Jiro Okuda

Task-specific signal transmission from prefrontal cortex in visual selective attention

Our voluntary behaviors are thought to be controlled by top-down signals from the prefrontal cortex that modulate neural processing in the posterior cortices according to the behavioral goal. However, we have insufficient evidence for the causal effect of the top-down signals. We applied a single-pulse transcranial magnetic stimulation over the human prefrontal cortex and measured the strength of the top-down signals as an increase in the efficiency of neural impulse transmission. The impulse induced by the stimulation transmitted to different posterior visual areas depending on the domain of visual features to which subjects attended. We also found that the amount of impulse transmission was associated with the level of attentional preparation and the performance of visual selective-attention tasks, consistent with the causal role of prefrontal top-down signals.

Neural Correlates of True Memory, False Memory, and Deception

We used functional magnetic resonance imaging (fMRI) to determine whether neural activity can differentiate between true memory, false memory, and deception. Subjects heard a series of semantically related words and were later asked to make a recognition judgment of old words, semantically related nonstudied words (lures for false recognition), and unrelated new words. They were also asked to make a deceptive response to half of the old and unrelated new words. There were 3 main findings. First, consistent with the notion that executive function supports deception, 2 types of deception (pretending to know and pretending not to know) recruited prefrontal activity. Second, consistent with the sensory reactivation hypothesis, the difference between true recognition and false recognition was found in the left temporoparietal regions probably engaged in the encoding of auditorily presented words. Third, the left prefrontal cortex was activated during pretending to know relative to correct rejection and false recognition, whereas the right anterior hippocampus was activated during false recognition relative to correct rejection and pretending to know. These findings indicate that fMRI can detect the difference in brain activity between deception and false memory despite the fact that subjects respond with “I know” to novel events in both processes.

Human posterior parietal cortex maintains color, shape and motion in visual short-term memory.

We used functional magnetic resonance imaging (fMRI) to investigate the neural substrate of visual short-term memory for objects defined by features processed in the dorsal and the ventral visual streams. Here we adopted the conventional delayed recognition task, whereas in addition to the more commonly used visual features of color and shape, motion direction was applied to define an item. Our behavioral results indicated that the capacity limit of visual short-term memory of motion direction was approximately two, which was significantly lower than those of color and shape, about three or four. We also found that storage capacity was significantly reduced when subjects were required to retain all three features superimposed in space. Meanwhile, fMRI results revealed that activity in the posterior part of the superior parietal lobe was memory-load dependent for all three features indicating that it collects and stores visual information from both the two visual processing streams, whereas the anterior part was load dependent only for motion.

Reactive Mechanism of Cognitive Control System

The prefrontal cortex (PFC) is thought to modulate the neural network state in favor of the processing of task-relevant sensory information prior to the presentation of sensory stimuli. However, this proactive control mechanism cannot always optimize the network state because of intrinsic fluctuation of neural activity upon arrival of sensory information. In the present study, we have investigated an additional control mechanism, in which the control process to regulate the behavior is adjusted to the trial-by-trial fluctuation in neural representations of sensory information. We asked normal human subjects to perform a variant of the Stroop task. Using functional magnetic resonance imaging, we isolated cognitive conflict at a sensory processing stage on a single-trial basis by calculating the difference in activation between task-relevant and task-irrelevant sensory areas. Activation in the dorsolateral PFC (DLPFC) covaried with the neural estimate of sensory conflict only on incongruent trials. Also, the coupling between the DLPFC and anterior cingulate cortex (ACC) was tighter on high-sensory conflict trials with fast response. The results suggest that although detection of sensory conflict is achieved by the DLPFC, online behavioral adjustment is achieved by interactive mechanisms between the DLPFC and ACC.

How sound symbolism is processed in the brain: a study on Japanese mimetic words.

Sound symbolism is the systematic and non-arbitrary link between word and meaning. Although a number of behavioral studies demonstrate that both children and adults are universally sensitive to sound symbolism in mimetic words, the neural mechanisms underlying this phenomenon have not yet been extensively investigated. The present study used functional magnetic resonance imaging to investigate how Japanese mimetic words are processed in the brain. In Experiment 1, we compared processing for motion mimetic words with that for non-sound symbolic motion verbs and adverbs. Mimetic words uniquely activated the right posterior superior temporal sulcus (STS). In Experiment 2, we further examined the generalizability of the findings from Experiment 1 by testing another domain: shape mimetics. Our results show that the right posterior STS was active when subjects processed both motion and shape mimetic words, thus suggesting that this area may be the primary structure for processing sound symbolism. Increased activity in the right posterior STS may also reflect how sound symbolic words function as both linguistic and non-linguistic iconic symbols.

The Nature of Age-Related Decline on the Reading Span Task

We examined the effect of age on working memory with a reading span task (RST) together with other verbal span tasks. Sixty-two participants were divided into three subgroups (young, middle-aged, and elderly). The RST performances were significantly different among all the subgroups. To elucidate which component of the working memory system is affected by age, we performed an analysis of covariance with the scores of simple and complex verbal span tasks as covariates. From the results, we conclude that the difference of the RST performance between the middle-aged and elderly groups reflects a decline in the capacity of the phonological loop, and the difference between the young and middle-aged groups reflects malfunctioning of the central executive system.

Different roles of the left and right parahippocampal regions in verbal recognition: a PET study

WE examined the role of the parahippocampal regions in humans during two types of verbal recognition process, i.e. delayed matching and non-matching, by measuring regional cerebral blood flow (rCBF) using PET. The results showed differential activation of each parahippocampal region during verbal memory tasks in which the side activated shifted depending on the nature of the task employed; an increase in rCBF in the left parahippocampal gyrus was associated with retrieval strategy of non-matching, and an increase in rCBF in the right parahippocampal gyrus was associated with retrieval strategy of matching. We conclude that lateralized parahippocampal activation may depend on the type of response required.

Retention of words in long-term memory : a functional neuroanatomical study with PET

We used PET to identify brain regions associated with retention of verbal materials in long-term memory. During a PET scan, subjects repeated many sets of words one after another. In a retention condition, they were simultaneously required to retain 10 key words that were irrelevant to the repetition task. Significant increases in regional cerebral blood flow during the retention condition were found in bilateral parahippocampal regions, the left prefrontal and parietal association cortices, the supplementary motor area, the neostriatum and the cerebellum. We clearly demonstrated that retention of verbal materials was accompanied by neural activities in the medial temporal lobes. We also showed that, in the early phase, retention of words in long-term memory recruited left cortical areas surrounding those relevant to verbal short-term memory.

Auditory evoked magnetic fields in patients with right hemisphere language dominance.

AUDITORY evoked magnetic fields for pure-tone stimuli were measured in seven subjects with right hemisphere dominance for language using a whole-head magneto-encephalography system linked to magnetic resonance imaging. N100m responses were observed in both hemispheres in five subjects. The N100m response latency to contralateral stimulation was significantly shorter on the right than on the left in all cases. Normal righthanded subjects with left hemisphere dominance showed exactly the same responses. Therefore, the shorter N100m latency in the right hemisphere is independent of the dominant hemisphere for language.

Stimulus-dependent adjustment of reward prediction error in the midbrain.

Previous reports have described that neural activities in midbrain dopamine areas are sensitive to unexpected reward delivery and omission. These activities are correlated with reward prediction error in reinforcement learning models, the difference between predicted reward values and the obtained reward outcome. These findings suggest that the reward prediction error signal in the brain updates reward prediction through stimulus–reward experiences. It remains unknown, however, how sensory processing of reward-predicting stimuli contributes to the computation of reward prediction error. To elucidate this issue, we examined the relation between stimulus discriminability of the reward-predicting stimuli and the reward prediction error signal in the brain using functional magnetic resonance imaging (fMRI). Before main experiments, subjects learned an association between the orientation of a perceptually salient (high-contrast) Gabor patch and a juice reward. The subjects were then presented with lower-contrast Gabor patch stimuli to predict a reward. We calculated the correlation between fMRI signals and reward prediction error in two reinforcement learning models: a model including the modulation of reward prediction by stimulus discriminability and a model excluding this modulation. Results showed that fMRI signals in the midbrain are more highly correlated with reward prediction error in the model that includes stimulus discriminability than in the model that excludes stimulus discriminability. No regions showed higher correlation with the model that excludes stimulus discriminability. Moreover, results show that the difference in correlation between the two models was significant from the first session of the experiment, suggesting that the reward computation in the midbrain was modulated based on stimulus discriminability before learning a new contingency between perceptually ambiguous stimuli and a reward. These results suggest that the human reward system can incorporate the level of the stimulus discriminability flexibly into reward computations by modulating previously acquired reward values for a typical stimulus.