Ewald Hering’s (1899) On the Limits of Visual Acuity: A Translation and Commentary

Abstract

Towards the end of the 19th Century, Hering and Helmholtz were arguing about the fineness of visual acuity. In a talk given in 1899, Hering finally established beyond reasonable doubt that humans can see spatial displacements smaller than the diameter of a foveal cone receptor, an ability we nowadays call ‘hyperacuity’ and still the topic of active research. Hering suggested that this ability is made manifest by averaging across the range of locations stimulated during miniature eye movements. However, this idea was made most clear only in a footnote to this (not well known) publication of his talk and so was missed by many subsequent workers. Accordingly, particularly towards the end of the 20th Century, Hering has commonly been mis-cited as having proposed in this paper that averaging occurs purely along the lengths of the edges in the image. Here, we present in translation what Hering actually said and why. In Supplementary Material, we additionally translate accounts of some background experiments by Volkmann (1863) that were cited by Hering.

Visual Neuropsychology in Development: Anatomo-Functional Brain Mechanisms of Action/Perception Binding in Health and Disease

“…This capacity is defined "vernier acuity" and refers to the ability to perceive spatial incongruence, e.g., misalignment, with a resolution even higher than simple visual acuity. The behavioral aspects of vernier acuity have been long known, since the seminal work by Ewald Hering at the end of the nineteenth century (Strasburger et al, 2018). As it goes beyond the physical features of the eye, the vernier acuity is one of the examples of the importance of cortical dynamics in supporting visual skills (Manny, 1988;Skoczenski and Norcia, 1999).…”

The Clinical Use of Vernier Acuity: Resolution of the Visual Cortex Is More Than Meets the Eye

“…The exact mechanism underlying the ability of vernier acuity, and other hyperacuities, to surpass this resolution limit of the eye and the retinal elements is still being clarified. Hering (1899) proposed that neural averaging of the signals of receptors along a target (the “local signs”), across minute eye movements, could explain the ability to precisely localise signals ( Figure 4 ; Strasburger et al, 2018 ). However, as he included the key concept regarding eye movements only in a footnote to his paper, he became widely miscited as having proposed that averaging occurred along the length of the line contour, an idea that in time was disproved by experiments showing that dot and curve stimuli could also act as vernier stimuli.…”

“…Excitation is depicted in green. (B) The offset edge completely falls along a single line of receptors (line 2) and is therefore perceived as “straight.” Small, repetitive eye movements between (A,B) would provide the location difference of the offset lines via higher order averaging ( Strasburger et al, 2018 ). (C) In a different orientation, an offset edge lies parallel to the boundaries between receptors.…”

Habitual higher order aberrations affect Landolt but not Vernier acuity

“…This is exemplified by our ability to detect the offset in a pair of lines or dots to a much finer degree than we are able to read. Psychophysical thresholds derived from Vernier and similar positional discrimination tasks are usually only a fraction of the eye's resolving power, and are well below the two-point sampling capacity of the retinal photoreceptor mosaic (Curcio, Sloan, Kalina, & Hendrickson, 1990; Hering, 1899; Hirsch & Curcio, 1989; Strasburger, Huber, & Rose, 2018). This is why the perceptual performance in these tasks was termed hyperacuity (Westheimer, 1975).…”