TEST OF DIFFERENTIAL SENSITIVITY/ LIMEN

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INTRODUCTION
The ear’s extremely wide range of sensitivity is one of the most striking aspects of audition. The ear is sensitive to range of intensities from about 0 dB SPL (which is amplitude of vibration of about size of a hydrogen molecule) to roughly 140 dB (at which pain and damage to the auditory mechanism ensue).This dynamic range of the approximately 140 dB corresponds to pressure ratio of 107:1. In other words, the most intense sound pressure that is bearable is on the order of 10 million times as great as the softest one that is perceivable under optimum listening condition. In terms of frequency, humans can hear tones as low as 2 Hz (although roughly 20 Hz is required for a perception of “tonality”) and as high as about 20 KHz. The frequency and intensity sensitivities of ear can interact, affecting each other to a greater or lesser degree. In addition, when duration the duration of sound is less than about half of a second, it affects both frequency and intensity sensitivity. Longer duration may be thought of as being infinitely long as far as auditory sensitivity is concerned.
The ear is able to discriminate small differences in a wide range of stimuli, i.e. it has a remarkable sensitivity- the ability to detect very small differences between similar sounds. This ability applies to all three parameters: intensity, frequency, and time.

DIFFERENTIAL SENSITIVITY   
It is the smallest perceivable difference between two sounds. This quantity is called either the difference limen (DL) or the just noticeable difference (JND). The DL is the smallest perceivable difference in db between two intensities (∆I) or the smallest perceivable change in Hz between two frequencies (∆f).
We may think of JND in two ways.
·         The absolute difference between two sounds
·         The relative difference between two sounds
·         Absolute difference is how much sound intensity is necessary for a typical, normally hearing person to just detect the presence of a stimulus or it is the least sound pressure that leads to a sensation of hearing.
·         Relative difference is obtained by dividing the absolute DL by the value of starting level. Thus, if the starting level is 1000 units and the DL ∆I/I is 50/1000=0.05. This ratio, ∆I/I, is called the Weber fraction.
Methods of measuring DLI:
      Luscher-Zwislocki Method (1948)
They used pure tones, presented at 40 dB SL and patients were asked to indicate when amplitude modulation of steady state signal resulted in pulsating sound. Those patients who could detect small intensity change were assumed to have cochlear lesions. That is persons having a critical percentage modulation of less than 80% were considered to have recruitment.
   Limitations:
1.      Persons with mild or non- steepy sloping audiometric configuration obtained critical modulation percentage greater than 8% and some normal hearing persons obtained  less than 8%
2.      Individual variability in judgment affected the reliability of the test.

Denes-Naunton Method ( 1950)
They developed a recruitment test based on the intensity DL for loudness. In the test, two tones of the same frequency are presented to an ear and the intensity of one of the tones is varied until a perception of a just-noticeable change in loudness results. Test was conducted at 4 and 44 dB SL.
The determination of recruitment was based on a relative than absolute measure of the intensity DL for loudness .If the intensity DL increased or remained the same as the SL increased, recruitment was assumed to be present. If the intensity DL decreased as the SL increased as the SL increased, recruitment was assumed to be absent. In case of cochlear hearing loss not much difference at 44 dB SL and 4 dB SL, where as in normal at 44 dB SL -1 dB increment enough, at 3-4 dB SL increment is needed to detect the change. Jerger (1952) modified the Luscher-Zwislock test. He used presentation level at 15 dB SL rather than 40 dB SL, since the literature shows that the intensity DL was most affected between  10 and 30 dB SL. He found that this showed good differentiation between recruiting and non recruiting ears.
He found that patient with reduced intensity DLs had non conductive hearing impairments; enlarged intensity DLs had functional hearing loss, normal intensity DLs had conductive or presbycusis.
Luscher (1951) modified the intensity DL test to employ a presentation level of 80 dB HL. He employed in person with hearing loss magnitude exceeding 60 dB HL, the rationale underlying this modification two folds.Recruitment for hearing loss was present at this level is small in normal hearing person, indicating that the task of masking loudness judgments was simplified. Complete recruitment was assumed to be present if hearing impaired person obtains an intensity DL of 4-6% at this presentation level

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