4/30/2013

Molecular steps in memory consolidation are associated with susceptibility to disruption by external novel stimuli


The question of how short-lived memories (~the momentary contents of one’s consciousness) become stored in long-term memory that lasts up to one’s lifetime is one of the most fundamental ones in cognitive neuroscience. This process is referred to as memory consolidation. While the underlying neural mechanisms have begun to be understood with increasing detail, open questions have remained. As one instance of this, there is research indicating that memory consolidation is not a unitary one-step process, but rather that there are lapses in memory even after the initial consolidation.

Thriving on recent advances in understanding the behavioral and molecular mechanisms underlying memory consolidation in Lymnaea during associative conditioning, Marra et al. (2012) studied the neural events underlying lapses in memory that take place during consolidation. Using a specific in vitro behavioral one-trial conditioning approach, the authors observed lapses in memory at 30 min and 2 hours after conditioning, but not at other tested time points of 10 min, one, three, and four hours. While the lapse at 30 min marked transition from short-term to intermediate-term memory, the lapse at two hours post-conditioning suggested that intermediate-term memory has early and late phases with distinct underlying molecular mechanisms. Furthre, these lapses constituted time points during which memory consolidation was susceptible to disruption by external stimuli. Disruption at two hours blocked long-term memory 24 hours later, and disruption at 30 min blocked recall at three and four hours post-conditioning. The underlying molecular mechanisms were studied by inspecting the effects of specific pharmacological agents modulating protein kinase activity on these different phases of consolidation.

The results suggested specific steps in memory consolidation, with changes in properties of ion channels resulting in increased excitability lasting for at least 10 min post-training, followed by enhancement of synaptic efficacy by proteins synthesized from existing RNA between 10 min and the first hour (e.g., insertion of more channel proteins) that lasts up to three hours. New synapses are then produced over at least the first 24 hours that correspond to consolidation to long-term memory. The long-term memory consolidation was observed to be dependent on protein kinases C and M, with early and late phases of long-term memory however differing in requirement of for duration of protein kinase A activity after training. Taken together, these results are highly exciting in illuminating specific molecular events underlying the distinct steps that lead to consolidation of memories from short-term through intermediate-term to long-term memory, and by showing how the transition periods between the processes of consolidation are specifically susceptible to disruption by distracting stimuli/events.

Reference: Marra V, O’Shea M, Benjamin PR, Kemenes I. Susceptibility of memory consolidation during lapses in recall. Nature Communications (2013) 4: 1578. http://dx.doi.org/10.1038/ncomms2591

4/21/2013

Automatic phonetic category processing in premotor cortex is functionally coupled with the dorsal auditory stream in human


Understanding how the human brain is able to extract phonetic information from acoustics of speech, given that it is highly variable due to a number of factors including phonetic context (e.g., in English “p” in “port” and “sport” are acoustically quite different) and voice characteristics of different speakers, has inspired both empirical and theoretical work. In the so-called auditory models, speech perception is accomplished solely based on analysis of the auditory information. In contrast, speech motor theory postulates that auditory speech inputs are mapped to motor schemas that form the basis for speech perception. While neuroimaging studies have provided support for both models, the precise role played by the speech motor system / dorsal processing stream (that involves auditory cortical structures posterior to the primary auditory cortex, inferior parietal and frontal cortical areas, as well the (pre)motor cortex) in speech perception has remained an open question.

In their recent study, Dr. Mark Chevillet et al. (2013) examined, in healthy volunteers using a specific functional magnetic resonance imaging adaptation paradigm, whether there is phonetic category selectivity in human premotor cortex and, further, using structural equation modeling, the authors sought answer to the question of whether the premotor cortex is functionally more coupled with anterior or posterior auditory cortical areas. Using pairs of morphed stimuli that varied equally in terms of acoustic distance but either belonged the same vs. different phonetic category, the authors indeed managed to see phonetic categorization taking place in premotor cortex that correlated with behavioral categorization performance. Further, as revealed by structural equation modeling, the premotor cortex was functionally coupled with posterior rather than anterior auditory cortical areas.

These findings are highly important in shedding light on the role of human premotor cortex in processing of phonetic information and, further, add to the growing pool of evidence linking the dorsal auditory processing stream with motor-schemata based speech perception. The authors elegantly utilize an adaptation functional neuroimaging paradigm combined with carefully designed continuums of auditory phonetic stimuli that retain constant acoustic distance with vs. without crossing of the phonetic category boundary. The structural equation modeling further provides important information about functional connectivity between the premotor cortex and auditory cortical areas. Since the subjects were actively engaged in a task other than phonetic categorization suggests that phonetic categorization that takes place in premotor cortical areas is highly automatic in nature, which further augments the importance of the findings.

Reference: Chevillet MA, Jiang X, Rauschecker JP, Riesenhuber M. Automatic phoneme category selectivity in the dorsal auditory stream. Journal of Neuroscience (2013) 33: 5208–5215. http://dx.doi.org/10.1523/JNEUROSCI.1870-12.2013

4/14/2013

Advantage for remembering object-context associations in subjects with Eastern-cultural background


Culturally determined differences in perception and cognition are rapidly emerging as a hot topic in modern cognitive neuroscience. Among the most interesting findings on this line of research are observations suggesting that Western people tend to think more analytically (e.g., focus attention on perceptual objects) and people from Eastern cultures tend to think more holistically (e.g., pay attention to contextual details). Naturally, any such effects are in absolute terms small and there is within-culture variability with overlapping distributions, however, given the potential impact such differences bear on, for example, inter-cultural understanding, such findings are highly exciting and significant.

In their recent study, Yang et al. (2013) measured context-dependent vs. context-independent object memory in 71 Canadian and 72 Chinese volunteers. These groups further contained both young and older volunteers to allow examination of interaction of age and culture effects. The memory-encoding task comprised of two blocks: in the first one, the participants rated line-drawing pictures of familiar objects either for their meaningfulness in context of independent living or the typicality of the objects in daily life. In the second block, the objects were rated for their meaningfulness in the context of fostering relationships with other persons or, akin to the control condition of the first block, the typicality of the objects in daily life.

The authors observed that while both groups exhibited similar age-related deterioration in item and context memory, the Chinese subjects significantly outperformed Canadians in context memory, and that this effects was equal in case of both the young and older subjects. These results suggest that due to cultural differences, Chinese have an advantage for memorization of socially meaningful object-context associations. Overall, these highly interesting behavioral findings add to the growing body of evidence indicating that cognitive functions are significantly shaped by the culture that one is brought up in.

Reference: Yang L, Li J, Hasher L, Wilkinson AJ, Yu J, Niu Y. Aging, culture ,and memory for socially meaningful item-context associations: an East-West cross-cultural comparison study. PLoS ONE (2013) 8: e60703. http://dx.doi.org/10.1371/journal.pone.0060703

4/07/2013

Ventral pallidum response to initial confirmation of bad advise predicts subsequent impediments in reinforcement learning


Bias to search for information that confirms one’s prior expectations and to dismiss hypothesis-incongruent information (until sufficient amount of contradictory information accumulates and forces one to abandon one’s initial hypothesis) is one of the most fundamental aspects of human information processing. As a specific example of this, it has been shown that after receiving initial expert advise, persons tend to be less likely to seek out alternative actions and, in case of bad advice, end up with less optimal outcomes. While this bias, known as the law of primacy in persuasion (i.e., out of two opposing arguments, the first one has a stronger effect), has been behaviorally demonstrated already in the 1920s, relatively little is known about the neural basis of learning based on bias due to initial advise by others and the possible role of initial confirmatory evidence in strengthening the bias.

In their recent study, Staudinger et al. (2013) had 35 right-handed volunteers carry out a probabilistic reinforcement-learning task on Japanese Hiragana symbols during functional magnetic resonance imaging, wherein the subjects received, prior to task onset, a misleading tip that two of the symbols would have the highest probability of being correct. The first trials were presented to the subjects non-randomly, providing two out of three advise-confirmatory trials for one of the advised symbols, and two out of three advise-contradictory trials for the other advised symbol. After these initial trials the task sequence became random.  After the scanning, the subjects completed a lottery task where the probability of winning 8€ was specified by the displayed symbol that the subjects were learning during their task in the fMRI scanner.

The results show that one-time misleading advise robustly biases subsequent learning and decision making; the initial advise-confirmatory trials strengthened the biasing effects of the misleading advise on learning. Furthermore, interestingly, stronger ventral pallidum responses to initial positive reinforcement of misleading advise predicted worse learning over the remainder of the experiment. Given that the ventral pallidum is part of the brain’s reinforcement-learning circuitry, these results are highly important in demonstrating how receiving a priori advise shapes the way that the reward circuitry processes subsequent advise-associated information, and thus biases the learning process.

Reference: Staudinger MR, B├╝chel C. How initial confirmatory experience potentiates the detrimental influence of bad advice. Neuroimage (2013) e-publication ahead of print. http://dx.doi.org/10.1016/j.neuroimage.2013.02.074