Dear Editor,

I should like to congratulate the authors for the article citing the original publication on peripheral fading by Troxler (1804). However, as editor of the website on the work of the Swiss philosopher Ignaz Paul Vital Troxler (1780-1866) who started as physician and ophthalmologist before engaging in research on anthropological and political subjects, I regret that after 201 years, the publication of 1804 is often cited in an incomplete and/or misleading way.

Troxler used to sign his publications in the world's first ophthalmological journal (i.e. "Ophthalmologische Bibliothek", edited by K. Himly and J.A.Schmidt) with "D." for "doctor". Therefore,the fact that the early neurophysiologist and neuropsychologist "D.Troxler" and the political philosopher I.P.V.Troxler are identical has often been overlooked in the medical literature, despite the fact that Troxler's concept of perception has had an important impact on the development of modern neuropsychological research.

I should like to suggest that, in references to "Über das Verschwinden gegebener Gegenstände innerhalb unseres Gesichtskreises" (On the disappearance of given objects from our visual field), the author be cited as "Troxler D.(I.P.V.)", to facilitate identification.

I kindly invite you to visit the website http://www.troxlerforum.ch which contains a biographical summary, a bibliography and some references on research related to Troxler's works on visual perception.

Sincerely Yours,

Hans U. Iselin M.D. CH-4310 Rheinfelden (Switzerland)

Dear Editor

In our paper entitled "Can fixation instability improve text perception during eccentric fixation in patients with central scotomas?",[1] we reported that patients with eccentric fixation can improve their perception of a text by performing rapid eye movements back and forth between two eccentric retinal areas, thus inducing a revival of eccentrically projected images. An experimental set-up allowed us to reproduce the perceptual phenomenon and the refixational eye movements in normal subjects. These subjects reported that a change in fixation position induced a refreshment of the letter image, immediately after the realisation of the eye movement. Improved perception lasted approximately one second. When fixation was maintained stable, a rapid fading effect was noted which reduced letter recognition. This suggested that performing refixational eye movements counteracts the occurrence of a form of perceptual filling-in, known as Troxler’s phenomenon.

We recently examined a patient with bilateral central scotomas and eccentric fixation who reported that blinking enabled him to refresh the projected image. This observation added to our understanding of mechanisms counteracting Troxler’s phenomenon. Moreover, it may have useful practical implications in rehabilitation procedures for low vision subjects. Therefore we believed this finding deserves to be briefly reported.

Case report

We investigated the reading strategies of a 44 year old man with bilateral central scotomas of about 20 degrees in diameter. His visual condition resulted from Stargardt’s disease, diagnosed at the age of 18. Visual acuity was 0.08 in the right eye and, 0.1 in the left eye. A Scanning Laser Ophthalmoscope (SLO, Rodenstock, Munich, Germany) was used to project letters, words and a paragraphed text onto his retina and to assess his fixation behaviour. We also asked the patient to read word of different length on a monitor.

We observed that while deciphering words he blinked in a voluntary and repetitive manner. He was aware of the phenomenon and he explained that blinking helped him to read because it induced a revival of word image. He developed the habit of blinking more frequently when he could not immediately read the presented text.

This phenomenon can be reproduced using the set up described in our above-mentioned study by maintaining fixation on a dot and rapidly blinking when the letter fades.

Comment

Our observations demonstrated the occurrence of two distinct clinical mechanisms counteracting fading of letters projected onto peripheral retina. The first consists of repeated changes of fixation whereas the second relies on repeated blinking. Both mechanisms involve intermittent suppression of letter projection on the concerned retinal area. Our findings corroborate a previous report emphasizing the influence of temporal variations of the visual stimulus on perceptual filling-in. Experimentally, normal individuals observe reduction of the filling-in process when exposed to a flickering background (de Weerd et al., 1995). This phenomenon should be kept in mind by low vision therapists when rehabilitating patients who report difficulties in distinguishing eccentrically located images.

Reference

(1) A Déruaz, M Matter, A R Whatham, M Goldschmidt, F Duret, M Issenhuth, and A B Safran. Can fixation instability improve text perception during eccentric fixation in patients with central scotomas? Br J Ophthalmol 2004; 88: 461-463.