Yasuki Noguchi

Temporal Dynamics of Neural Adaptation Effect in the Human Visual Ventral Stream

When the same visual stimulus is repeatedly presented with a brief interval, the brain responses to that stimulus are attenuated relative to those at first presentation [neural adaptation (NA)]. Although this effect has been widely observed in various regions of human brain, its temporal dynamics as a neuronal population has been mostly unclear. In the present study, we used a magnetoencephalography (MEG) and conducted a macrolevel investigation of the temporal profiles of the NA occurring in the human visual ventral stream. The combination of MEG with our previous random dot blinking method isolated the neural responses in the higher visual cortex relating to shape perception. We dissociated three dimensions of the NA: activation strength, peak latency, and temporal duration of neural response. The results revealed that visual responses to the repeated compared with novel stimulus showed a significant reduction in both activation strength and peak latency but not in the duration of neural processing. Furthermore, this acceleration of peak latency showed a significant correlation with reaction time of the subjects, whereas no correlation was found between the reaction time and the temporal duration of neural responses. These results indicate that (1) the NA involves the brain response changes in the temporal domain as well as the response attenuation reported previously, and (2) this temporal change is primarily observed as a rapid rising of “what” responses, rather than a temporal shortening of neural response curves within the visual ventral stream as considered previously.

Neural codes for somatosensory two-point discrimination in inferior parietal lobule: an fMRI study.

This is the first functional magnetic resonance imaging (fMRI) study to investigate the hemodynamic response related to somatosensory spatial discrimination, so-called two-point discrimination. During scanning, we examined two discrimination tasks using four types of electrical stimuli applied to one or two points with strong or weak intensity on the right and left forearm, respectively. In the two-point discrimination task (TPD), subjects reported whether they thought the stimulus was applied to one point or two. In the intensity discrimination task (ID), subjects were required to judge whether the stimulus was strong or weak. In each task, they pressed a button to report their choice. Comparing TPD with the control, we found activated regions in the inferior parietal lobule (IPL) around the supramarginal gyrus (SMG) (Brodmanns area 40) and anterior cingulate cortex (ACC). These areas were significantly activated irrespective of the forearm stimulated. Comparing ID with the control, there were no significantly activated regions. By comparing the TPD and ID, we identified that the left IPL was significantly activated, specifically in TPD, irrespective of the forearm stimulated. In contrast, there were no significantly activated regions in the ID task. Therefore, the left IPL is considered to play an important role in two-point discrimination.

Hiding true emotions: micro-expressions in eyes retrospectively concealed by mouth movements.

When we encounter someone we dislike, we may momentarily display a reflexive disgust expression, only to follow-up with a forced smile and greeting. Our daily lives are replete with a mixture of true and fake expressions. Nevertheless, are these fake expressions really effective at hiding our true emotions? Here we show that brief emotional changes in the eyes (micro-expressions, thought to reflect true emotions) can be successfully concealed by follow-up mouth movements (e.g. a smile). In the same manner as backward masking, mouth movements of a face inhibited conscious detection of all types of micro-expressions in that face, even when viewers paid full attention to the eye region. This masking works only in a backward direction, however, because no disrupting effect was observed when the mouth change preceded the eye change. These results provide scientific evidence for everyday behaviours like smiling to dissemble, and further clarify a major reason for the difficulty we face in discriminating genuine from fake emotional expressions.

Knowledge-based correction of flash-lag illusion

A flash is perceived to lag spatially behind a moving object even when the two retinal images are physically aligned (flash-lag effect, FLE). Here we show that this robust illusion can be diminished by a knowledge of letters in the observers brain. When moving and flashed segments in the FLE made the shape of a Kanji letter (ideographic characters used in Japan), the magnitude of the illusory lag perceived by Japanese subjects was significantly reduced compared to when conventional geometric (nonletter) segments were used. This diminishment was not observed when a pseudo-Kanji letter was presented to Japanese subjects or when non-Japanese English-speakers (who do not have a knowledge of Kanji) saw a real Kanji letter, indicating that the reduction in the FLE was induced by a retrieval of the knowledge (shapes of letters) stored in the observers brain. Furthermore, measurements of neural activities by magnetoencephalography showed that the initial brain response, in which the effect of the knowledge became evident, occurred as early as 160 msec after the appearance of the flashed segment. These results demonstrated a substantial influence of knowledge on the flash-lag illusion and further suggest a rapid response of the knowledge-based perceptual pathway in the human brain.

Association between salivary serotonin and the social sharing of happiness

Although human saliva contains the monoamine serotonin, which plays a key role in the modulation of emotional states, the association between salivary serotonin and empathic ability remains unclear. In order to elucidate the associations between salivary serotonin levels, trait empathy, and the sharing effect of emotions (i.e., sharing emotional experiences with others), we performed a vignette-based study. Participants were asked to evaluate their happiness when they experience several hypothetical life events, whereby we manipulated the valence of the imagined event (positive, neutral, or negative), as well as the presence of a friend (absent, positive, or negative). Results indicated that the presence of a happy friend significantly enhanced participants’ happiness. Correlation analysis demonstrated that salivary serotonin levels were negatively correlated with happiness when both the self and friend conditions were positive. Correlation analysis also indicated a negative relationship between salivary serotonin levels and trait empathy (particularly in perspective taking), which was measured by the Interpersonal Reactivity Index. Furthermore, an exploratory multiple regression analysis suggested that mothers’ attention during childhood predicted salivary serotonin levels. Our findings indicate that empathic abilities and the social sharing of happiness decreases as a function of salivary serotonin levels.

Direct behavioral and neural evidence for an offset-triggered conscious perception.

Many previous theories of perceptual awareness assume that a conscious representation of a stimulus is created from sensory information carried by an onset (appearance) of the stimulus. In contrast, here we provide behavioral and neural evidence for a new phenomenon in which conscious perception is directly triggered by an offset (disappearance) of a stimulus. When a stimulus made invisible by inter-ocular suppression physically disappeared from a screen, subjects reported an appearance (not disappearance) of that stimulus, correctly reporting a color of the disappeared stimulus. Measurements of brain activity further confirmed that the physical offset of an invisible stimulus evoked neural activity reflecting conscious perception of that stimulus. Those results indicate a new role of a stimulus offset to facilitate (rather than inhibit) an emergence of consciousness.

Non-uniform transformation of subjective time during action preparation

Although many studies have reported a distortion of subjective (internal) time during preparation and execution of actions, it is highly controversial whether actions cause a dilation or compression of time. In the present study, we tested a hypothesis that the previous controversy (dilation vs. compression) partly resulted from a mixture of two types of sensory inputs on which a time length was estimated; some studies asked subjects to measure the time of presentation for a single continuous stimulus (stimulus period, e.g. the duration of a long-lasting visual stimulus on a monitor) while others required estimation of a period without continuous stimulations (no-stimulus period, e.g. an inter-stimulus interval between two flashes). Results of our five experiments supported this hypothesis, showing that action preparation induced a dilation of a stimulus period, whereas a no-stimulus period was not subject to this dilation and sometimes can be compressed by action preparation. Those results provided a new insight into a previous view assuming a uniform dilation or compression of subjective time by actions. Our findings about the distinction between stimulus and no-stimulus periods also might contribute to a resolution of mixed results (action-induced dilation vs. compression) in a previous literature.

Retrospective triggering of conscious perception by an interstimulus interaction.

Attention facilitates conscious perception of a visual stimulus at an attended location. Interestingly, a recent study (using the Posner spatial-cueing task) reported that attention facilitated conscious perception even when it was cued after a stimulus was gone (postcued-attention or retroperception effect). Here, we show that this effect can be induced without any contribution of attention. Contrary to previous situations, we fixed a position of a target (Gabor patch) and cue (luminance change of a circle encompassing the target) across trials so that subjects always could allocate their full attention to the target position. The cue (luminance change) improved objective and subjective visibility of the nearby target even when it was given ∼200 ms after the targets offset. This retrospective improvement was diminished when a shape of the cue was changed from a circle to a dot pattern, suggesting that the improvement emerged from a visual interaction (combinations of shapes) between the circular cue and target. Those results indicated that a local visual interaction between the target and cue is sufficient to trigger consciousness of the target, revealing a new type of retroperception effect mediated by sensory (nonattentional) mechanisms.

Two speed factors of visual recognition independently correlated with fluid intelligence.

Growing evidence indicates a moderate but significant relationship between processing speed in visuo-cognitive tasks and general intelligence. On the other hand, findings from neuroscience proposed that the primate visual system consists of two major pathways, the ventral pathway for objects recognition and the dorsal pathway for spatial processing and attentive analysis. Previous studies seeking for visuo-cognitive factors of human intelligence indicated a significant correlation between fluid intelligence and the inspection time (IT), an index for a speed of object recognition performed in the ventral pathway. We thus presently examined a possibility that neural processing speed in the dorsal pathway also represented a factor of intelligence. Specifically, we used the mental rotation (MR) task, a popular psychometric measure for mental speed of spatial processing in the dorsal pathway. We found that the speed of MR was significantly correlated with intelligence scores, while it had no correlation with one’s IT (recognition speed of visual objects). Our results support the new possibility that intelligence could be explained by two types of mental speed, one related to object recognition (IT) and another for manipulation of mental images (MR).

Strongly-motivated positive affects induce faster responses to local than global information of visual stimuli: an approach using large-size Navon letters

Recent studies argue that strongly-motivated positive emotions (e.g. desire) narrow a scope of attention. This argument is mainly based on an observation that, while humans normally respond faster to global than local information of a visual stimulus (global advantage), positive affects eliminated the global advantage by selectively speeding responses to local (but not global) information. In other words, narrowing of attentional scope was indirectly evidenced by the elimination of global advantage (the same speed of processing between global and local information). No study has directly shown that strongly-motivated positive affects induce faster responses to local than global information while excluding a bias for global information (global advantage) in a baseline (emotionally-neutral) condition. In the present study, we addressed this issue by eliminating the global advantage in a baseline (neutral) state. Induction of positive affects under this state resulted in faster responses to local than global information. Our results provided direct evidence that positive affects in high motivational intensity narrow a scope of attention.