Background:
Hearing necessarily involves top-down influences on the sensory signals provided by bottom-up information from the ear. The top-down influences include elements of attention, memory, motivation, emotion, and learning, deriving from many regions of the cerebral cortex. They exert their influence via intra-cortical networks and auditory efferent pathways that extend back down the auditory system, right out to the ear. These ‘cognitive’ contributions to hearing affect sound detection, hearing-in-noise, and short- and long-term experiential modulation. Difficulty in speech perception in noisy environments (SiN) is the most common complaint that people of all ages and hearing levels make about their hearing. We review here aspects of those difficulties.

Methods:
Studies considered recruited children and older adults with normal audiograms. Tests included speech-in-noise, cognition, and remote delivery via the internet. Interventions included wireless devices and training.

Results:
For those with cochlear hearing loss, reduced sensitivity and broadened spectral and temporal processing contribute to poor speech perception in quiet and in noise. But for SiN, the nature of the noise is also important. Typically, able young adults can benefit from amplitude-modulated noise as it enables them to listen into the dips of the noise. They also benefit from a spatial separation between the target speech and the noise. However, those with reduced cognitive capabilities, notably children (especially those with learning difficulties), receive less benefit in these conditions. Older people have a high prevalence of both cochlear hearing loss and cognitive impairment. While these problems often occur together, and may be supraadditive and causally connected, they can also occur independently. We review studies showing that those (rare) older people with normal hearing sensitivity nevertheless have impaired SiN for both modulated and unmodulated noises, but older listeners show normal benefit from listening into the energetic minima of a fluctuating noise.

Discussion:
Effective interventions to improve SiN in older people are likely to include reduction of room reverberation, instruction on viewing important sound sources, improved signal to noise (e.g. Bluetooth, FM), onset enhancement, directional microphones on hearing devices, and auditory training. Training should emphasise engagement with the target sound and is best achieved through the use of highly motivating exercises. These may involve the use of social engagement and salient signals (e.g. talk radio) that are also likely to enhance general cognitive well-being.

Conclusions:
The reviewed studies – of development of hearing in children, of SiN perception in older adults, and of intervention – emphasise the role of top-down, cognitive factors in hearing, hearing impairment, and rehabilitation.