A new study reports a simple and non-invasive treatment method in the rehabilitation of maculopthy patients. Maculopathy is a pathological condition of a small area at the center of the retina called macula, that is associated with highly sensitive and accurate vision.
The neural mechanisms that shape our perception of elements in a visual scene depend on the activity of neurons in the Primary Visual Cortex. Although in the first stages the analysis of visual information is assumed to be local and independent, there are indeed some interactions that takes place even in the early stage. Depending on the distance between the receptive fields of neurons that codify similar stimuli, these interactions could be either excitatory, leading to a reduction of contrast thresholds, or inhibitory, resulting in an increasing of contrast thresholds. Similar stimuli share basic features such as orientation, dimension, spatial frequency and shape.
Crowding is expressed as the distance (arcmin) between the target and the flankers letters. Image credit Maniglia et al, PLoS ONE 6(10): e25568.
The possibility to modulate these interactions in the central visual field has been widely demonstrated and succesfully applied in treatments of central vision diseases such as amblyopia and myopia. In fact, using a training paradigm known as perceptual learning, a number of studies showed improvements not only in contrast thresholds, but also in non related, higher-level visual functions such as visual acuity or reading speed, abilities usually affected by central vision pathologies. The hypotheses for this transfer assume a spread of neural activation through a cascade of connections from lower to higher level of processing and the presence of the same linear filtering at the early stage for both contrast detection and visual discrimination. However, to date, there was no evidence for a similar possibility in the periphery of the visual field and it seemed unlikely to find facilitatory interactions even at small eccentricity, meaning that peripheral lateral interactions are mostly (if not only) inhibitory.
Stimuli used in the experiments. (A) the target (central patch) has the same orientation as that of the flankers. (B) the flankers are oriented horizontally with respect to the central vertical target. Image credit: Maniglia et al, PLoS ONE 6(10):e25568.
This stronger inhibitions seem to be the basis of a typical peripheric phenomenon known as crowding, generally defined as the deleterious influence of nearby contours on visual discrimination. Although not representing an issue for people with normal vision, crowding is indeed a practical problem for patients suffering of central vision loss, forced to use their periphery as a new fixation locus.
Recently, a team of researchers from the University of Padua, in a study published on PLoS ONE, showed that the possibility of modulating peripheral lateral interactions and demonstrated that crowding could be reduced. With a new perceptual learning procedure, an 8 week training on a wide range of spatial frequencies, they found an improvement in contrast thresholds along with the transfer to crowding effect.
The reduction of crowding is a striking evidence for the success of the treatment, and opens a new perspective in rehabilitation of patients suffering of central vision loss,who must use peripheral vision to perform tasks such as reading and refined figure-ground segmentation that normal sighted subjects perform in the fovea.
Reducing Crowding by Weakening Inhibitory Lateral Interactions in the Periphery with Perceptual Learning. Maniglia M, Pavan A, Cuturi LF, Campana G, Sato G, et al. PLoS ONE 2011. 6(10): e25568. doi:10.1371/journal.pone.0025568