Trends in Cognitive Sciences
OpinionStability of Sensory Topographies in Adult Cortex
Section snippets
Plasticity in Sensory Cortical Topographies
One of the key concepts in contemporary neuroscience is that experience shapes the central nervous system throughout life. The ability of the brain to adaptively change how it processes inputs based on new experience is termed ‘plasticity’ and underlies our ability to mature, learn new skills, and recover from injury. Our current understanding of neuroplasticity has been moulded by the work of Hubel and Wiesel 1, 2, 3 in the 1960s, who studied the visual cortex of cats following temporary
Functional Benefits of Reorganisation?
If deafferented cortex begins to process a new patch of the sensory sheet (on the retina or the skin), one would expect that the additional cortical volume would lead to perceptual gains for this ‘invading’ region (i.e., adaptive plasticity, see 14, 15). For example, SI remapping following digit amputation results in increased representation of the neighbouring digits, which in turn should lead to increased acuity for these digits [8]. Such perceptual gains would imply that signals arising to
Phantom and Referred Sensations
If remapping in SI does not result in direct perceptual gains, are there any other functional consequences to SI remapping? In other words, are these invading signals behaviourally relevant? The most extensively documented and captivating consequence relates to distorted phantom sensations following amputation. Even decades after injury, amputees report a continued sensation of the limb that is no longer there. These phantom sensations can be as vivid and as natural as the perception of one’s
Reorganisation in Humans
Results from neuroimaging studies in human amputees further challenge the view that neighbouring cortical representations invade the deafferented ones. While the lip representation encroaches somewhat on the limb representation following amputation, it does not annex it completely 36, 37, 38 in contrast to what is observed in electrophysiological recordings from amputated or deafferented monkeys 11, 12. Rather, the deafferented territory in human somatosensory cortex begins to respond to body
Persistent Representation Despite Input Loss
A further challenge to the notion that reorganisation causes functional consequences is provided by the perceptual correlates of nerve stimulation. Numerous studies have shown that, when the residual (injured) nerve is electrically stimulated, either directly 41, 42 or transcutaneously 43, 44, individuals experience the evoked somatosensory percepts as vividly and clearly arising from their phantom hand (Figure 2C), and not from other body parts such as the face. In fact, stimulation of the
Neural Basis of Reorganisation
The persistence of sensory experience despite peripheral input loss can be explained in part by nerve regeneration. Indeed, a severed sensory axon typically regrows and spontaneously reinnervates intact skin, for example, on the residual arm (see [47] for physiological review). As a result, touch applied to the reinnervated skin will produce signals that are mislabelled by the central nervous system as arising from the missing hand and result in a sensation projected to the missing hand. As
Stability of Sensory Topographies in Adult Cortex
In summary, loss of input from a body region in adulthood leads to the formation or potentiation of lateral connections in the brainstem, which gives rise to a new pathway from periphery to cortex. This new pathway alone can account for the face-elicited activity in monkeys’ hand cortex, and the contribution of cortical reorganisation per se remains unconfirmed. The original pathway seems to be relatively spared as evidenced by the elicitation of sensations evoked on the amputated or insensate
Neural Basis of Stable Cortical Representations
The stability of sensory topographies may be attributable to at least two factors. First, to form a new sensory representation requires reorganisation spanning a wide swath of cortex over which the representation is distributed in a functionally organised way, and the mechanisms of plasticity may not operate on sufficiently large spatial scales in the nervous system to allow for this. In fact, sensory topographies have been shown to be in part determined by genetically controlled patterning
Concluding Remarks and Future Perspectives for Brain Machine Interfaces
The aforementioned reinterpretation of the behavioural, imaging, and neurophysiological results implies a more nuanced view of cortical plasticity: while sensory cortices of adults are endowed with plasticity, this plasticity cannot result in the formation of completely novel representations, even under the extreme circumstance of deafferentation. To establish that aberrant activity in deprived cortex constitutes a new sensory representation of the displaced input requires causal evidence, for
Acknowledgements
The authors thank Jon Kaas, Murray Sherman, Jeffrey Yau, Patrick Haggard, and Jeremy Winberry for helpful comments on a previous version of this manuscript. T.R.M. was supported by a Henry Dale Fellowship jointly funded by the Wellcome Trust and the Royal Society (Grant No. 104128/Z/14/Z). S.J.B. was supported by NINDS grantsR01 NS095251 and NS 095162.
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