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Editor,—In the February 1999 issue of theBJO, we reported that subjects presenting with a right homonymous paracentral scotoma experienced “filling in” of field defects and, as a result did not perceive their scotoma.1 Moreover, the subjects described in our study noted that, after a few seconds of steady fixation, a perceptual distortion occurred in areas surrounding the defect, as if images bordering the scotoma were pulled toward the centre of the defect. We suggested that this phenomenon was due to a process of plasticity in the visual cortex.
In the same issue Dr Neil R Miller published a most interesting editorial devoted to our study, in which he raised a number of questions regarding the perceptual phenomena described.2In particular, he wondered whether distortion occurred with homonymous field defects on both sides of the visual space or only with right sided defects. A recent clinical observation has allowed us to clarify this issue. We therefore believe that it deserves to be briefly reported.
A 20 year old, right handed woman underwent occipital transtentorial excision of a right pineal cyst that caused triventricular hydrocephalus. The surgical procedure resulted in a small localised alteration in the right calcarine cortex, which was demonstrated on magnetic resonance imaging (MRI). The occipital abnormality clearly appeared on T2 post-contrast images as a small stroke localised in the middle calcarine cortex. The visual cortex alteration induced a small absolute scotoma, located paramedially in the left homonymous visual field. The scotoma was about two degrees in width and in height, and was centred 7 degrees below the horizontal meridian. Using a white on black Amsler grid, the patient was unable to note any defect, confirming that filling-in occurred in this patient. The scotoma was absolute when tested at 1 metre distance from the tangent screen, using a 10 W/1000 stimulus. However, the patient perceived no gap in the visual field. When she was asked to fixate steadily the nose of her examiner, who was facing her from a distance of 60 cm, she volunteered after about 8 seconds that her examiner's right shoulder appeared to be slightly contracted, and about 5 or 6 cm lower, and some 4 cm narrower, than the left shoulder. The homonymous scotoma was then delineated using a laser pointer, and was found to overlie the examiner's axillary area.
Our patient presented with a minute lesion in the right calcarine cortex, a common sequela of an occipital transtentorial approach to the pineal area.3 As a result, she was found to have a small left homonymous scotoma. Neuro-ophthalmological evaluation corroborated our previous findings that homonymous defects are “filled in”, and that images surrounding the defects are perceptually displaced towards the scotoma. This report answers one of the questions asked by Miller—namely, whether this perceptual phenomenon is restricted to scotomas located in the right homonymous field. We found that spatial distortion also occurred with defects located in the left homonymous field. The homonymous scotoma was re-delineated using a laser pointer, and was found to overly the examiner's axillary area. This finding has practical and theoretical implications.
Occurrence of a spatial distortion following alteration on either right or left homonymous visual field does not imply that the characters of changes in spatial perception are identical in both conditions. Indeed, specialisation of right and left cerebral hemispheres has been demonstrated in human normal subjects—for example, using functional magnetic resonance imaging (fMRI).4 In that fMRI study, it was shown that a stronger activation occurs in the left than in the right angular gyrus with categorical tasks (that is, when asking to judge whether a dot was presented above or below a horizontal line), whereas stronger activation occurred initially on the right than on the left angular gyrus in a coordinate task (that is, when subjects were asked to judge whether or not the distance between a dot and a bar was within a reference distance). In addition, the authors showed that, in the later task, the involvement of the right angular gyrus decreased with practice, while that of the left angular gyrus increased. Thus, obviously, although both hemispheres are involved in spatial function, there is theoretical evidence suggesting that some degree of hemispheric specialisation can be found in the process involved in visual perception of space. Within the frame of our clinical observation, however, these experimental data had no evident clinical implications.
This study was supported by the Swiss National Fund for Scientific Research, grants nos 3200-040780.94, 3200-0409594.96, and by the Aetas Foundation and the Sir Jules Thorn Foundation.
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