Neurobiology of Hearing (Salamanca, 2012) Auditory Cortex (2) Prof. Xiaoqin Wang

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1 Neurobiology of Hearing (Salamanca, 2012) Auditory Cortex (2) Prof. Xiaoqin Wang Laboratory of Auditory Neurophysiology Department of Biomedical Engineering Johns Hopkins University web1.johnshopkins.edu/xwang

2 Outline Lecture 1: Tonotopic organization and stimulus selectivity a) Anatomical structure of the mammalian auditory cortex b) Tonotopic organization of auditory cortex c) Firing patterns and tuning to preferred stimulus Lecture 2: Temporal processing a) Coding of time-varying signals b) Temporal-to-rate transformation in A1 c) Temporal-to-rate transformation outside A1 Lecture 3: Spectral and intensity processing a) Spectral processing b) Intensity processing Lecture 4: Spatial and auditory-feedback processing a) Spatial processing b) Auditory-feedback processing

3 Temporal processing: How does auditory cortex represent timevarying signals? 2-1

4 Spectral and Temporal Characteristics of Speech! Spectrum! Formant" Temporal! Fine structure ( carrier )" Coarse structure ( envelop )" 2-2

5 Reduction of spike timing precision in CN: Fine temporal structure AN Synchronization Index CN (primary-like cell) AN CN (chopper cell) (Blackburn and Sachs, 1989)

6 Reduction of spike timing precision in CN: Envelope CN (primary-like cell) AN CN (chopper cell) Modulation Frequency (Hz) Modulation Frequency (Hz) 2-4 Rohde and Greenberg (1994)

7 Further reduction of spike timing precision in IC: Low-pass to band-pass Modulation Frequency (Hz) 2-5 Langner and Schreiner (1988)

8 Thalamus (MGB) spiking faster than auditory cortex (AC) AC AC 10 Hz MGB MGB Increasing modulation frequency AC AC MGB MGB AC AC 140 Hz MGB MGB 2-6 Creutzfeldt et al. (1980)

9 Modulation Frequency Selectivity Independent of Spectral Contents rbmf tbmf Max sync. frequency 2-7 Liang et al. (2002)

10 Stimulus-locked responses are progressively slowed down along ascending auditory pathway Auditory cortex Slow Fast Thalamus Inferior colliculus Synchronization Index (Vector strength) Auditory cortex Thalamus Inferior colliculus Cochlear nucleus Auditory nerve Cochlear nucleus Auditory nerve (Characteristic Frequency) 2-8 (AN: Johnson, 1980, CN: Blackburn and Sachs, 1990)

11 Distributions of tbmf in different auditory cortical areas of the cat R C Imig and Reale (1980) tbmf: R f0 +R 2f0 2-9 Schreiner and Urbas (1988)

12 Awake versus Anesthetized Condition 2-10 Goldstein, Kiang and Brown (1959)

13 Click trains produce largely synchronized discharges in anesthetized condition Anesthetized Cat A1 ~ 80% ~ 20% 2-11 Lu and Wang (2000)

14 Click trains produce both synchronized and non-synchronized discharges in awake condition Stimulus Synchronized Responses! Non-Synchronized Responses! 2-12 Lu et al. (2001)

15 Synchronized and non-synchronized responses are also observed for dynamically changing stimuli Synchronized Responses Non-Synchronized Responses 2-13 Lu, Liang, Wang (Nature Neurosci., 2001)

16 Cortical Representations in Different Time Scales: A two-stage mechanism Perceptual boundary of discreteness VOT of speech Lower limit of pitch 2-14 Lu et al. (2001)

17 Greater extent of sustained firing in auditory cortex than in MGB MGB A Bartlett and Wang (J. Neurophysiol., 2007)

18 Progressive increase of non-synchronized firing from brainstem to cortex 2-16 Bartlett and Wang (J. Neurophysiol., 2007)

19 Summary: A1 representations to sequential events In awake condition: Synchronized population can explicitly represent slowly occurring sound sequences by their temporal discharge patterns. Non-synchronized population can implicitly represent rapidly occurring sound sequences by their firing rate. In anesthetized condition: Discharge synchronization rates are lower than those observed in awake condition. Non-synchronized responses are largely absent. 2-17

20 Why are auditory responses slowed down in cortex? For the purpose of multi-sensory integration. Other sensory systems (visual, tactile) are much slower at the periphery, but discharge synchrony rates are similar across sensory cortex. This slow-down allows auditory information to be integrated with information from other sensory modalities at the same clock rate. For the purpose of auditory object processing which requires temporal integration over longer time windows. 2-18

21 How does auditory cortex process sound sequences? Cortical processing of sound streams operates on a segment-by-segment basis rather than on a moment-by-moment basis as found in the auditory periphery Auditory cortex neurons mark sparse acoustic events (or onsets) with precise spike timing and transform rapidly occurring acoustic events into firing rate-based representations. 2-19

22 What else can firing rate-based representations encode? (There are always surprises ) 2-20

23 Temporal-to-rate transformation at longer time scales Non-synchronized (Negative monotonic) Synchronized Nonsynchronized (Positive monotonic) Lu et al. (Nat Neurosci. 2001) 2-21

24 Stimulus synchronization in the range of acoustic flutter 2-22 Bendor and Wang (Nature Neuroscience 2007)

25 Firing rate-based representations of acoustic flutter 2-23 Bendor and Wang (Nature Neuroscience 2007)

26 Non-synchronizing firing also encodes low repetition rates Flutter Pitch Non-syncrhonized responses ( Lu et al. 2001) 2-24 Bendor and Wang (Nat Neurosci. 2007)

27 Transformation of firing rate-based representations from A1 to rostral fields 2-25 Bendor and Wang (Nature Neuroscience 2007)

28 Spectral/temporal integration pathways in primate auditory cortex 2-26 Bendor and Wang (J. Neurophysiology, 2008)

29 What is the implication of these experiments? Non-synchronized responses are the results of temporal-to-rate transformations and represent processed (instead of preserved) information. The auditory cortex transforms stimulus features into internal representations that are no longer faithful replicas of acoustic dimensions. 2-27

30 Challenges in understanding cortical processing of acoustical information: 1) Transformation from isomorphic (faithful) to nonisomorphic representation of acoustic signals 2) Transformation from acoustical to perceptual dimension 2-28

31 Suggested readings: Temporal processing: 1. Lu, T., L. Liang and X. Wang. Temporal and rate representations of time-varying signals in the auditory cortex of awake primates. Nature Neuroscience, 4: , (2001). 2. Joris PX, Schreiner CE, Rees A. Neural processing of amplitude-modulated sounds. Physiol Rev (2004) 3. Bendor, D.A. and X. Wang. Differential neural coding of acoustic flutter within primate auditory cortex. Nature Neuroscience 10: (2007). 4. Wang, X., T. Lu, D. Bendor and E.L. Bartlett. Neural coding of temporal information in auditory thalamus and cortex. Neuroscience 154: (2008)