were able to demonstrate channel-like frequency tuning for all conditions studied, but the threshold elevations following adaptation were always independent of the phase shift between the test and adaptation gratings. Our results imply that the channels which are selectively tuned to spatial frequency are not selectively tuned to spatial phase.

Adaptation, Ocular↗
PubMed1997

Equivalent background speed in recovery from motion adaptation.

We measured, in the same observers, (1) the detectability, d, of a small rotational jump following adaptation to rotational motion and (2) the detectability of the same jump when superimposed on one of several background rotation speeds. Following 90 s of motion adaptation the detectability of the jump was impaired, and sensitivity slowly recovered over the course of 60 s. The detectability of the jump was also impaired by the background speed in a way consistent with a quadratic form of Weber's law. We propose that motion adaptation impairs the detectability of the small jump because it is as if an equivalent background speed has been superimposed on the display. We measured the equivalent background by finding the real background speed that produced the same d' at each instant in the recovery from motion adaptation. The equivalent background started at approximately one to two thirds the speed of the adapting motion, declined rapidly, rose to a small peak at 30 s, then disappeared by 60 s. Since the equivalent background speed corresponds to the speed of the motion aftereffect, we have measured the time course of the motion aftereffect with objective psychophysics.

Adaptation, Physiological↗
PubMed1997

Contrast adaptation and the spatial structure of natural images.

Natural images have a characteristic spatial structure, with amplitude spectra that decrease with frequency roughly as 1/f. We have examined how contrast (pattern-selective) adaptation to this structure influences the spatial sensitivity of the visual system. Contrast thresholds and suprathreshold contrast and frequency matches were measured after adaptation to random samples from an ensemble of images of outdoor scenes or of synthetic images formed by filtering the amplitude spectra of noise over a range of spectral slopes. Adaptation selectively reduced sensitivity at low-to-medium frequencies, biasing contrast sensitivity toward higher frequencies. The pattern of aftereffects was similar for different natural image ensembles but varied with large changes in the slope of the noise spectra. Our results suggest that adaptation to the spatial structure in natural scenes may exert strong and selective influences on perception that are important in characterizing the normal operating states of the visual system.

Adaptation, Physiological↗
PubMed1985

Discontinuity limits for the generation of visual motion aftereffects with sine- and square-wave gratings.

Visual motion aftereffects (MAE's) were produced with adapting gratings that underwent repeated, abrupt displacements in a uniform direction. MAE's could be generated with sine-wave gratings even when the magnitude of each displacement approached a phase angle as large as 1/2 cycle. The maximum spatial step for generating a MAE with a square-wave grating was less than 1/4 cycle. If a dark pause was introduced between the successive positions (phases) of the adapting grating, MAE's disappeared when the pause was longer than 60-70 msec. The results can be used to define the spatiotemporal-response limits of a short-range motion process (or system of directionally selective motion sensors), and the results also suggest that the individual Fourier components of complex spatial patterns are capable of producing independent signals for direction of motion.

Adult↗
PubMed1987

Some properties of the double-flash illusion.

When a field of light (an inducing field) is extinguished, and a subsequent probe pulse is presented (5-msec duration, interstimulus interval of 80-320 msec), the single probe pulse appears to be a double flash. We measured the frequency of seeing this double-flash illusion when it was induced by high-contrast flickering fields. The illusion is dependent on inducer-field frequency, showing a major peak and, for some interstimulus intervals, subpeaks. There are notable interobserver differences in the frequency location of the peaks. We evaluated whether the results represented the transient response of a mechanism analogous to a single-harmonic oscillator. A second experiment compares the threshold illuminance for detecting the probe pulse with the illuminance above threshold required to see the double-flash illusion. The illuminance required for a double-flash threshold is high, being 7-20 times the illuminance required for pulse detection.

Adult↗
PubMed1987

Magnification factor for adaptation of a visual transient mechanism.

After adaptation by an observer to a patch of gradually increasing (or decreasing) luminance, a steady test patch appeared to be gradually dimming (or brightening). These aftereffects did not transfer interocularly. Adaptation to a checkerboard, in which the white squares gradually dimmed while the black squares gradually brightened, gave an aftereffect that was a pattern of intersecting diagonal lines, that is, an extremely blurred checkerboard. The larger the squares of the checkerboard were, the farther into the periphery the aftereffect extended, because small squares were blurred out by the summation areas of the underlying visual channels, which were larger at increasing eccentricities and had diameters of 20 times the resolvable dot separation. The estimated visual acuity of these channels was as low as 20/400. These estimates were confirmed by manipulating separately the local and space-averaged luminances of the adapting stimulus.

Acclimatization↗
PubMed2005

Role of synaptic and intrinsic membrane properties in short-term receptive field dynamics in cat area 17.

We examined the mechanisms through which the prolonged presentation of either a high-contrast stimulus or an artificial scotoma [equivalent to the stimulation of the receptive field (RF) surround] induces changes in the RF properties of neurons intracellularly recorded in cat primary visual cortex. Discharge and synaptic RFs were quantitatively characterized using bright and dark bars randomly flashed in various positions. Compared with the lack of stimulation (0% contrast for 15-30 s), stimulation with high-contrast sine-wave gratings (15-30 s) was followed by a strong reduction in gain and a weak but significant reduction in width of spike discharge RFs. These reductions were accompanied by a membrane potential hyperpolarization, a decrease of synaptic RF width, and varying changes of synaptic RF gain. Passive hyperpolarization by DC injection also produced significant reduction in the width and gain of discharge RF. Mimicking, in single neurons, high-contrast stimulation with high-intensity current injection also induced a membrane potential hyperpolarization, whose amplitude was correlated with discharge RF gain and width changes. Recovery from adaptation to high-contrast stimulation during the period of gray screen or scotoma presentation was associated with an increase in gain and discharge RF size. Stimulation of the RF surround with an artificial scotoma did not have any additional aftereffects over those of adaptation to a gray screen, indicating that the contraction and expansion of RF gain and size are attributable to intrinsic and synaptic mechanisms underlying adaptation and de-adaptation to strong visual stimuli.

Action Potentials↗