User:YMMFan/sandbox

A binaural stimulus such as a dichotic pitch stimulus leads to a perception of a pitch that cannot be heard alone by one ear (Cramer & Huggins, 1958). In other words, the dichotic pitch stimulus, usually white noise must be presented to both ears at the same time to be heard. Neither of the stimulus alone allude to the pitch as it cannot be heard alone but has to be presented binaurally (to both ears).

A study conducted by Akeroyd and colleagues in 2001 with 49 undergraduate students suggested that no prior listening experience was necessary to hear the dichotic pitch stimulus. In the study, the participants had to recognize various known melodies in different types of dichotic pitches (Huggins pitch, binaural edge pitch and binaural coherence edge pitch).

Heschl’s Gyrus Activation
The Heschl’s gyrus is a part of the Temporal lobe, which is responsible for auditory processing. In relation to the dichotic pitch, Puschmann and colleagues investigated the activation of Heschl’s Gyrus using fMRI scans. In this study, Puschmann and colleagues presented pure tones and dichotic pitch stimuli’s and that both stimuli activated the lateral end of the Heschl’s guys in both hemispheres, right outside the primary auditory cortex. This indicates that the Heschl’s gyrus is involved in binaural processing and the perception of the dichotic pitch.

Variations of the Dichotic Pitch
There are many variations of the dichotic pitch including the Huggins Pitch, the Binaural edge pitch, the Fourcin pitch and the dichotic repetition pitch. These four variations fall into two classes as they are each created differently. The Huggins pitch and binaural edge pitch are generated though producing an interaural phase transition at a particular frequency with broadband noise. The manipulation of the interaural phase creates a singular pitch known as the dichotic pitch. On the other hand, the Fourcin Pitch and Dichotic repetition pitch fall into one class as to generate it there must be an interaural delay. The interaural manipulation for the Fourcin pitch occurs at multiple frequencies which produces complex dichotic pitches.

Huggin's Pitch (HP)
The Huggins pitch is the most common or generic form of the dichotic pitch and was put forward by W.H. Huggins. The dichotic pitch stimulus is presented to both ears and is perfectly correlated between the two ears except for a narrow frequency. With the Huggin’s pitch, the dichotic pitch stimulus is filtered with an all-pass filter. This allows an interaural phase change over 360 degrees, otherwise known as a shift from 0 to 2π radians, over a narrow frequency band.

This refers to when processing in the brain is dominant in one side, either the left or the right. A study by Michael Akeroyd and colleagues found that the dichotic pitch was lateralized to one or the other side of the brain during dichotic listening. In relation to the Huggin’s pitch, lateralization in the brain is dependent on the interaural configuration of the noise carrier, so whether it was diotic or interaurally inverted. When the noise carrier was interaurally inverted, the dichotic pitch was biased towards the left side of the brain. The lateralization of the perception of Huggin’s pitch was also dependent on the interaural time delay and the centre frequency of the narrow band.

Binaural Edge Pitch (BEP)
The binaural edge pitch is another form of the dichotic pitch. Similar to Huggins pitch, it is created by introducing an interaural phase shift varying from 0 to πradians, the phase boundary. The edge of the broadband noise produces a pitch near the phase boundary that an individual perceives as the dichotic pitch.

Different Theories
Different Theories

Modified equalization cancellation
The modified equalization cancellation model (EC model) suggests that after the dichotic pitch stimulus passes through an all-pass filter and enters the ear, it goes through a two-step process of equalization and cancellation. Equalization is the process in which the ear adjusts the differences in intensity and interaural time difference. Cancellation is where it cancels out the differences of the filter output. This adjusts the difference between two in interaural phase and the residual or left-over frequency is what we hear as the dichotic pitch.

It is also notable that this model fails to address the lateralization of the dichotic pitch.

Central-spectrum model
The Central-spectrum model or CAP was put forward by Raatgever and Bilsen which acknowledges both the pitch and lateralization of dichotic pitches. Raatgever and Bilson argued that “a mechanism recognises and selects the frequency spectrum information by making use of cues like harmonicity and depth of modulation or a priori knowledge of spectral features”.

However, it is notable that research by Culling and colleagues demonstrated that this model does not depict dichotic pitches accurately as it can give incorrect predictions of the pitch under certain “combinations of the interaural phase and the interaural time delay”. Therefore, they came up with the modified equalization cancellation model.

A study conducted by Santurette and Dau compared individuals with normal hearing and impaired hearing’s ability to perceive the dichotic pitch. It was concluded that the hearing-impaired individuals could either hear all types of binaural pitches, only perceive the strongest pitches or individuals with central auditory deficiencies were, in all, unable to perceive pitch at all. Reduced frequency selectivity and discrimination weakened the participant's ability to hear the dichotic pitch. The ability for certain hearing-impaired individuals to either perceive the dichotic pitch or not hear the dichotic pitch may make it a useful tool for clinical diagnosis. However, more in-depth research must be conducted into the use of the dichotic pitch stimuli in a clinical setting for diagnosing disorders such as dyslexia.