Visual extinction
[1] Research done by Pavlovskaya, Sagi, Soroker and Ring show that visual extinction is dependent on simple stimuli properties.
Their ability to report the stimuli correctly when presented singly indicates that this is not a problem with vision per se.
This bias against contralesional stimuli is evident even when patients are presented with two signals within the ipsilesional visual field, whose processing remains intact following the damage, since it is done in the opposite brain hemisphere.
[1] Historically, it was believed that the parietal damage weakened afferent neuron input to the visual cortex, and so the extinction event was caused by the signals originating in the contralesional field being lost during transmission.
Following parietal damage, all contralesional messages are lowered in priority for the brain's processing in some way – potentially by signal transmission delay – such that when presented with alternative stimuli, the contralesional stimulus doesn't arrive at the decision center simultaneously with the ipsilesional stimulus, and thus is denied conscious processing.
In the study, patients were shown to be able to focus their attention and improve reaction time, but still have subnormal performance.
[7] Extinction frequency may also be affected by reporting method, due to how visual processing interacts with the rest of the brain.
A positive diagnosis results from a patient's inability to correctly identify shapes or letters projected simultaneously in different areas of the screen.
Without any sort of treatment, visual extinction usually worsens in severity of symptoms or remains completely stagnant.
[4][10] Treatment methods for patients who have visual extinction generally involve use and training of an individual's vision.
In successful cases, generally forty hours of retraining the patients' vision were necessary as well as some additional work done at home with similar exercises.
[10] In another, single-stimulus rehabilitatory approach, the patient is shown a computer screen with a box in each corner and a fixation cross at the center.
Following thirty hour-long sessions, patients demonstrated a significant decrease in both extinction and hemispatial neglect.
[11] In patients whose brain damage was in the right hemisphere, left limb activation (LLA) was another treatment that proved effective.
This forces the patient to exercise spatial awareness using the damaged side of the brain, increasing activity around the lesion.
When two visual stimuli are presented to a patient, they can be processed as a single object due to the corresponding neuronal functions which are linked through long-range lateral interactions.
When presented with a figure containing illusory contours, patients were able to correctly report the presence of stimuli in both contralesional and ipsilesional hemispheres, due to their unconscious processing of the whole field to produce the illusion.
Combining the brain bias towards facial processing and surface discovery, a follow-up study by Dr. Vuilleumier exposed subjects to paired visual stimuli in three phases.
The effect did not occur when the stimuli were surrounded by a nonoval shape, pointing towards facial processing as being the key to this phenomenon.
Although the patient reported in such cases that he had not seen the indicator light on his hand, their performance was correspondingly better, similar to the results when visual feedback had been available.
[19] The ability to understand which factors in visual processing determine whether a contralesional event, occurring on the half of the patient's brain or body opposing the site of a lesion, is observed or eliminated can provide crucial insights connecting to the mechanisms of attention and operation.
