Fallacies in behavioural interpretation of auditory cortex plasticity

Sirs

The review by Weinberger¹ is of merit in pointing to the often neglected fact that learning-induced plasticity is found as early as primary sensory cortical areas. A main focus of the article is the attempt to establish a mechanistic relationship between suitably designed behavioural paradigms and plastic phenomena on the neuronal level of the primary auditory cortex. However, it should be pointed out that the article went too far in claiming the dependence of auditory memory on the type of neuronal plasticity that is described. Best frequency shifts as a form of retuning of neurons' receptive fields, and the reorganization of the tonotopic map measured by best frequencies are neither sufficient nor necessary for the described types of auditory learning and memory. They are not sufficient because map reorganization induced by alternative means (electrical microstimulation) does not alter frequency discrimination performance², and they are not necessary because tonal memory can develop even after bilateral ablation of the auditory cortex^3,4,5,6 (for a more detailed discussion, see Ref. 7). On the other hand, in training paradigms in which the relevance of the auditory cortex has been positively shown^7,8, many mechanisms are required which cannot be accounted for by simple retuning of a unit's best frequency. Among them, inhibitory mechanisms must have an important role as, in some cell types, they contribute to encoding of simple pure tones^9,10,11 In this vein it was shown that the change from a tonal-detection to a tonal-discrimination experiment involves both excitatory and inhibitory modification of the receptive field, which together constitute an enhancement of contrast sensitivity^12,13 instead of retuning by best frequency shift. Discovery of more complex types of receptive field reorganization such as these requires appropriate analysis. The criterion for frequency-specific plasticity used by Weinberger, namely the requirement that learning-induced changes of firing probability at the training frequency must exceed all other changes, leads a strong bias towards the particular type of retuning described in the article.