Vision Research 102 (2014) 19–25
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Stimulus motion improves spatial contrast sensitivity in budgerigars (Melopsittacus undulatus) Nicola Kristin Haller a,b, Olle Lind c, Stephan Steinlechner a, Almut Kelber b,⇑ a
Institut für Zoologie, Stiftung Tierärztliche Hochschule Hannover, Germany Department of Biology, Lund University, Sweden c Department of Optometry and Vision Science, University of Auckland, New Zealand b
a r t i c l e
i n f o
Article history: Received 26 May 2014 Received in revised form 8 July 2014 Available online 27 July 2014 Keywords: Bird vision Budgerigar Spatial contrast sensitivity Motion vision Motion perception
a b s t r a c t Birds are generally thought to have excellent vision with high spatial resolution. However, spatial contrast sensitivity of birds for stationary targets is low compared to other animals with similar acuity, such as mammals. For fast flying animals body stability and coordination are highly important, and visual motion cues are known to be relevant for flight control. We have tested five budgerigars (Melopsittacus undulatus) in behavioural discrimination experiments to determine whether or not stimulus motion improves contrast sensitivity. The birds were trained to distinguish between a homogenous grey field and sine-wave gratings of spatial frequencies between 0.48 and 6.5 cyc/deg, and Michelson contrasts between 0.7% and 99%. The gratings were either stationary or drifting with velocities between 0.9 and 13 deg/s. Budgerigars were able to discriminate patterns of lower contrast from grey when the gratings were drifting, and the improvement in sensitivity was strongest at lower spatial frequencies and higher drift velocities. Our findings indicate that motion cues can have positive effects on visual perception of birds. This is similar to earlier results on human vision. Contrast sensitivity, tested solely with stationary stimuli, underestimates the sensory capacity of budgerigars flying through their natural environments. Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
1. Introduction Vision is thought to be one of the most important senses for birds, not the least for flight control and obstacle avoidance. To find a collision-free path through a cluttered environment, birds make use of optic flow cues (Bhagavatula et al., 2011). Perception of object motion is therefore a key feature of flight. Motion perception is very likely an achromatic visual task (Burton, 2000; Osorio, Miklosi, & Gonda, 1999), and mediated by one class of cones in birds, the double cones. Their special engagement in motion vision has been suggested earlier (Campenhausen & Kirschfeld, 1998), although the ultimate function of double cones is yet unclear (Bennett & Théry, 2007). In behavioural tests with stationary visual stimuli, birds were shown to have surprisingly low contrast sensitivity for achromatic gratings, compared to other vertebrates (Ghim & Hodos, 2006; Harmening et al., 2009; Lind & Kelber, 2011; Lind et al., 2012). However, for understanding flight control, stationary stimuli are of nominal importance.
⇑ Corresponding author. E-mail address:
[email protected] (A. Kelber).
While it is known how moving patterns influence contrast sensitivity in humans (Barten, 1993; Kelly, 1979; Robson, 1966) contrast sensitivity for drifting gratings has been studied only rarely in birds. Chickens have been tested in optomotor experiments with large-field drifting gratings (Schmid & Wildsoet, 1998; Shi & Stell, 2013), but we are not aware of tests of spatial contrast sensitivity with small-field drifting stimuli in birds. In this study we investigate the influence of motion on spatial contrast sensitivity in budgerigars (Melopsittacus undulatus). Budgerigars were chosen as model species because they have been shown to use optic flow for flight control (Bhagavatula et al., 2011), and their contrast sensitivity has been determined with stationary stimuli before (Lind & Kelber, 2011; Lind et al., 2012). 2. Methods 2.1. Animals Five budgerigars (two females and three males) took part in the experiment. Their age varied between six months and seven years, and two of them had previous experience of behavioural experiments. The birds were fed on mixed seeds supplemented with minerals and vegetables – mainly iceberg lettuce and carrots
http://dx.doi.org/10.1016/j.visres.2014.07.007 0042-6989/Ó 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
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– for vitamin sustenance. Birds were kept in cages of similar size (ca. 80 45 70 cm, length width height) in pairs in the same conditions as during at least half a year before experiments. On experiment days, budgerigars were fed with seeds during the experimental sessions, at least twice a day. Additionally they received vegetables in their housing cages. Birds had a resting phase over the middle of the day – like budgerigars in natural environment (Wyndham, 1980). On two days every week, when no experiments were performed, birds were allowed to eat seeds ad libitum. The experiments were performed during approximately five months. Each animal was trained and tested in the experimental cage individually, but could always communicate with the other birds by vocalisations. The animals were kept in accordance with the ethical guidelines stated by the Swedish Board of Agriculture and the experiments were approved by the local ethical committee (permit number M 405-12). 2.2. Experimental set-up We used the same experimental cage as in earlier studies (Lind & Kelber, 2011; see Fig. 1). The cage had a length of 1580 mm, a width of 860 mm and a height of 670 mm and was made of grey metal mesh, except for grey Perspex panels that covered the floor and replaced the mesh at one of the short ends. At this short end, the grey panel had two windows for stimulus presentation, each 150 mm 150 mm in size and 250 mm apart from each other. 45 mm below each stimulus window, a grey feeder box with a lid and a landing perch was inserted into a small hole of 60 mm 95 mm. The cage was partially divided by an opaque white plastic board, separating the stimuli windows up to a distance of 1268 mm. The budgerigars started stimulus approach from a perch 144 mm from the end of the dividing board. From the perch each stimulus window thus obtained a visual angle of 6.7°, with a distance between both stimuli of 11°. The experimental cage was placed in a lightproof compartment and illuminated from above by four white light emitting diodes (LEDs; LZC-00NW40, LED Engin Inc., San Jose, USA), powered with a 175 W dual power supply (CPX200, Thurlby Thandar instruments Ltd., Huntingdon, England). To produce homogenous illumination, the LEDs were directed upwards, and light was reflected into the
cage by wrinkled aluminium foil covering the entire area of the cage. The grey perspex board surrounding the stimulus windows had a luminance of 8–10 cd/m2, and a white standard placed on the cage floor in a 45°angle had a luminance of 125 cd/m2, measured with a radiometer (ILT1700 with detector SPM068-01, International Light). The birds were filmed with a video camera positioned on the end of the cage opposite to the stimuli, and observed by the experimenter on a separate monitor, invisible for the birds (see Fig. 1). 2.3. Stimulus presentation and behavioural procedure Budgerigars were trained to discriminate a homogenous grey stimulus from sine-wave gratings (tilted 45°) of varying spatial frequency that were presented simultaneously. Flying into the right or left cage division was counted as a choice. Landing on the feeder under the homogenous grey stimulus was rewarded with access to food for 2–4 s, whereas flying to the grating had no consequences. After each trial both stimulus windows turned black again. The bird had to fly back to the starting perch and wait for the next stimulus presentation. All stimuli were presented on a large colour LCD screen (EIZO SX3031W-H) using a modified script from psychotoolbox3 (version 3.0.10, http://psychtoolbox.org) in matlab (version 7.12.0.635, The MathWorks Inc.). We tested the birds with sine-wave gratings of five spatial frequencies: 0.48, 0.95, 1.9, 4.7, 6.5 cyc/deg. At each frequency Michelson contrast C varied between 99% and 0.7%. Michelson contrast is given as
C¼
ðImax Imin Þ ðImax þ Imin Þ
ð1Þ
where Imax and Imin are the maximal and minimal intensities of the grating (Michelson, 1927). The mean stimulus luminance was 63 cd/m2, and gratings were presented either stationary or drifting. Measurements of stimulus luminance and cage illumination were repeated on a regular basis, and no changes were detected throughout the experimental period. Experiments started with a frequency of 1.9 cyc/deg, because budgerigars were known to be most sensitive for stationary pattern of spatial frequencies between 1 and 2 cyc/deg (Lind & Kelber, 2011), and gratings were presented either stationary or drifting.
A
cage divider video camera
stimulus windows and feeder boxes
starting perch LCD Screen cage divider
B
stimulus window
starting perch
feeder box
Fig. 1. Experimental cage. View from above (A) and from the side (B) (for details see Section 2).
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We presented the rewarded homogenous stimuli in two shades of grey with approximately 20% lower and 20% higher intensity than the unrewarded stimuli, in an equal number of trials, to make sure that the animals could not use brightness to make a decision. We first tested each spatial frequency with drifting stimuli, then with stationary stimuli, followed by additional experiments with faster or slower drift velocities. Some of the earliest tests were repeated in the end to test for possible learning effects during the experimental period. Drift velocity varied between 0.9 and 13 deg/s, which results in contrast frequencies between 1.5 and 9.1 cyc/s. Contrast frequency describes the ratio of angular velocity (deg/s) of a moving pattern and the spatial wavelength (1/spatial frequency) (deg), and thus defines the number of grating cycles passing a given point in space per second (see, e.g. Eckert & Hamdorf, 1981). The LCD screen did not allow us to test high frequency gratings drifting at high velocities due to restrictions in spatial and temporal resolution. Table 1 shows which velocities and spatial frequencies were used. For each bird, we created testing series of 20 trials with five different contrasts, allowing for reliable fit of psychometric functions. The order of unrewarded patterns and the side of presentation were varied pseudorandomly (Gellerman, 1933), such that rewarded stimuli appeared in no more than three consecutive trials on the same side, and equally often on the left and right sides. A bird was tested with between two and four series a day. After finishing 10 series with a total of 200 trials, which is 40 trials for each contrast, spatial frequency or drifting velocity were changed. Before tests with a new spatial frequency were started, each bird had to reach 80% correct choices in two consecutive training series of 20 trials with high contrast gratings to favour similar preconditions. 2.4. Data analysis Contrast threshold values were estimated by fitting a logistic function to the pooled data of all five birds using the Palamedes toolbox (version 1.6.2, Prins & Kingdom, 2009) in matlab:
1 wðxÞ ¼ y þ ð1 y kÞ 1 þ e ax b
ð2Þ
where w is the correct choice frequency at stimulus contrast x, c is the lower asymptote of the psychometric function (fixed to 0.5), k is the lapse rate, i.e. the difference between the upper asymptote and 1 (limited to vary between 0 and 0.2), and a and b are unrestricted fit parameters. To estimate the accuracy of the fitted function, we used a non-parametric Bootstrapping procedure (500 simulations) with Palamedes toolbox in matlab. As in earlier test of contrast sensitivity in budgerigars (Lind & Kelber, 2011), we set the threshold to 72.5% correct choices, which corresponds to the choice frequency that is significantly different from random behaviour (one-sided binomial distribution, n = 40, p < 0.005). To describe the contrast sensitivity as a function of spatial frequency, we fitted a double-exponential function with a Table 1 Contrast frequencies (cyc/s) resulting from the tested combinations of spatial frequency and drift velocity (deg/s). Spatial frequency (cyc/deg)
Drift velocity (deg/s) 0.0
0.48 0.95 1.9 4.7 6.5 a
0.0 0.0 0.0 0.0 0.0
0.9
6.0
1.6a
3.0 7.5 9.1
3.2
6.3
12.6
1.5 3.0 6.1
3.0 6.0
6.0
For the spatial frequency of 6.5 cyc/deg a drift velocity of 1.4 deg/s was used.
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non-linear least square procedure to the data as suggested by Uhlrich, Essock, and Lehmkuhle (1981):
SðmÞ ¼ 100ðK 1 e2pam K 2 e2pbm Þ
ð3Þ
where S(m) is the contrast sensitivity for the spatial frequency m, and K, a and b are free fitting parameters. A two-way ANOVA was used in matlab to determine whether spatial contrast sensitivities for stationary and drifting gratings were significantly different. 3. Results 3.1. Learning performance All birds learned the discrimination task with high-contrast gratings rapidly and made between 90% and 100% correct choices with stationary and drifting patterns of 1.9 cyc/deg. Training the animals to gratings of lower and higher frequencies required slightly more time and lapse rates were higher, but all five birds met the criterion of 80% correct choices in two consecutive trials and could be tested with lower contrasts. Inter-individual variation was generally not very large, although three birds (Milou, Lucky and Bud) performed slightly better during all experiments (see Supplementary material, Tables S1 and S3). 3.2. Contrast thresholds for stationary stimuli Fig. 2 shows the mean performance of all budgerigars in contrast threshold experiments with stationary stimuli of spatial frequencies between 0.48 and 6.5 cyc/deg. The lowest threshold was 7.1% Michelson contrast with a spatial frequency of 1.9 cyc/ deg, corresponding to a contrast sensitivity of 14. Thresholds for the lowest and highest tested spatial frequency were 19% and 61% contrast, respectively (see Supplementary material, Table S2). 3.3. Contrast thresholds for drifting stimuli With drifting stimuli we found the lowest threshold with a drift velocity of 3.2 deg/s (6 cyc/s) at a spatial frequency of 1.9 cyc/deg (Fig. 3). The mean threshold of all birds for this conditions was 5.8% Michelson contrast and a contrast sensitivity of 17.4, compared to the threshold of 7.1% with stationary stimuli. Two budgerigars had a threshold below the average, discriminating 4.4% Michelson contrast, and a corresponding contrast sensitivity of 22.5 (Lucky and Milou; see Supplementary material, Table S1). Increasing drift velocity (and contrast frequency) led to significantly higher contrast sensitivity for patterns of all tested spatial frequencies (Fig. 4, p < 0.05, see ANOVA, Supplementary material, Table S3), except for 1.9 deg/s, where one bird surprisingly had a lower threshold with the stationary patterns. The largest differences were seen with very low and very high spatial frequencies. The threshold for the lowest spatial frequency was 6.6% Michelson contrast with a drift velocity of 12.6 deg/s (6 cyc/s), compared to 19% contrast with the stationary pattern. The threshold for the highest spatial frequency was 42% contrast, with a drift velocity of 0.9 deg/s (6 cyc/s) and 32% contrast with a higher velocity of 1.4 deg/s (9.1 cyc/s), compared to 61% contrast with stationary stimuli (see Supplementary material, Table S2). 4. Discussion Our investigations show that stimulus motion significantly improves contrast sensitivity of budgerigars for achromatic gratings at spatial frequencies between 0.48 and 6.5 cyc/deg
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1.0
1.0
0.8
0.8
0.6
0.6
0.48 cyc/deg 12.6 deg/s 6.3 deg/s 3.2 deg/s 0.0 deg/s
0.48 cyc/deg 0.4
0.4
1.0
1.0
0.8
0.8
0.6
0.6
0.95 cyc/deg 6.3 deg/s 3.2 deg/s 0.0 deg/s
0.4
0.4
1.0
1.0
0.8
0.6 1.9 cyc/deg 0.4
fraction correct choices
fraction correct choices
0.95 cyc/deg
0.8
0.6
0.4
1.0
1.0
0.8
0.8
0.6
1.9 cyc/deg 3.2 deg/s 1.6 deg/s 0.0 deg/s
0.6 4.7 cyc/deg 1.6 deg/s 0.0 deg/s
4.7 cyc/deg 0.4
0.4
1.0
1.0
0.8
0.8
0.6
0.6
6.5 cyc/deg 1.4 deg/s 0.9 deg/s 0.0 deg/s
6.5 cyc/deg 0.4
0.4
0.2
0.4
0.6
0.8
1.0
Michelson Contrast Fig. 2. Contrast thresholds of budgerigars and psychometric functions for stationary gratings with spatial frequencies between 0.48 and 6.5 cyc/deg. Filled circles are choice frequencies of the birds. Circle size is related to the number of birds tested on each particular Michelson contrast. The smallest circles represent one bird (40 choices) and the largest circles five birds (200 choices). Dashed lines give the logistic functions fitted to the data (see Section 2), squares with error bars represent threshold values with standard errors as estimated from bootstrapping (see Section 2).
(Figs 3 and 4). The maximum sensitivity for any of the drifting gratings was 17, for gratings with a spatial frequency of 1.9 cyc/deg and a drift velocity of 3.2 deg/s (6.1 cyc/s). 4.1. Contrast sensitivity of budgerigars for stationary stimuli Maximum contrast sensitivity for stationary gratings in budgerigars has earlier been measured as about 10 (Lind & Kelber, 2011), and other birds show similar values (Barn owl: 13, Harmening et al., 2009; Pigeon: 12, Hodos et al., 2002; Chicken: 12, Jarvis et al., 2009; Wedge-tailed eagle: 14, Reymond & Wolfe, 1981). The only bird, for which higher contrast sensitivity has been
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Michelson Contrast Fig. 3. Influence of motion on contrast threshold. Logistic functions fitted to the psychophysical data from all birds (as in Fig. 2) for spatial frequencies between 0.48 and 6.5 cyc/deg at various drift velocities. Solid lines: stationary gratings, dashed lines: drifting gratings. Average thresholds of all five birds and standard errors are represented by squares with error bars. Each bird performed 200 trials, which is 40 trials for five contrasts each, at each drift velocity (see Section 2).
determined, was one American kestrel (31, Hirsch, 1982). Among mammals, many species have higher contrast sensitivity (see Lind et al., 2012; for a list and references) and primates show the highest values (see Barten, 1999, for a list and references; Burr & Ross, 1982; De Valois, Morgan, & Snodderly, 1974). All these species were tested with stationary stimuli. In our experiments with stationary stimuli we found higher maximum contrast sensitivity (Fig. 5), compared to the earlier study. The interpolated contrast sensitivity maximum in our data is 13.3, for a spatial frequency of 1.7 cyc/deg, while Lind and Kelber (2011) found a sensitivity maximum of 10 at 1.4 cyc/deg. The extrapolated cut-off frequency is slightly lower with 7.7 cyc/deg compared to 10 cyc/deg (Lind & Kelber, 2011). The value
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Contrast Sensitivity
*
* *
101
12.6 deg/s 6.3 deg/s 3.2 deg/s 1.5 deg/s 0.9 deg/s 0.0 deg/s
*
100
100
present study, 2014 X: 1.7 Y: 13.3
Contrast Sensitivity
A
101
Spatial Frequency (cyc/deg)
101 X: 1.4 Y: 10.2 LIND & KELBER, 2011
LIND & KELBER, 2011 x: 10.0
present study, 2014 x: 7.7
100 100
101
Spatial Frequency (cyc/deg)
B
Contrast Sensitivity
contrast frequency: 6 cyc/s 101
stationary
100
100
101
Spatial Frequency (cyc/deg) Fig. 4. Influence of drift velocity on spatial contrast sensitivity. (A) Filled circles represent contrast sensitivities (inverse contrast thresholds) for stationary stimuli and the solid line is a double exponential function fitted with a non-linear least square procedure to the data. Triangles represent inverse contrast thresholds for drifting gratings with darker colours for higher drift velocities (in deg/s: light yellow 0.9, dark yellow 1.4–1.6, light orange 3.2, dark orange 6.3, red 12.6). Error bars give standard errors as in Figs. 2 and 3. Stars indicate at which spatial frequencies drift velocity had a significant influence on contrast sensitivity. (B) Fitted curves for experiments with stationary stimuli (black line) and stimuli drifting with approximately the same contrast frequency, 6 cyc/s (dashed line; see Table 1, bold letters).
is similar to the anatomically determined spatial resolution in the area centralis of the budgerigar (6.9 cyc/deg; Mitkus et al., 2014), which is looking laterally. Although our experimental conditions were similar to those described by Lind and Kelber (2011), four parameters differed between the studies: (i) we used a slightly higher stimulus luminance of 63 cd/m2 compared to 50 cd/m2, (ii) instead of square-wave gratings we presented sine-wave gratings, (iii) we used different birds, and (iv) the angular size of the stimuli was 6.7°, compared to 3.6° in the earlier study. If the difference in stimulus luminance had an effect we would expect a shift of maximum sensitivity towards higher frequencies (chicken: Jarvis et al., 2009; human: Kelly, 1977), and with the use of sine-wave instead of square-wave gratings, we would expect lower sensitivity for low spatial frequencies (De Valois & De Valois, 1990). However, we observed neither of these effects, but rather we found trends in the opposite directions. We cannot exclude differences in visual capabilities of the experimental animals in this and the previous study. Studies in budgerigars and Bourke´s parrots indicate a relatively high interindividual variation of performance in behavioural studies (Lind
Fig. 5. Contrast sensitivity functions for stationary gratings. Each circle represents the inverse of the contrast threshold at a particular spatial frequency. The bold line stands for a double exponential function fitted to data of the present study, and error bars represent standard errors. The thinner line refers to data of Lind and Kelber (2011), and error bars represents 95% confidence intervals. Filled squares indicate peak sensitivities (Y), peak frequencies (X) and cut-off frequencies (x) of both studies. Exact values are shown in boxes.
et al., 2012) and this is in line with our findings. Two of our birds (Milou and Bud) also participated in the earlier study (Lind & Kelber, 2011). In the present experiments, both birds have higher maximum contrast sensitivity for stationary gratings but slightly lower extrapolated spatial resolution, which may indicate that other factors have influenced experimental performance. Finally, changes in stimulus size are known to modify contrast sensitivity (chicken: Jarvis et al., 2009; human: Cohen, Carlson, & Cody, 1976). Jarvis et al. (2009) found improved contrast sensitivity in chicken as grating size was increased from 7.6° to 32° of visual angle, especially for low spatial frequencies. Our findings in budgerigars look very similar (Fig. 5), letting us assume that larger stimulus size is the main reason for the higher contrast sensitivity that we observed, compared to the earlier study. 4.2. Stimulus motion improves contrast sensitivity We found a general improvement of contrast sensitivity for drifting gratings at all spatial frequencies (Fig. 4A), but the greatest improvement was found at low spatial frequencies. Results in humans show that stimulus motion improves contrast sensitivity selectively at lower spatial frequencies so that the contrast sensitivity function is shifted towards lower frequencies without any change in form or peak frequency (Burr & Ross, 1982). This means that in humans, unlike in budgerigars, lower contrast sensitivity for high spatial frequencies was found with moving stimuli, compared to stationary stimuli, likewise at slow and fast drift velocities (1–800 deg/s). To further investigate these similarities (low-frequency bias) and differences (contrast sensitivities for high frequencies with moving stimuli) in birds and humans, more experiments are needed. Contrast frequency (cyc/s) is assumed to be more adequate to describe motion detection of the visual system than drift velocity (deg/s) (e.g. human: Breitmeyer, 1973; Burr & Ross, 1982; Kelly, 1979; Tolhurst, Sharpe, & Hart, 1973). Thus, we included both dimensions in our data analysis. At a contrast frequency of 6 cyc/s we found higher sensitivities at all spatial frequencies, compared to experiments with lower contrast frequencies (Fig. 4B). A bird experiences this contrast frequency when flying at 12 m/s and passing a row of trees in a distance of 6 m, that
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are 1 m wide and equally spaced. For humans a contrast frequency of 6 cyc/s is suggested as optimal for all spatial frequencies (Tolhurst, 1973). However, we could not test higher drift velocities, and we did not look for an optimal contrast frequency in this study. Thus, its existence is merely speculative until now. Further experiments with higher drift velocities would be necessary. Our results show that contrast sensitivity, tested solely with stationary stimuli, likely underestimates the sensory capacity of budgerigars flying through their natural environments. Although they may not discriminate as small details as predators with their higher spatial resolution (Gaffney & Hodos, 2003; Hirsch, 1982), budgerigars are capable of effective collision-avoidance and control of take-off and landing (Bhagavatula et al., 2011). Having higher contrast sensitivity for moving objects (present study) and better visual skills for motion perception, compared to poor response to steady state stimuli (Jarvis et al., 2001), may help them to deal with these complex situations. Together with budgerigars’ ability to use optic flow cues for observing and regulating flight velocity in narrow passageways (Bhagavatula et al., 2011), these adaptions may contribute to sufficiently good vision for a flying animal. Whether higher contrast sensitivity in some individuals or, more generally spoken, in some species of birds directly affects their abilities during flight, for example in allowing a higher maximum flight speed, would be interesting to examine. 4.3. Mechanisms of motion processing and response to drift velocity It would also be interesting to understand the neural pathways underlying the observed behaviour. In pigeons, the existence of at least two neural pathways, dealing with motion vision, has been demonstrated: (i) the accessory optic system and (ii) the tectofugal system. The accessory optic system is known to analyse large-field motion resulting from self-motion (Frost, Wylie, & Wang, 1990; Pakan & Wylie, 2006; Wylie, 2013), and thus helps to control body posture and locomotion (Frost, Wylie, & Wang, 1990). In addition the nucleus of the basal optic root, a component of the accessory optic system, is responding to slowly drifting pattern between 1 and 5 deg/s (Frost, Wylie, & Wang, 1990). The tectofugal system appears to be specialized for the analysis of object motion. In pigeons moderate to fast object motion is processed by the tectofugal system (Bischof & Watanabe, 1997; Frost, Wylie, & Wang, 1990). Until now the contrast sensitivity of neural pathways for motion vision has not been studied intensively in birds. In pigeons, neurons in the nucleus of the basal optic root responded to movement of large-field gratings with a spatial frequency of 0.1 cyc/deg, even when tested with contrasts as low as 5% (Wolf-Oberhollenzer & Kirschfeld, 1994), giving evidence for good contrast sensitivity, similar to the present study. In the present experiments, budgerigars were required to react to motion in small-field stimuli while sitting on a starting perch. They could change body position and direction of view at any time. Thus, the experimental setup does not provide a sufficient basis to allow us a statement about the pathways that may have controlled the behavioural response of our birds. Additional experiments are needed to understand the neural basis of the differences discovered in our study. 5. Conclusions The results of our study with drifting stimuli strongly support the hypothesis that motion influences contrast sensitivity in birds. Perched, but freely moving birds, discriminate drifting gratings better than stationary gratings at all tested spatial frequencies.
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