Neuroimaging paradigms for tonotopic mapping (II): The influence of acquisition protocol

Abstract

Numerous studies on the tonotopic organisation of auditory cortex in humans have employed a wide range of neuroimaging protocols to assess cortical frequency tuning. In the present functional magnetic resonance imaging (fMRI) study, we made a systematic comparison between acquisition protocols with variable levels of interference from acoustic scanner noise. Using sweep stimuli to evoke travelling waves of activation, we measured sound-evoked response signals using sparse, clustered, and continuous imaging protocols that were characterised by inter-scan intervals of 8.8, 2.2, or 0.0 s, respectively. With regard to sensitivity to sound-evoked activation, the sparse and clustered protocols performed similarly, and both detected more activation than the continuous method. Qualitatively, tonotopic maps in activated areas proved highly similar, in the sense that the overall pattern of tonotopic gradients was reproducible across all three protocols. However, quantitatively, we observed substantial reductions in response amplitudes to moderately low stimulus frequencies that coincided with regions of strong energy in the scanner noise spectrum for the clustered and continuous protocols compared to the sparse protocol. At the same time, extreme frequencies became over-represented for these two protocols, and high best frequencies became relatively more abundant. Our results indicate that although all three scanning protocols are suitable to determine the layout of tonotopic fields, an exact quantitative assessment of the representation of various sound frequencies is substantially confounded by the presence of scanner noise. In addition, we noticed anomalous signal dynamics in response to our travelling wave paradigm that suggest that the assessment of frequency-dependent tuning is non-trivially influenced by time-dependent (hemo)dynamics when using sweep stimuli.

Intracortical depth analyses of frequency-sensitive regions of human auditory cortex using 7T fMRI

“…This has lead to a pessimistic evaluation that AC properties are so complex that they cannot be reasonably averaged across the subjects. This may be partially explainable by the limited SNR of single-subject analyses of auditory fMRI (Langers and van Dijk, 2012),, which can be alleviated by optimizing the stimulation and acquisition parameters (Langers et al, 2014a; Langers et al, 2014b). A more fundamental issue, however, is that even the cytoarchitectonally defined AC area boundaries vary greatly across subjects in relation to volumetrically defined macroanatomical landmarks such as HG (Humphries et al, 2010; Rademacher et al, 1993; Rademacher et al, 2001).…”

“…While the present study was specifically concentrated on optimizing imaging parameters and data analysis, a number of other factors need to be optimized during AC frequency sensitivity studies (Langers et al, 2014a; Langers et al, 2014b). Auditory neurons tend to respond in a non-specific way for loud stimuli, even if they are tuned to a specific feature at low stimulus intensities.…”

“…Whereas recent studies provide exhaustive information on the strengths and weaknesses of various stimulation and statistical analysis approaches (Da Costa et al, 2011; Hertz and Amedi, 2010; Langers et al, 2014a; Langers et al, 2014b; Langers and van Dijk, 2012; Moerel et al, 2014; Seifritz et al, 2006; Striem-Amit et al, 2011; Talavage et al, 2004; Tanji et al, 2010), the present study specifically concentrates on addressing physiological and vascular anatomy confounds. To this end, we utilize a technique previously applied in visual cortices (Polimeni et al, 2010) to sample 1 mm isotropic fMRI voxels from all depths of AC and to exclude those intersecting the pial surface, in order to avoid the large draining vessels that bias the BOLD signal away from the actual sites of neuronal activity.…”

Population receptive field estimates of human auditory cortex

“…This “traveling wave” improves signal to noise: goodness-of-fit index values were higher for ascending/descending tone progressions than for random tone sequences . However these effects also result in an over-emphasis of frequencies near the beginning or the end of the sweep (Binda et al, 2013; Dumoulin and Wandell, 2008; Duncan and Boynton, 2003; Haak et al, 2012; Langers et al, 2014a; 2014b). We therefore do not report bandwidth estimates for the ascending/descending sequences as these effects can result in an overestimation of receptive field size, probably due to spatiotemporal blurring.…”

“…Thus, frequency progression stimuli, especially when analyzed using ‘winner take all’ methods, have the potential to result in an overrepresentation of frequencies near the beginning or the end of the sweep. Analogous concerns have been described for visual retinotopic mapping methods (Binda et al, 2013; Dumoulin and Wandell, 2008; Duncan and Boynton, 2003; Haak et al, 2012), and recently discussed for tonotopic mapping methods (Langers et al, 2014a; 2014b). …”