The next metaphor I examine is the brain is plastic. In particular, synapses are plastic: they change with activity. But this is not the same thing as saying that synapses are dynamic. Synapses are plastic means that synapses are objects that can change shape while keeping the same substance. Specifically, they can be manipulated into different shapes. Plasticity is a possibility for change that is 1) limited in that only the shape and not the substance is changed, 2) persistent, 3) reversible, 4) mediated by an external actor. For example, cell death is a change but it is not plasticity; developmental changes are also not considered as plasticity even though they can be activity-dependent. These two examples are irreversible changes and therefore not cases of plasticity. Internal changes entirely mediated by intrinsic events would not normally be called plasticity. Transient changes would also not be called plasticity: for example a change in spike threshold after firing is called adaptation or accommodation, not plasticity.

This is quite clearly a metaphor, which carries a particular view on how neural structures change. For example, part of what we describe as synaptic plasticity actually corresponds to the elimination of synapses or of receptors (synaptic pruning), and therefore might be better described by the sculpting metaphor. The metaphor also hides the fact that the substance that makes all those structures is continually renewed (protein turn-over), and this is quite different from a plastic object. This is in fact quite different from an object. The persistence of shape (e.g. of a synapse) is mediated by active processes (which involve gene expression), as opposed to passive persistence of a plastic object. Changes of shape then involve interaction with those processes, rather than direct manipulation.