A.G.M. van Vliet

Åland eye disease: no albino misrouting

Electrophysiological studies showed that a patient with Åland eye disease had no misrouting of the optic pathways which is always found in all forms of albinism as a consequence of the retinogeniculate anomaly. Also the spontaneous and optokinetic nystagmus did not resemble that of the large majority of human albinos. The marked asymmetry found in this patient seems to be typical for humans with a defective development of foveal binocular vision. These findings are in agreement with clinical, nystagmographic and EM findings that Åland eye disease is distinct from the Nettleship‐Falls type of X–linked ocular albinism. Furthermore, Åland eye disease is different from X‐chromosomal congenital stationary night blindness with myopia by the fact that the scotopic functions are only moderately affected and there is no restriction of the peripheral photopic visual fields. In addition, there is latent nystagmus of extraocular type that appears also in female carriers. There is no ophthalmoplegia, there is a progression of the myopia and the dyschromatopsia is of secondary type.

On the Central Mechanism of Latent Nystagmus

The eye movements of patients with latent nystagmus were recorded during monocular gaze intention following acoustic stimulation in complete darkness and in a series of experiments with the aid of the katoptric pseudoscope of Ewald. It appears to be the intention of “looking‐with‐one eye” which elicits latent nystagmus. Moreover, it can be shown that the moment of appearance and the direction of the nystagmus are determined not by the illuminated retina but by the act of “pointing” an object with one eye. The preponderance of motor over sensory factors, and especially of intentional motor behaviour, is clearly demonstrated. The possible central mechanism of latent nystagmus is discussed.

Nystagmographical Studies in Åland Eye Disease

An extensive electronystagmographical examination was made of 25 members of a family with Åland eye disease (Forsius‐Eriksson syndrome). It appears that the nystagmus belongs to the syndrome of latent nystagmus. Out of seven investigated male subjects with Åland eye disease, five showed classical latent nystagmus and two had slight manifestations of latent nystagmus. Of nine investigated female subjects, only two revealed a very slight latent nystagmus. According to our studies we can postulate two modes by which latent nystagmus could have been inherited in this family:

Optokinetic nystagmus in patients with defects of the central visual field.

Optokinetic nystagmus (OKN) was studied in patients with central visual field defects. Apart from a classic OKN, an inverted nystagmus was observed in some cases. A prerequisite for eliciting this paradoxic OKN was that the attention of the subject was directed to the field defect. Although other factors seem to be co-determining, a central mechanism is thought to process the visual information according to spatial-selective attention.

Optokinetic nystagmus and spatial-selective attention: an experimental study

The influence of spatial-selective attention on the direction of the optokinetic nystagmus (OKN) was investigated in normal humans. During horizontal optokinetic stimulation of the peripheral retina, the central visual field was neutralized by ambivalent stimulation. The results showed that the direction of the OKN was modulated by the direction of the attention. A central mechanism that processes the optic information according to spatial-selective attention is proposed.