Christian Lorenzi

Sounds such as speech, music and natural soundscapes are decomposed by the peripheral auditory system of humans (the cochlea) into narrow frequency bands. The resulting signals convey information at different time scales to more central auditory structures. A dichotomy between slow "temporal envelope" cues and faster "temporal fine structure" (TFS) cues has been proposed to explore several aspects of auditory perception including speech intelligibility in quiet or against competing sound sources.

Starting from the late nineties, Lorenzi conducted a research program on auditory perception combining signal processing, psychophysical, electrophysiological and computational methods based on this envelope/TFS dichotomy. He examined the role of these two cues in sound discrimination and auditory scene analysis,^[9][10] how these cues are processed at each stage of the auditory system, and the effects of peripheral (cochlear)^[11] or central damage,^[12] development,^[13][14] ageing^[15] and rehabilitation systems (e.g., hearing aids^[16] or cochlear implants^[17]) on the perception of these temporal envelope and TFS cues.

His early work on the perception of temporal-envelope information corroborated the existence of tuned (selective) modulation filters at central stages of the human auditory system, consistent with the notion that the auditory system computes some form of modulation spectrum of incoming sounds.^[18][19] He then showed that dynamic information in sounds not only is carried by so-called first-order characteristic of sounds (e.g., onset and offset cues, slow amplitude modulations composing the envelope of sounds), but also can be carried by “second-order” characteristics such as variations in the temporal-envelope contrast (modulation depth). His psychophysical conducted on normal-hearing people and patients with cochlear or brain lesions is consistent with the idea that, as in vision, nonlinear mechanisms along the auditory pathway generate an audible distortion component at the 2nd-order AM frequency in the internal modulation spectrum of sounds.^[20]

His more recent work on the perception of TFS information suggested that TFS cues convey as much spectro-temporal information as temporal-envelope cues do for complex sounds such as speech.^[21] He also showed that TFS cues are less vulnerable than temporal envelope cues when sounds are masked by competing sounds such as noise or presented at high intensities, and may play a role in robust sound coding at the low (brainstem) level.^[21][22] His work conducted with people with sensorineural hearing loss and computational models of auditory perception showed how cochlear lesions may alter the neural representation of TFS cues in the early stages of the auditory system, even in regions of the pure-tone audiogram where hearing is clinically considered as normal.^{[23][24][25][26]}

Since 2020, his work has reoriented towards human auditory ecology, a new field promoting research on the human ability to perceive ecological processes at work in natural environments.^[27] This line of research invites hearing scientists to collaborate actively with soundscape ecologists or ecoacousticians, earth scientists and geographers in order to characterise acoustic statistics of natural scenes,^[28] and study how humans listen to natural soundscapes and perceive subtle/transient auditory attributes of natural scenes such as habitat, moment of the day, season or biodiversity^[29] and effects of sensorineural hearing loss on these capacities.^[30]

• Temporal envelope and fine structure