Discrimination of frequency ramps in subjects with cochlear hearing loss.

The auditory sensitivity for detecting linear frequency sweeps of a continuous pure tone has been studied in ten young subjects with cochlear hearing loss. The mean thresholds were elevated by a factor of 2.8 as compared with a normal group over the whole range of ramp durations studied (10-500 msec). The results show that this elevation is most likely caused mainly by the cochlear lesion per se, other possible factors having only a minor effect. No clear correlations could be found between thresholds for frequency change and results of other pure tone audiometric tests. Such tests thus cannot predict a subject's frequency discrimination.

Thresholds for linear frequency ramps of a continuous pure tone.

The human auditory sensitivity in detecting linear frequency ramps of a continuous pure tone has been studied. It is shown that for short ramp durations (less than 200 msec) discrimination depends on the difference between base and plateau frequencies, the mean threshold being about 3 Hz at 1 kHz. For longer ramp durations (greater than 200 msec), discrimination was found to be based on detection of the actual frequency sweep. No significant difference was found between thresholds for upward and downward sweeps. Expressed in Hz, the threshold for frequency change was approximately constant for base frequencies up to 1 kHz, above which it increased, reaching approximately 14 Hz at 4 kHz. There was no significant difference in the threshold for frequency change from 40 to 80 dB HL but at 20 dB HL the threshold was significantly higher than at 40 dB HL. Intra-individual variation in thresholds was found to be smaller than inter-individual variation. The results are discussed in relation to previous frequency discrimination data, where either tone pulse pairs, continuous frequency modulation or frequency ramps were used as stimuli.

Slow evoked cortical responses to linear frequency ramps in subjects with cochlear lesions.

Slow evoked cortical potentials in response to linear frequency ramps of a continuous pure tone with a 1 kHz base frequency have been recorded from ten relatively young subjects with hearing loss of cochlear origin. At small frequency ramps, the N1-latency of their responses to the three ramp durations studied (20, 100, 500 msec) was significantly longer than those of a group with normal hearing. As the ramps are made larger, the difference between the latencies of the pathological group and the normal group becomes smaller; above a certain rate of frequency change, the latency of N1 becomes smaller in the pathological cases than in the normal group. This crossover occurs at frequency change rates around 1-3 kHz/sec. It is concluded that recording of evoked cortical responses to frequency ramps may provide an additional tool in the differential diagnosis of hearing disorders.

Thresholds for linear amplitude change of a continuous pure tone.

The human auditory sensitivity in detecting linear amplitude change of a continuous pure tone has been studied in normal-hearing subjects. It is shown that for short glide durations (less than 100 ms) the duration of the following plateau exerts a significant influence on the DLI. The average DLI at 1 kHz and 60 dB HL was found to be of the order of 0.8 dB when the intensity glide had a duration of 10 ms and was followed by a much longer plateau. For longer glide durations (greater than or equal to 200 ms) the DLI increased significantly as compared with shorter durations. There was no significant difference between increasing and decreasing intensity change. Significantly larger DLIs were found at 250 and 500 Hz than at 1, 2 and 4 kHz. The sound level was found to have a significant influence on the DLI. At low levels of 40 dB HL, and lower, the increase in DLI for detecting sound levels is highly significant. A falling exponential function offers a mathematical description of the relationship with good fit. It is concluded that an integrating mechanism with an integration time of approx. 200 ms could explain the auditory ability to detect linear amplitude glides of a continuous tone. The results are discussed in relation to previous intensity discrimination data, where pulse pairs, continuous intensity modulation or intensity glides were used as stimuli.

Slow evoked cortical responses to linear frequency ramps of a continuous pure tone.

Slow evoked cortical potentials from ten young normal-hearing subjects have been recorded as responses to linear frequency ramps of a continuous pure tone. Frequency changes from 10 to 500 Hz were studied; the rate of frequency change was varied from 0.02 to 50 kHz/s while the duration of the change was varied from 10 to 500 ms. The rate of frequency change was shown to have the greatest bearing on the responses except for frequency ramp durations below 50 ms and frequency changes below 50 Hz. The base frequencies (250-4000 Hz) and sound levels (20-80 dB HL) exerted an influence on the evoked responses that was qualitatively similar to their influence on behavioral thresholds. The direction of the frequency sweep had no significant influence on the evoked responses. A functional model is proposed in which the time derivate of the signal frequency is integrated with an adaptable integration time that is controlled by the rate of the frequency change.