4.3.1. Mechanical stimulation
A clear relationship was demonstrated between the intensity o f the stimulus and the mean latency, amplitude, duration, and area o f the flexion reflex response. A n analysis was made o f the effect o f stimulus intensity evoked with von Frey hairs at threshold, 2.8
X threshold, 7.6 x threshold, 21x threshold, and finally with a heel lance, on the latency, amplitude, duration, and area, o f the control group flexion reflex response across the entire PCA range from 28 - 42 weeks.
N o relationship was demonstrated between any o f the parameters and PCA when either latency, amplitude, duration, or area were plotted against PCA at each stimulus intensity relative to threshold, despite the clear relationship o f stimulus intensity to EMG
parameters across the whole sample. The plots o f the data from individual infants showed random scatter in all cases. A full record o f these plots is shown in Appendix 1. However, the plots did reveal some outlying values w hich were traced back to the individual infants whose parameters they were. In all cases, there was no apparent reason for the aberrant values. When plots were made o f EMG parameters versus PCA for the observer groups at each stimulus intensity, they also show ed random scatter in a similar range to those for the control group. Therefore, it was decided to combine all the groups and plot EMG parameters versus PCA again for each stimulus intensity. This process did reveal som e more outlying values w hich were again traced back to individual infants in the observer groups, but the values for each observer group as a whole did not appear as a separate group on any o f the plots for any o f the parameters. The statistician (Dr R Brooks, Department o f Statistical Science, University College London), was consulted and it was considered that the outliers m ay not represent valid observations, and could be part o f a separate population. H owever, they have been included in the data analysis, and the subsequent figures arising from that analysis on the advice o f the examiners, because the converse could also be true, these values possibly representing extremes o f the biological range.
For each o f the EMG parameters; latency, amplitude, duration, and area, there were some random m issing values for each stimulus intensity, w hich meant that the numbers for each stimulus intensity across all EMG parameters were uneven when the initial summary statistics were performed. However, subsequent tw o w ay analyses o f variance for each EMG parameter were only performed on com plete data sets across the first 4 stimulus intensities (see figure 4.4A-D ). The analyses o f variance did not include responses to heel lance as this stimulus was not quantifiable numerically. The analyses
w ere also performed in a way which dealt with the ‘infant effect’ across all stimulus intensities. Subsequently, line plots were made o f the data from individual infants at stimulus intensities from threshold - 21 x threshold, to see whether the patterns evident in the graphs o f means ± SEM were reflected in these data (see figure 4.5A-D). The data from the heel lance stimulus were not included in these line graphs because this stimulus was given to a reduced number o f infants (n=13), and it seemed important to show the data from the majority o f infants over the first 4 levels o f stimulation.
In addition, the possibility was investigated o f a relationship between PCA and the EMG parameters o f the flexion reflex response in terms o f absolute stimulus intensity, rather than in relation to threshold. The latency, amplitude, duration, and area o f the flexion reflex response to 3 von Frey hair weights, 1.66g, 2.75g, and 4.57g, were examined across the full PCA range from 28 - 42 w eeks (n=30, 25, & 27, for the 3 w eights respectively). The reduced numbers were due to the fact that, because o f differences in initial threshold, not all infants were tested at these weights, and for each w eight, there were infants for whom the weight was the threshold for the response, and others for whom it was 2.8 or 7.6 x threshold. For the heaviest weight, there was one infant for w hom it was 21 x threshold.
Plots were made o f the latency, amplitude, duration, and area o f the response versus PCA for each o f the von Frey hair weights, and a relationship was demonstrated between the duration o f the response and PCA for 1 o f the weights, 2.75g, which is described in greater detail in section 4.3.1.3. Apart from this, no relationships were demonstrated, and the plots showed random scatter for the other parameters.
4 .3 .1.1. Latency
There w as a clear decrease in the latency o f the flexion reflex response w ith increasing m echanical stimulus intensity from 2.8 x threshold to heel lance. The m ean latency at 2.8 X threshold was 790.4m sec (SEM =74, n=35), whereas the m ean latency on heel
lance was 302m sec (SEM =67.9, n=13, see figure 4.4A ). H ow ever, this pattern did not extend to the latency at threshold (649.4m sec, SE M =67.1, n=35), w hich w as low er than that at 2.8 x threshold. Nevertheless, two way analysis o f variance over the first 4 levels o f stimulation showed that the effect o f stimulus intensity on latency w as significant (F=3.21, D F= 3/102, P=0.03). This trend is reflected in the line plot o f data from individual infants o f stimulus intensity versus latency (see figure 4.5 A ), in w hich many o f the infants showed an increase in latency from threshold to 2.8 x threshold, follow ed by a subsequent decrease in latency to 21 x threshold.
4.3.1.2. Amplitude
There was a clear increase in the peak amplitude o f the flex io n reflex response with increasing stimulus intensity from threshold to heel lance. The m ean peak amplitude at threshold w as 78.6pV (SEM =13.4, n=38), whereas the m ean peak amplitude on heel lance w as 2 7 0.4p V (SEM =33.3, n=13, see figure 4.4B ). The tw o w ay analysis o f variance over the first 4 levels o f stimulation showed that there w as a highly significant effect o f stimulus intensity on peak amplitude (F=7.36, D F = 3 /1 11, P < 0.001). This is also mirrored in the data from individual infants (see figure 4 .5 B ), in w hich the greatest amplitudes are shown in response to stimulation at 21 x threshold.
Again, as with the mechanical stimulation data, there were random m issing values for each o f the EMG parameters at each stimulus intensity above initial threshold. This made the numbers for each stimulus intensity uneven when the initial summary statistics were performed. However, it was not necessary to eliminate cases wdth m issing values from the data in order to perform further statistical analysis, as none was made.
N o relationship was demonstrated between electrical stimulus intensity and either the latency, amplitude, duration or area o f the flexion reflex response. The group means and standard errors o f the mean for each EMG parameter at each stimulus intensity are displayed in figure 4.8A -D . It was decided not to proceed w ith analysis o f variance as no relationships between the variables were evident on initial data analysis. Nevertheless, line plots were made o f complete sets o f data from individual infants across all levels o f stimulation (n=5), and these plots mirrored the lack o f trends seen in the graphs o f means ± SEM (see figure 4.9A-D).
When, as had been performed for mechanical stimulation, the latency, amplitude, duration, and area o f the flexion reflex response were plotted against PCA at 3 absolute stimulus intensities (5, 5.5, and 6mA) rather than relative to threshold, no relationships were demonstrated between these parameters and PCA at any o f the stimulus intensities.