In daily life our brain is constantly exposed to a huge amount of sensory information which our perceptual system has to deal with. Due to limited processing capacities the sensory input has to be selected and structured, so that the brain becomes able to effectively cope with its environment. Selective attention is a key mechanism realizing these selection processes and has been intensively studied in the past Giard et al. (2000
). Accordingly, neuronal activity of the relevant stimuli or stimuli features needs to be enhanced and processing facilitated whereas activity of distracting components ought to be suppressed. Indeed, attention to auditory stimuli was shown to induce an amplification of neuronal activity elicited by relevant stimuli (Petkov et al., 2004
; Woldorff et al., 1993
) and an inhibition of neuronal activity related to irrelevant stimuli (Bidet-Caulet et al., 2007
) in auditory brain regions. Even though abundant evidence exists demonstrating the modulation of neuronal activity by attention in the auditory system, it is not clear at what level of the auditory system this happens. Empirical data from neuroimaging as well as electrophysiology has not been conclusive and only few studies were able to find involvement of auditory cortex (Bidet-Caulet et al., 2007
; Fujiwara et al., 1998
; Jäncke et al., 1999
). Thus, for a better understanding of the mechanisms involved in auditory attention, it remains interesting if and how primary and secondary auditory cortices are contributing.
An elegant way of studying attentional effects in the auditory cortex is examining the auditory steady state response (aSSR). aSSRs are evoked by rapid periodic stimulus sequences in contrast to transient evoked responses. Usually, aSSRs are elicited by sequences of clicks (Galambos et al., 1981
), amplitude modulated tones (Picton et al., 1987
) or tone pulses (Pantev et al., 1996
The generation of the aSSR was pinpointed to the auditory cortex. Studying this more precisely, it turned out that different parts of the auditory cortex are activated according to different modulation frequencies (Giraud et al., 2000
). Liégeois-Chauvel et al. (2004
) systematically studied the origin of steady-state responses generated by different AM frequencies: when looking at the left and right primary
auditory cortex the amplitude of the steady-state response decreases continuously with increased modulation frequencies. The power of aSSRs modulated by 16 Hz or more are marginal in the right secondary
auditory cortex. In the left secondary
auditory cortex, however, the major power decline starts at 30 Hz – pointing to an impact of the left secondary auditory cortex in the generation of aSSRs with frequencies below 30 Hz. These findings are in line with the intracranial studies conducted by Bidet-Caulet and colleagues who localized 21 and 29 Hz responses to primary and
secondary areas of the auditory cortex. Furthermore, magnetic source imaging studies pinpoint the origin of the 40 Hz aSSR to the primary
auditory cortex (Gutschalk et al., 1999
; Pantev et al., 1996
; Weisz et al., 2004
; Wienbruch et al., 2006
). Thus, steady-state responses of higher frequencies (gamma range) seem to be mainly generated in the primary auditory cortex whereas aSSRs of lower frequencies (<30 Hz) seem to have an origin in primary as well as secondary auditory cortex.
The analysis of the aSSR entails several advantages due to the characteristics of the resulting neuronal response: aSSRs closely follow the rhythm of the tone. Hence, knowing the modulation frequency, data analysis can be based on this predefined frequency. Since the noise is not phase-locked to the modulation frequency of the stimulus, averaging several responses leads to suppression of noise and thus to a strong signal-to-noise ratio. For MEG data in the auditory system it has been found that modulation frequencies around 40 Hz result in the strongest signal-to-noise ratio (Ross et al., 2000
). According to the original work of Galambos et al. (1981
), the steady-state response amplitude peaks between 15 and 20 Hz and again between 30 and 50 Hz with the major peak at 40 Hz. Thus, the signal-to-noise ratio and the power of steady-state responses vary with the modulation frequency.
A further advantage of the aSSR is that multiple auditory stimuli with different modulation frequencies can be presented simultaneously leaving different traces in the recorded signal at the respective modulation frequencies. This technique termed ‘frequency tagging’ has been successfully employed in auditory neuroscience such as binaural integration (Fujiki et al., 2002
), aversive conditioning (Weisz et al., 2007
) or auditory stream segregation (Bidet-Caulet et al., 2007
). This approach is especially interesting as in real life situations usually various auditory stimuli reach both ears at the same time, so that our brain has to focus on essential parts of the auditory information while ignoring distractor auditory stimuli. Using frequency-tagged stimuli, it is possible to simultaneously expose the auditory system to different tones and estimate the accordant power changes in dependence of the attentional load. Hence, for a better understanding of attentional processes in the primary and secondary auditory cortex, the investigation of if and how the aSSR is modulated by attention is essential.
Until now, little evidence exists in favour of an attention-mediated influence on the aSSR. In a pioneering EEG study, Linden et al. (1987
) have not been able to disclose an attentional impact on the amplitude of the aSSR (stimulus rates 37–41
Hz) despite a large variety of different employed paradigms. After this first authorative attempt, it took >20
years to show that it is indeed possible to modulate the 40
Hz aSSR by directed attention. In a MEG study, Ross et al. (2004
) found an enhancement of the aSSRs amplitude by attention in the left hemisphere, contralateral to the auditory stimulation. Though the work of Ross and colleagues represents a significant step concerning the investigation of the attentional affect on the aSSR, their results are only informative to some extent. Thus, in their experimental setting, the aSSR may be affected by attentional changes that are not specific to the processed information but could result from more general changes in arousal or alertness. Moreover, selective attention could not be investigated within the auditory modality as the control task merely required attention to the visual domain. Finally, as they exclusively stimulated monaurally hemispheric differences could not be derivated.
Recently, Bidet-Caulet et al. (2007
) did an illuminating study clarifying most of these open questions. Recording intracranial EEG in epilepsy patients they studied the mechanisms of selective attention in the primary auditory cortex. Their subjects were exposed to two competing auditory streams (stimulus rates 21 and 29
Hz) and had to indicate the spatial direction of one of these two streams. The authors found an enhancement of the aSSR elicited by the attended stream and a reduction for the ignored stream. Interestingly, these results were restricted to the left hemisphere while the findings in the right hemisphere were more ambiguous. In line with this, accumulating evidence demonstrated that the left hemisphere appeared to be more sensitive to attentional modulation than the right hemisphere (Bidet-Caulet et al., 2007
; Petkov et al., 2004
). Furthermore, Skosnik et al. (2007
) recently performed a study that investigated the impact of attention on 20 and 40
Hz responses. Click trains were presented binaurally in an oddball discrimination task and participants had to count targets (20% of the stimuli). When the 40
Hz clicks were defined as targets the 40
Hz responses were enhanced while the amplitude of the 20
Hz responses did not change at frontocentral electrodes. In contrast, when participants were attending the 20
Hz responses, no significant power changes were observed for none of the responses.
Based on these recent results, it becomes clear that the aSSR is indeed modifiable by attention contrary to former assumptions of an insusceptibility of the aSSR to attention. Thereby, the modulation frequency, kind of task, experimental design or hemispheric differences turned out to be crucial for the attentional modulation of the aSSR. Nevertheless, various questions, clarifying the complex pattern of aSSRs and attention, are still open: the role of contralateral and ipsilateral activations contributing to the changes in the aSSR amplitude by attention is not solved yet. Furthermore, the susceptibility of the aSSR to attention is likely to change according to different modulation frequencies. This is interesting with regard to the varying impact of primary and secondary auditory cortex in the generation of steady-state responses elicited by different modulation frequencies.
In the present study, subjects were exposed to tones modulated by 20 and 45
Hz which were delivered to the right and left ear simultaneously. Subjects were asked to attend to a cued ear. In this way, both hemispheres were activated at the same time and changes in the aSSR amplitude could be derivated. These changes in amplitude were exclusively due to whether the respective AM tone was attended or not. Thus, our experimental design allowed for studying auditory selective attention within the auditory system in a situation of sound rivalry and to scrutinize on a possibly different behaviour of ipsilateral and contralateral activations. Furthermore, a possibly different sensitivity of the two hemispheres to attentional processes could be investigated. Finally, we were able to study attentional differences of the 20 and 45
Hz responses what is especially informative with respect to the different generators of these two steady-state responses.