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• Double simultaneous stimulation (DSS) is the presentation of paired sensory stimuli to the two sides simultaneously. This can be visual, aural or tactile.
• Light touch stimuli presented rapidly, simultaneously, and at minimal intensity to homologous areas on the body (distal and proximal samplings on extremities) may pick up very minor threshold differences in sensation. Additionally, this testing can also detect neglect phenomena (inability to report, respond or orient to stimuli generally on the contralesional side) due to damage of the parietal association cortex.
• Neglect may be hard to distinguish from involvement of the primary sensory systems. However, neglect usually can be demonstrated in multiple sensory systems (i.e., visual, auditory, and sensory), confirming that it is not simply damage to one sensory system. Association cortex lesions, particularly involvement of the right posterior parietal cortex, may become apparent only on double simultaneous stimulation.
• The face-hand test as shown by Dr Blumenfeld, takes advantage of the fact that stimuli delivered to the face dominate over stimulation elsewhere in the body. This dominance is especially prominent in children and in demented patients.
• Before the age of ten, stimuli presented simultaneously to the face and ipsilateral or contralateral hand are frequently (more than three in ten stimulations) perceived at the face alone. Perception of the hand, and, if tested, other parts of the body is extinguished. In an older child or adult, several initial extinctions of the hand may occur, but very quickly both stimuli are correctly perceived. In the patient with diffuse hemispheric dysfunction (dementia), a regression to consistent bilateral extinction of the hand stimuli is frequently seen.
• This test therefore can be doubly useful, first as an indication of diffuse hemispheric function and second by stimulating the face and opposite hand, a means of detecting minor hemisensory defects
• For example, if the patient consistently extinguishes only the right hand and not the left, a sensory threshold elevation due to primary sensory system or association cortex involvement on the left is suspect.