Subjects
Twenty individuals participated in the study: 10 young adults (age range = 19–23, M ± SD = 21 ± 4 years, 5 females) and 10 elderly adults (age range = 61–80, M = 69 ± 6, 5 females). Participants gave written informed consent approved by the University of California Irvine’s Institutional Review Board and were monetarily compensated. The methods were in accordance with principles set forth in the Belmont Report and Declaration of Helsinki. Participants were screened prior to the start of the study to ensure no known severe hearing dysfunction, medical, or mental health illnesses. Participants were also given hearing tests to screen for hearing loss. Young adult participants had normal hearing with thresholds ≤ 20 dB HL (Hearing Level) at octave frequencies between 0.125 and 8 kHz. Elderly participants had normal hearing (≤ 20 dB HL) at octave frequencies between 0.125 and 2 kHz, but mild-to-moderate hearing loss at 4 kHz (30 dB HL) and 8 kHz (45 dB HL). To minimize the influence of hearing loss, all three tasks had stimulus frequencies below 2 kHz (see Experimental protocol below).
To ensure little to no nicotine dependence from use or exposure, participants were required to have a score of 0–2 out of 10 on the Fagerstrӧm index of smoking dependency 27,28. Eighteen participants were non- or social-smokers, defined as smoking no more than 100 cigarettes in their lifetime and none in the past year 24. Two subjects reported quitting smoking more than 20 years ago. To avoid chemical interactions, participants were instructed to abstain from alcohol consumption for 24 h and food consumption for ≥ 1 h prior to testing. To avoid caffeine withdrawal in regular caffeine consumers, ½ cup of a caffeine-containing beverage was permitted ≥ 1 h prior to testing 29.
Experimental protocol
Three tasks were selected for their lower performance in older than younger adults, including frequency and intensity discrimination 17 and frequency modulation identification 18. The previously described experimental protocols were closely followed in the present study. Briefly, the frequency discrimination experiment measured the just-noticeable-difference in pitch for a 400-ms, 500-Hz pure tone presented at 55 dB SPL. The intensity discrimination experiment measured the just-noticeable-difference in loudness for a 400-ms, 500-Hz pure tone presented at 55 dB SPL. The just-noticeable-difference was obtained by a three-interval, two-alternative, forced-choice adaptive procedure using a two-down, one-up rule to yield 71% correct performance 30.
The frequency modulation identification experiment measured the signal-to-noise ratio required to identify an up-down or down-up frequency modulation “signal” in the presence of a ‘noise” 18. The 400-ms signal consisted of harmonics with a fundamental frequency of 189 Hz and a single spectral peak or formant at 1000 Hz. The formant frequency was dynamically modulated by a 5-Hz triangular wave with a 55% modulation swing. The 800-ms noise also consisted of similarly-modulated harmonics but with a different fundamental frequency of 107 Hz. The signal was presented at the temporal center of the noise. The signal level was constant at 55 dB SPL, while the noise was adaptively varied. The participant had to report whether the modulation was up-down or down-up in a single-interval, two-alternative, forced-choice task. A three-down, one-up rule estimated the signal-to-noise ratio, at which the participant correctly identified 79% of the frequency modulated signal 31.
All experiments took place in a double-walled, sound-attenuated booth. Stimuli were presented binaurally through circumaural headphones (Sennheiser HAD-200, Wedemark, Germany). The reported result for each participant was the arithmetic mean of the estimate obtained in three to five runs. In all three tasks, lower values reflected better performance.
Study design
All procedures leading up to experiments, including method of drug delivery and time between sessions to allow for drug clearance followed those specified by Pham et al. 12. Briefly, six mg of nicotine was delivered in the form of two pieces of mint-flavored polacrilex gum (4 mg and 2 mg; Nicorette®, Johnson & Johnson, Inc). Two mint-flavored gum (Eclipse®), resembling the nicotine gum in size, shape, color, and texture, served as the placebo. Furthermore, subjects wore a blindfold during both administrations to mask any potential visual differences between placebo and nicotine gums; a drop of Tabasco sauce was added to each gum piece to disguise taste bias 32. Test occurred between 8:00 am and 6:00 pm and took place at a consistent time across sessions to avoid confounding arousal and attention effects. All three experiments were completed in two sessions, in which a treatment (nicotine or placebo) was give while pre- and post-treatment data were collected. Audiograms were measured prior to pre-treatment testing in the first session. Afterwards, participants received either nicotine or placebo gum in a randomized design. The protocol was repeated with the alternate treatment (nicotine or placebo) in the second session adhering to a single-blind intra-subject design. Treatment sessions were separated by ≥ 48 h to allow for treatment clearance. Timing was carefully controlled to assure that nicotine plasma concentration at this dosage reached and maintained peak levels for the duration of the three experiments 12,33.
Pulse oximetry, mood changes, and side effects
Neither nicotine nor placebo treatment significantly changed blood oxygen saturation in either group (mean ± SD: young, pre-nicotine 97.9 ± 0.3%; post-nicotine 98.2 ± 0.6; pre-placebo 97.9 ± 0.9; post-placebo 98.1 ± 1.1; elderly, pre-nicotine 96.0 ± 1.8; post-nicotine 96.8 ± 1.5; pre-placebo 97.0 ± 1.8; post-placebo 97.0 ± 1.0). Nicotine treatment did not significantly change pulse rate in either group (young, pre-nicotine 71.0 ± 4.5/min; post-nicotine 73.9 ± 8.1; elderly, pre-nicotine 71.0 ± 8.2; post-nicotine 70.6 ± 6.4). Placebo treatment did not significantly alter pulse rate for the young (pre-placebo 70.5 ± 8.2; post-placebo 68.8 ± 8.5) but significantly decreased pulse rate for the elderly (pre-placebo 72.2 ± 8.2; post-placebo 66.6 ± 8.0, p < 0.01).
Participants also provided subjective pre- and post-treatment ratings using a 9-category mood profile, where responses for each category were binary (e.g., tense/relaxed) and using a 5-point side effects scale, where 1 corresponded to no symptoms and 5 corresponded to severe symptoms such as jittery, headache, nausea, or vomiting 10. Ratings were averaged across the three experiments. No significant pre- versus post-treatment change in mood was found for nicotine or placebo treatment in either group (both p > 0.05). No post-treatment difference between placebo and nicotine was observed in either group for ratings of side effects (both p > 0.05).
Data analysis
A paired two-sample t-test was used to assess differences in baseline performance between older and younger groups for each task. Baseline performance was the average of the two sets of pre-treatment data from nicotine and placebo conditions. Justification for combining pre-treatment data was performed via a Kolmogorov-Smirnov test, revealing that the combined data follow a normal distribution for each experiment (frequency discrimination, p = 0.34; frequency modulation identification, p = 0.83; intensity discrimination, p = 0.83). Effect sizes were determined with Cohen’s d (small effect: ~0.2, medium effect: ~0.5, large effect: ~0.8). To test the primary hypothesis that nicotine improves auditory performance, post-treatment performance was compared as a function of age and drug using a two-way mixed effects analysis of variance (ANOVA) for each auditory task, with age as a between-subjects factor and drug as a within-subjects factor. Two-tailed, two-sample t-tests were used to test if combined (i.e., young and elderly) post-nicotine minus placebo scores were significantly different from zero, and effect sizes were again determined with Cohen’s d. To test the secondary hypothesis that participants with lower baseline performance would benefit more from nicotine treatment, a linear regression was conducted between the nicotine-placebo post-treatment difference and the baseline performance for each of the three auditory tasks.