Auditory middle-latency components to fusion of speech elements forming an auditory object

The purpose of this study was to define early brain activity associated with fusion of speech elements to form an auditory object in the middle-latency range preceding the F-Complex.

Stimuli were binaural formant transition and base, that were presented separately or fused to form the vowel-consonant-vowel sequence /ada/. Eleven right-handed, adult, native Hebrew speakers listened to 2/s presentations, and the brain potentials from C(z) during the 250 msec following transition onset (in the responses to transition and to the fused word) or following the time it would have been presented (in the response to base alone) were recorded. The net-fusion response was extracted by subtracting the sum of potentials to the base and the formant transition from the potentials to the fused sound.

Auditory middle-latency components, comprising of 9 peaks and troughs were recorded in response to the base, to the formant transition and to the fused /ada/. In general, the responses to the fused object were significantly smaller in peak amplitude and in total activity (area under the curve) resulting in the difference waveform of the net-fusion response that also included 9 peaks, but with opposite polarities.

The early middle-latency components to fusion indicate that the fusion of speech elements to a word involves inhibition, occlusion or both. The results are in line with the uniqueness of speech perception and the early role of the auditory cortex in speech analysis.