INSTANCIAS SOP (SOP INSTANCES)

See section 2.4.4 for recording parameters. The stimuli were presented at an ISI of 800 ms using linked mastoids as a reference.

2.5.3.6 Construction o f ERPs.

Epochs were constructed as explained in section 2.4.7.1 with a duration o f 800 ms including a pre-stimulus base line o f 50 ms.

2.5.3.7 Voltage maps.

The topography o f the N1 was compared by visual inspection o f voltage maps constructed as described in section 2.4.7.2.

2.5.3.8 Statistical analysis.

A two way within subjects ANOVA was used to analyse the amplitude and latency data o f the standard AERP component and the MMN. The first factor was stimulus type (duration, frequency, duration and frequency) and the second factor was electrode location (F4, Fz, F3, Fc4, Fc3), (see 2.4.7.4).

2.5.4 Results.

An auditory response was recorded in all subjects in all trials consisting o f a P l- N l- P2 waveform.

The standard AERP N1 component did not differ in latency as a function o f deviant stimulus type (F(2,i4) = 2.19; p =0.15) or across electrodes (F(4,2g) = 0.13; p = 0.87) (Table 2. 3,Figure 2. 4). Similarly, there were no significant changes o f peak to peak amplitude as a function o f deviant stimulus type (P l-N l = (F(2,i4) = 0.14; p = 0.87), N 1-P2 = (F(2,i4) = 3.60; p = 0.055) or across electrodes (P l-N l = F(4,2g) = 3.23; p = 0.059), N1-P2 = (F(4,28)= 1.38; p = 0.27).

-2 uV 50 ms

Figure 2. 4. S tandard A E R Ps (700 Hz 75 m s) from Fz recorded using an oddball paradigm em ploying different deviant stim uli. B lue, ‘D eviant 1’ 700 H z 25 m s, Red,

‘D eviant 2 ’ 1000 H z, Black ‘D eviant 3 ’ 1000 Hz 25 ms. D ashed line represents stim ulus onset.

Trial N1 (ms) P l/N l (p,V) N1/P2 (\i\)

Duration M ean 101.80 6.45 &23

±SD &68 2 2 3 2.40

Frequency M ean 97.12 6.42 7.99

±SD 11.22 2.57 2.47

Both M ean 98.43 6.27 6.72

±SD 7.10 2.46 3.13

T able 2. 3. S tandard (700 Hz, 75 m s) N1 A m plitude ()laV) and latency (m s) at Fz for each recording paradigm .

2-D voltage m aps o f the standard A E R P N1 com ponent show ed no obvious differences in topography regardless o f the deviant stim ulus used (Figure 2. 5). The m aps consisted o f a negative field covering m ost o f the fronto-central regions o f the

head, in accordance with previous studies on the topography o f the scalp recorded N1 (S c h e rg e t al., 1989).

1 0 2 ms law ms _{9 7 m s}

(a) (b) (c)

F igure 2. 5. G rand average 2-D voltage m aps o f the standard A E R P ‘ peak latency N i c o m p o n en t’ evoked in an oddball paradigm with (a) duration deviant stim uli, (b) frequency deviant stim uli and (c) deviant stim uli varying in duration and frequency.

M M N s w ere analysed by com paring the standard and deviant A E R Ps as a subtraction w aveform , constructed by subtracting the standard E R P from that o f the deviant ERP. D uration deviant stim uli ‘trial 1’ evoked a late M M N com ponent that w as independent o f the N1 potential. Frequency deviant stim uli ‘trial 2 ’ evoked a M M N that appeared to include the region o f the descending lim b o f the N l . The M M N response evoked in ‘trial 3 ’ to both frequency and duration deviant stim uli included both a late M M N as in the duration paradigm and an early M M N due to the frequency deviance (Figure 2. 6). M easured at Fz the m ean (± SD ) latencies for duration, frequency and 'both' M M N across subjects w ere, 173 (± 53) m s, 139 (± 42) m s and 169 (± 55.6) ms. A N O V A revealed that there w ere no significant differences in M M N latency as a function o f deviant stim ulus type (F(2,m)= 0.79; p= 0.448) or

across electrodes (F(4,28) = 1 29; p = 0.30). H ow ever, inspection o f the cell m eans

MMN and duration MMN at electrode sites, F3 (t = 2.56; p = .0.038), FC3 (t = 2.49; p = 0.041) FC4 (t = 2.40; p = 0.051) and between frequency M MN and duration/frequency MMN at electrodes sites, FC3 (t = 2.49; p = 0.041), FC4 (t = - 2.44; p = 0044). Because o f these pairwise differences in some o f the means the main effects o f the ANO VA were not significant. However, the interaction o f stimulus type and electrode location approached significance (F(g, 56) = 3.142; p = 0.08). There were no significant differences in MMN amplitude as a function o f deviant stimulus type (F(2,i4) = 3.59; p = 0.06) or across electrodes (F(4,28) = 138; p = 0.27) (Figure 2. 6). Statistical analysis o f the predicted and obtained MMN revealed a close similarity between the waveforms at Fz using an epoch o f 0-350m s (Pearson’s correlation coefficient = 0.91).

11

a

b

c

F igure 2. 6. (i) S tandard and deviant w aveform s evo k ed to standard stim uli o f 700 Hz 75 m s and deviant stim uli o f (a) 700 Hz 25 m s, (b) 1000 H z 75 m s and (c) 1000

Hz 25 ms. (ii) deviant m inus standard subtraction w aveform s, (d) C om parison o f

predicted M M N (addition o f M M N evoked by tw o deviant stim uli varying in one characteristic) and M M N evoked by a deviant stim ulus varying from the standard in tw o characteristics, red w aveform represents predicted M M N .

2.5.5 Discussion.

This study has shown that the auditory stimulation techniques are successful in generating stimuli that allow AERPs to be recorded by off-line data analysis of the raw BEG. The stability o f the response to the standard stimuli has been investigated, revealing that the standard AERP is not affected by the characteristics o f the deviant stimulus.

It has also been confirmed that MMNs can be evoked in response to a number of stimulus characteristics and can be separated on the basis o f the morphology of the MMN. The MMN is evoked in response to the difference between the deviant stimulus characteristics and the memory trace o f the standard stimulus characteristics (for example see reviews by Naàtânen (1995a) and Sams (1991). The stability o f the standard AERP enables us to state confidently that any differences in the MMN analysed using a subtraction waveform, evoked by different deviant stimuli (with identical standard stimuli) are due to the deviant stimulus characteristics alone. The peak latency o f the frequency MMN was statistically shorter from that o f the

‘duration’ and combined M M N ’s. The combined MMN contained contributions from the frequency and duration MMN. The duration sub-component was larger in amplitude resulting in a peak latency similar to that o f the duration MMN.

Our finding that summation o f MMN evoked by two deviant stimuli varying in one stimulus feature predicts the MMN to a single stimulus varying in both are in agreement with the hypothesis that standard stimuli leave neuronal representations of its acoustic characteristics in areas o f auditory cortex that are compared independently to deviant stimuli. In addition these findings also support the opinion

that the comparison o f these neuronal representation o f acoustic characteristics can be accessed simultaneously i.e. in parallel.

Significant MMNs evoked by a frequency difference has been demonstrated to be stable in-group analysis and even at individual subject levels (Escera, Grau, 1996). A study that investigated the variability and replicability o f different types o f MMN found that the ‘duration’ M MN was more stable than the ‘frequency’ M M N both within and across subjects (Pekkonen et al., 1995). Although somewhat lower than that o f adults, significant MMNs in individual paediatric subjects have been recorded (Uwer, Von Suchodoletz, 2000). The presence or absence o f an M M N is dependent to a considerable extent on the methods employed during data analysis (McGee et al., 1997). The stability o f the MMN recorded in our adult study validates our methodology of auditory stimulation, EEG recording and data analysis which will be applied to paediatric clinical populations in later chapters.

3 Intracranial recordings of auditory event related