Neurociencia de la Baja Visión y formación de LPR en personas con pérdida de visión central Parte 2

Introduction to Low Vision and Central Vision Loss

Understanding Low Vision

• Low vision refers to a condition where individuals can use their vision for planning or executing tasks, despite having significant visual impairment.

• It is crucial not to treat individuals with low vision as if they are completely blind; rehabilitation can enhance their ability to utilize remaining vision.

Common Retinal Diseases Causing Central Vision Loss

• The most prevalent retinal diseases leading to low vision include diabetic retinopathy and age-related macular degeneration (AMD).

• Other conditions such as degenerative myopia and specific types of retinal thrombosis also contribute to central vision loss.

Key Concept: Preferred Retinal Locus (PRL)

Definition and Importance of PRL

• The preferred retinal locus (PRL) is the area in the peripheral retina that individuals with central vision loss adaptively use for fixation.

• When the fovea is damaged, patients systematically select a region in the peripheral retina for visual tasks, demonstrating neural plasticity.

Characteristics of PRL

• Patients do not randomly fixate around their scotoma; they consistently choose specific regions (e.g., P1 and P2) for fixation.

• The location of PRLs varies among individuals, influenced by personal preferences and possibly other factors yet to be fully understood.

Formation and Adaptation of PRL

Speed of PRL Formation

• Research indicates that PRLs form rapidly within months after diagnosis, often within three months post-diagnosis of AMD.

Performance Correlation with PRL Location

• There is a correlation between the location of the PRL relative to the scotoma and reading performance; those who fixate above their scotoma tend to perform worse due to obstructed lines while reading.

Changes Over Time in PRL

Evolution of Reading Performance

• Over four years, some regions used for fixation may disappear while others become more prominent, particularly areas left or below the scotoma which correlate with better reading speeds.

Stability in Fixation Locations

• A significant percentage (81%) maintain their initial chosen fixation point over time, indicating stability in how patients adapt visually.

Hypotheses on PRL Formation

Overview of Hypotheses

• Three main hypotheses explain how patients develop their preferred retinal loci: function-driven formation, performance-driven selection, and cortical reorganization.

Functionally Driven Hypothesis

• This hypothesis suggests that certain regions are selected based on functional needs like reading efficiency. However, evidence shows this may not always align with optimal visual acuity locations.

Performance-Based Selection

• Some studies indicate that patients might choose areas based on where they have better attentional capabilities rather than just visual acuity alone.

Cortical Reorganization Theory

• While less discussed here due to time constraints, this theory posits that changes in brain processing could influence how patients adapt their visual strategies following damage.

The Role of Neuroplasticity in LPR Formation

Understanding LPR and Visual Function

• The hypothesis suggests that the formation of the LPR (Locus of Preferred Retinal) is influenced by regions retaining visual function, making cortical reorganization easier. This occurs when certain areas stop receiving stimuli from the retina.

• Neuroplasticity allows nearby regions to send their afferents to areas of the cortex that have become underutilized, facilitating adaptation. However, this plasticity is more effective with closer afferents from the retina.

Cortical Reorganization and Attention

• Another theory posits that cortical reorganization drives LPR formation, potentially co-directed by visual attention as well. A central question arises: does LPR behave like a phobia? Some aspects align with phobic behavior while others do not.

Eye Movements and Their Implications

Ocular Control Mechanisms

• Eye movements are largely controlled by the superior colliculus; normal vision aligns fovea with objects of interest through saccades (rapid eye movements). In patients with central vision loss, fixation stability is significantly compromised.

• Individuals with low vision exhibit larger fixation movements compared to those with normal vision due to instability in fixation points, leading to broader movement patterns around focal points.

Fixation Instability in Low Vision Patients

• Studies show that patients with macular degeneration experience greater fixation instability than controls, indicating a larger area for fixations compared to individuals without visual impairments. This highlights significant differences between normal and impaired visual systems regarding fixation stability.

Comparing LPR and Phobia in Eye Movement

Oculomotor Referencing

• While LPR exhibits some similarities to phobia concerning oculomotor referencing—where eye movements aim to align either fovea or LPR with objects—the precision differs significantly between them. Errors in saccade targeting reveal these discrepancies clearly.

• An experiment demonstrated that errors in saccade positioning relative to LPR are consistently smaller than those related to fovea alignment, suggesting variability in how these locations are utilized during visual tasks.

Visual Acuity Differences Between LPR and Fovea

Agudeza Visual Analysis

• Unlike foveal vision, which has distinct characteristics such as higher receptor density affecting acuity, the visual acuity at the level of LPR does not match that of fovea despite being used for object centralization tasks during low vision scenarios. This indicates inherent limitations within peripheral processing capabilities compared to central processing abilities found at the fovea level.

Crowding Phenomenon: Implications for Reading

Understanding Crowding Effects

• The phenomenon known as "crowding" refers to difficulties distinguishing closely spaced letters or stimuli; it varies based on orientation (horizontal vs vertical). Research shows that crowding effects differ significantly between normal peripheral vision and conditions involving central vision loss like those seen in patients using an LPR strategy for reading tasks.

Comparative Analysis of Crowding

• In healthy individuals' foveal vision, letter spacing requirements appear circular (similar distances needed both vertically and horizontally), whereas peripheral conditions necessitate greater spacing vertically than horizontally for clear distinction among characters or words—a critical factor impacting reading efficiency among visually impaired populations utilizing alternative strategies like an adapted use of their remaining functional sight areas such as an established locus preference region (LPR).