Superior memorizers employ different neural networks for encoding and recall.

Superior memorizers often employ the method of loci (MoL) to memorize large amounts of information. The MoL, known since ancient times, relies on a complex process where information to be memorized is bound to landmarks along mental routes in a previously memorized environment. However, functional magnetic resonance imaging data on groups of trained superior memorizer are rare. Based on the memorizing strategy reported by superior memorizers, we developed a scheme of the processes successively employed during memorizing and recalling digits and relate these to brain activation that is specific for the encoding and recall period. In the examined superior memorizers several regions, suggested to be involved in mental navigation and digit-to-word processing, were specifically activated during encoding: bilateral early visual cortex, retrosplenial cortex, left parahippocampus, left visual cortex, and left superior parietal cortex. Although the scheme suggests that some steps during encoding and recall seem to be analog, none of the encoding areas were specifically activated during the recall. Instead, we found strong activation in left anterior superior temporal gyrus, which we relate to recalling the sequential order of the digits, and right motor cortex that may be related to reciting the digits.

On the definition and interpretation of voice selective activation in the temporal cortex.

Regions along the superior temporal sulci and in the anterior temporal lobes have been found to be involved in voice processing. It has even been argued that parts of the temporal cortices serve as voice-selective areas. Yet, evidence for voice-selective activation in the strict sense is still missing. The current fMRI study aimed at assessing the degree of voice-specific processing in different parts of the superior and middle temporal cortices. To this end, voices of famous persons were contrasted with widely different categories, which were sounds of animals and musical instruments. The argumentation was that only brain regions with statistically proven absence of activation by the control stimuli may be considered as candidates for voice-selective areas. Neural activity was found to be stronger in response to human voices in all analyzed parts of the temporal lobes except for the middle and posterior STG. More importantly, the activation differences between voices and the other environmental sounds increased continuously from the mid-posterior STG to the anterior MTG. Here, only voices but not the control stimuli excited an increase of the BOLD response above a resting baseline level. The findings are discussed with reference to the function of the anterior temporal lobes in person recognition and the general question on how to define selectivity of brain regions for a specific class of stimuli or tasks. In addition, our results corroborate recent assumptions about the hierarchical organization of auditory processing building on a processing stream from the primary auditory cortices to anterior portions of the temporal lobes.

The temporal lobes differentiate between the voices of famous and unknown people: an event-related fMRI study on speaker recognition.

It is widely accepted that the perception of human voices is supported by neural structures located along the superior temporal sulci. However, there is an ongoing discussion to what extent the activations found in fMRI studies are evoked by the vocal features themselves or are the result of phonetic processing. To show that the temporal lobes are indeed engaged in voice processing, short utterances spoken by famous and unknown people were presented to healthy young participants whose task it was to identify the familiar speakers. In two event-related fMRI experiments, the temporal lobes were found to differentiate between familiar and unfamiliar voices such that named voices elicited higher BOLD signal intensities than unfamiliar voices. Yet, the temporal cortices did not only discriminate between familiar and unfamiliar voices. Experiment 2, which required overtly spoken responses and allowed to distinguish between four familiarity grades, revealed that there was a fine-grained differentiation between all of these familiarity levels with higher familiarity being associated with larger BOLD signal amplitudes. Finally, we observed a gradual response change such that the BOLD signal differences between unfamiliar and highly familiar voices increased with the distance of an area from the transverse temporal gyri, especially towards the anterior temporal cortex and the middle temporal gyri. Therefore, the results suggest that (the anterior and non-superior portions of) the temporal lobes participate in voice-specific processing independent from phonetic components also involved in spoken speech material.

Determining language laterality by fMRI and dichotic listening.

For imaging studies on hemispheric specialization of the human brain, data about known functional asymmetries other than handedness would be valuable for a reliable interpretation of lateralized activation in individuals or groups of subjects. As certain aspects of language processing are observed to be a function of primarily the left, it can be used as a reference for other asymmetric processes such as sensory or cognitive skills. For analyzing language laterality, there are a variety of methods, but these differ in application or accuracy. In this study, we tested the reliability of two widely used methods - dichotic listening and fMRI - to determine language dominance in 30 individual subjects. The German adaptation of a dichotic listening test (Hättig, H., Beier, M., 2000. FRWT: a dichotic listening test for clinical and scientific contexts, Zeitschr f Neuropsychologie 11. 233-245.) classified 54% of the 26 right-handed subjects as left hemispheric dominant. The results of the fMRI paradigm (Fernández, G., de Greiff, A., von Oertzen, J., et al., 2001. Language mapping in less than 15 min: real-time functional MRI during routine clinical investigation. Neuroimage 14, 585-594.) tested on the same subjects, however, classified 92% of the right-handed subjects as left dominant. The main reason for this discrepancy was that the ear dominance score of many subjects in the dichotic listening test was too low to determine a reliable ear advantage. As a consequence, this specific dichotic listening test cannot be used to determine language laterality in individual subjects. On the other hand, the fMRI results are consistent with numerous studies showing left dominant language processing in more than 90% of right-handers. In some subjects, however, language laterality critically depends on the areas used to determine the laterality index.