Defining Suppressive Interactions: Methodology

Defining the null and experimental hypothesis

Since the present paper is devoted to methods of analyzing cochlear non-linearity, it is necessary to define the preferred method that should be used in order to achieve efficient results of the experiment. The issue is to define two-tone suppression with the help of a practical experiment. Houtgast (1974) and Duifhuis (1976) agreed that psychoacoustical suppression can only be shown if the signal is not presented simultaneously to the same ear to which supressee and suppressor are presented (this is the null hypothesis for the present paper). In the present context there are two types of defining the suppression in the described way – through forward masking and pulsation threshold method. The task of the present paper is to demonstrate the experiment conducted by Duifhuis, which is the investigation of interrelations of the effect and alterations of the suppressor and suppressee in case of the probe signal interference as well as deviations of their ratio under different combinations of these ratio variables:

If a small probe signal is presented simultaneously with the suppressee, then both will be suppressed by the suppressor. The ratio of probe and suppressee is thus left unaffected. Since the masked threshold of the probe happens to be determined largely by this ratio, the suppression effect does not show up. In the case of non-simultaneous masking, only the suppressee undergoes suppression, and the probe is then unaffected (Duifhuis, 1980: 915).

Judging from the conditions described by Duifhuis, it is reasonable to assume that the experimental hypothesis should sound as follows: using non-simultaneous representation of the probe signal and the suppressor together with the suppressee it will be possible to demonstrate psychoacoustical suppression. The paper will deal primarily on the auditory non-linearity and will define the peculiarities of two-tone suppression in a more particular way.

Description of subjects, inclusion criteria and explanation of the rationale for selecting the number of subjects that were tested

The chief inclusion criteria to be used is the subjects’ having normal audiograms, since it has been proved that two-tone suppression may be witnessed only with the normal, not damaged ear. The subjects were chosen among the students of our university, being participants on a legal basis and gaining credits for their studies due to the participation in the experiment. They all took part in the experiment on a voluntary basis and participated for quite a long period of time to ensure credibility of results.

The number of participants is limited to 10 because of the individual peculiarities of suppression shown in every case that cannot be averaged, thus should be represented and analyzed individually.

Description of equipment

First of all, it is necessary to mention the place where all experiments took place – it was a sound-proof booth to ensure that no more sounds would interfere with the process. Secondly, the equipment through which the sounds were transformed to the ear of the participant should be mentioned – following the experiment of Duifhuis, headphones KOSS/600 AA and Pioneer SE 700 were used to accomplish the present experiment. Calibration was based on a B&K artificial ear type 4153 of the defined headphones, for this reason all levels were given in SPL (Duifhuis, 1980: 915).

One more piece of equipment to be used in the present experiment is the computer on which the listener had to choose the answers ‘yes’ or ‘no’ in the process of the fulfilling the prescribed task. The model of the computer is irrelevant since its characteristics could in any way influence the results of the experiment.

Description of the procedure and task

The procedure included seating the listener in the sound-proof booth and started the series of signals him-/herself. There were two different schemes on which the experiment took place, referring to the project proposed by Duifhuis (1980). Model 1 included 10 series of stimuli that were initiated by the listener. However, in the process of the experiment Duifhuis found a more comfortable method of conducting the experiment – during the experiment the pulsed masker stimulus, i.e. the one of the suppressor and suppressee, was repeated for an undefined period and was manually started and stopped by the participant of the experiment. However, the frequency of the signal was chosen by the creators of the experiment, so it appeared once during three consecutive cycles out of every eight (Duifhuis, 1980).

Having investigated the matter, we have decided to refer to the second mode of conducting the experiment at once. Listeners will be tested individually while seated in a Sound proof booth. The Listener’s main task is to observe a computer screen in front of them. The listener himself got the subjective responsibility to control the ongoing experiment by choosing to press “Yes” or “No”. Pulsation threshold method as the preferred one for organizing the present experiment facilitated the participant’s perception of the probe signal, being continuously repeated and thus being easier for the participant’s to focus on.

The choice of the second mode of conducting the experiment may be viewed as follows:

Figure 1. The mode initially used by Duifhuis in his experiment.

Figure 2. The mode used by Duifhuis later on and chosen for the present experiment.

The figures shown above explain why the second method is more suitable for the present work – it affords the participant of the experiment better distinguish the probe signal because of its continued repetition.

The next step in the present methodology is to define the major concepts which will become the guidance for the completion of the project and analyzing its results. They are the suppressor, the suppressee, the two-tone suppression, pulsation threshold method and the rationale for preferring this method to forward masking.

Suppressor

Hawkins (1995) explains the concept of suppressor as the second tone introduced wile testing the response to one probe tone. If in such a situation the response to the probe is reduced, the result is referred to as two-tone suppression.

Brian Moore (2003) in his work An Introduction to the physiology of hearing explicitly gave out the mechanism of activity of the suppressor in the relation to the listener’s perception of the suppressed tone. He states that the suppressor causes the reduction of influence of the suppressed sound on the pattern of phase locking, thus resulting in the response of the neuron solely to the suppressor. The effect of the suppressor on phase locking is evident even in the excitatory area of the neuron. The author admits that the suppressor causes a response of excitatory character, producing an “increase in firing rate”. Nevertheless, he mentions one case in which it does not happen – “when the suppressor is outside or at the edges of the response area” (Moore, 2003: 47).

Robinette and Glattke (2007) also investigated the concept of the suppressor in detail and dealt with its influence on the probe signal’s perception by the listener. They state that the suppressor

can be recorded by introducing a suppressor stimulus into the same, opposite, or both ears…When suppression is studied while presenting a suppressor stimulus to the same ear from which TEOAE is recorded it is not possible to present the emission-evoking stimulus and the suppressor stimulus simultaneously without producing unwanted interactions between the OAE evoking stimulus and the suppressor stimulus that is used to elicit the efferent effect (299).

It should be noted from the following citation that TEOAE stands for transient evoked otoacoustic emission, and OAE stands for plainly otoacoustic emission.

Suppressee

In the present study, suppressee is defined as the sound that is suppressed in the experiment dealing with non-linearity and in the context of investigation of two-tone suppression. It is the initial sound represented for the listener in which the suppressor is further introduced.

In the comment of Duifhuis (1989) to his paper he issued in 1980 publishing the results of his two-tone suppression research, the author stipulates the following interrelations between the suppressor and suppressee in the situation when the suppressor signal is more powerful than the suppressee tone:

Suppression increased with increasing level of the suppressor. However, this does not imply the increase of suppression with level of suppressee, or with overall level. The real prediction from my data, and from my interpretation, is that for a fixed suppressor level, suppression with increase with increasing suppressee level up to the point where the suppressee will dominate and overtake the response. Then the suppression will decrease with further increase of suppressee level (Comments, n.d.: 1).

However, in the reply to this comment it is stated that the pulsation thresholds in the described study were measured for a fixed-level suppressee with the varying level of only the suppressor. The opinion is also voiced that “the suppression will decrease (and not increase) with increasing level of suppressee even when the suppressee level is considerably lower than the level of the suppressor” (Comments, n.d.: 1). Thus, it becomes clear that the role of the suppressee in the process of two-tone suppression is still not defined and should be checked in an experimental way.

Two-tone suppression

Moore (1995) describes the two-tone suppression as the phenomenon that can be witnessed through presenting a tone at the CF of a neuron or close to it, after that presenting another tone with varied frequency and intensity and recording the influence of this tone on the neuron response. The influence of the second tone is usually viewed as its interaction with the excitatory area, while the characteristics of the second tone fall bounded by the tuning curve. The result of the interference is the increase in the firing rate.

The suppression actually occurs when the fall takes place outside that area:

The effect is usually the greatest in two suppression regions at frequencies slightly above or below the area of the unit’s excitatory response to a single tone… The suppression effect begins and seizes very rapidly, within a few milliseconds of the onset and termintation of the second tone, consistent with its likely origin as a mechanical effect on the BM (Moore, 1995: 46).

It should be noted that in the present quotation the abbreviation BM stands for basilar membrane. It is also essential to understand that two-tone suppression may be witnessed only with the listeners with a normal ear as well as other nonlinearities:

The normal BM shows several nonlinearities, including compressive input-output functions, two-tone suppression, and combination-tone generation (Moore, 1995: 22).

Pulsation threshold

The Pulsation Threshold method is similar to the one used by Duifhuis(1979) and Houtgust (1972 , 1973) where the tested subject has pulsation thresholds measured by presenting a suppressor and suppressee at the same time. The suppressee and the stimuli presented by the probe hold the same frequency (frequency in probe = frequency in suppressee).

During the experiment the suppressor frequency is gradually altered from low to high in order to avoid “adaptation effects”. The subject is required to give constant feedback to the machine and all changes in function are the direct results of the feedback that is given in.

In his description of the pulsation threshold method W.M. Hartmann (1997) refers to the discovery of Thurlow in 1957 that the listener will perceive the weaker tone as a continuous sound if the weak tone and strong tone alternate repeatedly. Thurlow draw an analogy with visual perception of fragments of lighter and darker color, appearing one on the background and one on the foreground, thus creating a visual impression that the lighter fragments on the background are uninterrupted.

Thus, Hartmann goes further in his outline of the pulsation threshold method, giving a couple of examples of its efficiency in the investigation of listeners’ responses to different types of thresholds. This way, he explores an interesting peculiarity of the human ear to perceive sounds as much more intelligible when the silent intervals used as a pulsation threshold are filled with a stronger foreground noise (masking). As soon as intervals are not filled and remain silent, the overall perception of sound remains much lower than in the previously described situation. The essence of the method is described by the author as follows:

The experiment arranges a weak signal to alternate with a strong masker, for example noise. The experiment looks for the weakest signal that can just barely be heard as pulsing, because, according to Houtgast’s interpretation, there then exists a neural channel in which the excitation caused by the signal is just barely greater than the excitation caused by the masker (Hartmann, 1997).

The choice of pulsation method compared to forward masking.

The usage of pulsation threshold method suggests more credibility in determination of suppression effects than the forward marking method. Duifhuis also supports this idea:

Since the effects of suppression measured in terms of threshold differences are greater in pulsation threshold, it makes it a preferred method to use” (Duifhuis , 1979).

Sid P. Bacon, Richard R. Fay and Arthur N. Popper (2004) measured the forward-masking level “required to mask the signal at different masker requences”. The result they achieved was the range from 400 and 750 Hz and 1,944 to 5,000 Hz. However, the pulsation threshold method allowed to conduct the investigation of two-tone suppression at the levels from 250 to 8,000 Hz (Bacon et al., 2004). Consequently, with the purpose of obtaining credible and multi-aspect results the preferred method for the present research is pulsation threshold.

References

1. Bacon, SP et al. (2004). Compression: from cochlea to cochlear implants. Springer, 228 pp.
2. Comments (n.d.). [Online].
3. Duifhuis, H. (1980). ‘‘Level effects in psychophysical two-tone suppression,’’ J. Acoust. Soc. Am. 67, pp. 914–927.
4. Hartmann, W.M. (1997). Signals, sound, and sensation. Springer, 647 pp.
5. Hawkins, H.L. (1995). Auditory computation. Springer, 517 pp.
6. Moore, B.C.J. (1995). Perceptual consequences of cochlear damage. Oxford University Press, 232 pp.
7. Moore, B.C.J. (2003). An introduction to the psychology of hearing. Emerald Group Publishing, 413 pp.
8. Robinette, M.S., and Glattke, T.J. (2007). Otoacoustic emissions: clinical applications. Thieme, 436 pp.