A basic process in the build up of conceptual knowledge is the formation of categories involving the abstraction of shared features from the specific sensory experiences. Our previous work in the rodent (gerbil) auditory system has demonstrated that the formation of auditory categories is accompanied by the emergence of category-specific spatiotemporal activity patterns in auditory cortex (Ohl et al., Nature, 2001). Here, the investigation of the formation of category-specific activity patterns was extended to the multisensory domain. We have examined whether perceptual categories, after being formed in one sensory modality (audition or vision), can be transferred to another sensory modality (vision or audition, resp.), and have suggested a physiological basis for this audiovisual category transfer.
We trained Mongolian Gerbils (Meriones unguiculatus) to associate a slow (0.5 Hz) and a fast (5 Hz) presentation rate of stimuli with the Go response and NoGo response, respectively, in an active avoidance paradigm (shuttle box). One group of animals was trained with auditory tone pips, the other with visual flashes as stimuli. After sufficient training with the first sensory modality, a second training phase was initiated in which the sensory modality of the stimuli was changed from auditory to visual, or from visual to auditory, respectively. In this second training phase, groups were further split into a group with congruent training (contingency between presentation rate and required response remained unchanged) and a group with incongruent training (contingency between presentation rate and required response was reversed).
After the modality switch, the congruent groups showed a higher acquisition rate of the conditioned responses than the incongruent groups indicating a crossmodal transfer of the rate-response association.
During training, the electrocorticogram was recorded from two 16 -electrode arrays chronically implanted onto the epidural surface of primary auditory and the visual cortex.
Cortical activity patterns in the ongoing electrocorticogram (ECoG) associated with the Go- and the No-Go stimuli were determined in the spatial distribution of signal power using a multivariate pattern classification procedure (Barrie et al., J. Neurophysiol., 1996; Freeman, J. Neurophysiol., 2000).
During auditory training, in animals discriminating the auditory stimuli, patterns in auditory cortex developed with learning in accordance with our previous results. In addition, patterns could also be observed in the visual cortex in later training sessions, and at later post-stimulus time points within a trial. During visual training, in animals not showing correct responses during the first training sessions, we suspect that there was no transfer learning. Also, in these animals no significant patterns could be detected in the early training sessions. Nevertheless, some animals started to develop correct responses in later phases of the training. For these animals patterns could be identified in the visual cortex. For animals showing correct discrimination already during the first visual training sessions we suspect that they transferred the rate-response association learned during auditory training to the visual training. In these animals patterns could be detected in both the auditory and the visual cortex already during the first training sessions. We suggest that in these animals activity in both auditory and visual cortex was instrumental for achieving the crossmodal transfer of learned associations.