In total, we analyzed 5357 slow-phase segments from the responses of six birds. The data resulted from a manifold of conditions: the individual birds (Table 1), clockwise and counter-clockwise stimulation, binocular and monocular stimulation, stimulus velocity and age of the birds, given in PHDs.
Table 1
Distribution of number of cases in respect to individual birds
owl
|
All
|
F
|
G
|
H
|
I
|
J
|
K
|
#
|
5357
|
1095
|
302
|
411
|
1454
|
1154
|
941
|
Since responses to clockwise and counter-clockwise stimulation were equivalent in binocular adults (Wagner et al. 2021), we pooled the responses in these two conditions for the further analyses. Table 2 shows that 1380 data points came from binocular stimulation, 2335 from monocular stimulation in the T-N direction, while monocular stimulation in the N-T contributed 1642 data points. With respect to age, we attempted to record data at certain PHDs for most velocities and at the remaining PHDs only for stimulus velocities of 10, 15 and 30 deg/s. Thus, number of cases at the different PHDs (Table 3) and the different velocities (Table 4) differ. Therefore, the much higher number of data we obtained with stimulus velocities of 10, 15 and 30 deg/s than with the other stimulus velocities should yield the most reliable results that may serve as critical benchmarks for interpretation. We chose to present the data from the other stimulus velocities in the following as well, because no other data from juvenile owls is available so far and because they illustrate the development more broadly. In this sense, we regard them as supplementary data that complete the picture (for more discussion see below). With respect to individual birds, we concentrated on certain velocities for certain birds (owl I: 10 deg/s; owls G + H: 15 deg/s, owls J + K: 30 deg/s). Owl F was tested with all velocities.
Table 2
Distribution of the number of cases on different conditions (binocular, N-T, T-N)
|
binocular
|
owls tested
|
N-T
|
Owls tested
|
T-N
|
Owls tested
|
#
|
1380
|
6
|
1642
|
6
|
2335
|
6
|
Table 3 Distribution of number of cases in respect to age
age
|
11
|
12
|
13
|
14
|
15
|
16
|
17
|
18
|
19
|
#
|
14
|
81
|
65
|
252
|
354
|
303
|
297
|
498
|
348
|
owls
|
H, K
|
F, H, K
|
F, H
|
F, G, I, J, K
|
F, G, I, J, K
|
H, J, I, K
|
H, I, J
|
F, G, H, I, J, K
|
F, G, H, I, K
|
|
age
|
20
|
21
|
22
|
23
|
24
|
25
|
26
|
27
|
28
|
#
|
188
|
319
|
259
|
119
|
174
|
459
|
194
|
212
|
44
|
owls
|
G, H, I, J
|
F, G, I, J, K
|
F, I, J
|
H, I, K
|
F, I, J
|
F, G, H, I, J, K
|
F, H, I
|
G, I, J, K
|
I
|
|
age
|
29
|
30
|
31
|
32
|
33
|
35
|
36
|
37
|
38
|
39
|
40
|
49
|
50
|
56
|
65
|
#
|
102
|
117
|
14
|
52
|
186
|
35
|
124
|
96
|
27
|
71
|
80
|
102
|
55
|
31
|
85
|
owls
|
I, J
|
K
|
H
|
H
|
I, J, K
|
I
|
J, K
|
J
|
H
|
J
|
G, J
|
J
|
K
|
I
|
K
|
Table 4
Distribution of number of cases in respect to velocity
Velocity (deg/s)
|
5
|
10
|
15
|
20
|
30
|
40
|
60
|
80
|
#
|
172
|
2039
|
713
|
246
|
1586
|
302
|
233
|
66
|
owls
|
F,J, K
|
F, I, J, K
|
G, H
|
F, J, K
|
F, J, K
|
F, J, K
|
F, J, K
|
F, J, K
|
In the following we first describe general observations of the juveniles in the stimulus set-up during the recordings, then present the temporal development of binocular responses, and finally report responses to monocular stimulation.
General observations of juvenile barn owls during recording
Tests with three owls started before the birds showed a reaction to the stimulus, and before they presumably opened their eyes. The eye lids are closed at birth. Then a small slit can be seen, but it is not clear whether the birds really see something. The latter can only be inferred from behavioral reactions, if not electrophysiological methods like EEG or invasive methods are to be used. We did not use the latter methods, but relied on behavioral testing. We tried several stimuli apart from the wide-field stimulus later used for recording optocollic data. Amongst these were stimulation with a moving stick or moving hand. During these attempts, the owls were typically sitting in the drum on different platforms. Stimulation always lasted several minutes. Since the birds had been removed from the parents at this time and were held in a comfortable environment close to the experimenters, tests with the very young birds could be repeated several times a day.
While owl K did not react to the optomotor stimulus on PHDs 9 (see video 1 in supplements) and 10, it showed the first following behaviors on PHD 11 while it was sitting in a beaker in the drum and was stimulated by wide-field motion (see video 2 in supplements; Fig. 3a-c). Likewise, owl F did not follow stimulus motion on PHD 11, but did so on PHD 12. Thus, in these two birds the very first reactions to the wide-field stimulus could be documented. Owl J was tested every day from PHD 10 on. It first reacted to the stimulus on PHD 13, but the first data available is from PHD 14. In the other three birds, testing started also at PHD 14. All six owls showed persistent reactions from PHD 14 on (see video 3 in supplements). However, periods during which the birds followed the stimulus were typically interrupted by periods during which the birds did not react (see video 3 in supplements). Also, apart from the rotational movements, sometimes translational movements of the head were observed (see video 3 in supplements). The latter were not further analyzed. Across owls, quantitative data were obtained from PHD 11 to PHD 65.
Typically, very young birds were placed in a staining dish or a beaker, supported by soft paper for comfort, but otherwise free to move during the recording (see video 3 in supplements). It was obvious that very young birds (approximately up to PHD 13) had problems with stabilizing the head. Nevertheless, high-gain responses were observed. The head was above the upper rim of the dish, with the lower jaw often touching the rim. In this situation, the head rotated, following the rotation of the stimulus. From about PHD 14, the birds could hold their head (Fig. 2a). Although the birds were not yet standing on their feet, now the head did no longer touch the rim of the staining dish. The birds were calm and typically followed the stimulus. Again, after a few more days (around PHD 20), the birds were also standing on the feet (Fig. 2b). At this time, the birds became more agile, and they sometimes started to negotiate the staining dish. Thus, we started to keep the birds in a beaker that was adapted to the size of the birds, and did no longer use the staining dish (Fig. 2c). Note that the birds were free to move in the staining dish or the beaker and were not restrained in any other way than being placed in a container. While the birds tolerated well being seated in a beaker, the responses of the birds became more variable, especially after PHD 30. The untrained birds often behaved in an agitated way, for longer periods they seemed to show no interest in the stimulus pattern and seemed to be distracted (see video 4 in supplements). Nevertheless, it was possible to record data after PHD 30 and up to PHD 65, the last day of juvenile life covered in this work.
Binocular optocollic responses of juvenile barn owls
Binocular data were obtained from all owls and for all stimulus velocities (Table 2). Binocular stimulation with a wide-field pattern very reliably elicited the OCR in juvenile owls of all ages. The birds showed persisting reactions for all stimulus velocities tested (Fig. 3a, d, g, j, m). Specifically, gains were adult-like from the first day of responding to the stimulus for all stimulus velocities tested (Fig. 4). In the following, we discuss five typical examples that provide a picture of the variability of the responses (Fig. 3a, d, g, j, m), and present a quantitative analysis (Figs. 4, 5).
The typical reaction of an owl to visual wide-field stimulation was that it followed the stimulus by head rotation. Stimulus movement in the counter-clockwise direction elicited a counter-clockwise head rotation during the slow-following phase (Fig. 3d). Opposite (clockwise) head turning was observed with opposite (clockwise) stimulus movement (Fig. 3a, g, j, m). A slow-phase segment ended with a saccadic turn in opposite direction to the slow-phase movement. The angular velocity of the head was almost constant while the owl followed the stimulus. This may be concluded from the almost linear change of head azimuth with time (Fig. 3a, d, g, j, m). The gains were often 80% or higher, and only 1 out of 19 slow-phase segments shown for binocular stimulation in Fig. 3 had a gain below 70% (see numbers close to the single slow-phase segments in Fig. 3a, d, g, j, m and Fig. 4).
Before analyzing the typical behavior of the birds presented so far, we point to some rare behavior. For example, a special situation is shown in Fig. 3g. Here, after the first following movement with a high gain and low-amplitude saccade, the owl ceased to follow the stimulus for about 3 secs, before it started the next following movement (see arrow in Fig. 3g). As may be seen from the gain values noted in Fig. 3g (83.6 and 72.2), the period during which the owl was not following the stimulus was not included in the analysis. Another peculiarity occurred in the sequence shown in Fig. 3m. Here, the return saccade starting at 3.68 s was followed by a head movement that was much faster than the stimulus movement for more than half a second (3.92 to 4.64s, see arrow in Fig. 3m). Then a movement in the opposite direction occurred with a low velocity (4.72 to 4.96), before the bird started to follow the stimulus with a gain of 86% at 5.12s. Both, the fast head rotation from 3.92 to 4.64s and the movement in the opposite direction were also not included in the analysis. In the other 3 examples shown (Fig. 3a, d, j), the owl followed the stimulus during the total time sequence shown, as it did in the vast majority of cases. Note, however, that the amplitudes of the following movements varied considerably. We did not further analyze amplitudes and durations of the slow-phase segments, but concentrated here on the development of gains.
The quantitative analysis of the data sets for stimulus velocities of 10, 15, and 30 deg/s (Fig. 4) demonstrated that adult-like gain values were reached very early. The median gains reached an adult-like value from the first day of responding. For example, the first day of responding for 30 deg/s was on PHD 11 in owl K (Fig. 4f). Already at this day, the gain was not statistically different from the gain at PHD 33 (Mann-Whitney U-Test, number of cases PDH 11: 6, PHD 33: 11, U = 32, z-score = 0.05025, p = 0.96012). The data of all owls (data were recorded with owls F, K, J) showed a similar picture (Fig. 4e). Median gains stayed pretty much constant during the whole period of recording from PHD 11 to PHD 40. The data recorded in the whole period were pooled and tested against the data from adult birds as published in Wagner et al. (2021). There was no difference between the two data sets (Mann-Whitney U-Test, number juvenile: 407, number adult: 73, U = 15073, z-score = 0.1989, p = 0.8424; see also Fig. 5). The time course of development was fitted by a sigmoidal function (which was chosen as it describes also the monocular data (Figs. 6, 7), see Material and methods). The function fitting the 30 deg/s data demonstrated that the 90%-PHD corresponded to the first day of responding (Fig. 4e, f).
Similar observations were made for a velocity of 15 deg/s for which data from owls G and H were available. The earliest recording in owl H, at PHD 13, already yielded data (median gain value 87.8) that was statistically not different from the data at PHD 32 (median gain value 91.1) (Mann-Whitney U-Test, number of cases PDH 13: 15, PHD 32: 20, U = 141, z-score = 0.28333, p = 0.77948). Again, this was confirmed, if the data of owls G and H were pooled (Fig. 4c), and also here the juvenile and adult data were not different from each other (Mann-Whitney U-Test, number juvenile: 211, number adult: 11, U = 1046.5, z-score=-0.5224, p = 0.6015; see also Fig. 5).
The data for a stimulus velocity of 10 deg/s were mainly based on recordings with owl I (Fig. 4b), with some data also from owls F, J, and K (Fig. 4a). Again, the very first recordings, on PHD 14, showed a median gain (84.3) close to that measured at PHD 29 (85.4) or PHD 33 (92.2), and much higher than that determined at PHD 56 (72). However, for 10 deg/s stimulus velocity, the juvenile data yielded significantly lower gains than measured in adults (Mann-Whitney U-Test, number juvenile: 634, number adult: 64, U = 9867, z-score= -6.7781, p = 1.218*10− 11; see also Fig. 5). The reason for this difference is not clear. For all other stimulus velocities tested, the juvenile and the adult responses were not different (Fig. 5).
Median gains with binocular stimulation were close to 100% for velocities up to 20 deg/s (Fig. 5). The median gains decreased to 70% for velocities up to 60 deg/s and to about 40% at a stimulus velocity of 80 deg/s (Fig. 5). Gain values did not change in the course of development, apart from some extraordinary recording days, where median values were either below (Fig. 4a, b, PHDs 17 and 18) or above (Fig. 4e, PHD 36) the rest of the values. The differences between the 1st and the 3rd quartiles were between 14.3 and 24.2 percent of gain in absolute terms or, relative to the median gain values, between 15 and 29 percent. In total, binocular gains measured in juvenile birds were not statistically different from adult gains for 6 out of 7 stimulus velocities tested that ranged from 5 to 60 deg/s (Fig. 5).
Monocular optocollic responses of juvenile barn owls
While an adult-like response behavior was observed from the first PHD of responding for binocular stimulation, the response pattern for monocular stimulation was more complex. Major differences occurred in the responses to N-T and T-N stimulation. First, at the first PHD at which the bird responded (PHD 11 in owl K, stimulus velocity: 30 deg/s), the responses to both T-N and N-T stimulation were short and of low gain (Fig. 3b, c; see Fig. 6i, j for a quantitative analysis of the reaction with a stimulus velocity of 30 deg/s). This changed fast for the responses to T-N stimulation not only for low stimulus velocities (10 deg/s, Fig. 3n, Fig. 6d), but also for 30 deg/s (Fig. 3e, h, k; Fig. 6j). By contrast, gains to N-T stimulation remained low for several days. These gains gradually increased during development. At PHD 19 responses to N-T stimulation were of high gain for a stimulus velocity of 10 deg/s (Fig. 3o: single gain values 81.5 and 85, quantitative analysis in Fig. 6c: median gain: 76.8), but still low for 30 deg/s (Fig. 3i: single gain values: 26 and 31, quantitative analysis in Fig. 6i: median gain: 47). At PHD 27, gains for N-T stimulation had increased also for a stimulus velocity of 30 deg/s (Fig. 3l: single gain values: 71, 56, 71, 70.7, 64.5, quantitative analysis in Fig. 6i: median gain: 66).
The fitting of the responses offered a possibility to gain insight into the duration of the development. The inflection points of the fit function as determined from the data were all between 9 and 13 PHDs. This suggested to us that the development started at similar times for all velocities and conditions. The duration of development may be derived from the 90 − 50 differences and the 90%-PHDs that are both related to the factor c of the fitting function. These two parameters varied a lot with stimulus velocity (range 10–26 PHD for 90%PHD, Fig. 7b). They yielded highly correlated values (7 data points, correlation coefficient: 0.988, p < 0.00003). In the following we use the 90%-PHDs as a measure for the duration of the development (Fig. 7b). The 90%-PHDs for N-T stimulation were 26, 17, and 24 for stimulus velocities of 10, 15, and 30 deg/s, respectively (Fig. 7b). The responses to T-N stimulation were high from very early on. The 90%-PHDs for T-N stimulation were between PHD 11 and 14 for all stimulus velocities tested (Fig. 7b). In other words, the 90%-PHD was reached almost immediately after the first day of responding (Fig. 7b). The longest time necessary to reach 90% of the final values with T-N stimulation was three days which occurred for a stimulus velocity of 30 deg/s (Fig. 7b).
The fitting of the data did not only make it possible to quantify the duration of development, but also helped to obtain insight into the differences in upper gain values for the different stimulus types (binocular, monocular T-N, monocular N-T). For binocular stimulation sufficient data for fitting was available for 10, 15 and 30 deg/s. The comparisons showed that the upper values for binocular stimulation were very close to the upper values for T-N stimulation (compare dashed and dotted lines in Fig. 7a). Larger differences were seen between the responses to binocular and T-N stimulation on the one and the responses to N-T stimulation on the other side (Fig. 7a). The upper values for N-T responses were significantly lower than the upper values for T-N responses (7 pairs of upper values, Wilcoxon Matched Pairs signed rank test, z=-2.418; p = 0.016).
Figure 7a also shows that the differences in the upper values for T-N and N-T responses increased with stimulus velocity. This resulted in an increase of the T-N/N-T factors with stimulus velocity (Fig. 7c). A comparison with the adult T-N/N-T-factors showed that the juvenile T-N-N-T factors derived from the fits are very similar to the measured T-N/N-T-factors in adults for velocities up to 20 deg/s, while they are slightly larger for higher velocities.
While the upper values of the fits yielded data that reflected the final result of development, it was also interesting to examine the temporal change of the gains and specifically the T-N/N-T factors during development. To this end, we pooled data from three distinct age periods (PHD 11–18, PHD 19–25, and PHD 26–65) and compared the results with the result from the adults (Fig. 7d-f). While this approach coarsens the time resolution of the data compared to the data shown in Fig. 6, the resulting curves are less bumpy and allow better insight into the underlying mechanisms than the plots shown in Fig. 6. Figure 7d demonstrates that the T-N gains were high from early on. T-N gains for a stimulus velocity of 60 and 80 deg/s decreased in the last period (Fig. 7d). Note, however, that the latter data points are based on low numbers (Table 4). In the course of development, gains for N-T stimulus did not change much for stimulus velocities up to 20 deg/s and also not for 40 and 60 deg/s (Fig. 7e). The gain for a stimulus velocity of 30 deg/s increased in the last period ranging from 26–65 PHDs compared to the gains in the earlier two periods and reached an adult-like value (Fig. 7e). Figure 7f summarizes the data shown in Fig. 7d and e and demonstrates that the measured factors T-N/N-T for velocities up to 20 deg/s were close to 1 and adult-like from the first period on. By contrast, there were developmental changes of the factors T-N/N-T for velocities above 20 deg/s. The values were larger than the adult values for the first two time-averaging periods from PHD 11–18 and PHD 19–25. The T-N/N-T factors derived from the measured gain data reached adult-like values for the last analysis period (PHD 26–65) (Fig. 7f) consistent with the T-N/N-T factors derived from the fitted data shown in Fig. 7c.