After restricted cochlear lesions in adult animals, the frequency selectivity of neurons in the cortical region deprived of its normal input by the lesion is changed such that the region is occupied by expanded representations of adjacent (perilesion) frequencies. Analogous changes in cortical frequency selectivity and organization are seen as a consequence of behavioral training that enhances the significance of particular acoustic stimuli. The occurrence of such reorganization in a wide range of species (including simian primates) suggests that it would also occur in humans. Direct evidence in support of this suggestion is provided by a small body of functional imaging evidence. Although such reorganization almost certainly does not have a compensatory function, such a profound change in the pattern of cortical activation produced by stimuli exciting perilesion parts of the receptor epithelium would be expected to have perceptual consequences and, perhaps, clinical implications.

Adult↗

PubMedDate not supplied

Injury- and use-related plasticity in the primary sensory cortex of adult mammals: possible relationship to perceptual learning.

1. Restricted cochlear lesions in adult animals result in a reorganization of auditory cortex such that the cortical region deprived of its normal input by the lesion is occupied by expanded representations of adjacent cochlear loci (and thus of the frequencies represented at those loci). Analogous injury-induced reorganization is seen in somatosensory, visual and motor cortices of adult animals after restricted peripheral lesions. 2. Rather than constituting a central compensation for the peripheral loss, such reorganization appears to be an extreme form of changes in cortical organization that occur as a consequence of altered patterns of input such as arise from differential use of restricted regions of receptor surfaces ('use-related' reorganization). Thus, the frequency organization of auditory cortex is modified in animals trained to perform a frequency discrimination task and analogous changes in the frequency selectivity of cortical neurons are produced by classical conditioning procedures. 3. Recent evidence from the visual system suggests that changes similar to those involved in injury- and use-related cortical reorganization may underlie some forms of what has been called 'perceptual learning', the improvement in sensory/ perceptual discriminative performance with practice. Some forms of such learning are highly specific to the particular stimuli used in training (i.e. do not generalize to other stimuli), suggesting that the improved performance reflects a change in neural circuitry at a relatively early level of sensory processing. The limited available evidence supports the occurrence of such learning in the auditory system. 4. Recent studies using functional imaging and related techniques indicate that injury- and use-related reorganization occurs in human sensory and motor cortex.

Animals↗

PubMedDate not supplied

Neuronal responses across cortical field A1 in plasticity induced by peripheral auditory organ damage.

The adult auditory cortex is capable of a plastic reorganization of its tonotopic map after damage to restricted parts of the cochlear sensory epithelium. We examine the precise conditions of cochlear damage required to demonstrate such plasticity in the primary auditory cortex (A1) of the cat and the changes observed in neuronal responses in the A1 which has reorganized in plasticity of the tonotopic map. From these data we attempt to predict the conditions required for similar plasticity to occur in humans after cochlear damage.

Animals↗

PubMedDate not supplied

Injury- and use-related plasticity in adult auditory cortex.

After restricted cochlear lesions in adult animals the frequency selectivity of neurons in the cortical region deprived of its normal input by the lesion is changed such that the region is occupied by expanded representations of adjacent (perilesion) frequencies. These changes reflect a dynamic process of reorganization (plasticity) and are not explicable as passive consequences of the lesion. Analogous plasticity of cortical frequency selectivity and organization is seen following behavioural training that enhances the significance of particular acoustic stimuli. The occurrence of injury- and use-related auditory cortical plasticity gives rise to a number of questions relating to the mechanisms involved, the perceptual consequences and functional significance of such plastic changes, and their implications for the central processing of input from prosthetic devices. Evidence relating to these issues is briefly summarized in this review, and the directions of future research are considered.

Auditory Cortex↗

PubMedDate not supplied

A developmental study of the sound pressure transformation by the head of the cat.

The interaural intensity difference (IID) created by the imposition of a cat's body in a free sound field was measured in two age groups of kittens and one group of adult cats. For all animals significantly larger IIDs were obtained for high than for low frequency stimulation. The frequency at which peak IID occurred was higher for the younger group of kittens than for either the Old Kittens or the Adults. Above 4 kHz the Adults showed a marked variation of IID with azimuth, whilst the Young Kittens showed little variation of IID with azimuth at frequencies below 10 kHz. It was found that Adult-like IIDs are achieved before physical maturation is complete. The results are discussed with reference to the physiological development of sensory systems and the implications for a possible post-natal plasticity of the auditory system are examined.

Acoustic Stimulation↗

PubMedDate not supplied

Effects of reflex middle-ear muscle contractions on cochlear responses to bone-conducted sound.

The effects of contralaterally elicited middle-ear muscle (MEM) reflexes on cochlear microphonic responses to air- and bone-conducted tones were examined in decerobrate cats. Stapedius effects on bone condn air conduction were almost identical in configuration and amplitude to those on air conduction at all frequencies. However, tensor tympani effects were more complex, the configuration of the bone-conduction effects varying with the location of the transducer on the skull and with frequency. The relative contributions of the two muscles to the effects of joint contractions varied markedly between animals. It is suggested that non-reflex MEM contractions associated with activity of the facial musculature might provide protection against masking of environmental sounds by the low-frequency bone-conducted sound generated by such activity.

Air↗