In an unpublished manuscript datable to 1956-1957, La vision comme modèle de la connaissance (Vision as a model of knowledge), the philosopher of medicine and biology Georges Canguilhem referred to specific biological-adaptive mechanisms compensating for some human vision anomalies, particularly hemianopic anomalies. In individuals in whom it tends to occur, hemianopsia (or hemianopia) involves the loss of one half of a visual field. A complete right hemianopsia, as David Hubel (1988) pointed out, entails for instance the dimming of the center of gaze, with a decreased efficiency of the foveal region: it is as if, viewing the word far and trying to focus on the a, you could not see the r, but only the f and the left side of the a.
Canguilhem stressed that, in the case of hemianopic patients, macular vision, that is, precise and distinct vision usually due to the action of a tiny part of the eye called fovea, tends to remain approximately intact despite the total blurring of the central part of a visus. In a nutshell, several patients seem to perceive no sensation of blindness: “no sensation of dimming in the blind field. The absence of light is not perceived” (mn. p. 58). This is due, according to Canguilhem, to a veritable reorganization of the damaged visual field: the hemianopic patient’s eye develops a new “functional fovea” (mn. p. 59) replacing the diminished or compromised action of old fovea centralis.
Let’s read the manuscript in more detail. In 1795 Sömmering made it clear that the perceptual role of vision is mainly due to this eyeball micro-area, fovea, in which a “reciprocal correspondence of the cells it contains and the nerve fibers can be observed”, entailing “the fineness of the sensory and cerebral analysis of light impressions at this point” (mn. p. 59). In short, by quoting the French philosopher Maurice Pradines, one could say that “we can benefit from optimal vision through the macula, and also through a specific point, the fovea centralis. This part […], measured in a vertical line, does not go beyond 1.2 mm. It is therefore, in all senses, a point; and so, since it is thanks to this point that we have optimal vision, we channel into it, through a sliding movement of the eye, all that we want to see” (Pradines 1943-1946/1986, vol. I, p. 540, my trans.). What are the symptoms displayed by patients affected by hemianopsia and subsequent loss of usual sensory action of fovea? With a reference to David Katz’s account, Canguilhem indicates that “often a patient demonstrates hemianopsia by mistaking his handling of objects. His movements go to the right or left of the objects when he wants to frame them, depending on the [visual] half affected by disease. The patient does not fix his gaze directly on the eyes of the observer when the latter asks to do so, but seems to look a little to the side” (Katz 1955, p. 161-162, my trans.). Hence, what is interesting to note is that the patient, instead of maintaining a straight gaze, looks sideways at the interlocutor. This kind of behavior is because the hemianopic patient “gives to him a functional fovea. That is, the subject creates for his own use, a not anatomical but physiological fovea, and each point of the retina changes from how it was previously” (mn. p. 59, my italics). To further clarify this point, Canguilhem refers once again to Katz, according to which, in hemianopic patients, the new functional fovea responds to the subjective needs of focusing the external objects. “The damaged visual field” Katz wrote, “is transformed in such a way as to become optimal again. As a result of this reorganization, the patient has the impression of looking straight ahead, accommodating [the gaze] no longer from the fovea centralis, but from a point more or less close to it. A new fovea centralis is formed on the intact half of the retina. Thus, the area in which [the new fovea centralis] is located benefits from a higher visual acuity than before. The new fovea centralis is not automatically static like the old one: it does not exist except from a functional point of view, its position changes according to needs” (Katz 1955, p. 161-162, my trans.).
This is why, for Canguilhem (mn. p. 59, following Goldstein 1934), distinct vision is not the exclusive prerogative of a healthy eye: an hemianopic person can continuously move himself in order to gain the center of visual field to obtain an optimal vision. Hence, Canguilhem’s claim: “and this is not a pathological behavior, but a normal one” (mn. p. 59), since focusing on an object means moving oneself with respect to the object location, and moving it (by moving oneself) with respect to other objects. So, in cases where the saccadic movements of fovea centralis are inhibited by hemianopic phenomena, the development of a new “functional fovea” (mn. p. 59) within the retinal surface corresponds to a normal and by no means pathological state: the organism carries out an adaptive reorganization of its retinal components in order to maintain, even in anatomically abnormal conditions, the interaction with its vital context. It is therefore the environment, and the way a specific organism interacts with it, what determines (among other things) the adaptive regulation processes of biological organisms.
These Canguilhem’s findings show analogies with some recent theories of biology and medicine.
That’s the case, for instance, of Saborido’s theory of malfunction in philosophy of medicine. As indicated by Saborido et al. (2016), establishing what is normal or pathological, that is malfunctional, is not a matter stemming from a statistic view point, or from a simply observation of organism’s physiological states. It stems from an assessment of the concrete relationship between organism and environment. For example, a Gilbert’s syndrome involves no malfunction in those patients who do not experience any obstacles during daily activities, despite an excessive and uncontrolled increase of bilirubin, a pigment contained in bile (Saborido et al. 2016, p. 113). On the one hand, of course, malfunction is something objectively related to the physical physiology of an organism. On the other hand, if the organism does not perceive any obstacles during its interaction with a context (despite an objective anomaly that does not fall within the statistical norm, like an uncontrolled increase of bilirubin), the organism does not present malfunctions. This is precisely due to those internal adaptive regulation mechanisms to which Canguilhem also referred. These mechanisms can successfully compensate for an excess of bilirubin, which is why the subject feels healthy. In general conditions, “when environmental conditions change, the adaptive system triggers actions in order adequately to modulate the functioning of the organs, namely, to set them in those specific regimes of functioning that satisfy the adaptive norms. This is precisely an adaptive reaction”, which is triggered by the way a specific living being concretely interacts with an environment (Saborido et al. 2016, p. 109). This is precisely what Canguilhem meant with respect to the generative process of a functional fovea, as well. Fovea is functional insofar as it arises from practical, day-to-day needs of a specific individual in a given context.
After all, a similar remark was made by Amundson (2000). “If some trauma happened to relocate an optic vesicle to an unusual position on the head”, he wrote (Amundson 2000, p. 39), “lens induction would still proceed and result in a functioning eye. A more familiar aspect of developmental plasticity is the ontogenetic adaptation of an organism to its external environment. Development of use-enlarged muscles and protective calluses are customary examples of this kind of phenomenon”.
To conclude, we say that adaptive mechanisms compensating for the onset of eye pathological states are, therefore, a particular case of a more general adaptive capacity that we can extend to biological organism as a whole. This general capacity, allowing you to react to an unexpected situation generated by context variations, is called “normativity” (Canguilhem 1956-57).
 Which can be consulted at the CAPHÉS (Centre d’Archives en Philosophie, Histoire et Édition des Sciences) in Paris, location: GC. 13. 2. For a further analysis of this text, see Sfara, E. 2016. Una filosofia della prassi: organismi, arte e visione in Georges Canguilhem, Torino, NuovaTrauben; and Sfara, E. 2018. Georges Canguilhem inédit. Essai sur une philosophie de l’action, Paris, L’Harmattan.
Amundson, R. (2000). Against normal function. Studies in History and Philosophy of Biologicaland Biomedical Sciences, 31, 33–53.
Canguilhem, G. (1956-57). La vision comme modèle de la connaissance. Unpublished work. CAPHÉS, location: GC. 13. 2.
Goldstein, K. (1934). Der Aufbau des Organismus. Einführung in die Biologie unter besonderer Berücksichtigung der Erfahrungen am kranken Menschen, Den Haag, Nijhoff (fr. trans. by D. E. Burckhardt and J. Kuntz, La structure de l’organisme, Paris, Gallimard, 1951).
Hubel, D. (1988). Eye, brain and vision, New York, W. H. Freeman & Co.
Katz, D. (1944). Gestaltpsychologie, Basel, Schwabe (fr. trans. by M. David and S. Voute, Introduction à la psychologie de la forme, Paris, Marcel Rivière et Cie, 1955).
Pradines, M. (1943-1946). Traité de psychologie générale, 3 vol., ed. 1986, Paris, P.U.F.Saborido, C. et al. (2016). Organizational malfunctions and the notions of Health and Disease. In Giroux (ed.), Naturalism in the Philosophy of Health, History, Philosophy and Theory of the Life Sciences, 17, 101-120.