Data of all participants were included in the analysis.
4.3.2.1 Primary Outcome
163 Delayed Word Recall Correct
There was no significant main effect of the treatment on correctly recalled words in the Delayed Word Recall task, F (3, 28.74) = 1.88, p = .16. There was no main effect of
period, F (3, 29.30) = 0.43, p = .73; or treatment x period interaction, F (9, 35.26) = 1.20, p
= .33. No significant differences in performance were observed following administration of active treatment drinks compared to placebo.
4.3.2.2 Secondary Outcomes - Memory
Immediate Word Recall Correct
There was no significant main effect of treatment on correctly recalled words, F (3, 33.78)
= 1.87, p = .15. There was a significant main effect of period, F (3, 34.72) = 2.85, p = .05, with participants remembering fewer words at their first visit, but no significant interaction between treatment and period, F (9, 32.22) = 1.07, p = .41. Comparison with placebo showed no significant differences following administration of any of the active treatment drinks.
Immediate Word Recall Errors
There was no significant main effect of treatment on number of incorrectly recalled words, F (3, 34.77) = 0.58, p = .63. There was no main effect of period, F (3, 25.29) = 1.68, p = .20, and no significant interaction between treatment and period, F (9, 28.19) = 1.03, p = .44. There were no significant differences observed following administration of active treatment drinks compared to placebo.
164 Delayed Word Recall Errors
There was no significant main effect of the treatment drinks on incorrectly recalled words, F (3, 19.61) = 0.19, p = .90. There was no main effect of period, F (3, 20.20) = 0.98, p = .42, and no interaction between treatment and period, F (9, 21.16) = 1.23, p = .33.
However, participants performed significantly better following the placebo drink (M = 1.67) compared to after the caffeine only drink (M = 1.71), p = .04.
Word Recognition Accuracy
There was no significant main effect of treatment on word recognition accuracy, F (3, 33.71) = 1.08, p = .37. There was no main effect of period, F (3, 35.94) = 1.42, p = .25; or interaction between treatment and period, F (9, 35.86) = 1.60, p = .15. There were no significant differences observed following administration of active treatment drinks compared to placebo.
Word Recognition Speed
There was no significant main effect of treatment on reaction time for correctly recognised words, F (3, 29.14) = 0.23, p = .88; no main effect of period, F (3, 28.95) = 0.52, p = .67;
and no significant treatment x period interaction, F (9, 32.12) = 0.84, p = .59. There were no significant differences observed following administration of active treatment drinks compared to placebo.
Picture Recognition Accuracy
165 There was no significant main effect of treatment on the percentage correct responses on the picture recognition task, F (3, 48.67) = 0.29, p = .84. There was no main effect of period, F (3, 43.20) = 0.31, p = .82; or interaction between treatment and period, F (9, 32.99) = 0.66, p = .74.
Picture Recognition Speed
There was no significant main effect of treatment on the reaction time of correctly
recognised pictures, F (3, 15.36) = 1.34, p = .30. There was no main effect of period, F (3, 23.55) = 2.61, p = .08; or interaction between treatment and period, F (9, 23.05) = 1.57, p
= .18. There were no significant differences observed following administration of active treatment drinks compared to placebo.
Face Recognition Accuracy
There was no significant main effect of the treatment drinks on the correct responses on the face recognition task, F (3, 29.04) = 0.78, p = .51, and no main effect of period, F (3, 31.48) = 1.57, p = .22, or treatment x period interaction, F (9, 20.36) = 1.63, p = .17.
Comparison with placebo revealed no significant effects.
Face Recognition Speed
There was no significant main effect of treatment on the correct reaction time responses on the Face Recognition task, F (3, 39.04) = 0.17, p = .92. There was no main effect of
period, F (3, 32.65) = 0.35, p = .79; or interaction between treatment and period, F (9,
166 26.65) = 1.86, p = .10. No significant differences were observed following administration of active treatment drinks compared to placebo.
Table 4.3 Memory Results, means and standard errors
Treatment Combined Glucose Caffeine Placebo
Immediate Word
*Significant compared to placebo at p < .05
167 4.4 Discussion
The current study aimed to provide further insight into the effects of caffeine and glucose containing drinks using a more realistic testing paradigm with greater ecological validity.
More specifically, we assessed the effects of drinks on memory performance when administered in the afternoon prior to retrieval, primarily on delayed free recall. Whereas previous studies have shown that in isolation, caffeine and glucose improve memory when administered prior to retrieval, in particular when measured by free recall (Angelucci et al., 1999; Angelucci et al., 2002; Kopf & Baratti, 1996; Flint & Riccio, 1998; Manning, et al., 1992; Manning, Stone, Korol & Gold, 1998; Sünram-Lea, Foster, Durlach & Perez, 2002a), the results of the current study provided no evidence for such facilitation effect as neither the delayed free recall task, nor any of the other tasks showed glucose facilitation.
For caffeine and for both substances in combination there was no evidence of any
enhancement to performance when they were administered prior to retrieval. Indeed there was evidence to show that participants’ memory performance was impaired following caffeine, as participants’ performance was better following placebo compared to after the caffeine and combination drinks.
Previous research has found that pre-test administration of glucose attenuated retention loss indicative of infantile amnesia in rats (Flint & Riccio, 1998). Manning et al., (1998) found that 50g glucose improved memory performance when administered immediately prior to testing memory of a previously heard passage. It may be that whilst 40g glucose is a similar dose to 50g used in Manning et al.,’s study, it is not effective for enhancement of recognition memory. The dose response profile for different tasks may be quite sensitive to changes in dose levels. The dose response profile for glucose has been previously shown to be different for different types of tasks (Sünram-Lea, Owen, Finnegan & Hu, 2011), and so
168 it may be that the specific stages of the memory process i.e. learning, storage and retrieval are also modulated by different doses.
The caffeine drink was found to have detrimental effects on some of the performance measures, and so it seems that when consumed in isolation, caffeine has a negative effect.
This finding does not support previous literature as caffeine has previously been found to impair memory acquisition, but have beneficial effects on memory consolidation and retrieval (Angelucci et al., 1999; Angelucci et al., 2002). One explanation for this finding may be state-dependent memory effects; whereby memory retrieval requires the state is the same as when encoding took place (Bruins Slot & Colpaert, 1999; Ceretta, Camera, Mello
& Ruben, 2008). Sanday et al., (2013) found that pre-training caffeine-induced amnesia could be abolished by the administration of pre-test caffeine. This was further supported by the finding that whilst pre-training administration of caffeine impaired memory performance on a subsequent test, it did not alter the way mice learned to avoid the
aversive enclosed arm compared to saline (Sanday et al., 2013). Therefore, the pre-training amnesia was not related to impaired learning of the task (Sanday et al., 2013). In the current study caffeine was only administered at recall and not at encoding, whereas in previous studies where caffeine was found to have beneficial effects on memory (see Chapter 2) caffeine was administered prior to encoding; therefore caffeine was present at both encoding and retrieval and this could have resulted in state-dependent memory effects. Another reason why no effects were observed might have been that the time scale between encoding and retrieval was too short to see the beneficial effects of these
substances on memory retrieval. In this study the interval was only 6hrs from the start of the encoding session to the start of the retrieval session, whereas in previous research there has been a gap of 24hrs (Flint & Riccio, 1998; Manning et al., 1998) or 48hrs (Angelucci et al., 1999; Angelucci et al., 2002). The longer timeframe would increase the cognitive
169 demand of the task as participants would have to maintain the to-be-remembered material for longer.
In conclusion, this study aimed to examine the effects of administration of glucose and caffeine and their combination prior to memory retrieval, primarily as measured by the performance of correctly recalled words on the delayed word recall task. No facilitation of memory performance after glucose administration was evident on this task or any of the secondary outcome measures. Consequently, the result of this study do not support previous research which found more robust effects following retrograde glucose
administration (Flint & Riccio, 1998; Manning et al., 1998) and beneficial effects when caffeine was administered pre-retrieval (Angelucci et al., 1999; Angelucci et al., 2002).
Moreover, there were no effects when caffeine and glucose were administered in
combination. However, there are a number of factors that might have led to the failure to observe any effects. These include dose, time of day, level of fasting. For example, it would be useful to establish the dose-response profile of retrograde administration in order to see whether there is an optimal combination dose that is specific to enhancing retrieval.
The optimal time for drink administration should also be investigated.
170