Rearing differences in novelty seeking behavior
All IS monkeys (11 of 11) and all but one of the NS monkeys (8 of 9) approached the threshold of the novel test box and peered into it during testing (X2=1.287, df=19, p=0.257). Monkeys from both of the rearing conditions were therefore aware of the test box, but IS monkeys contacted the box threshold faster (F1,18=7.425, p=0.014), were in more frequent contact with the box threshold (F1,18=12.876, p=0.002), and spent more time on the box threshold (F1,18=5.584, p=0.030) compared to NS monkeys. More IS (11 of 11) than NS (5 of 9) monkeys likewise chose to enter the box (X2=6.111, df=19, p=0.013). IS monkeys entered faster (F1,18=13.124, p=0.002), did so with greater frequency (F1,18=9.008, p=0.008), and spent more time in the box (F1,18=4.863, p=0.041) compared to NS monkeys.
Latencies to contact the box threshold and enter the box did not differ significantly over repeated test sessions, but rearing condition-by-test session interactions were discerned for other measures. Over the five repeated test sessions, IS monkeys less often contacted the box threshold (F4,40=5.834, p=0.001), spent less time on the box threshold (F4,40=10.447, p<0.0001), entered the box less often (F4,40=7.615, p<0.0001), and spent less time in the box (F4,40=4.013, p=0.008). These same measures for NS monkeys consistently remained at low levels ().
Figure 1 Rearing differences in novelty seeking at 2.5 years of age. Measures of (A) frequency to contact the novel box’s threshold, (B) frequency to enter the novel box, and (C) duration spent in the novel box across the five test sessions are presented (more ...)
More IS (9 of 11) than NS (2 of 9) monkeys engaged in object exploration in the test box (X2=7.103, df=19, p=0.008). Because only two of the NS monkeys engaged in object exploration, further analysis of object exploration was restricted to IS monkeys. A significant preference for exploration of novel vs. familiar objects was discerned for the IS monkeys. Specifically, IS monkeys explored novel vs. familiar objects more frequently (80 ± 22 vs. 50 ± 17 contact counts; t10=2.280, p=0.046) and for longer durations (504 ± 156 vs. 204 ± 67 seconds; t10=3.012, p=0.013). Exploration latencies for novel vs. familiar objects were not significantly different. Latencies, frequencies, and durations did not differ significantly across repeated test sessions for object exploration in IS monkeys.
Consistency of novelty seeking measures within and between test sessions
Kendall’s coefficient of concordance computed separately for six measures of behavior exhibited by IS and NS monkeys indicated that individual differences were consistent across repeated test sessions (threshold latency: W=0.71, df=19, p<0.0001; threshold frequency: W=0.71, df=19, p<0.0001; threshold duration: W=0.80, df=19, p<0.0001; box entry latency: W=0.75, df=19, p<0.0001; box entry frequency: W=0.80, df=19, p<0.0001; box duration: W=0.81, df=19, p<0.0001). The mean for each measure was therefore used for the PCA to generate a single summary score of novelty seeking behavior for each of the monkeys. All six measures were highly correlated with one another (rs range: 0.89 to 0.98), and all six measures loaded onto Factor 1 in the PCA which explained 87% of the total variance. Based on the following factor loadings (threshold latency = 0.84, threshold frequency = 0.98, threshold duration = 0.97, box entry latency = 0.93, box entry frequency = 0.96, box duration = 0.92), Factor 1 provided a suitable summary index of novelty seeking.
Plasma cortisol measures
Novelty seeking behavior occurred in the absence of adrenocortical activation in both IS and NS monkeys. Cortisol levels immediately after test sessions 1 and 5 did not differ significantly from baseline levels measured in undisturbed home cage conditions for monkeys from both rearing conditions (F2,36=0.847, p=0.434). This outcome was evident whether or not the time of blood sample collection was included in the statistical analysis as a covariate. Baseline cortisol levels in undisturbed home cage conditions before initiation or after completion of the novelty test sessions did not differ significantly in IS compared to NS monkeys (F1,18=1.012, p=0.328). Neither baseline nor post-test cortisol levels correlated with the summary index of novelty seeking behavior derived by PCA.
CSF measures of monoamine metabolites and CRF
CSF levels of 5HIAA, HVA, and CRF did not differ significantly in IS compared to NS monkeys (). A rearing condition-by-age interaction was discerned for CSF levels of MHPG (F1,18=4.527, p=0.047). IS compared to NS monkeys had lower levels of MHPG at 1.25 years of age (F1,18=4.731, p=0.043), but rearing differences were no longer evident at 2.25 years of age (). From 1.25 to 2.25 years of age, MHPG, 5HIAA, and HVA levels declined significantly in monkeys from both rearing conditions, while similar declines in CRF levels did not reach statistical significance (). Despite these age-related declines in monoamine metabolite levels, the relative rank order of individual differences was consistent across repeated sampling points for CSF levels of 5HIAA, HVA, and CRF, but not MHPG (). None of the CSF measures correlated with the summary novelty seeking index derived by PCA.
Rearing differences, age effects, and individual consistency in cerebrospinal fluid (CSF) monoamine metabolite and corticotropin-releasing factor (CRF) measures.
Consistency of individual differences in behavior across situations
In an earlier experiment, at 35 weeks of age, novelty seeking behavior was assessed in the same monkeys under inescapable conditions that involved involuntary separation from the home cage and occurred in the presence of each monkey’s mother (Parker et al 2004
). The relative rank order of individual differences in novelty seeking in the earlier experiment and the summary index of novelty seeking behavior described above was remarkably consistent. Specifically, the summary novelty seeking index derived by PCA at 2.5 years of age correlated with object exploration latencies (rs
= −0.479, df=18, p<0.05) and novel object exploration frequencies (rs
= 0.491, df=18, p<0.05) at 35 weeks of age.