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.

BackgroundPrepulse inhibition (PPI) of the startle response is an important tool to investigate the biology of schizophrenia. PPI is usually observed by use of a startle reflex such as blinking following an intense sound. A similar phenomenon has not been reported for cortical responses.ResultsIn 12 healthy subjects, change-related cortical activity in response to an abrupt increase of sound pressure by 5 dB above the background of 65 dB SPL (test stimulus) was measured using magnetoencephalography. The test stimulus evoked a clear cortical response peaking at around 130 ms (Change-N1m). In Experiment 1, effects of the intensity of a prepulse (0.5 ~ 5 dB) on the test response were examined using a paired stimulation paradigm. In Experiment 2, effects of the interval between the prepulse and test stimulus were examined using interstimulus intervals (ISIs) of 50 ~ 350 ms. When the test stimulus was preceded by the prepulse, the Change-N1m was more strongly inhibited by a stronger prepulse (Experiment 1) and a shorter ISI prepulse (Experiment 2). In addition, the amplitude of the test Change-N1m correlated positively with both the amplitude of the prepulse-evoked response and the degree of inhibition, suggesting that subjects who are more sensitive to the auditory change are more strongly inhibited by the prepulse.ConclusionsSince Change-N1m is easy to measure and control, it would be a valuable tool to investigate mechanisms of sensory gating or the biology of certain mental diseases such as schizophrenia.

No abstract

Temporal regularity provides an important cue for the identification of natural sounds. Here, we measured auditory evoked cortical magnetic fields to investigate the neural processing of temporal regularity that cannot be tonotopically represented in the auditory periphery. Auditory steady state responses (ASSR) and sustained fields (SF) elicited by 40 Hz amplitude modulated periodic and non-periodic noises were analyzed. Periodic noises of 40-, 20-, and 5-Hz were prepared in the form of repeating frozen noises where the same noise segment appears at either each period (40 Hz), every second period (20 Hz), or every eighth period (5 Hz) of amplitude modulation. Compared to non-periodic white noises, periodic noises with repetition rates of 5-, 20-, and 40-Hz caused significantly increased SF amplitudes in both hemispheres. ASSR amplitudes were significantly enhanced for 20- and 40-Hz periodic noises in the right hemisphere while no enhancement was observed for periodic noises in the left hemisphere. The observed variation of the regularity effect between evoked response components and hemispheres may reflect the differences in the temporal integration window lengths adopted between ASSR and SF generators and also between the right and left auditory pathways.