Psychophysical studies indicate that structural features of odorants differentially influence their perceived odor. In the olfactory bulb (OB), odorants are represented by ensembles of activated glomeruli. Here we used optical imaging of intrinsic signals to examine how these structural features are represented spatially in the sensory map of the rat OB. We found that the dorsal OB contained two topographically fixed domains; constituent glomeruli in each domain could be activated by odorants with particular functional groups. Within each domain, other structural features such as carbon chain length and branching were represented by local differences in patterns. These results suggest that structural features are categorized into two classes, primary features (functional groups) that characterize each domain, and secondary features that are represented by local positions within each domain. Such hierarchical representations of different structural features correlate well with psychophysical structure-odor relationships.

Odor signals are conveyed from the olfactory bulb to the olfactory cortex (OC) by mitral cells (MCs) and tufted cells (TCs). However, whether and how the two types of projection neuron differ in function and axonal connectivity is still poorly understood. Odor responses and axonal projection patterns were compared between MCs and TCs in mice by visualizing axons of electrophysiologically-identified single neurons. TCs demonstrated shorter onset latency for reliable responses than MCs. The shorter latency response of TCs was maintained in a wide range of odor concentrations, whereas MCs responded only to strong signals. Furthermore, individual TCs projected densely to focal targets only in anterior areas of the OC, whereas individual MCs dispersedly projected to all OC areas. Surprisingly, in anterior OC areas, the two cell types projected to segregated sub-areas. These results suggest that MCs and TCs transmit temporally distinct odor information to different OC targets.

Intrinsic signal imaging from inferotemporal (IT) cortex, a visual area essential for object perception and recognition, revealed that visually presented objects activated patches in a distributed manner. When visual features of these objects were partially removed, the simplified stimuli activated only a subset of the patches elicited by the originals. This result, in conjunction with extracellular recording, suggests that an object is represented by a combination of cortical columns, each of which represents a visual feature (feature column). Simplification of an object occasionally caused the appearance of columns that were not active when viewing the more complex form. Thus, not all the columns related to a particular feature were necessarily activated by the original objects. Taken together, these results suggest that objects may be represented not only by simply combining feature columns but also by using a variety of combinations of active and inactive columns for individual features.

To investigate the functional organization of object recognition, the technique of optical imaging was applied to the primate inferotemporal cortex, which is thought to be essential for object recognition. The features critical for the activation of single cells were first determined in unit recordings with electrodes. In the subsequent optical imaging, presentation of the critical features activated patchy regions around 0.5 millimeters in diameter, covering the site of the electrode penetration at which the critical feature had been determined. Because signals in optical imaging reflect average neuronal activities in the regions, the result directly indicates the regional clustering of cells responding to similar features.