Forward masking patterns produced by intracochlear electrical stimulation of one and two electrode pairs in the human cochlea.

Three psychophysical forward masking studies were conducted on a multichannel cochlear implant patient. The first study investigated the masking pattern produced by a bipolar electrode pair at different stimulus currents. It was found that the masking pattern for a single-masker bipolar electrode pair had a maximum located at an electrode position where the masker and probe coincided. The spread of the masking pattern was not symmetrical about the maximum. The amount of masking decreased very rapidly toward the apical direction and less rapidly toward the basal direction from the position of the maximum. As the stimulus current increased, the amount of masking at the maximum increased and the masking pattern broadened toward the base. The second study investigated the masking pattern produced by the activation of single bipolar electrode pairs with different spatial extents. The spatial extent of a bipolar electrode pair is defined as the distance between the apical and basal electrode members of the bipolar pair. With a small spatial extent (1.5 mm), the more basal electrode pairs (higher threshold and smaller dynamic range) produced broader masking patterns than the more apical electrode pairs (lower threshold and wider dynamic range), suggesting that there was more current spread at the basal region. With a larger spatial extent (4.5 mm), an additional secondary masking maximum was observed in the vicinity of the apical electrode member of the masker; this was observed only when the apical electrode member lay within the low-threshold apical region. The third study investigated the masking patterns produced by two loudness balanced bipolar masker electrode pairs activated within a stimulus period (inverse of the pulse repetition rate). The biphasic current pulses delivered to the two electrode pairs were nonoverlapping in time. It was found that, at any probe electrode position, the amount of masking produced by the two combined bipolar electrode pairs approximately followed the greater of the two maskings produced respectively by the two individual bipolar masker electrode pairs.