Psychoacoustical contribution of each lateral lemniscus

Although each lateral lemniscus is required for sound localization in its contralateral hemifield, no auditory function is yet known for the neural activity evoked in the lemniscus ipsilateral to a sound source. In an attempt to assess the role played by the ipsilateral lemniscus, monaural cats were tested on an array of psychoacoustical tasks before and after surgical section of one or the other lateral lemniscus. The results show that the lemniscus contralateral to the remaining intact ear is either necessary or sufficient for 24 of the 26 tests administered. However, the lemniscus ipsilateral to the intact ear is both necessary and sufficient (or alternatively, the contralateral lemniscus makes no obvious contribution) to normal thresholds in two of the tasks: detection of low-frequency tones (< 4 kHz) and detection of low-frequency AM modulation. Because of their projections to the ipsilateral inferior colliculus via the ipsilateral lemniscus, the anatomical substrate of these two unusual tasks is probably the fibers from the MSO and possibly, the LSO, ipsilateral to the intact ear.