Figure 9.

Odor processing and learning in olfactory network. A, Heat maps depicting 10 × 10 arrays of odor response patterns in the OB and piriform cortex. Each image shows the average responses of Mi cells or Pyr cells during a 2 s odor presentation with warmer colors representing higher responses. Each column shows responses to one simulated odor. The odors in the left and middle columns together form a dissimilar pair, while the odors in the middle and right columns constitute a similar odor pair, according to the definitions in Figure 5. Odor representations conveyed to the OB are processed in the OB network to create bulbar output. The three upper rows show examples of bulbar output patterns under control conditions (ACh ON) and partial receptor blockade conditions (Ni OFF or Mu OFF). In the bulb, average response patterns are modulated by nicotinic, but not muscarinic, receptor activation. Bulbar output patterns are conveyed to the cortical network (PC before learning), where these are modulated by synaptic plasticity in the cortical association fibers (PC after learning). In the piriform cortex, neural response patterns are transformed by learning. Learned patterns in response to OB patterns under the different drug conditions are shown in the last three rows of panels. Note that learning under control conditions (ACh ON) leads to well defined and separated pairs of odor patterns, whereas learning with Ni OFF (in the OB) creates high-overlap pattern and learning with Mu OFF (in the OB) leads to very reduced learning. B, Average change in pairwise distances between dissimilar and similar bulbar output patterns relative to the distance between these patterns at the input to the OB. Any changes within a category are due to bulbar processing. The graph shows distances between odor representations conveyed to the bulb (OB input), and at the output of the OB under full or partial modulation (OB output). Bulbar output representations are rendered more dissimilar when nicotinic receptors are active (OB output, ACh ON, Mu OFF). In contrast, at this level of representation—based on firing rate only—blockade of muscarinic receptors does not affect bulbar output (OB output, Mu OFF). C, Average change in pairwise Euclidean distance of simulated cortical representations for dissimilar and similar odorants relative to the prelearning distance between dissimilar odorants. The graph shows pairwise distances between cortical odor representations before learning (Pre learning) or after learning with bulbar modulation on (Post learning, OB Ach ON) or partially on (Ni OFF, Mu OFF). Cortical learning of dissimilar odorants is only impaired by blockade of muscarinic receptors in the OB (OB Mu OFF), whereas cortical learning of similar odorants is impaired by blockade of both muscarinic and nicotinic receptors (Ni OFF, Mu OFF).