Cortical mechanisms in hearing

Current understanding of neural processing in the auditory cortex has been shaped by a variety of experimental approaches in animals and humans. It remains a daunting challenge to reconcile data as diverse as synaptic properties recorded in a rodent brain slice and functional images of auditory cortex in a behaving human. Nevertheless, the gaps are narrowing through a renewed focus on humans and other primates, a continuing interest in evidence for functional pathways, a broader application of modern imaging techniques, a growing awareness of cortical sensitivity to dynamic features of sounds, and an improved understanding of auditory cortical circuitry.     

Cortical mechanisms and cues for action

Monkeys have more highly developed brains and are more intelligent than rats; yet rats learn some tasks as efficiently as monkeys. For example, rats are as quick at discovering which of two doors hides food or how to open the doors. Presumably tasks of this sort do not greatly tax cortical associative mechanisms since the animals have only to cumulate facts about objects. It is argued that cortical mechanisms are crucial for the ability to relate together information that is presented at different times or in different places. After removal of parts of frontal cortex monkeys can still associate cues that are presented together but they are poor at relating cues that are presented apart.     

Cortical mechanisms of sensory learning and object recognition

Learning about the world through our senses constrains our ability to recognise our surroundings. Experience shapes perception. What is the neural basis for object recognition and how are learning-induced changes in recognition manifested in neural populations? We consider first the location of neurons that appear to be critical for object recognition, before describing what is known about their function. Two complementary processes of object recognition are considered: discrimination among diagnostic object features and generalization across non-diagnostic features. Neural plasticity appears to underlie the development of discrimination and generalization for a given set of features, though tracking these changes directly over the course of learning has remained an elusive task.     

Training attentional control in infancy

Several recent studies have reported that cognitive training in adults does not lead to generalized performance improvements [1, 2], whereas many studies with younger participants (children 4 years and older) have reported distal transfer [3, 4]. This is consistent with convergent evidence [5-8] for greater neural and behavioral plasticity earlier in development. We used gaze-contingent paradigms to train 11-month-old infants on a battery of attentional control tasks. Relative to an active control group, and following only a relatively short training period, posttraining assessments revealed improvements in cognitive control and sustained attention, reduced saccadic reaction times, and reduced latencies to disengage visual attention. Trend changes were also observed in spontaneous looking behavior during free play, but no change was found in working memory. The amount of training correlated with the degree of improvement on some measures. These findings are to our knowledge the first demonstration of distal transfer following attentional control training in infancy. Given the longitudinal relationships identified between early attentional control and learning in academic settings [9, 10], and the causal role that impaired control of attention may play in disrupting learning in several disorders [11-14], the current results open a number of avenues for future work.     

Cortical domains and the mechanisms of asymmetric cell division

Asymmetric cell divisions are central to the generation of cell-fate diversity because factors that are present in a mother cell and distributed unequally at cell division can generate distinct daughters. The process o f asymmetric cell division can be described as consisting of three steps: setting up an asymmetric cue in the mother cell, localizing factors with respect to this cue, and positioning the plane o f cell division so that localized factors are partitioned asymmetrically between daughters. This review describes how specialized cortical domains play a key role in each of these steps and discusses our current understanding of the molecular nature o f cortical domains and the mechanisms by which they may orchestrate asymmetric cell divisions.     

Cortical control of reaching movements

Recent studies provide further support for the hypothesis that spatial representations of limb position, target locations, and potential motor actions are expressed in the neuronal activity in parietal cortex. In contrast, precentral cortical activity more strongly expresses processes involved in the selection and execution of motor actions. As a general conceptual framework, these processes may be interpreted in terms of such formalisms as sensorimotor transformation and ‘internal models’.     

Cortical control of plant microtubules

The cortical microtubule array of plant cells appears in early G(1) and remodels during the progression of the cell cycle and differentiation, and in response to various stimuli. Recent studies suggest that cortical microtubules are mostly formed on pre-existing microtubules and, after detachment from the initial nucleation sites, actively interact with each other to attain distinct distribution patterns. The plus end of growing microtubules is thought to accumulate protein complexes that regulate both microtubule dynamics and interactions with cortical targets. The ROP family of small GTPases and the mitogen-activated protein kinase pathways have emerged as key players that mediate the cortical control of plant microtubules.     

Cortical control of motor sequences

The neural substrate of sequence learning is well known. However, we lack a clear understanding of the detailed functional properties of many of the areas involved. The reason for this discrepancy lies, in part, in the fact that two types of processes, implicit and explicit, subserve motor sequence learning, and these often interact with each other. The most significant recent advances have been the elucidation of the very complex relationships between medial motor areas and the temporal and ordinal control of sequences, and the demonstration that motor cortex is an important site for sequence storage and production. The challenge for the future will be to develop a coherent and internally consistent theory of sequence control.     

Prospective timing under dual-task paradigms: attentional and contextual-change mechanisms

The effect of a concurrent memory task on prospective time estimates by human participants was investigated in two experiments. The objective was to isolate task effects from those of participant timing strategy (self-paced counting) and number of contextual changes during the temporal stimulus. Accordingly, self-paced counting was suppressed by requiring participants to perform a word-reading task during the temporal stimuli, while number of stimulus changes presented during temporal stimuli was controlled. Presence versus absence of the concurrent memory task was manipulated in Experiment 1, and instruction to focus on timing or to focus on memory was manipulated in Experiment 2. There was no significant effect of presence versus absence of the concurrent memory task on time estimates; however, time estimates were shorter when participants were instructed to focus on memory versus timing. In both experiments, time estimates were positively correlated with participants' estimates of the number of words presented during the interval, even though number of words presented was invariant. These findings were generally consistent with resource-allocation attentional accounts of concurrent task effects; however, support for a contextual-change model of timing was also obtained.     

Cortical bone failure mechanisms during screw pullout

An experimental and computational study of screw pullout from cortical bone has been conducted. A novel modification of standard pullout tests providing real time image capture of damage mechanisms during screw pullout was developed. Pullout forces, measured using the novel test rig, have been validated against standard pullout tests. Pullout tests were conducted, considering osteon alignment, to investigate the effect of osteons aligned parallel to the axis of the orthopaedic screw (longitudinal pullout) as well as the effect of osteons aligned perpendicular to the axis of the screw (transverse pullout). Distinctive alternate failure mechanisms, for longitudinally and transversely orientated cortical bone during screw pullout, were uncovered. Vertical crack propagation, parallel to the axis of the screw, was observed for a longitudinal pullout. Horizontal crack propagation, perpendicular to the axis of the screw, was observed for a transverse pullout. Finite element simulation of screw pullout, incorporating material damage and crack propagation, was also performed. Simulations revealed that a homogenous material model for cortical bone predicts vertical crack propagation patterns for both longitudinal and transverse screw pullout. A bi-layered composite model representing cortical bone microstructure was developed. A unique set of material and damage properties was used for both transverse and longitudinal pullout simulations, with only layer orientations being changed. Simulations predicted: (i) higher pullout forces for transverse pullout; (ii) horizontal crack paths perpendicular to screw axis for transverse pullout, whereas vertical crack paths were computed for longitudinal pullout. Computed results agreed closely with experimental observations in terms of pullout force and crack propagation.     

Time-of-day variation in sustained attentional control

Sustained attention is a fundamental cognitive function underlying many activities in daily life including workplace safety, but its natural variation throughout the day is incompletely characterized. To examine time-of-day variation, we collected a large online data set (N = 6,363) with participation throughout the day and around the world on the gradual-onset continuous performance task, a sensitive measure of sustained attention. This allowed us to examine accuracy, attentional stability, and strategy. Results show that both accuracy and attentional stability peak between 9:00 and 11:00 a.m. and progressively decline throughout the day, whereas strategy is more stable.     

Cortical control of microtubule stability and polarization

In both dividing and interphase cells, microtubules are remodeled in response to signal transduction pathways triggered by a variety of stimuli. Members of the Rho family of small GTPases have emerged as key intermediates in transmitting signals to cortical factors that mediate capture of dynamic microtubules at specific sites. The specificity of cortical capture appears to be controlled by microtubule tip proteins and cortical receptors that bind these proteins. Recent studies suggest that some of the proteins interacting with microtubule tips behave as bridging proteins between the microtubule tip proteins and their cortical receptors. Such bridging proteins may enhance cortical capture of microtubules directly or indirectly through interactions with the actin cytoskeleton.     

Cortical mechanisms specific to explicit visual object recognition

The cortical mechanisms associated with conscious object recognition were studied using functional magnetic resonance imaging (fMRI). Participants were required to recognize pictures of masked objects that were presented very briefly, randomly and repeatedly. This design yielded a gradual accomplishment of successful recognition. Cortical activity in a ventrotemporal visual region was linearly correlated with perception of object identity. Therefore, although object recognition is rapid, awareness of an object's identity is not a discrete phenomenon but rather associated with gradually increasing cortical activity. Furthermore, the focus of the activity in the temporal cortex shifted anteriorly as subjects reported an increased knowledge regarding identity. The results presented here provide new insights into the processes underlying explicit object recognition, as well as the analysis that takes place immediately before and after recognition is possible.     

Cortical mechanisms for reinforcement learning in competitive games

Game theory analyses optimal strategies for multiple decision makers interacting in a social group. However, the behaviours of individual humans and animals often deviate systematically from the optimal strategies described by game theory. The behaviours of rhesus monkeys (Macaca mulatta) in simple zero-sum games showed similar patterns, but their departures from the optimal strategies were well accounted for by a simple reinforcement-learning algorithm. During a computer-simulated zero-sum game, neurons in the dorsolateral prefrontal cortex often encoded the previous choices of the animal and its opponent as well as the animal's reward history. By contrast, the neurons in the anterior cingulate cortex predominantly encoded the animal's reward history. Using simple competitive games, therefore, we have demonstrated functional specialization between different areas of the primate frontal cortex involved in outcome monitoring and action selection. Temporally extended signals related to the animal's previous choices might facilitate the association between choices and their delayed outcomes, whereas information about the choices of the opponent might be used to estimate the reward expected from a particular action. Finally, signals related to the reward history might be used to monitor the overall success of the animal's current decision-making strategy.