Encoding and Retrieval of Artificial Visuoauditory Memory Traces in the Auditory Cortex Requires the Entorhinal Cortex

Abstract

Damage to the medial temporal lobe impairs the encoding of new memories and the retrieval of memories acquired immediately before the damage in human. In this study, we demonstrated that artificial visuoauditory memory traces can be established in the rat auditory cortex and that their encoding and retrieval depend on the entorhinal cortex of the medial temporal lobe in the rat. We trained rats to associate a visual stimulus with electrical stimulation of the auditory cortex using a classical conditioning protocol. After conditioning, we examined the associative memory traces electrophysiologically (i.e., visual stimulus-evoked responses of auditory cortical neurons) and behaviorally (i.e., visual stimulus-induced freezing and visual stimulus-guided reward retrieval). The establishment of a visuoauditory memory trace in the auditory cortex, which was detectable by electrophysiological recordings, was achieved over 20 -30 conditioning trials and was blocked by unilateral, temporary inactivation of the entorhinal cortex. Retrieval of a previously established visuoauditory memory was also affected by unilateral entorhinal cortex inactivation. These findings suggest that the entorhinal cortex is necessary for the encoding and involved in the retrieval of artificial visuoauditory memory in the auditory cortex, at least during the early stages of memory consolidation.

Time course of the dependence of associative memory retrieval on the entorhinal cortex

“…However, a recent study by Chen et al (2013) demonstrated that the neocortex is involved in the process of information storage from the beginning of memory encoding. Therefore, combining these results with the results of our current experiment, we propose a new model in which a new memory is encoded in both the neocortex and the hippocampus that are associated via the EC.…”

“…The parameters of the paradigm were set based on a previous research (Chen et al, 2013). A combined stimulus of a 100-ms light and a 20-ms sound (200-ms delay from the onset of the light) was used as the conditioned stimulus (CS) and a 500-ms foot shock (500-ms delay from the onset of the light) was used as the unconditioned stimulus (US), as illustrated in Fig.…”

“…The freezing response was scored by the experimenter using the time frame provided by Windows Media Player. The amount of time that each rat spent in a freezing response within 30 s after the presentation of light was measured to calculate the freezing percentage (Chen et al, 2013). Associative memory was assessed on two trials for each test.…”

“…Audiovisual interactions and representations enable the cross-modal processing of the visual modulation of neurons in the auditory cortex (Romanski, 2007;Kayser et al, 2008). In one of our recent studies (Chen et al, 2013), we conducted an experiment with electrodes implanted in the auditory cortex and a drug cannula in the EC of behaving rats. Reversible inactivation of the EC impaired not only the encoding of an artificial associative memory between a visual stimulus and bilateral auditory cortex stimulation but also the retrieval of the memory within a week after the establishment of the memory.…”

Separating lexical-semantic access from other mnemonic processes in picture-name verification

“…In addition these primary sensory areas, posterior and prefrontal association cortices also play a role in representing cross-modal associations (Fuster et al, 2000). The arbitrary associations are believed to be initially mediated via medial temporal cortex connecting the unimodal sensory cortices (McClelland et al, 1995; Miyashita et al, 1996, 1998; Frankland and Bontempi, 2005; Eichenbaum et al, 2007; Chen et al, 2013). Over time, these medial temporally mediated memory traces become independent of medial temporal structures and dependent upon posterior and prefrontal association cortices (Frankland and Bontempi, 2005).…”