The SSAEP has been used to study binaural interactions, showing evidence of interaural suppression 23, 24 and increased responses from binaurally summed stimuli 17, 22, 25. For low signal modulation frequencies (<55 Hz), brain responses are thought to reflect cortical processes 15, 20, 21, 22. This steady-state auditory evoked potential (SSAEP) is greatest around 40 Hz 20, 21 and increases monotonically with increasing modulation depth 17, 18.
When a carrier stimulus (typically either a pure-tone or broadband noise) is modulated in amplitude, neural oscillations at the modulation frequency can be detected at the scalp 15, 16, 17, 18, 19, being typically strongest at the vertex in EEG recordings 20.
Furthermore, binaural sounds are perceived as being slightly louder than monaural sounds, though typically less than twice as loud 11, 12, 13, 14. This difference persists above threshold, with intensity discrimination performance being better binaurally than monaurally 10. Subsequent studies have provided similar or slightly lower values 6, 7, 8, and there is general agreement that two ears are better than one at detection threshold 9.
This accounted for any differences in sensitivity (or audibility), and revealed that summation (the improvement in sensitivity afforded by binaural presentation) was approximately 3.6 dB (a factor of 1.5). 1 presented signals to the two ears that were equated for each ear’s individual threshold sound level when presented binaurally. This study uses complementary techniques (psychophysics, steady-state electroencephalography (EEG) and computational modelling) to probe the neural operations that underpin binaural summation of amplitude-modulated signals.Ĭlassical psychophysical studies demonstrated that the threshold for detecting a very faint tone is lower when the tone is presented binaurally versus monaurally. But what precisely is the algorithm that governs the combination of sounds across the ears? The nonlinearities inherent in sensory processing mean that simple linear signal addition is unlikely. In some animals, such as bats and dolphins, echolocation can be precise enough to permit navigation through the environment, and there are reports of visually impaired humans using a similar strategy 3, 4, which requires both ears 5. This operation confers several benefits, including increased sensitivity to low intensity sounds 1 and inferring location and motion direction of sound sources based on interaural time differences 2. The auditory system integrates information across the two ears. These findings position our understanding of binaural summation in a broader context of work on sensory signal combination in the brain, and delineate the similarities and differences between vision and hearing. We suggest that the distinct ecological constraints on vision and hearing can explain this difference, if it is assumed that the brain avoids over-representing sensory signals originating from a single object. Both data sets were well-fit by a computational model originally derived for visual signal combination, but with suppression between the two channels (ears) being much weaker than in binocular vision. The EEG responses were greater for binaural than monaural presentation of modulated tones, and when a masker was presented to one ear, it produced only weak suppression of the response to a signal presented to the other ear. Discrimination thresholds followed a ‘dipper’ shaped function of pedestal modulation depth, and were consistently lower for binaural than monaural presentation of modulated tones. The brain combines sounds from the two ears, but what is the algorithm used to achieve this summation of signals? Here we combine psychophysical amplitude modulation discrimination and steady-state electroencephalography (EEG) data to investigate the architecture of binaural combination for amplitude-modulated tones.
0 kommentar(er)
