For the amplitude-modulated stimulus, it was expected that both a transient and steady-state gamma-band response would be present. A 2 × 2 × 2 × 2 mixed design ANOVA (group by type of gamma by ear by hemisphere) was evaluated separately for evoked source strength, induced source strength and for PLF.
For evoked source strength, all of the main effects were significant. First, a significant main effect of diagnosis (F(1,28)=7.20, p<.02) indicated that the control group had higher gamma source strengths than the schizophrenia group. There was also a significant hemisphere main effect, F(1,28)=8.21, p < .01, indicating greater strength in the right hemisphere than the left. A significant main effect of ear of stimulation demonstrated that the left ear produced greater evoked source strength than right ear stimulation, F( 1,28)=8.84, p=.006. Finally, there was a main effect of gamma type, F(1,28)=21.37, p < .001, indicating that steady state gamma responses were larger than transient responses. Several interaction terms were significant, however, warranting caution in a straightforward interpretation of the main effects. First, a significant ear of stimulation by hemisphere interaction was noted, F(1,28)=18.70, p<.001, suggesting that although contralateral stimulation led to greater normalized source strength in the right hemisphere compared to right ear stimulation, there was little, if any, difference for the left hemisphere between ipsilateral and contralateral stimuli. Further, there was a significant ear of stimulation by hemisphere by type of gamma interaction, indicating that the previous effect was driven primarily by the steady state gamma, F(1,28)=20.03, p<.001. There were no significant interactions for the group factor at .05 alpha.
For induced source strength, there were significant main effects of diagnosis (F(1,28)=7.01, p <.02), gamma type (F(1,28)=28.80, p<.001) and hemisphere (F(1,28)=9.88, p=.004), indicating greater induced power in the schizophrenia group than in controls, in the transient window relative to the steady state window, and in the left compared to right hemispheres. Induced power appeared to be higher for ipsilateral than contralateral responses, as indicated by a significant ear by hemisphere interaction, F(1,28)=58.85, p<.001. The diagnosis by ear by hemisphere term was not significant, F(1,28)=3.81,p=.06, but might suggest that the ipsilateral dominance of induced power was primarily seen in control subjects. Finally, a significant gamma type by hemisphere term (F(1,28)=31.15, p<.001) indicated that greater induced power in the left hemisphere was observed only during the steady state window.
For PLF the schizophrenia group had significantly reduced values relative to controls, F(1, 28)=6.56, p < .02. As with source strength, we observed a significant effect of hemisphere on PLF, F(1,28)= 13.73, p < .001, indicating greater right than left hemisphere phase-locking. A significant ear by hemisphere effect, F(1,28)=86.57, p < .0001, indicated greater phase-locking in the hemisphere contralateral to ear of stimulation, relative to ipsilateral stimulation. A significant diagnosis by hemisphere by ear effect, F(1,28) = 6.09, p < .02, indicated that the contralateral ear advantage was reduced in the schizophrenia group, however. A significant main effect of gamma type indicated that the steady-state gamma PLF was stronger than the transient PLF, F(1,28)=28.88, p < .0001. However, a significant hemisphere by type interaction, F(1,28)=26.31, p<.0001, indicated that this hemisphere difference was significantly more pronounced for the steady-state PLF region than the transient region. Finally, a significant ear by gamma type interaction, F(1,28)=4.39, p < .05, combined with a significant 3-way interaction between ear, hemisphere and gamma-type (F(1,28)=14.99, p < .001), indicated that the differences between left and right hemispheres were restricted to the steady-state PLF region and most pronounced for right ear stimulation. and show the group mean contra-lateral PLF and evoked source strength waveforms across time for both hemispheres.
Contralateral Source Space Projected Inter-trial Phase Locking Factors at 40 Hertz to Steady State Stimulation. Data are group means.
Contralateral Source Space Projected Evoked Source Strength at 40 Hertz to Steady State Stimulation. Data are group means.
Pure Tone Stimuli
A 2×2×2 mixed design ANOVA (group by ear by hemisphere) was evaluated separately for the transient gamma band evoked source strength and PLF. A significant ear by hemisphere effect, F(1,25) = 7.93, p < .01, suggested that contralateral ear stimulation produced higher evoked amplitudes than ipsilateral stimulation. No other significant effects were noted for source strength. Note that due to either poor data quality or subject dropout the number of subjects in the schizophrenia group was reduced by two, and the number of controls reduced by one, for the pure tone condition.
For the pure tone induced power data, no significant main effects were observed (all p>.15). A significant diagnosis by hemisphere interaction was noted, F(1,25)=4.71, p<.05, indicating that while there were no differences in induced power between the two hemispheres for control subjects, induced power was higher in the left than right hemisphere in the schizophrenia group. In addition, there was a significant ear by hemisphere term, F(1,25)=8.17, p<.01, indicating that ipsilateral induced response power was higher than contralateral induced response power.
For the pure tone transient PLF, a significant interaction effect for ear by hemisphere was also noted, F(1, 25)=6.90, p < .02, indicating a contralateral ear of stimulation advantage. A strong trend for a significant diagnosis by hemisphere term was also observed, F(1,25) = 4.13, p < .06, suggesting the possibility of a reduction in PLF in the left, but not right, hemisphere for the schizophrenia group. No other effects reached significance at .05 alpha. and show the group mean PLF data for the pure tone contra and ipsi-lateral sources.
Contralateral Source Space Projected Inter-trial Phase Locking Factors at 40 Hz to Pure Tone Stimulation. Data are group means.
Ipsilateral Source Space Projected Inter-trial Phase Locking Factors at 40 Hz to Pure Tone Stimulation. Data are group means.
There were no significant correlations between the BPRS-24 total scores or chlorpromazine equivalents and any of the 40 Hz measures for the AM stimulus or pure tone stimulus after Bonferoni correction for multiple comparisons. Further, no significant correlations, before or after Bonferoni correction, were present for smoking pack-years and any of the 40 Hz measures.