A study of systematic visual perseveration involving central mechanisms.
The subject of this investigation experiences vivid after-images which persist for tens of seconds following stimulation by light insufficiently intense to produce after-images in normal subjects. The after-images occur at all points in the visual field, and have spatial structure closely similar to that of the stimuli which elicit them, the only distortion being the absence of high (greater than or equal to 10 cycles X deg-1) spatial frequencies. The duration of the after image is dependent on the spatial structure and spectral content of the eliciting stimulus, being maximum for low, nonzero spatial frequencies (1-2 cycles X deg-1) and yellow or green wavelengths. The after-images appear 'khaki' or 'mustard-green' in colour, regardless of the spectral composition of the light stimulus, and those produced by flickering or moving targets do not themselves exhibit temporal fluctuations, although movement produces a streak along the path described by the moving object. After six weeks of treatment with the anticonvulsant drug, carbamazepine, the duration of the after-images fell by about 30%, but no further reduction occurred over the following three months. In spite of the prolonged after-images, the subject has normal sensitivity for detection of flicker and movement, but that for detection of a single flash is raised abnormally at low background illumination. Neither CT nor MRI brain scans revealed any abnormality. These responses are discussed in relation to those found in other subjects suffering visual perseveration or palinopsia, and we examine their possible causes.
D000328:Adult; D000360:Afterimage / Q000502:physiology*; D005260:Female; D006801:Humans; D010775:Photic Stimulation; D014786:Vision Disorders / Q000503:physiopathology*; D014796:Visual Perception / Q000502:physiology