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Responses of neurons in the marmoset primary auditory cortex to interaural level differences: comparison of pure tones and vocalizations.

Lui LL, Mokri Y, Reser DH, Rosa MG, Rajan R - Front Neurosci (2015)

Bottom Line: ILD sensitivity was heavily dependent on ABL: changes in ABL by ±20 dB SPL from the optimal level for ILD sensitivity led to significant decreases in ILD sensitivity for all stimuli, although ILD sensitivity to pure tones and Ock calls was most robust to such ABL changes.Our results demonstrate differences in ILD coding for pure tones and vocalizations, showing that ILD sensitivity in A1 to complex sounds cannot be simply extrapolated from that to pure tones.They also show A1 neurons do not show level-invariant representation of ILD, suggesting that such a representation of auditory space is likely to require population coding, and further processing at subsequent hierarchical stages.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, Monash University Clayton, VIC, Australia ; Australian Research Council, Centre of Excellence for Integrative Brain Function, Monash University Clayton, VIC, Australia.

ABSTRACT
Interaural level differences (ILDs) are the dominant cue for localizing the sources of high frequency sounds that differ in azimuth. Neurons in the primary auditory cortex (A1) respond differentially to ILDs of simple stimuli such as tones and noise bands, but the extent to which this applies to complex natural sounds, such as vocalizations, is not known. In sufentanil/N2O anesthetized marmosets, we compared the responses of 76 A1 neurons to three vocalizations (Ock, Tsik, and Twitter) and pure tones at cells' characteristic frequency. Each stimulus was presented with ILDs ranging from 20 dB favoring the contralateral ear to 20 dB favoring the ipsilateral ear to cover most of the frontal azimuthal space. The response to each stimulus was tested at three average binaural levels (ABLs). Most neurons were sensitive to ILDs of vocalizations and pure tones. For all stimuli, the majority of cells had monotonic ILD sensitivity functions favoring the contralateral ear, but we also observed ILD sensitivity functions that peaked near the midline and functions favoring the ipsilateral ear. Representation of ILD in A1 was better for pure tones and the Ock vocalization in comparison to the Tsik and Twitter calls; this was reflected by higher discrimination indices and greater modulation ranges. ILD sensitivity was heavily dependent on ABL: changes in ABL by ±20 dB SPL from the optimal level for ILD sensitivity led to significant decreases in ILD sensitivity for all stimuli, although ILD sensitivity to pure tones and Ock calls was most robust to such ABL changes. Our results demonstrate differences in ILD coding for pure tones and vocalizations, showing that ILD sensitivity in A1 to complex sounds cannot be simply extrapolated from that to pure tones. They also show A1 neurons do not show level-invariant representation of ILD, suggesting that such a representation of auditory space is likely to require population coding, and further processing at subsequent hierarchical stages.

No MeSH data available.


Related in: MedlinePlus

Percentage of cells sensitive to ILD and ABL for each stimulus type. The total percentages of cells sensitive to ILD and to ABL (including interaction effect) for each stimulus are shown in the four sub-plots. The color code indicates whether cells are sensitive to only one variable, or have a significant interaction effect. Cells are considered to be sensitive to ILD if there is a significant main effect for ILD and/or a significant interaction of ILD and ABL (Two-Way ANOVA, p < 0.01). The corresponding rule applies to ABL. These categories are not mutually exclusive, i.e., a cell can be sensitive to both ILD and ABL and therefore would contribute to both bars; however, each cell can only contribute to a single bar only once, i.e., cells with interaction effects will appear in gray regardless of main effect(s).
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Figure 4: Percentage of cells sensitive to ILD and ABL for each stimulus type. The total percentages of cells sensitive to ILD and to ABL (including interaction effect) for each stimulus are shown in the four sub-plots. The color code indicates whether cells are sensitive to only one variable, or have a significant interaction effect. Cells are considered to be sensitive to ILD if there is a significant main effect for ILD and/or a significant interaction of ILD and ABL (Two-Way ANOVA, p < 0.01). The corresponding rule applies to ABL. These categories are not mutually exclusive, i.e., a cell can be sensitive to both ILD and ABL and therefore would contribute to both bars; however, each cell can only contribute to a single bar only once, i.e., cells with interaction effects will appear in gray regardless of main effect(s).

Mentions: As demonstrated by the examples shown in Figure 2, the pattern of ILD sensitivity for each stimulus type almost always depended on ABL. This is summarized for our population of cells in Figure 4; each subpanel presents data for one stimulus type. For all stimuli, the majority of cells were sensitive to ILD (first bar in each subpanel). However, the proportion of ILD-sensitive units was always marginally lower than that of cells sensitive to ABL (second bar in each subpanel), and more importantly, the majority of ILD-sensitive cells showed a significant interaction with ABL (the gray segment of each bar in these subpanels) irrespective of stimulus. This is exemplified by Cell C in Figure 2: its sensitivity (or the lack thereof) always depended on the ABL for every stimulus to which it was responsive. Cells that showed an interaction effect represented a large majority of the total cells tuned for ILD (76–81%); these percentages were not different between stimuli [χ2(3) = 0.02, p > 0.05; Figure 4]. The percentage of cells that showed an interaction between ILD and ABL was about 50–60% across all stimuli (gray bars in Figure 4), and this difference corresponded to the total number of cells that were sensitive to ILD.


Responses of neurons in the marmoset primary auditory cortex to interaural level differences: comparison of pure tones and vocalizations.

Lui LL, Mokri Y, Reser DH, Rosa MG, Rajan R - Front Neurosci (2015)

Percentage of cells sensitive to ILD and ABL for each stimulus type. The total percentages of cells sensitive to ILD and to ABL (including interaction effect) for each stimulus are shown in the four sub-plots. The color code indicates whether cells are sensitive to only one variable, or have a significant interaction effect. Cells are considered to be sensitive to ILD if there is a significant main effect for ILD and/or a significant interaction of ILD and ABL (Two-Way ANOVA, p < 0.01). The corresponding rule applies to ABL. These categories are not mutually exclusive, i.e., a cell can be sensitive to both ILD and ABL and therefore would contribute to both bars; however, each cell can only contribute to a single bar only once, i.e., cells with interaction effects will appear in gray regardless of main effect(s).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4403308&req=5

Figure 4: Percentage of cells sensitive to ILD and ABL for each stimulus type. The total percentages of cells sensitive to ILD and to ABL (including interaction effect) for each stimulus are shown in the four sub-plots. The color code indicates whether cells are sensitive to only one variable, or have a significant interaction effect. Cells are considered to be sensitive to ILD if there is a significant main effect for ILD and/or a significant interaction of ILD and ABL (Two-Way ANOVA, p < 0.01). The corresponding rule applies to ABL. These categories are not mutually exclusive, i.e., a cell can be sensitive to both ILD and ABL and therefore would contribute to both bars; however, each cell can only contribute to a single bar only once, i.e., cells with interaction effects will appear in gray regardless of main effect(s).
Mentions: As demonstrated by the examples shown in Figure 2, the pattern of ILD sensitivity for each stimulus type almost always depended on ABL. This is summarized for our population of cells in Figure 4; each subpanel presents data for one stimulus type. For all stimuli, the majority of cells were sensitive to ILD (first bar in each subpanel). However, the proportion of ILD-sensitive units was always marginally lower than that of cells sensitive to ABL (second bar in each subpanel), and more importantly, the majority of ILD-sensitive cells showed a significant interaction with ABL (the gray segment of each bar in these subpanels) irrespective of stimulus. This is exemplified by Cell C in Figure 2: its sensitivity (or the lack thereof) always depended on the ABL for every stimulus to which it was responsive. Cells that showed an interaction effect represented a large majority of the total cells tuned for ILD (76–81%); these percentages were not different between stimuli [χ2(3) = 0.02, p > 0.05; Figure 4]. The percentage of cells that showed an interaction between ILD and ABL was about 50–60% across all stimuli (gray bars in Figure 4), and this difference corresponded to the total number of cells that were sensitive to ILD.

Bottom Line: ILD sensitivity was heavily dependent on ABL: changes in ABL by ±20 dB SPL from the optimal level for ILD sensitivity led to significant decreases in ILD sensitivity for all stimuli, although ILD sensitivity to pure tones and Ock calls was most robust to such ABL changes.Our results demonstrate differences in ILD coding for pure tones and vocalizations, showing that ILD sensitivity in A1 to complex sounds cannot be simply extrapolated from that to pure tones.They also show A1 neurons do not show level-invariant representation of ILD, suggesting that such a representation of auditory space is likely to require population coding, and further processing at subsequent hierarchical stages.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, Monash University Clayton, VIC, Australia ; Australian Research Council, Centre of Excellence for Integrative Brain Function, Monash University Clayton, VIC, Australia.

ABSTRACT
Interaural level differences (ILDs) are the dominant cue for localizing the sources of high frequency sounds that differ in azimuth. Neurons in the primary auditory cortex (A1) respond differentially to ILDs of simple stimuli such as tones and noise bands, but the extent to which this applies to complex natural sounds, such as vocalizations, is not known. In sufentanil/N2O anesthetized marmosets, we compared the responses of 76 A1 neurons to three vocalizations (Ock, Tsik, and Twitter) and pure tones at cells' characteristic frequency. Each stimulus was presented with ILDs ranging from 20 dB favoring the contralateral ear to 20 dB favoring the ipsilateral ear to cover most of the frontal azimuthal space. The response to each stimulus was tested at three average binaural levels (ABLs). Most neurons were sensitive to ILDs of vocalizations and pure tones. For all stimuli, the majority of cells had monotonic ILD sensitivity functions favoring the contralateral ear, but we also observed ILD sensitivity functions that peaked near the midline and functions favoring the ipsilateral ear. Representation of ILD in A1 was better for pure tones and the Ock vocalization in comparison to the Tsik and Twitter calls; this was reflected by higher discrimination indices and greater modulation ranges. ILD sensitivity was heavily dependent on ABL: changes in ABL by ±20 dB SPL from the optimal level for ILD sensitivity led to significant decreases in ILD sensitivity for all stimuli, although ILD sensitivity to pure tones and Ock calls was most robust to such ABL changes. Our results demonstrate differences in ILD coding for pure tones and vocalizations, showing that ILD sensitivity in A1 to complex sounds cannot be simply extrapolated from that to pure tones. They also show A1 neurons do not show level-invariant representation of ILD, suggesting that such a representation of auditory space is likely to require population coding, and further processing at subsequent hierarchical stages.

No MeSH data available.


Related in: MedlinePlus