In one set of studies, Morrell reported auditory responses in primary visual cortex of animals that had been trained to associate auditory and visual stimuli (Morrell et al., 1957). While highly

controversial at the time, these results now seem consistent with the common substrates hypothesis. Fulvestrant in vivo Similarly, using cross-modal associative learning, Joaquin Fuster and colleagues (e.g., Zhou and Fuster, 2000) have provided several electrophysiological demonstrations of recall-related activity in the auditory and somatosensory cortices. As summarized above, the neuronal plasticity in IT cortex that accompanies paired-association learning is likely to be mediated via local circuit changes within this visual area (Figure 4A), which in turn provide the foundation for associative recall. Evidence indicates that this retrieval process takes two basic forms: automatic and active (Miyashita, 2004). In the automatic case, a bottom-up cue stimulus directly activates the neuronal representation of an associated stimulus, via the pre-established links in IT cortex. In the active case, retrieval is presumed to occur under executive control mediated by the prefrontal cortex. In this scenario, prefrontal cortex maintains stimulus and task-relevant information in working memory. Top-down signals from prefrontal cortex reactivate associative memory

Galunisertib mouse circuits Phenibut in IT cortex as dictated by the behavioral context at hand (Tomita et al., 1999). The situation in MT differs primarily in that the paired stimuli are unlikely to be associated via changes in local connections within this visual area. One possibility is that the visual associations learned in the experiment of Schlack and Albright (2007) are stored via circuit changes in IT cortex, in

a manner no different from that seen in earlier studies of pair-coding responses in IT (Messinger et al., 2001 and Sakai and Miyashita, 1991). According to this hypothesis, the recall-related activity observed in MT reflects a backward spread of feature-specific activation, originating with the memory trace in IT (via automatic or active processes) and descending through visual cortex (Figure 4B). Whatever the source of the feedback, there are several provocative features of the recall event that may inform an understanding of the underlying mechanism. To begin with, the neurophysiological data indicate that recall-related signals are highly specific. Indeed, in area MT the selectivity for stimuli associated with directions of motion is nearly indistinguishable from the selectivity for the motions themselves (Schlack and Albright, 2007). This selectivity suggests a high degree of anatomical specificity in the feedback signals that activate MT neurons under these conditions.