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Publication Detail
Sensitivity to frequency-pattern violation - evidence for perceptual informational masking
  • Publication Type:
  • Authors:
    Barascud N, Chait M
  • Publication date:
  • Name of conference:
    International Conference on Auditory Cortex
  • Conference start date:
In this study, we use psychophysics and MEG functional brain imaging in order to assess listeners' ability to automatically 'learn' rapidly presented frequency patterns. Pattern learning is assessed by briefly exposing listeners to frequency-based sound patterns and measuring behavioural/brain responses to pattern violation. Specifically, our stimulus set consists of transitions between regularly alternating sequences of three short (60ms) tones (ABCABC...; new frequencies on each trial; duration between 1260 and 1440ms overall, i.e. the pattern is repeated 7 times before the transition) and a long (540ms) constant frequency tone. In the 'expected' (EXP) condition, the post transition frequency is expected based on the on-going frequency pattern. In this case, the change is a 'duration deviant', detectable from one pip-duration (60ms) after the transition. In the 'unexpected' condition (UN-EXP), the transition is to an unexpected frequency (always the frequency following the one that is expected: e.g. ...ABCB...). If the system is sensitive to frequency pattern violation, the change should be detectable once the post-transition frequency is extracted. Otherwise both kinds of transitions should be detected simultaneously. Importantly, in both conditions the transition was to a frequency that appeared equally often in the preceding pattern, such that the detection of pattern violation could not be based on adaptation. In the first magnetoencephalography (MEG) experiment (N=16), naïve subjects listened to EXP and UN-EXP stimuli while performing an unrelated decoy task. We demonstrate that brain responses to both stimuli are characterized by identical dynamics, except that the UN-EXP response occurs slightly earlier (~10ms). Albeit small, this latency difference is robust and withstands both average and single-trial based analyses, suggesting that the auditory cortex is sensitive to frequency pattern violations and that these are detected rapidly and automatically. On the behavioural side however, early pilot experiments indicated that listeners have difficulty in discriminating between EXP and UN-EXP (unable to indicate whether the post-transition, long pure-tone constituted a violation of the preceding frequency pattern) . Why are listeners unable to access this information' We hypothesized that this is due to perceptual masking: information about violation of frequency expectation is available briefly, but is subsequently perceptually masked by the second violation (change in tone duration). To test this hypothesis, we compared (with separate MEG and behavioural experiments) the original stimuli (termed 'LONG') with two other variants: one where the last tone was shortened to 60ms ('SHORT'), the other where the deviant tone was directly followed by a regular sequence ('CONTINUOUS': ...ABCBABC...). In the behavioural study listeners (N=22) had to determine whether the last tone was expected based on the on-going REG sequence (EXP) or violated the regularity pattern (UN-EXP) by pressing one of two keyboard keys (or in the CONTINUOUS case, whether they could detect a 'missing' tone in the sequence). The CONTINUOUS condition contains only a single violation and was hypothesized to be significantly more detectable. The SHORT condition contains two violations (frequency, and the subsequent cessation of the sequence) but since offset events are known to be less attention grabbing than onset events we hypothesized better performance on this condition relative to LONG. As expected, performance on CONTINUOUS was at ceiling. However, many participants could not perform either SHORT or LONG tasks. Importantly, those who could do the task (d'>0.5; 14 listeners) showed significantly better performance for SHORT than for LONG (p=0.01). 11/14 listeners presented this effect. Overall, our data support the 'perceptual masking' hypothesis. The auditory cortex can detect violations in frequency patterns rapidly and automatically. Information about the violation is available in Planum Temporale at about 200ms post violation. However, in certain cases such violations do not reach perceptual awareness due to 'perceptual masking' by other events which occur within the same sound stream but appear to be perceptually prioritized by the auditory system.
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