Proceedings paper

 

 

INSIGHTS INTO THE FUNCTIONAL ORGANIZATION OF MUSIC PROCESSING REVEALED USING CONTINUOUS ACROSS-SUBJECT EVENT-RELATED POTENTIAL AVERAGING

Douglas D. Potter (1,2), Helen Sharpe (1), Deniz Basbinar (1), Susan Jory (1)
(1) Keele University, UK (2) Now at University of Dundee, UK

d.d.potter@dundee.ac.uk
http://www.dundee.ac.uk/psychology/ddpotter/

Introduction

Recent functional imaging studies have shown that pitch and rhythm processing utilizes resources primarily in the left hemisphere and that timbre processing utilizes resources primarily in the right hemisphere . We report here preliminary findings using single trial across subject averaging of electrical brain activity associated with passive listening to unfamiliar pieces of music. The advantage of this technique is that one can look at unique and transient changes in the operation of perception, memory and attention mechanisms that are occurring over the time scale of seconds and minutes, but with a resolution of milliseconds.

Method.

In this study participants listened to 3 different and unfamiliar pieces of music while the ongoing electroencephalogram was recorded at 500hz from 19 standard 10/20 locations on the head. A continuous, single trial across subject, topographic map of activation (using an average reference) was generated from these continuous samples.

Results / Discussion

Only brief samples of the continuous recording are presented here. These can be viewed in the attached shockwave files y7s8, f8s4 and f9s4 by clicking on the links when using internet explorer 5 or another browser with shockwave capability. In simple terms the blue negative patches indicate areas of high negative potential and the red areas high positive potential. In general terms negative potentials often indicate sustained enhanced activation and positive potentials indicate transient inhibitory processes but such simple interpretations do not cover the full range of possibilities. In the present images blue and red are best treated as crude indicators of regions in which more activity is occurring or has occurred. Representative frames from these movies are illustrated below in Figures 1-3. It is clear from the movies that musical pieces with different structures evoke quite different patterns of activation. However there are common features in these patterns of activation that would be expected given the specific structural features of these stimuli. Both y7s8 and f9s4 have an abrupt onset to the music and in these movies the first prominent feature is a fronto-central P3a that is associated with the brain rapidly orienting attention to this new stimulus. In the case of f8s4 the piece starts slowly and so a P3a is not obvious in this recording. In all the movies a more posterior positive feature is observed following the P3a and this would typically be classed as a P3b. In the examples given here this positive feature is quite variable in distribution. This is a probably a result of the differing structure of the pieces. In standard experiments that evoke P3b deflections and involve averaging of several trials within subject, the distribution is relatively diffuse. The P3b is believed to be made up of a number of distributed sources in the cortex as well as the hippocampus. In the present results it appears that more evidence of multiple sources can be discerned possibly as a result of the trial unique nature of the response. The P3b deflection is generally regarded as marking the operation of certain long-term memory processes.

The next class of distinctive features in the movies are those that seem to be related to the processing of music. No specific manipulation of pitch, rhythm or timbre was made in the present study and comparison of regions of activation will be made with previous neuropsychological and imaging findings. Neuropsychological studies indicate an important role for the right hemisphere in timbre processing and recent imaging evidence highlights precentral and inferior frontal regions (BA 4,6) in particular . There is clear evidence in f8s4 and f9s4 of activity in this right frontal area.

In the present study there is evidence of 3 discrete loci of activation in the region of the left parietal / temporal / occipital (PTO) junction. This is most clearly seen in f8s4. This observation is consistent with PET imaging findings and is primarily attributed to analytical pitch processing. A further locus of activation on the right parieto/temporal/occipital junction can be seen most prominently in y7s8 but also in f8s4 and f9s4. This is associated most clearly with synchronized putative pitch processing activity in the region of the left PTO junction in f8s4. Activation of this region is consistent with the findings in passive pitch processing conditions . This pattern of bilateral activation in these posterior regions with a bias towards the left hemisphere is in accordance with recent PET observations of pitch processing . Zatorre and co-workers have reported activation in temporal and fronto-temporal regions in the processing of pitch. This apparent conflicting result can be resolved by considering the dynamic patterns of activation seen in the present study. In the movies presented here it can be seen that right frontal regions are active during some of the times that the posterior regions are active, suggesting that the areas associated with timbre processing participate in pitch processing. In addition Platel and co-workers note that these right frontal regions were also active during pitch and rhythm processing .

The one feature that is not well represented in the present data is the activation of more anterior left frontal regions (BA44/6) when attending to rhythmic structure in music . However in the musical piece with the strongest rhythm, f9s4, there is evidence of activation at more anterior locations on the left hemisphere. Again, the level of detail seen in these single trial averages appears to exceed that typically seen using more traditional brain electrical activity averaging techniques.

The final class of feature that can be seen is a distinct pattern of activation over lateral anterior frontal and ventral posterior regions. This is most clearly seen in y7s4. This pattern of activation may be consistent with PET functional imaging studies that show activation of frontal and cerebellar regions when subjects are unfamiliar with stimulus or task parameters or frontal activations associated with long term memory retrieval/encoding and working memory operation .

Conclusion

These are preliminary findings and much further work needs to be done to relate specific aspects of these patterns of brain electrical activity to more specific aspects of music processing and memory function. These initial findings do, nevertheless, seem to fit well with current PET studies of music processing. Continuous visualization of patterns of event-related potentials to each element in a continuous stimulus stream may offer significant advantages in trying to understand the dynamic interactions of regions of activation as well as dissociating the relative contribution of overlapping memory and attention processes. While Positron Emission Tomography and functional Magnetic Resonance Imaging have identified regions of the brain involved in specific aspects of music processing single trial across-subject averaging may allow the possibility of looking in far greater detail at the response properties of specific regions and may provide important insights into the functional properties of these regions.

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Figure 1. y7s8 Single-trial across-subject average (n=32) based on 19 standard 10-20 positions. Potential distribution is projected onto a 3-d model of head viewed from the rear. Example frames of main event related potential features observed during first 8 seconds in passive listening to music stimulus. Main features are central P3a at 440 msec, parietal P3b at 640 msec, right posterior occipito-parietal ?P3R? at 800/1720 msec and right ventral-occipital / left frontal activation in 800-3560 examples. Voltage Range +/- 10 microvolts.

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Figure 2. f8s4. Parameters are as stated in Figure 1, except that potential distribution projection is onto a convex surface that allows a view of the entire potential map. (n=20)

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Figure 3 f9s4. Parameters as stated in Figure 2. (n=18)

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