Petri Paavilainen

Auditory frequency discrimination and event-related potentials

Auditory stimulus blocks were presented to 6 subjects. 80% of the stimuli in each block were standards of 1000 Hz and 20% were deviants of either 1002 Hz, 1004 Hz, 1008 Hz, 1016 Hz or 1032 Hz, one deviant type in each block. The constant interstimulus interval was 1 sec and the order of the stimuli was randomized. The subject was instructed either to ignore the deviant stimuli (ignore condition) or to press a response key to them (discrimination condition). In the ignore condition, an ERP component called the mismatch negativity (MMN), with a peak latency of approximately 170 msec, was elicited by those deviants exceeding the discrimination threshold (1016 Hz and 1032 Hz) and also those at the threshold (1008 Hz) tended to elicit a small MMN. In the discrimination condition, in addition to MMN, another negative component, N2b, was elicited by the detected deviants. This component had a somewhat longer latency than, and its midline distribution was posterior to, the MMN. The present results are in line with the hypothesis according to which the MMN component reflects the activation of cerebral mechanisms of passive discrimination, those which cause us to become aware of occasional changes in unattended stimulus sequences. In the discrimination condition, N2b and the slow parietal positivity were dominant features of the ERPs elicited by the detected suprathreshold deviants. The data obtained at the discrimination threshold specifically associate the parietal positivity with becoming aware of stimulus change since those deviants which were detected elicited this positivity whereas there was none to those (physically identical) deviants which remained undetected.

Right hemisphere dominance of different mismatch negativities

Auditory stimulus blocks were presented to 10 reading subjects. Each block consisted of 2 types of stimulus, standard (P = 90%) and deviant (P = 10%), delivered in a random order with short constant inter-stimulus intervals. The standard stimuli were 600 Hz. 80 dB SPL 50 msec sine wave bursts. In different blocks, the deviant stimuli differed from the standards either in frequency (650 Hz), intensity (70 dB) or duration (20 msec). Left- and right-ear stimulations were used in separate blocks. Event-related brain potentials (ERPs) were recorded with 16 electrodes over both hemispheres. All the different types of deviant stimuli elicited an ERP component called the mismatch negativity (MMN). The MMN was larger over the right hemisphere irrespective of the ear stimulated whereas the N1 component, elicited by both standards and deviants, was larger over the hemisphere contralateral to the ear stimulated. The results provide further evidence for the view that the MMN reflects a neural mismatch process with a memory trace which automatically codes the physical features of the repetitive stimuli.

Mismatch negativity to change in spatial location of an auditory stimulus

Auditory stimulus blocks were presented to 12 reading subjects. Each block consisted of 2 types, standard (P = 90%) and deviant stimuli (P = 10%), delivered in a random order. The only difference between these stimuli was their spatial location of origin. The subject always heard the standards as coming straight in front and the deviants from an angle of either 10, 45, or 90 degrees to the right of the standards. The spatial locations were produced via earphones by introducing for low-frequency (600 Hz) tones an interaural phase difference and for high-frequency (3000 Hz) tones an interaural intensity difference. Standard and deviant stimuli were also delivered in more natural, free-field, conditions via differently positioned loudspeakers. The deviant tones elicited an event-related brain potential component called the mismatch negativity (MMN), followed by a P3a component. Thus changes in spatial location of an auditory stimulus produced by following either one of the two main principles of human sound localization elicited the MMN. Consequently, it was concluded that the spatial location of a sound source is coded in the hypothesized neuronal stimulus traces reflected by the MMN and, further, that a change in this location is automatically detected by the brain by means of the MMN generator process.

Frequency and location specificity of the human vertex N1 wave

Test tones of 1000 Hz subjectively located in the middle of the head were randomly presented with equiprobable intervening tones. The latter stimulus was constant within a stimulus block. The frequency of the intervening stimulus varied between different blocks from 578 Hz to 1728 Hz and its location varied in parallel with the frequency along the left-right dimension through 7 different locations. The constant inter-stimulus interval was 460 msec. The EEG was recorded at Cz and Fz. The N1 wave elicited by the test stimuli was smaller the smaller was the separation between the two stimuli in frequency or location. These results were interpreted in terms of stimulus-specifically adapted detector activity. The more the test and intervening stimuli resemble each other the greater is the overlap between the respective feature-detector populations activated and, therefore, the smaller is the N1 amplitude. Thus the sensory-specific component of the N1 wave generated in the primary auditory areas at least in part reflects detector activity. The selective frequency adaptation was much more specific in the present study than in the previous ones and suggested that the N1 component recorded was generated by highly frequency-specific neurons. The frequency and location effects were independent, i.e., the frequency effect was rather similar for different location separations, and vice versa. Thus, evidence for separate detectors for frequency and location of an auditory stimulus was obtained.

Representation of abstract attributes of auditory stimuli in the human brain

Representations of abstract attributes of auditory stimuli in the human brain were demonstrated using the mismatch negativity (MMN), an event-related potential component elicited by a change in a repetitive sound. Stimuli were pairs of sinusoidal tones. There were two types of tone pairs in each block, standard (p = 85%) and deviant pairs (p = 15%), delivered in a random order. Standard and deviant tone pairs differed only in the direction of within-pair frequency change. In addition, the frequency levels of both the standard and deviant pairs varied randomly within a wide range in a block; thus the standard pairs shared the direction of the within-pair frequency change but not the absolute frequency level. Correspondingly, the deviant pairs only shared the opposite direction of the within-pair change. Nevertheless, the deviant tone pairs elicited MMN, implying that even the direction of the within-pair frequency change of the standard stimuli, and not just their absolute frequencies, developed a neural representation.

Temporal integration of auditory information in sensory memory as reflected by the mismatch negativity

Event-related potentials (ERPs) to different types of infrequent change in a tone pair composed of two closely spaced tones of different frequencies were recorded. The mismatch negativity (MMN), a change-specific component of the ERP, was elicited by reversing the order of the two tones, by repeating the first tone, by replacing the first tone with the second tone, or by omitting the second tone. The omission of the second tone, however, elicited the MMN only when the interval between the two tones was very short (offset to onset 40 or 140 ms) but did not when this interval was somewhat longer (240 or 340 ms). The pattern of the present results suggests that sensory-memory traces, as reflected by the MMN, integrate information about two closely spaced stimuli into a unitary sensory event.

Preattentive extraction of abstract feature conjunctions from auditory stimulation as reflected by the mismatch negativity (MMN).

Brain mechanisms extracting invariant information from varying auditory inputs were studied using the mismatch-negativity (MMN) brain response. We wished to determine whether the preattentive sound-analysis mechanisms, reflected by MMN, are capable of extracting invariant relationships based on abstract conjunctions between two sound features. The standard stimuli varied over a large range in frequency and intensity dimensions following the rule that the higher the frequency, the louder the intensity. The occasional deviant stimuli violated this frequency-intensity relationship and elicited an MMN. The results demonstrate that preattentive processing of auditory stimuli extends to unexpectedly complex relationships between the stimulus features.

Event-related potentials to repetition and change of auditory stimuli

The major intent of this study was to compare the role of stimulus repetition and change in the elicitation of the MMN, an ERP component specific to stimulus change, and N2b, usually partially overlapping the MMN when stimuli are attended. Event-related potentials were recorded in one set of conditions where subjects ignored the stimuli and read a book, and in another set of conditions where subjects counted stimuli designated as targets. Stimuli were delivered in 4 ways, the common feature between all these conditions being the occurrence of infrequent events at a probability of 0.20: (1) an oddball paradigm with 1 deviant, (2) an oddball paradigm with 2 deviants, each with a probability of 0.10, (3) a regular alternation of tones of 2 pitches where either of the 2 tones infrequently repeated (P = 0.20), and (4) a random presentation of tones of 5 different pitches, where any of the 5 tones infrequently repeated (P = 0.20). In the count conditions, the infrequent events were designated as targets. It was found that the MMN was elicited by stimulus change and not stimulus repetition in the ignore and count conditions, whereas the N2b was elicited by both stimulus changes and repetitions in the count conditions. It was also possible, in the count conditions, to disentangle the part of the late positive complex which is related to stimulus deviation and the part which is related to stimulus significance (target).

Neural plasticity in processing of sound location by the early blind: an event-related potential study

Event-related potentials (ERPs) to a change in the locus of origin of a repetitive sound were studied in early blind human subjects. It was found that the N2b component of the ERP was posteriorly distributed on the scalp to that in the sighted control subjects. This suggests that the blind might use, to a larger extent than the sighted, parietal, or perhaps even occipital, brain areas in sound localization. The present results thus appear to demonstrate plastic changes in neural populations involved in processing of auditory space following early loss of vision.

Mismatch negativity and behavioural discrimination in humans as a function of the magnitude of change in sound duration.

This study investigated how duration changes are processed in the human brain as indexed by the mismatch negativity (MMN), a component of the auditory event-related potential (ERP) reflecting sensory memory. Subjects were presented with sequences of repetitive 100-ms white noise bursts interspersed by occasional duration deviants which were 1, 10, or 50 ms (decrements) or 110, 150, or 200 ms (increments) in duration. In a separate task, subjects were asked to detect deviants within the sequence via a button-press. MMN was elicited by both stimulus decrements and increments and increased in amplitude as a function of the amount of deviation from the standard duration except for the shortest, 1-ms deviant. Behavioural detection paralleled the MMN responses, suggesting a link between the processes underlying MMN and behavioural measures.