A method to dynamically control unwanted loudness cues when measuring amplitude modulation detection in cochlear implant users

BACKGROUND

Amplitude modulation (AM) detection is a measure of temporal processing that has been correlated with cochlear implant (CI) users' speech understanding. For CI users, AM stimuli have been shown to be louder than steady-state (non-AM) stimuli presented at the same reference current level, suggesting that unwanted loudness cues might contribute to CI users' AM sensitivity as measured in a modulation detection task. In this paper, a new method is introduced to dynamically control unwanted AM loudness cues when adaptively measuring modulation detection thresholds (MDTs) in CI users.

METHODS

MDTs were adaptively measured in 9 CI subjects using a three-alternative, forced-choice procedure, with and without dynamic control of unwanted AM loudness cues. To control for AM loudness cues during the MDT task, the level of the steady-state (non-AM) stimuli was increased to match the loudness of the AM stimulus using a non-linear amplitude scaling function, which was obtained by first loudness-balancing non-AM stimuli to AM stimuli at various modulation depths. To further protect against unwanted loudness cues, ±0.75dB of level roving was also applied to all stimuli during the MDT task.

RESULTS

Absolute MDTs were generally poorer when unwanted AM loudness cues were controlled. However, the effects of modulation frequency and presentation level on modulation sensitivity were fundamentally unchanged by the availability of AM loudness cues.

CONCLUSIONS

The data suggest that the present method controlling for unwanted AM loudness cues might better represent CI users' MDTs, without changing fundamental effects of modulation frequency and presentation level on CI users' modulation sensitivity.

Voluntary exercise regionally augments rates of cerebral protein synthesis

Exercise is a natural form of neurophysiologic stimulation that has known benefits for mental health, maintenance of cerebral function, and stress reduction. Exercise is known to induce an upregulation of brain-derived neurotrophic factor and this is thought to be involved in associated increases in neural plasticity. Protein synthesis is also an essential component of adaptive plasticity. We hypothesized that exercise may stimulate changes in brain protein synthesis as part of its effects on plasticity. Here, we applied the quantitative autoradiographic L-[1-(14)C]leucine method to the in vivo determination of regional rates of cerebral protein synthesis (rCPS) in adult rats following a seven day period of voluntary wheel-running and their sedentary counterparts. In four of 21 brain regions examined, the mean values of rCPS in the exercised rats were statistically significantly higher than in sedentary controls; regions affected were paraventricular hypothalamic nucleus, ventral hippocampus as a whole, CA1 pyramidal cell layer in ventral hippocampus, and frontal cortex. Increases in rCPS approached statistical significance in dentate gyrus of the ventral hippocampus. Our results affirm the value of exercise in encouraging hippocampal and possibly cortical neuroplasticity, and also suggest that exercise may modulate stimulation of stress-response pathways. Ultimately, our study indicates that measurement of rCPS with PET might be used as a marker of brain response to exercise in human subjects.