Tomokazu Urakawa

Cortical dynamics of the visual change detection process.

In this study, the cortical dynamics of the visual change detection process were investigated using an oddball paradigm similar to that used in auditory mismatch negativity studies. When subjects watched a silent movie, color stimuli were presented using 280 dual color LEDs arranged along the frame of the video screen. Task-irrelevant red and blue color stimuli were presented randomly at a probability of 10% and 90%, respectively, in one session and vice versa for the other one, and we traced brain responses using magnetoencephalography. Results show that activation in the middle occipital gyrus (MOG) was significantly enhanced for the infrequent stimulus, while early activities in Brodmanns area 17/18 were comparable for the frequent and infrequent stimuli. These results suggest that automatic visual change detection is associated with the MOG activity.

Non-linear laws of echoic memory and auditory change detection in humans

BackgroundThe detection of any abrupt change in the environment is important to survival. Since memory of preceding sensory conditions is necessary for detecting changes, such a change-detection system relates closely to the memory system. Here we used an auditory change-related N1 subcomponent (change-N1) of event-related brain potentials to investigate cortical mechanisms underlying change detection and echoic memory.ResultsChange-N1 was elicited by a simple paradigm with two tones, a standard followed by a deviant, while subjects watched a silent movie. The amplitude of change-N1 elicited by a fixed sound pressure deviance (70 dB vs. 75 dB) was negatively correlated with the logarithm of the interval between the standard sound and deviant sound (1, 10, 100, or 1000 ms), while positively correlated with the logarithm of the duration of the standard sound (25, 100, 500, or 1000 ms). The amplitude of change-N1 elicited by a deviance in sound pressure, sound frequency, and sound location was correlated with the logarithm of the magnitude of physical differences between the standard and deviant sounds.ConclusionsThe present findings suggest that temporal representation of echoic memory is non-linear and Weber-Fechner law holds for the automatic cortical response to sound changes within a suprathreshold range. Since the present results show that the behavior of echoic memory can be understood through change-N1, change-N1 would be a useful tool to investigate memory systems.

Cortical dynamics of visual change detection based on sensory memory

Detecting a visual change was suggested to relate closely to the visual sensory memory formed by visual stimuli before the occurrence of the change, because change detection involves identifying a difference between ongoing and preceding sensory conditions. Previous neuroimaging studies showed that an abrupt visual change activates the middle occipital gyrus (MOG). However, it still remains to be elucidated whether the MOG is related to visual change detection based on sensory memory. Here we tried to settle this issue using a new method of stimulation with blue and red LEDs to emphasize a memory-based change detection process. There were two stimuli, a standard trial stimulus and a deviant trial stimulus. The former was a red light lasting 500 ms, and the latter was a red light lasting 250 ms immediately followed by a blue light lasting 250 ms. Effects of the trial-trial interval, 250 approximately 2000 ms, were investigated to know how cortical responses to the abrupt change (from red to blue) were affected by preceding conditions. The brain response to the deviant trial stimulus was recorded by magnetoencephalography. Results of a multi-dipole analysis showed that the activity in the MOG, peaking at around 150 ms after the change onset, decreased in amplitude as the interval increased, but the earlier activity in BA 17/18 was not affected by the interval. These results suggested that the MOG is an important cortical area relating to the sensory memory-based visual change-detecting system.

Effects of prior sustained tactile stimulation on the somatosensory response to the sudden change of intensity in humans: an magnetoencephalography study

The rapid detection of sensory changes is important to survival. The change-detection system should relate closely to memory since it requires the brain to separate a new stimulus from past sensory status. To clarify effects of past sensory status on processing in the human somatosensory cortex, brain responses to an abrupt change of intensity in a train of electrical pulses applied to the hand were recorded by magnetoencephalography (MEG). In Experiment 1, effects of the magnitude of deviance (1.0, 0.5, 0.3, 0.2, and 0.1 mA) between conditioning and test stimuli were examined. In Experiment 2, effects of the duration of the conditioning stimulus (3, 1.5, 1.0, and 0.5 s) were examined. The abrupt change in stimulus intensity activated the contralateral primary (cSI) and secondary somatosensory cortex (cSII). The amplitude of the cSI and cSII activity was dependent on not only the magnitude of the change in intensity but also the length of the conditioning stimulus prior to the change, suggesting that storage of prior tactile information was involved in generating these responses. The possibility that an activity of onset (with no conditioning stimulus) would be involved in the change-related activity was also discussed.

Echoic memory of a single pure tone indexed by change-related brain activity

BackgroundThe rapid detection of sensory change is important to survival. The process should relate closely to memory since it requires that the brain separate a new stimulus from an ongoing background or past event. Given that sensory memory monitors current sensory status and works to pick-up changes in real-time, any change detected by this system should evoke a change-related cortical response. To test this hypothesis, we examined whether the single presentation of a sound is enough to elicit a change-related cortical response, and therefore, shape a memory trace enough to separate a subsequent stimulus.ResultsUnder a paradigm where two pure sounds 300 ms in duration and 800 or 840 Hz in frequency were presented in a specific order at an even probability, cortical responses to each sound were measured with magnetoencephalograms. Sounds were grouped to five events regardless of their frequency, 1D, 2D, and 3D (a sound preceded by one, two, or three different sounds), and 1S and 2S (a sound preceded by one or two same sounds). Whereas activation in the planum temporale did not differ among events, activation in the superior temporal gyrus (STG) was clearly greater for the different events (1D, 2D, 3D) than the same event (1S and 2S).ConclusionsOne presentation of a sound is enough to shape a memory trace for comparison with a subsequent physically different sound and elicits change-related cortical responses in the STG. The STG works as a real-time sensory gate open to a new event.

Neural basis of stable perception of an ambiguous apparent motion stimulus.

Although it has been shown that an alternative dominant percept induced by an ambiguous visual scene has neural correlates in various cortical areas, it is not known how such a dominant percept is maintained until it switches to another. We measured the primary visual response to the two-frame bistable apparent motion stimulus (stroboscopic alternative motion) when observers continuously perceived one motion and compared this with the response for another motion using magnetoencephalography. We observed a response component at around 160 ms after the frame change, the amplitude of which depended on the perceived motion. In contrast, brain responses to less ambiguous and physically unambiguous motions in both the horizontal and vertical directions did not evoke such a component. The differential response evoked by the bistable apparent motion is therefore distinct from directionally-selective visual responses. The results indicate the existence of neural activity related to establish and maintain one dominant percept, the magnitude of which is related to the ambiguity of the stimulus. This is in the line with the currently proposed idea that dominant percept is established in the distributed cortical areas including the early visual areas. Further, the existence of the neural activity induced only by the ambiguous image suggests that the competitive neural activities for the two possible percepts exist even when one dominant image is continuously perceived.

The development of the perception of facial emotional change examined using ERPs

OBJECTIVE The development of the perception of changes in facial emotion was investigated using event-related potentials (ERPs) in children and adults. METHODS Four different conditions were presented: (1) N-H: a neutral face that suddenly changed to a happy face. (2) H-N: reverse of N-H. (3) N-A: a neutral face that suddenly changed to an angry face. (4) A-N: reverse of N-A. RESULTS In the bilateral posterior temporal areas, a negative component was evoked by all conditions in younger children (7-10 years old), older children (11-14 years old), and adults (23-33 years old) within 150-300 ms. Peak latency was significantly shorter and amplitude was significantly smaller in adults than younger and older children. Moreover, maximum amplitude was significantly larger for N-H and N-A than H-N and A-N in younger children and for N-H than the other three conditions in adults. CONCLUSION The areas of the brain involved in perceiving changes in facial emotion have not matured by 14 years of age. SIGNIFICANCE Our study is the first to clarify a difference between children and adults in the perception of facial emotional change.

Temporal window of integration in the somatosensory modality: An MEG study

OBJECTIVE We sought to clarify the presence of a temporal window of integration (TWI) in the somatosensory modality by manipulating the inter-stimulus interval (ISI). METHODS We recorded cortical activity following the last of a train of electric pulses (stimulus offset) applied to the left hand in nine healthy volunteers using magnetoencephalography (MEG). Somatosensory evoked magnetic fields (SEFs) were elicited by the offset of a train of pulses 3s in total duration with four ISIs (25, 50, 75, and 100 ms). RESULTS Results show that (i) off-M100 was clearly elicited by the ISI-25 and 50 ms conditions but not ISI-75 and 100 ms conditions, and (ii) the generator for off-M100 was mainly located in the contralateral primary and secondary somatosensory cortex (SI and SII). CONCLUSION The upper limit of the TWI in the somatosensory modality is between 50 and 75 ms, and the contralateral SI and SII play an important role in integrating temporal information. SIGNIFICANCE The present study clarifies the presence of the TWI in the somatosensory modality.

VISUAL MOTION DIRECTION IS REPRESENTED IN POPULATION-LEVEL NEURAL RESPONSE AS MEASURED BY MAGNETOENCEPHALOGRAPHY

We investigated whether direction information is represented in the population-level neural response evoked by the visual motion stimulus, as measured by magnetoencephalography. Coherent motions with varied speed, varied direction, and different coherence level were presented using random dot kinematography. Peak latency of responses to motion onset was inversely related to speed in all directions, as previously reported, but no significant effect of direction on latency changes was identified. Mutual information entropy (IE) calculated using four-direction response data increased significantly (>2.14) after motion onset in 41.3% of response data and maximum IE was distributed at approximately 20 ms after peak response latency. When response waveforms showing significant differences (by multivariate discriminant analysis) in distribution of the three waveform parameters (peak amplitude, peak latency, and 75% waveform width) with stimulus directions were analyzed, 87 waveform stimulus directions (80.6%) were correctly estimated using these parameters. Correct estimation rate was unaffected by stimulus speed, but was affected by coherence level, even though both speed and coherence affected response amplitude similarly. Our results indicate that speed and direction of stimulus motion are represented in the distinct properties of a response waveform, suggesting that the human brain processes speed and direction separately, at least in part.

Effects of stimulus field size and coherence of visual motion on cortical responses in humans: An MEG study

Among various kinds of visual motion, wide field coherent visual motion should have characteristic physiological significance regarding the relationship between the external world and us. To detect veridical visual motion in the surrounding environment, specific mechanisms are necessary to differentiate it from the wide field coherent motion due to ones own movement. To disclose whether and how the neural process of wide field coherent motion is different from that of other motions, we measured cortical responses to visual motions in humans using magnetoencephalography (MEG) manipulating both field size and coherence. Results showed that an increase in field size enhanced the response at sensors around the parieto-occipital area, and that the difference in activity between coherent and incoherent motion tended to be larger for the wide field. These findings suggest that wide field coherent and incoherent motion is detected differently at least in part in the parieto-occipital area, and suggest the neural process of wide field coherent motion could be pronouncedly tapped by a combination of field size and coherence.