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were defined before

were defined before measurements by digitizing the location of the nasion and the preauricular points of the left and the right ear as well as of four head-position indicator coils located behind the ears and on the forehead. During the digitizing and subsequent 40-min measurement, the subject lay on a research bed. The supine position minimized head movements during the measurement. Subjects were instructed to avoid moving and excessive blinking during the measurement. The stimuli were delivered to the subject’s left and right ear via plastic tubes and earpieces. The onset-to-onset interstimulus interval (ISI) varied randomly between 1 and 4 s. Sinusoidal and speech stimuli were presented in separate sequences, in a counterbalanced order, systematically rotating the presentation of the stimulus sequences (Latin square design) in order to avoid learning and exhaustion effects. Evoked responses were recorded and averaged online over a period of 1050 ms including a 100-ms pre-stimulus baseline. The sampling rate was 600 Hz. Four electro-oculogram electrodes monitored vertical eye movements from above and beneath the left eye and horizontal eye movements from the temples. Measurement epochs contaminated by activation from the electrooculograms exceeding |150| lV or |3000| fT/cm at any MEG sensor were discarded from the averaging. The recording was continued until 80 responses were averaged. In the active recording condition, behavioral reaction times (RTs) were measured. 2.4. Data analysis Responses for each stimulus type (sinusoid and speech) and recording condition (active and passive) were recorded and averaged separately during the measurements. The averaged data were then analyzed offline in order to study the transient brain response for possible effects in response amplitude, latency and source location. The responses were analyzed individually for each subject and separately for the temporal lobe of each hemisphere using the planar gradiometer sensor displaying the brain response with maximum amplitude. This analysis was carried out separately for each measurement condition and stimulus type. In single-sensor analyses, the peak amplitude and latency of the response were determined. A lowpass filter of 20 Hz was used. Source locations of the responses were quantified through equivalent current dipole (ECD) analyses. These were carried out using sets of 44 sensors over the left and right hemisphere. ECD locations were determined in a 3D coordinate system where the x-axis passes through the preauricular points (positive values towards the right ear), the y-axis passes through the nasion (positive values to the anterior direction), and the z-axis is perpendicular to the x–y plane (positive val- Amplitude (fT/cm) 120 0 50 L.E. Matilainen et al. / Clinical Neurophysiology 121 (2010) 902–911 905 0 850 Time (ms) ues upwards). For ECD analyses, the response data were filtered with a 2–20 Hz passband. Raw data from the active recording condition, including stimulus presentation times, brain responses, and the behavioral RTs were analyzed. RTs were analyzed in a 0 to 2000 ms time window. In the young subject group, RTs were analyzed for each of the nine subjects. In the aged subject group, analyses were carried out for eight subjects. The within-group effects on response amplitude, latency, ECD location, and RT were analyzed using repeated measures ANOVA. The factors were stimulus type (sinusoid and speech), recording condition (active and passive), hemisphere (left and right), and ECD location (x, y and z). The between-group effects were analyzed with one-way ANOVA. Newman–Keuls post hoc tests were used when appropriate (p < 0.05). The ECD location of the responses to sinusoidal stimuli in one aged subject could not be defined in the right hemisphere in the active recording condition. The missing values were replaced by the average values of the subject group in this instance. For illustration purposes, the response curve of each subject was shifted in time so that the peak amplitude coincided with the grand-averaged response latency of each subject group (see Fig. 1). The time-shifted response curves were then averaged with the purpose of elucidating the grand-averaged response curve of each subject group. Time-shifting removes the effect of inter-individual latency variations and is a useful method for enabling illustrations of grand-averaged brain responses while having no effects on the statistical analyses. 3. Results As shown in Figs. 2 and 3, the sinusoidal and speech stimuli elicited prominent transient brain responses in both the young and the aged subject group. This was followed by sustained activity and a stimulus-offset response at the latency of approximately 850 ms, some 100 ms after stimulus offset. 3.1. Young subjects When the young subjects were presented with the sinusoidal stimuli in the passive recording condition, the amplitude of the resulting transient brain response was, on the average, 47 and 43 fT/cm in the left and the right hemisphere, respectively (F(1,8) = 0.21, p = n.s.; see Table 2). In the active recording condition, the amplitude of the response elicited by the sinusoids was 65 and 64 fT/cm in the left and the right hemisphere, respectively (F(1,8) = 0.02, p = n.s.). For speech sounds, the average response amplitudes in the passive recording condition were 74 and 77 fT/ Amplitude (fT/cm) 50 0 5 0 850 Time (ms) Fig. 1. (Left) Left-hemispheric responses elicited by the sinusoidal stimuli in the passive recording condition for each subject in the aged group (thin lines), and the corresponding grand-averaged response curve without the time-shifting procedure (thick line). (Right) The grand-averaged response curve with (gray line) and without (black line) the time-shifting procedure. Note the difference in the amplitude scale.

906 L.E. Matilainen et al. / Clinical Neurophysiology 121 (2010) 902–911 Passive Active Amplitude ( fT/ cm) 120 Amplitude ( fT/ cm) 120 cm in the left and the right hemisphere, respectively (F(1,8) = 0.10, p = n.s.). In the active condition, the corresponding amplitudes were 86 and 90 fT/cm (F(1,8) = 0.08, p = n.s.). Thus, the speech stimuli elicited more prominent responses than did the sinusoids in both the left (F(1,8) = 10.60, p < 0.05 and F(1,8) = 11.52, p < 0.01 in the passive and the active recording condition, respec- 0 0 0 Left hemisphere Right hemisphere 120 Time (ms) 850 0 120 0 0 Young subjects Time (ms) Fig. 2. Grand-averaged left- and right-hemispheric responses of the young subjects elicited by sinusoidal (gray) and speech stimuli (black) in the passive (top row) and active (bottom row) recording condition. Speech stimuli elicited more prominent responses than sinusoids. The source locations of the responses are shown as arrows (with the size and orientation of the arrow indicating source strength and direction, respectively) in the ECD models obtained at response maxima, from which the upper ones depict activation elicited by sinusoids and the lower ones that elicited by speech. Contours indicate inward flux (black) and outward flux (gray), with a contour step of 10 fT/cm. The shaded area describes the stimulus sound pressure corresponding to a linear increase on the dB-scale from 30 to 60 dB (see Section 2.2.). Passive Active Amplitude ( fT/ cm) Amplitude ( fT/ cm) 120 0 120 0 0 speech sinusoid tively) and the right hemisphere (F(1,8) = 28.62, p < 0.001 and F(1,8) = 55.83, p < 0.001; see Fig. 2). Interestingly, attentional engagement enhanced the response amplitudes for sinusoids by approximately 40% in the left hemisphere (F(1,8) = 5.82; p < 0.05) and 50% in the right hemisphere (F(1,8) = 8.20; p < 0.05) when compared to responses obtained in Left hemisphere Right hemisphere 120 Time (ms) speech sinusoid 120 0 0 850 0 Aged subjects Time (ms) Fig. 3. Grand-averaged left- and right-hemispheric responses of the aged subjects elicited by sinusoidal (gray) and speech stimuli (black) in the passive (top row) and active (bottom row) recording condition. Speech stimuli elicited more prominent responses than did sinusoids. The source locations of the responses are shown as arrows (with the size and orientation of the arrow indicating source strength and direction, respectively) in the ECD models obtained at response maxima, from which the upper ones depict activation elicited by sinusoids and the lower ones that elicited by speech. Contours indicate inward flux (black) and outward flux (gray), with a contour step of 10 fT/cm. The shaded area describes the stimulus sound pressure corresponding to a linear increase on the dB-scale from 30 to 60 dB (see Section 2.2.). 850 850

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