Finally, it is shown in a study of monkeys that removal of the ventral prefrontal area 12 (including 45 A) impairs the ability of monkeys to relearn a visual associative task (visual matching), even though there were no demands on working memory. Evidence is also presented from an event-related fMRI study that activity in this area is time-locked both to the presentation of the visual stimuli and also to the time of the motor response. Imaging with both positron-emission tomography (PET) and functional magnetic-resonance imaging (fMRI) reveals learning-related increases in activity when normal subjects learn visual associative tasks. This paper provides evidence that the ventral prefrontal cortex plays a role in the learning of tasks in which subjects must learn to associate visual cues and responses. Alternatively, each synesthete is only different from a non-synesthete in the binding of his or her specific inducer to his or her specific concurrent.

Furthermore, it allows examining whether synesthetes are, as a group, categorically different from all non-synesthetic subjects. This allows studying which of the previously obtained white-matter and grey-matter differences in synesthesia are specific to that (i.e., linguistic–color) type of synesthesia, and which are related more generally to having synesthesia. In this subject group, different types of synesthetes were present the inducers and concurrent ranged across perceptual modalities (e.g., visual, auditory, taste, tactile) but also involved non-sensory concepts (e.g., ‘personality’). Furthermore, the subjects were presented with an extensive synesthesia questionnaire. In this project, brain structure measurements (connectivity and volume) were obtained from a large group of subjects (over 350 individuals). We do not yet know whether the processes involved in multisensory binding are similar or different across different types of synesthesia. Currently, the majority of synesthesia research is aimed at a few types of synesthesia colors evoked by written or spoken language and spatial arrangements evoked by sequential concepts. The condition synesthesia offers an extraordinary opportunity to study mechanisms involved in (hyper)-binding across different modalities.

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