The brain works out sound direction by comparing the times of when sound reaches the left versus the right ear. This cue is known as interaural time difference, or ITD for short. But how exactly the brain decodes this information is still unknown. The brain contains nerve cells that each show maximum activity in response to one particular ITD.
One idea is that these nerve cells are arranged in the brain like a map from left to right, and that the brain then uses this map to estimate sound direction. This is known as the Jeffress model, after the scientist who first proposed it.
There is some evidence that birds and alligators actually use a system like this to localize sounds, but no such map of nerve cells has yet been identified in mammals. In contrast, the cells in the lateral superior olive LSO neurons prefer high-frequency sounds. Extracellular recordings — which measure the electrical signals around the cells — indicated that these neurons respond more slowly, meaning that they could integrate the inhibitory and excitatory signals from the two ears , over a long time window — more than a millisecond Figure 1C.
This would make them able to detect interaural level differences, which could be relayed via changes in the firing rate of the neurons. In an extremely challenging set of experiments, the researchers latched microscopic glass electrodes directly onto individual LSO neurons in the brain of Mongolian gerbils to obtain recordings from within the cells.
This was done without knowing whether the cells were principal neurons or neurons of other types. They then characterized how the neurons responded to sound and to interaural level differences. Finally, they identified which of these cells were principal neurons by injecting the neurons with a dye and examining their morphology under an electron microscope.
Together, these data revealed that, contrary to what was observed in the extracellular studies, the principal neurons of the LSO do not exhibit the type of slow response that integrates multiple inputs. Rather, their responses are similar to those shown by MSO neurons that is, a fast response just to the onset of sound.
However, these fast principal cells did still appear to convey information about interaural level differences. Even before the work by Franken et al. LSO neurons are inhibited by another area in the brainstem, which encodes the timing of sound with extraordinarily high precision Figure 1C. In fact, decades ago LSO cells were observed to be sensitive to interaural level differences as well as interaural time differences, which is consistent with this well-timed inhibitory input Finlayson and Caspary, ; Joris and Yin, ; Tollin et al.
Nonetheless the view that LSO principal cells are slower and relatively better at integrating signals compared to the MSO has persisted Remme et al. Franken et al. Like MSO cells, LSO principal neurons fire off small action potentials, which are difficult to detect through extracellular measures. Thus, prior studies that used extracellular recordings were likely biased towards the other classes of LSO neurons that fire large action potentials; ironically, these cells can integrate signals over time, and also respond to interaural level differences.
To this day, behavioral observations largely support the duplex theory of sound localization Macpherson and Middlebrooks, If this framework is not directly reflected in the organization of the superior olivary complex, how is the processing of time and level differences distributed across the LSO and MSO? Yet, in contrast to the MSO, the LSO detects the coincidence of excitation from the ear on the same side and inhibition from the opposite side. Rather than conveying information about level differences through changes in their firing rates, the fast LSO cells do so via shifts in the delay of the interaction between excitation and inhibition, which is dependent on the intensity of the sound.
Indeed, computational modeling of LSO neurons that detect the coincidence of excitation and inhibition from the two ears was recently shown to effectively create sensitivity to time differences for high-frequency sounds, but also to preserve sensitivity to level differences Ashida et al. It is also possible that level differences are processed at the next stage of the brain auditory system; in particular, the neurons in the auditory midbrain integrate inputs over longer time windows, long enough to compute the level-related signals Brown and Tollin, ; Li and Pollak, The study by Franken et al.
Only systematic and rigorous experimental approaches can help detect such anomalies, and rightly cast doubt on well-established scientific dogma. The new findings now need to be replicated across the species where LSO neurons have been studied to determine whether they will stand the test of time.
In the meantime, these results encourage us to pause and rethink how the duplex theory can be implemented in the superior olivary complex. As hearing is primarily processed by a pair of organs the two ears , a distinction is made between listening with a single ear monaural hearing and with both ears binaural hearing.
In natural environments, only dichotic hearing exists, allowing binaural processing to occur. If both ears received the same information, we would be unable to follow conversations in noisy situations, locate sound sources and accurately define our sound environment. Localisation occurs mainly on primary sound sources which dominate the auditory environment. It becomes less precise when multiple sound sources are superimposed, and when reverberations create second sources.
In the vertical plane or elevation , the primary cues are monaural — they are generated by the modification of a sound by the torso, head, and the outer ear of the listener. The time difference between the arrival of a sound wave at each ear is an important cue for estimating the position of a sound in the horizontal plane. In this case, the distance SL is greater than SR and the sound will therefore arrive at the right ear sooner than at the left. The difference in the arrival time of the sound wave at each ear is called the interaural time difference ITD.
At this point, it is equal to around 0. ITD is a fundamental cue for localising the source of a sound that has a frequency of less than Hz. Above Hz, ITD cues become ambiguous. However, in the case of complex sounds, the ITD of the envelope slow modulation of the high frequencies can be perceived. This is known as the interaural envelope time difference.
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