Leun J. Otten

Separating the brain regions involved in recollection and familiarity in recognition memory

The neural substrates of recognition memory retrieval were examined in a functional magnetic resonance imaging study designed to separate activity related to recollection from that related to continuous variations in familiarity. Across a variety of brain regions, the neural signature of recollection was found to be distinct from familiarity, demonstrating that recollection cannot be attributed to familiarity strength. In the prefrontal cortex, an anterior medial region was related to recollection, but lateral regions, including the anterior and dorsolateral prefrontal cortex, were related to familiarity. Along the lateral parietal cortex, two functionally distinct regions were also observed: a lateral parietal/temporal region related to recollection and a more superior parietal region involved in familiarity. Similarly, in medial parietal regions, the posterior cingulate was related to recollection, whereas the precuneus was related to familiarity. The hippocampus was related to recollection, but also exhibited an inverse relationship to familiarity-driven recognition confidence. The results indicate that recollection and familiarity rely on different networks of brain regions and provide insights into the functional roles of different regions involved in episodic recognition memory.

Mismatch Negativity: Different Water in the Same River

The mismatch negativity (MMN) is a frontal negative deflection in the human event-related potential that typically occurs when a repeating auditory stimulus changes in some manner. The MMN can be elicited by many kinds of stimulus change, varying from simple changes in a single stimulus feature to abstract changes in the relationship between stimuli. The main intracerebral sources for the MMN are located in the auditory cortices of the temporal lobe. Since it occurs whether or not stimuli are being attended, the MMN represents an automatic cerebral process for detecting change. The MMN is clinically helpful in terms of demonstrating disordered sensory processing or disordered memory in groups of patients. Improvements in the techniques for measuring the MMN and in the paradigms for eliciting it will be needed before the MMN can become clinically useful as an objective measurement of such disorders in individual patients.

Context effects on the neural correlates of recognition memory: an electrophysiological study.

Event-related potentials (ERPs) were recorded during a recognition memory test for previously studied visual objects. Some studied objects were paired with the same context (landscape scenes) as at study, some were superimposed on a different studied context, and some were paired with new contexts. Unstudied objects were paired with either a studied or a new context. Three ERP memory effects were observed: an early effect elicited by all stimuli containing at least one studied component; a second effect elicited only by stimuli in which both object and context had been studied; and a third effect elicited by stimuli containing a studied object. Thus, test stimuli engaged three distinct kinds of memory-related neural activity which differed in their specificity for task-relevant features.

State-related and item-related neural correlates of successful memory encoding

Neuroimaging studies show that the efficacy of long-term memory encoding of a stimulus is indexed by transient neural activity elicited by that stimulus. Here, we show that successful memory encoding is also indexed by neural activity that is tonically maintained throughout a study task. Using functional magnetic resonance imaging (fMRI), transient and sustained neural activity were dissociated with a mixed event-related and blocked design. In a series of short task blocks, human subjects made semantic or phonological decisions about visually presented words. After statistically removing item-related activity, we found that the mean level of activity across a task block was correlated with the number of words subsequently remembered from that block. These correlations were found in inferior medial parietal and left prefrontal cortex for the semantic task, and in superior medial parietal cortex for the phonological task. Our findings suggest that state-related activity in these brain regions is involved in memory encoding.

Episodic Encoding Is More than the Sum of Its Parts: An fMRI Investigation of Multifeatural Contextual Encoding

Episodic memories are characterized by their contextual richness, yet little is known about how the various features comprising an episode are brought together in memory. Here we employed fMRI and a multidimensional source memory procedure to investigate processes supporting the mnemonic binding of item and contextual information. Volunteers were scanned while encoding items for which the contextual features (color and location) varied independently, allowing activity elicited at the time of study to be segregated according to whether both, one, or neither feature was successfully retrieved on a later memory test. Activity uniquely associated with successful encoding of both features was identified in the intra-parietal sulcus, a region strongly implicated in the support of attentionally mediated perceptual binding. The findings suggest that the encoding of disparate features of an episode into a common memory representation requires that the features be conjoined in a common perceptual representation when the episode is initially experienced.

When more means less: Neural activity related to unsuccessful memory encoding

The neural correlates of memory encoding have been studied by contrasting neural activity elicited by items at the time of learning according to whether they were later remembered or forgotten [1]. Previous studies have focused on regions where neural activity is greater for subsequently remembered items [2-8]. Here, we describe regions where activity is greater for subsequently forgotten items. In two experiments that employed the same incidental learning task, activity in an overlapping set of cortical regions (posterior cingulate, inferior and medial parietal, and dorsolateral prefrontal) was associated with failure on a subsequent memory test.

Brain activity before an event predicts later recollection

Neural activity elicited by an event can predict whether the event is successfully encoded into memory. Here we assessed whether memory encoding relies not only on neural activity that follows an event, but also on activity that precedes it. In two experiments we found that human brain activity elicited by a cue presented just before a word could predict whether the word would be recollected in a later memory test.

Electrophysiological correlates of memory encoding are task-dependent

Event-related potentials (ERPs) were used to investigate whether the neural correlates of successful episodic encoding differ according to the nature of the study task. At study, 16 subjects were cued to make either animacy or alphabetic decisions about visually presented words. A recognition memory test with confidence judgements followed after a delay of 30 min. For the animacy task, words that were subsequently confidently recognised were associated with a positive-going ERP modulation. By contrast, for the alphabetic task, confident recognition was associated with a negative-going ERP modulation. Both types of subsequent memory effects started shortly after word onset. These findings suggest that the neural correlates of memory encoding differ qualitatively, rather than quantitatively, according to the nature of the study task. Episodic encoding thus seems to be supported by multiple, task-specific, neural systems. The early onset of these memory effects suggests that episodic encoding can be facilitated by processes that start before the onset of the to-be-encoded item.

Voluntary Control over Prestimulus Activity Related to Encoding

A new development in our understanding of human long-term memory is that effective memory formation relies on neural activity just before an event. It is unknown whether such prestimulus activity is under voluntary control or a reflection of random fluctuations over time. In the present study, we addressed two issues: (1) whether prestimulus activity is influenced by an individuals motivation to encode, and (2) at what point in time encoding-related activity emerges. Electrical brain activity was recorded while healthy male and female adults memorized series of words. Each word was preceded by a cue, which indicated the monetary reward that would be received if the following word was later remembered. Memory was tested after a short delay with a five-way recognition task to separate different sources of recognition. Electrical activity elicited by the reward cue predicted later memory of a word. Crucially, however, this was only observed when the incentive to memorize a word was high. Encoding-related activity preceded high-reward words that were later recollected. This activity started shortly after cue onset and persisted until word onset. Prestimulus activity thus not only signals cue-related processing but also an ensuing preparatory state. In contrast, reward-related activity was limited to the time period immediately after the reward cue. These findings indicate that engaging neural activity that benefits the encoding of an upcoming event is under voluntary control, reflecting a strategic preparatory state in anticipation of processing an event.

Effects of visual attentional load on auditory processing.

Auditory evoked potentials were recorded while participants attended to visually presented digits. The difficulty of the visual task was manipulated by requiring participants to process only the current digit (0-back) or both the current and the preceding digit (1-back). Tones deviating in frequency from standard tones elicited a frontal mismatch negativity peaking around 200 ms which did not vary with visual task. However, decreasing the visual task load enhanced a right-temporal positive wave peaking around 200 ms when tones were presented slowly, and a frontocentral negative wave peaking around 450 ms when tones were presented more rapidly. The degree to which task-irrelevant sounds are processed therefore depends on the degree to which a visual task engages attentional resources.