ناتوانی در ادراک بیماری برای کور رنگی؛ یک مطالعه موردی طولی مغزی

Abstract Cerebral achromatopsia is a rare disorder of colour vision caused by bilateral damage to the occipito-temporal cortex. Patients with cerebral achromatopsia are commonly said to suffer due to their disturbed colour sense. Here, we report the case of a patient with cerebral achromatopsia who was initially unaware of his deficit, although three experiments with eye movement recordings demonstrated his severe inability to use colour information in everyday tasks. During two months, the evolution of his colour vision deficit was followed with repeated standardized colour vision tests and eye movement recordings. While his performance continuously improved, he became more and more aware of the deficit. Only after colour vision had almost normalized, his subjective colour sensation was inconspicuous again. The simultaneous occurrence of achromatopsia and the corresponding anosognosia and their parallel recovery suggest that both deficits were due to dysfunction of the same brain region. Consequently, the subjective experience of colour loss in achromatopsia may depend on the residual function of the damaged colour centre.

. Introduction Cerebral achromatopsia denotes the complete or partial loss of colour vision after cortical damage (Zeki, 1990). Although the existence of a colour centre in the human brain has been debated for over a century (Zeki, 1990), it is now widely accepted that bilateral damage to the ventral occipito-temporal cortex causes this disorder (Brazis, Masdeu, & Biller, 2007). Achromatopsia is frequently associated with other deficits, most prominently visual field defects and prosopagnosia (Beck et al., 1978, Meadows, 1974 and Zeki, 1990). Achromatopsia is a rare condition and most reported patients with achromatopsia either did not recover or were not followed up (for an overview see Bartels & Zeki, 2000, but see Bornstein and Kidron, 1959 and Rondot et al., 1967). Therefore, the natural history of the disorder is not well known. It has been stated that patients with achromatopsia usually notice their deficit (Bauer and Demery, 2003 and Hodges, 2007). Moreover, the confrontation with a visual world drained of colour has often been vividly described as a frightening and distressing experience (e.g. Pallis, 1955). Interestingly, however, there are a few published reports of patients who either did not notice their colour perception deficit (Green and Lessel, 1977 and Grüsser and Landis, 1991) or did so only some time after brain damage, suggesting unawareness for a loss of colour vision (Sacks, 1996 and Steffan, 1881). The unawareness of a handicap after brain damage is known as “anosognosia” and has been described for the motor, sensory and visual system (Prigatano & Schacter, 1991). Anosognosia for achromatopsia and its evolution over time, however, has not been studied in depth. The study of anosognosia is important since it can inform concepts of higher brain functions, in particular models of consciousness (Bisiach & Geminiani, 1991). Along these lines, cases of anosognosia for achromatopsia may add to our understanding of visual consciousness. We had the rare opportunity to repeatedly analyse consciousness of colour perception in a patient who suffered from cerebral achromatopsia due to bilateral ischemic damage to the occipito-temporal cortex. Using standardized colour vision tests and eye movement recordings we could show that four days after the stroke the deficit rendered him completely unable to make use of colours in order to discriminate and identify objects. Yet, despite the severity of achromatopsia, he was completely unaware of his handicap. Over a period of eight weeks, an almost complete recovery of achromatopsia could be demonstrated. During this recovery phase, objective improvement of his colour sense was accompanied by increased subjective awareness of his deficit. This observation points towards a possible role of the cerebral colour centre not only in processing but also in consciously perceiving colour.

. Results 4.1. Oddity search task In the oddity search task, the patient failed to report any target at T1. He told that he could not recognise anything unusual in the colour pictures shown. In contrast, the controls’ performance was almost flawless, only one of them missing out on a single target. This difference in search performance is reflected in the oculomotor data (Figs. 3B and 4): Healthy subjects spent a significantly greater proportion of the total viewing time on the target ROIs (44.7%; S.D. 10%) compared to the patient (15.6%; modified t-test: t = −2.656, pone-tailed = 0.014). In contrast, the patient did not significantly differ from controls with regard to the time spent fixating the target region on the respective grey-scale images (patient: 9.4%; control group: 10.5%, S.D. 2.4%; modified t-test: t = −0.435, pone-tailed = 0.338). Oddity search task. The mean percentage of total viewing time spent in the ... Fig. 4. Oddity search task. The mean percentage of total viewing time spent in the target ROI is shown for coloured (left) and corresponding grey-scale pictures (right), respectively. Figure options One month later (T2), the patient found four out of five targets. The improved performance was also reflected in the oculomotor behaviour, which had become similar to that of controls (time on ROI: 45.7%; t-test: t = 0.199, pone-tailed = 0.424; Fig. 4). With regard to his fixation pattern on grey-scale images the patient remained similar to controls (time on ROI: 12.7%; t-test: t = 0.87, pone-tailed = 0.205). 4.2. Object-colour matching task In the object-colour matching task, the patient promptly identified all objects at T1, a result arguing for intact form perception. However, only in two out of 10 trials did he choose the appropriately coloured drawing (i.e. eight errors). This performance is about equal to chance level and considerably below that of healthy controls who gave 98% correct answers. At that time, the patient spent a significantly lower percentage of the total viewing time looking at the correct drawing (30.8%) compared to controls (46.8%; S.D. 6.4%; modified t-test: t = −2.371, pone-tailed = 0.023; Figs. 3A and 5). Object-colour matching task. Percentage of viewing time spent in the target ROI. Fig. 5. Object-colour matching task. Percentage of viewing time spent in the target ROI. Figure options One month later, at T2, the patient's performance on the object-colour matching task had considerably improved, resulting in only one error. Concurrently, his oculomotor behaviour was now found to be similar to that of controls (percentage of time on target: 40.7%; modified t-test: t = −0.912, pone-tailed = 0.194). 4.3. Ishihara task At T1 the patient declared that he was unable to discern the differently coloured path on the pseudoisochromatic colour plates. In line with this statement, his visual fixations were seemingly unsystematically distributed, while controls clearly followed the path (Fig. 6). Statistical analysis confirmed that the patient spent a significantly shorter proportion of the viewing time looking at the ROIs comprising the differently coloured paths (patient: 30%; controls: 78.8%, S.D. 11.1%; modified t-test: t = −4.145, pone-tailed = 0.002; Fig. 7). However, in the control condition, which does not require colour vision, the patient performed well, making 94% of all fixations within the specified ROI. Scanpaths for one example of the Ishihara task. Upper left: control subject. ... Fig. 6. Scanpaths for one example of the Ishihara task. Upper left: control subject. Upper right: patient at T1. Lower left: patient at T2. Lower right: patient at T3. Figure options Ishihara task. Percentage of viewing time spent in the target ROI. Fig. 7. Ishihara task. Percentage of viewing time spent in the target ROI. Figure options One month later, at T2, the patient hesitatingly reported recognising the paths. Consequently, his oculomotor behaviour showed some improvement, but was still significantly different from that of the controls (time on ROI: 41%; modified t-test: t = −3.211, pone-tailed = 0.007). In the control condition, 87% of fixation time fell within the ROI. Two months after stroke, at T3, the patient's performance had again improved. Although he still performed worse than controls, the difference was no longer statistically significant (time on ROI: 63.4%; modified t-test: t = −1.308, pone-tailed = 0.116).