Stimulus-specific adaptation

Stimulus-specific adaptation  ::  Maria N. Geffen et al, University of Pennsylvania  ::  http://www.uphs.upenn.edu/news/News_Releases/2015/10/geffen/

Summary and review of the above article

This research relates to the ability of the auditory system to recognise sounds as the same word, even at widely different pitches and speeds, and also the greater sensitivity of hearing for unexpected as opposed to expected sounds. In the wild, the unexpected or infrequent sound might warn of the approach of a predator, in contrast to the more general background noises of nature.

Stimulus-specific

A process known as stimulus-specific adaptation occurs in all sensory processes. In the auditory system, there is a desensitisation towards expected sounds, and a correspondingly increased sensitisation towards unexpected sounds that might have greater significance than the run of background noise. Figure 1This has been known as a general principle for a good time, but Geffen et al used a process called optogenetics to study the mechanism in greater deal. Optogenetics is a recently developed process, in which particular neurons can be switched on or off by a burst of light passed down an optical fibre.

Inhibitory neurons

It was found that two types of inhibitory neuron drove separate mechanisms involved in this process. Somatostatin-positive interneurons inhibit excitatory neurons during expected noises. In contrast, parvalbumin-positive interneurons inhibit both expected and unexpected noises. The net effect is that expected noises are more inhibited, and unexpected noises correspondingly more noticeable.

Neuroscience has tended to assume multiple stages of sensory cortex processing in which noise and irrelevant material was progressively removed from a signal. This is referred to as invariant representation, by which the brain can extract the same interpretation from what are actually a range of different sensory inputs, such as varying pitch and speed of sound. nn.3382-F4In confirmation of this, Geffen et al showed neurons in the supra-rhinal auditory field varied significantly less in their response to distorted sounds than neurons in the primary auditory cortex. As a higher region of the auditory cortex, the supra-rhinal was better placed to generalise across different versions of a vocalisation.

In terms of consciousness studies, this can be related to findings that only the processing of the final stages of sensory cortex processing are involved in consciousness, while processing in the primary sensory cortices is unconscious.

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