Introduction To Auditory Cognitive Neuroscience

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Temporal Lobe

Back 1

– Auditory, speech processing

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• White matter:

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Long-range mostly myelinated axons –
facilitating global communcation.

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• Grey matter:

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Local networks of cell bodies and dendrites
and mostly unmyelinated local axons.

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Herscl's Gyrus**

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Individual temporal lobes, with primary auditory cortex shaded.

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Phrenology:

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a 19thC pursuit of assigning specific personality traits and functions to specific brain “organs”.
It was assumed that these could be described by
measuring skull shape and size.

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Brain lesions can occur from:

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• Trauma (accidents, etc.)
• Stroke
– Results from such patients can be very informative, but damage is often poorly localized, making specific conclusions difficult.

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A “gold standard” for cognitive neuropsychology.

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• Double dissociations help to make a case for specificity
– Subject 1 performs well at task A but poorly at task B.
– Subject 2 performs well at task B but poorly at task A.
• Double dissociations have become what?

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• Broca’s and Wernicke’s areas

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Speech production and perception, respectively. Both in left hemisphere.

*deficits in musical tests after removal of right temporal lobe.

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Left-right distinction no longer so clear

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• Music processing not confined to right hemisphere, even for individuals without musical training.
• Some evidence for modality specific processing, with rhythm dominated more by left hemisphere, and melody by right

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• Patient IR, studied by Peretz and colleagues (Montreal), showed severe music impairments following damage to both temporal lobes and right frontal lobe.

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Results:

No effect on verbal abilities, and was still able to distinguish emotional content of music.

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• Damage to temporal lobes can affect pitch-contour and/or pitch-interval perception and tune agnosia (inability to name previously familiar tunes).

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• Conversely, left hemisphere damage can affect speech while leaving music unimpaired

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Limitations of Lesion-based studies

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By definition, these brains are not normal. Effects of plasticity may have altered the normal place-function relations.
• Effects can be difficult to localize precisely

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PET – Positron Emission Tomography

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• Measures cerebral blood flow using a (weak) radioactive tracer that is injected into the blood stream.

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(+) PET

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• Good spatial resolution
(images usually superimposed
on MRI images).

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(-) PET

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• Poor temporal resolution

• Limit to number of repetitions a subject can do (because of radioactive tracer).

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fMRI – Functional Magnetic Resonance
Imaging

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• Measures blood oxygenation level differences

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Pros of fMRI:

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• Less invasive than PET – no radioactive (or other) injection needed.
• Excellent spatial resolution.

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Cons of fMRI:

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• Very noisy – levels during each scan can exceed 100 dB SPL.
• Poor temporal resolution –seconds.

*fMRI can only ever measure differences
between conditions: baselines are too
variable across subjects and time to make
absolute measures meaningful.

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Addressing challenges to using fMRI for auditory experiments

Clustered image acquisition:

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A method for removing the impact of scanner acoustic noise on auditory fMRI activation

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• Voxel-based Morphometry:

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Assesses anatomical (structural) aspects, using slight differences in magnetic properties of white and grey matter in the brain.

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Diffusion Tensor Imaging (DTI):

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Used to identify tracts and connectivity within cortex. Measures direction of water diffusion, taking advantage of the fact that axon bundles, and their myelin sheaths facilitate the diffusion
of water molecules preferentially along their main direction. May be useful in identifying circuits and pathways within cortex.

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Resting state fMRI

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Correlations in spontaneous fluctuations suggest functional connectivity.

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EEG and MEG
Electro- and Magneto-encephalography

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• Measures of electrical or magnetic fields
generated within the brain.

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Pros of EEG and MEG

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• Excellent temporal resolution (milliseconds).
• Quiet – more suitable for auditory experiments.

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Cons of EEG and MEG:

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• Poorer spatial resolution.

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Recent Trend in Brain Imaging

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Combining spatial resolution of (f)MRI with
temporal resolution of EEG/MEG.

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EEG

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• Measures summed activity of at least 1,000,000 neurons under each cm2 of scalp.
• Population response.
• “Wave” measured in response to stimulus

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Auditory evoked potentials (AEPs)

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• “Automatic” responses found in Auditory brainstem response and early-latency
waves.
• Later waves depend more on attention and task
performance.