Experiments performed

A series o f either 3 or 5 experiments w as performed using m echanical stimuli at each testing session on individual infants. 2 experiments w ere performed at each session w hen electrical stimuli were used. Som e infants were tested w ith both m echanical and electrical stimuli in one session. The entire testing period lasted betw een 1 and 3 hours depending on how many experiments w ere performed, and whether the infant was settled. In addition to the flexion reflex EM G responses being recorded on floppy disk, sample data and von Frey hair thresholds were recorded on a chart specially designed for this project (see figure 2.6 for a com pleted exam ple).

2.7.1. Mechanical stimulation

2.7.1.1. Initial threshold

The initial threshold for the flexion reflex w as established at the beginning o f the testing session by applying a von Frey hair perpendicularly downwards on the plantar surface o f the foot towards the outer aspect o f the heel (see plate 2.10). The spot w as always marked with a pen to ensure that stim uli were subsequently delivered to the same point. A very fine hair w as used to begin with, and working upwards in force, the initial threshold w as considered to have been reached w hen there w as EM G activity in one or other o f the m uscles being tested, accompanied by visible ^withdrawal m ovem ents o f the lim b away from the stimulus. Som etim es, EMG activity w as present w hen the visible withdrawal m ovem ents were very small.

Follow ing the establishment o f the initial threshold, subsequent von Frey hair experiments were conducted in random order, the randomisation being performed using

a scientific calculator. A period o f 3-5 minutes was allowed between each experiment in order that the effect o f the previous stimulation was minimised. Experiments involving heel lance and dummy heel lance were always performed at the end o f the testing session.

2.7.1.2. Repeated stimulation

In exactly the same place on the foot as had been used for initial threshold, the von Frey hair applying a weight 2.8 times threshold was applied in the same w ay, at 10 second intervals (see Amistrong-James 1975) for 10 times. Additionally, in a small group o f 10 ‘control’ infants aged 33-37 weeks PCA, repeated stimuli were applied according to the same protocol, but at 7.6 times threshold, for comparison with control infants in the 33- 37 w eek age group who had received repeated stimuli at 2.8 tim es threshold. The threshold follow ing repeated stimulation was then ascertained, starting with the von Frey hair 0.6 tim es threshold (1 hair in the series below that which had evoked the threshold response), to see if the threshold had been lowered follow ing repeated stimulation. The EMG response was recorded following each stimulation, and the response was noted visually, although it was not graded as the quantitative data were provided by the EMG recordings.

2.7.1.3. Sunrathreshold stimulation with single stimuli

Once again in the same place on the foot, single stimuli were delivered using the von Frey hairs applying a weight 7.6 times threshold (4 grades o f hair above initial threshold) and 21 times threshold (6 grades o f hair above initial threshold). A n interval o f 5 minutes was left between these 2 stimuli. As before, the flexion reflex EMG response was recorded and leg movements were noted visually.

2.7.1.4. Dummy heel lance and heel lance

The final experiments at the end o f the testing session were those involving dummy heel lance and heel lance. These were not performed on every infant in the study, only on those from whom a capillary blood sample was clinically required. The dummy heel lance was performed first, and involved warming the heel by holding it in m y hand, and then swabbing it on the outer aspect o f its plantar surface with an alcohol swab. Then, holding the heel as little as possible to minimise the stimulus being given, the 'Autolet' device without a lancet inserted, was applied to the skin, and the spring-loaded trigger released, delivering a stimulus whose 'touch' component w as very much like that felt when a heel lance was given, but without the noxious component o f the lancet.

After a waiting period o f 5 minutes, the heel lance was delivered. The procedure for warming and swabbing the heel and using the 'Autolet' was exactly the same as for the dummy heel lance, except this time a lancet was inserted, and the noxious stimulus was delivered. The period o f the EMG sweep (5 seconds), was allowed to elapse, to allow the flexion response to be recorded on the 'Sapphire' before the heel was squeezed and the blood sample was collected.

2.7.2. Electrical stimulation

2.7.2.1. Initial threshold

The initial threshold for the electrically-evoked flexion reflex w as established by giving trains o f electrical pulses via the small bipolar stimulator in the manner already

described (see section 2.6.2 o f this chapter). The stimuli commenced at a current o f 1mA, working upwards in intensity at intervals o f 0.5m A until the threshold o f the reflex was reached. Trains o f 5 stimuli were given. A s with mechanical stimulation, once the initial threshold was established, experiments were conducted in random order, the randomisation being performed using a scientific calculator. Again, a period o f 3-5 minutes w as left between each train o f stimuli in experiments involving only one train at a particular stimulus strength, for reasons already stated (see section 2.6.2 o f this chapter).

2 .1 2 .2 . Repeated stimulation

Trains o f electrical stimuli 1.85 times threshold (2-3m A above threshold), were delivered at intervals o f 10 seconds 10 times, in order to provide a comparison with m echanically-evoked flexion reflexes. The threshold follow ing repeated electrical stimulation was then ascertained by decreasing the stimulus intensity to 1mA b elow the initial threshold level in order to see if a flexion reflex could be evoked at this intensity. I f no reflex could be evoked at this strength, the current was increased in increments o f 0.5m A until the threshold following repeated stimulation was reached. The flexion reflex responses to each train were not averaged, in order to provide a direct comparison with m echanically-evoked withdrawal reflexes.

2.7.2.3 Suprathreshold stimulation with single trains o f stimuli

Follow ing the establishment o f initial threshold, single trains o f electrical stimuli were delivered in increments o f 0.5m A from 1mA above threshold, but in random order, to 2- 3m A above threshold, at which repeated stimuli were given (see above). Possibly because o f the more synchronous nature o f electrical stimuli, it was not possible to

increase the level o f stimulation above 3-3.5mA above threshold, as stimuli given above this level generally caused the infants to wince slightly.