THE INFLUENCE OF STIMULUS-INTENSITY AND INTER-STIMULUS INTERVAL ON THE DETECTION OF PITCH AND LOUDNESS CHANGES

This study illuminates processes underlying change detection for diffe rent features (detection of pitch versus loudness changes) and differe nt amounts of attentional allocation (automatic versus attentive chang e detection). For this reason, the influence of important stimulus cha racteristics (intensity and inter-stimulus interval (ISI)) on these di fferent types of change detection was determined. By varying intensity , it should be clarified whether these processes are mainly sensitive to the informational content of the change or to the total amount of s timulus energy. By varying ISI, it should be determined whether they a re differentially sensitive to manipulations of encoding time and/or s tate of sensory refractoriness. Automatic change detection was indexed by the mismatch negativity (MMN), which is a component of the event-r elated brain potential (ERP). Attentive change detection was indexed b y the N2b and P3 components of the ERP and by behavioral performance. Human subjects were presented with a high-probability standard tone an d a low-probability deviant-tone, which differed from the standard ton e in frequency (Experiment I) or intensity (Experiment II). In separat e blocks, the intensities of the standard stimuli were of 55 and 70 dB SPL and ISIs were of 350 and 950 ms. During the first part of the exp eriments, subjects were engaged in silent reading, whereas they tried to discriminate deviants from standards in the second part. The MMN el icited by a frequency change was invariant to variations in intensity and ISI, whereas the MMN elicited by an intensity change was significa ntly modulated by both intensity and ISI. This implies functional diff erences between the neural traces underlying the frequency MMN and the intensity-MMN. In addition, there were larger effects of the ISI on t he N2b and P3 amplitudes as compared with the effects on the MMN ampli tudes, suggesting stronger capacity limitations for attentive change d etection than for automatic change detection.