Functional Retinal Locus Rather than Multiple PRLs?

When viewing Figure 1 in Crossland et al., ¹ one cannot escape an obvious observation: The PRLs depicted in black overlap the same retinal regions as the PRLs depicted in yellow in virtually all cases. Out of the approximately 900 deg² of retinal surface, both black and yellow, PRLs reside more or less on the same 10 deg² of retina. This is no coincidence. Could it be that in fact what is described in this article is the same retinotopic oculomotor effect displaying different fixation stability patterns when the patient is presented with different tasks? Could it be that our terminology for describing these observations needs to be changed? 

In fact, we suggested exactly this in a recent paper ² in which we introduced the concept of a functional retinal locus defined as a retinal region hosting the area with best retinal sensitivity and the preferred retinal loci used for fixation. The implication of our statement was that the same regional retinal area may host multiple PRLs, as presented vividly in Figure 1 in Crossland et al. ¹ Hence, it is erroneous for the authors to claim that the study subjects used “a different retinal region for point fixation and for word fixation.” 

In fact, it is obvious that the same retinal region was used; however, various overlapping fixation stability patterns were displayed for various tasks, a distinction overlooked by the authors. Fixation span and fixation span patterns graphically depicted in Figure 1 for black and yellow PRLs suggest also that they are expressions of the same continuum of oculomotor skills. This concept is further supported if one attempts regression analysis from data presented between the black and yellow PRL BCEAs (bivariate contour ellipse areas) showing a high relationship between the two (R = 0.615, P ≤ 0.02685; [t = 2.587, df = 11]; y = 1.083x + 689.4). The differential in fixation stability shown in Crossland et al. ¹ is also in line with other observations that better fixation stability is not the optimal desirable performance for all tasks in patients with loss of macular function. ³  

We congratulate the authors for the compelling evidence they present in describing the various fixation stability patterns as related to specific tasks. We view this article, however, as evidence of the need to move away from mentioning one representative fixation point, but rather to emphasize regional fixation areas when referring to the topography of the residual functional retina in patients with macular function loss.