February 8, 2007

The Medial Temporal Lobe Distinguishes Old from New Independently of Consciousness

There is an interesting paper, The Medial Temporal Lobe Distinguishes Old from New Independently of Consciousness in The Journal of Neuroscience. The novel part is that the MTL novelty distinction can operate at an unconscious level. From one perspective the MTL is traditionally thought to be part of a declarative memory system, suggesting that a majorityif not allof the processing here involves consciousness. This result thus suggests that at least for this function, consciousness does not need to be an issue. What seems amiss in this paper is the more detailed account of MTL regions. Several studies have documentedboth for humans and non-human primatesthat different parts of the MTL make different contributions to the novelty distinction. Specifically, the perirhinal cortex is thought to be the primary processor of old/new distinctions. Clickthrough for abstract.

Abstract

Although it is widely accepted that the medial temporal lobes (MTLs) are critical for becoming aware that something happened in the past, there is virtually no evidence whether MTL sensitivity to event oldness also depends on conscious awareness.

Using event-related functional magnetic resonance imaging, we show that activity in posterior MTL tracks whether an item is actually old (true oldness), regardless of participants’ awareness of oldness (perceived oldness). Confirming its sensitivity to the objective nature of the stimulus, activity in this region was strongly correlated with individual memory performance (r_0.74). At the same time, we found that memory errors (misses) were associated with activity in an anterior MTL region, which signaled whether an item was consciously experienced as new (perceived novelty).

Logistic regression analyses based on individual trial activity indicated that the two MTL regions showed opposing relationships with behavior, and that memory performance was determined by their joint activity. Furthermore, functional connectivity analyses showed that perceived novelty activity in the posterior MTL inhibited true oldness activity in the anterior MTL. These findings indicate that participants’ behavior reflected the combined effects of multiple MTL regions. More generally, our results show that parts of MTL can distinguish old from new independently of consciousness.

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