On Seeing With Defective Vision: 3

Homonymous deficits

 Scotomata, quadrant, and hemifield deficits

Static representations of homonymous visual field defects are straightforward [ A] but cannot capture an aspect of crucial importance to vision of daily life, namely, the role of eye movements. The following simulation aims to do so. Select laterality, defect type, and defect depth. The selected defect will be superposed on the background together with an eye symbol, indicating the direction of gaze. Move the mouse cursor over the display to change the direction of gaze. The eye can easily be moved into visible parts of the scene, reflecting the normal ease of directing gaze to new fixation points. It is much more difficult to direct gaze to those parts of the scene that are hidden inside field defects. Actually, "blind" saccades require a conscious effort and they commonly fail to land on the desired targets. These difficulties are mimicked in the display by the erratic behavior of the eye symbol when attempting to drag it in the direction of a field defect. Hold down the keyboard shift key to disable this feature.

Patients with field defects that are severe enough to endanger their driving licenses often claim that they easily can compensate for their defects by actively looking around. The display may help to demonstrate how demanding, erratic and time-consuming searching eye movements may be.

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Arguably, an "egocentric" representation, that is, a simulation where the background view moves relative to a stationary fixation point, may be more realistic. Click the to open such simulation a in a separate window. Place the cursor near the fixation point and use small cursor movements to induce background movements. Again, the movement will be erratic when attempting to look in the direction of a field defect. Hold down the shift key to over-ride this feature.

 Homonymous field defects and vision in depth

Like all other patients with visual field defects, those who suffer from homonymous defects will experience problems with depth perception. Although perimetric maps may suggest that binocular vision is possible within an extended area on the normal side (to the left in this representation of a right homonymous hemianopia), a visual space plot reveals that the binocular region is quite restricted (yellow area in the diagram). Binocular vision cannot take place either inside or outside the fixation point, which regions are covered by one eye only. It is easy to understand that critical depth judgements near the fixation point will be very difficult. Everyday examples include the handling of cutlery and the cutting of nails.

 Migraine aura

Both the popular and the scientific literature abounds with drawings and paintings of classical migraine auras and so does the Internet (e g, [ 1]) but animated representations are very rare. Animation is useful to highlight aspects that are important for a confident diagnosis. The display below attempts to capture The display runs at a much accelerated pace: the real thing usually runs its course in some 20 minutes. Durations of this order are well compatible with the rate of movement of Leão's spreading depression [2] and the size of the primary visual cortex.

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Left-click on the display to start an aura simulation expanding left-ward from the center. Right-click to make the aura expand towards the right. The animation can be stopped in flight by clicking again.

The display may be particularly useful when evaluating a first attack of a possible aura, where the patient commonly finds it difficult to describe details that are important for the diagnosis. Ask the patient if he or she does or does not recognize any of the features shown in the display. A negative answer is a strong indication for considering other mechanisms, including

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