Behavioural Processes 118 (2015) 28–33

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Effect of stimulus and response separation in a matching-to-sample task in the brushtail possum (Trichosurus vulpecula) Kristie E. Cameron ∗ , James S.A. McEwan, Bill Temple ¯ The University of Waikato/Te Whare Wananga o Waikato, New Zealand

a r t i c l e

i n f o

Article history: Received 22 September 2014 Received in revised form 7 May 2015 Accepted 8 May 2015 Available online 12 May 2015 Keywords: Stimulus location Response manipulanda Distance Brushtail possum

a b s t r a c t This study seeks to investigate the impact of changing the proximity of stimulus and response manipulanda on matching-to-sample performance in possums. Possums were presented with five rows of blue and yellow stimuli arranged vertically 25 mm apart above response levers. Generally, peak performance occurred at the distance from the lever currently being trained. Performance generalized to distances close to the currently trained distance and decreased in accuracy at distances further from the trained level. The findings from this experiment provide evidence for placing stimuli and response manipulanda close together to improve acquisition of a task, and increase the responding accuracy in MTS experiments. This suggests that spatial contiguity in the relative location of stimuli and response manipulanda is critical to animals performing complex operant tasks. © 2015 Elsevier B.V. All rights reserved.

1. Introduction It has been reported that the location of the stimulus relative to the response manipulanda has an impact on responding performance. This relationship, between the stimulus and response manipulanda, was first described by Kohler (1925) and is termed ‘spatial contiguity’ (e.g., McClearn and Harlow, 1954). In studies using primates it was reported that the spatial displacement of the stimulus, response manipulanda and reinforcer delivery system slowed learning of object discrimination tasks in monkeys, but not when the stimulus was contiguous with either the response manipulanda or reinforcer delivery system (e.g., Miller and Murphy, 1964; Murphy and Miller, 1955, 1958). Jenkins (1943) found monkeys could perform a discrimination task with greater than a 150 mm gap between the stimulus and response manipulanda and Stollnitz and Schrier (1962) reported performance was accurate at up to 450 mm. The effect of spatial contiguity was also found in responding in children where the stimulus separated from the response manipulanda affected learning of a discrimination between two stimuli (Murphy and Miller, 1959). The children learned discrimination tasks when the stimulus and response manipulanda were contiguous (Jeffrey and Cohen, 1964). Stimuli, response manipulanda and reinforcer delivery system that are spatially contiguous or combined have been utilized in

∗ Corresponding author at: School of Natural Sciences Unitec – Institute of Technology Carrington Road, Auckland, New Zealand Tel: +64 9 8154321x8578. E-mail address: [email protected] (K.E. Cameron). http://dx.doi.org/10.1016/j.beproc.2015.05.006 0376-6357/© 2015 Elsevier B.V. All rights reserved.

operant tasks. Jarvik (1953) combined the stimulus, response apparatus and delivery of reinforcement in a flavor discrimination task with primates using flavored bread dyed either red or green. When a choice between the colors was made, the bread was allowed to be consumed as the reinforcer. These types of combinations have also been used in tasks such as the Matching-to-Sample (MTS) procedures where the subject is required to make a response in the presence of a discriminative stimulus. The subject is then presented with two comparison stimuli of which one is the same as the discriminative stimulus, and one is different. Comparison stimuli may be presented simultaneously with the sample stimuli or after a programmed delay, this is known as Delayed Matching to Sample (DMTS). Reinforcement is delivered following selection of the matching comparison stimulus (Blough, 1959; Adamson et al., 2000). The combination of stimuli and response manipulanda has been reported using species such as humans using touch screens (e.g., Robbins et al., 1997), rats responding via bar biting (e.g., Porter, Burk and Mair, 2000), and primates (e.g., Paule et al., 1998) and pigeons activating lit keys (e.g., White, 1985,2001) to gain a reinforcer. Most of this research utilizes the traditional laboratory animal, such as the pigeon, where the eye and the beak are in the same area of the body. This type of animal is not as useful to study the impact on responding performance in an operant task, such as MTS, when the stimulus and response manipulanda are separated by a physical distance. This requires a large subject with limbs and opposable thumbs such as the primate to activate the response manipulanda (e.g., Murphy and Miller, 1955). As primates are in short supply in New Zealand, the brushtail possum made a convenient laboratory

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Table 1 The sex, mean weight over the experimental period (kg), approximate attainment year, the method of attainment and previous experimental experience for each possum including DMTS Experiment 1: conditional discrimination and DMTS using flickering versus still light, and Experiment 2: conditional discrimination and DMTS using horizontal versus vertical LED lights. Possum

Sex (M or F)

Mean weight (kg)a

Approximate attainment year

Method of attainment

Previous experimental experience

Sparky Hasty Marmite Tom Ishan Charlotte Bonnie Gus Elroy Taylor

F M M M M F F M M F

3.89 3.58 3.78 2.68 4.10 3.99 2.95 4.11 3.26 4.55

2004 2005 2000 2004 2003 2003 2000 2002 2005 2006

Wild caught Wild caught Lab raised From other lab Wild caught From other lab Lab raised Wild caught Wild caught Lab raised

DMTS (Exp 1 & 2) Naïve DMTS (Exp 1 & 2) DMTS (Exp 1 & 2) Naïve DMTS (Exp 1 & 2) DMTS (Exp 1 & 2) Naïve Naïve Naïve

a

Mean weight is calculated over experimental period 25/7/08–19/5/09. Mean weight the total weight divided by 24 weigh sessions.

subject to study the impact on responding of separated stimuli and response manipulanda on remembering performance in a MTS task. The brushtail possum, Trichosurus vulpecula, is a nocturnal, omnivorous pest in New Zealand affecting agriculture as carriers of bovine tuberculosis and consumers of crops; and act as compete with indigenous wildlife resulting in the destruction of native forest and bush (Cowan, 1992; Landcare Research NZ, 2008). Therefore the study, and subsequent exploitation, of possum natural behaviour and psychophysical abilities is important for identifying strategies to contribute to pest control. Several experiments have been conducted to study the psychophysical abilities of the possum. For example, Webster (1975) examined how visual information is transferred in the possum brain using a visual discrimination task. He combined the stimulus, response apparatus and reinforcer into one vertically displayed piece of carrot in comparison to a horizontal piece. More recently, possums have been trained to discriminate flickering light, shape, and color followed by the measurement of the remembering ability of the possum using a DMTS procedure. The first experiment required a discrimination between a still and flickering light (e.g., Blough, 1959); known as the Critical Flicker Fusion frequency (Signal, Foster & Temple., 2001). Possums could discern a flickering light from still at 22.4 Hz compared to the 10 Hz of humans (Campos and Bedell, 1978). The second experiment used three small panels of green LED lights that were arranged in a 5 × 5 matrix that presented five horizontal lights and five vertical green lights (Hardaker, 2006 unpub.). The third experiment used a tested single blue and yellow LED light stimuli. Results showed that across all three experiments, the majority of possums could discriminate between stimuli sets and could generally perform a DMTS task for each set; however, this was at only very short delays and only after many experimental sessions. Due to these limitations, it was necessary to investigate reasons for poor performance. One possible explanation for poor performance is whether the animals could discriminate between the two choices within each set of stimuli. The DMTS task for each of the three experiments, however, was introduced only after each animal could reliably discriminate the stimuli for all sets at or above an 85% correct criterion (Signal et al., 2001). A more probable explanation for poor performance is that the apparatus used in all experiments where the stimuli were presented 55 mm above the response levers. This is not in line with published MTS or DMTS procedures of which combine the stimulus and response manipulanda in humans (e.g., Robbins et al., 1997), primates (e.g., Jenkins, 1943) or rats (e.g., Porter et al., 2000). Stimuli and response manipulanda have been semi-combined on a touch screen where the subject is able to select a portion of the screen which is not the stimuli to indicate a choice in pigeons (Wright et al., 1998). This study used the MTS procedure to investigate the impact on performance when the distance between the stimulus and response

manipulanda was varied. Initially, the response levers were immediately below the stimuli to represent the previous studies that combine stimulus and response manipulanda. Then the distance between response levers and stimuli was increased systematically by 25 mm to observe changes in matching performance. It was predicted that performance competency would be improved when the stimuli were immediately above the lever compared to greater distances above the response lever. 2. Method 2.1. Subjects Ten brushtail possums (Tricosurus vulpecula) (three female and seven male) served in this experiment. Five possums had previous experience in a DMTS procedure and five possums were experimentally naïve (Table 1). Possums were weighed fortnightly and supplementary food amounts were adjusted according to these weights. Rations consisted of dock leaves (Rumex obtusifolius), apple, and food pellets (Camtech Manufacturing Ltd.® , New Zealand) delivered after the completion of scheduled experimental sessions. Possums had constant access to water throughout the experiment. 2.2. Housing Experiments were carried out in the home cage consisting of wire mesh cages (540-mm wide × 850-mm high and 470-mm deep), with a nest box (450-mm wide × 300-mm high sloping from 360-mm to 195-mm) above the cage. All possums were housed together in one room where a 12:12 hour reversed light/dark cycle was in effect. Cleaning and maintenance occurred during the light rotation at about the same time each day. Experimental sessions were carried out during the dark rotation where three 60-watt red-light bulbs semi illuminated the room. 2.3. Apparatus Individual home cages functioned as the operant chambers with the response panel mounted as the cage doors. A response panel (350-mm wide × 450-mm high) consisting of three vertical rectangular slots for colored stimuli and three round holes for response levers. Clear perspex covered the slots on the cage side to prevent access to the stimuli by the possum. The lever equipment consisted of three micro switches, Honeywell BZ-2RW863/A2, with the lever reduced in length to extend 15 mm through the hole in the panel. Three pairs of stimuli were located above each lever. The lights were located in a strip of nylon (115- mm long × 7- mm wide × 5mm deep) with individual lights 25 mm apart (Fig. 1). For each pair, one strip had blue LED lights and the other had yellow lights. The

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Fig. 1. The graphic on the left is a scale diagram (25:1) of the operant chamber response panels. The stimuli pairs made up of two nylon strips slid into the three vertical slots. The graphic on the right is a diagram of one nylon strip. The 5 mm circles represent the LED lights. For each response panel there would be three blue/yellow pairs of nylon strips.

order of the color strips was changed every five sessions to prevent possums performing according to the order and position of the color strips. 2.4. Procedure During MTS training, a trial began when the center light was turned on showing either a blue or yellow color. Five presses (FR 5) on the center lever turned on two side lights, one side of the same color as the center light (S+ ), and the other side was the alternative color (S− ). A single response on the S+ lever led to 3 s access to reinforcement and elicited a short feedback beep. Responses to the S− lever terminated the trial and led to 3 s blackout and a 6 s inter-trial interval, before commencing a new trial. During the MTS 0 s delay procedure the center light turned off after the FR 5 was completed at the same time the side lights were illuminated. An equal number of reinforcers were distributed to the left and right levers and blue and yellow stimuli to avoid learned biases to color or side. For example, 25% of reinforcers were delivered on trials

when the S+ was the left lever regardless of the color of the light and 25% of reinforcers when the S+ was the right lever regardless of the color of the light. The same was true for the left and right levers regardless of the color of the light. All sessions ended after 3600 s or 100 reinforcements, which ever occurred first. A computer system running MEDTM software and interface located in an adjacent room, recorded all data and controlled the experimental events. Each possum was trained on the simultaneous MTS procedure at 0 mm above the lever followed by MTS 0 s delay at the distances of 0 mm, 25 mm, 50 mm, 75 mm and 100 mm above the response levers. Each possum was required to respond with 85% correct or above for five consecutive sessions on the training level to proceed to probe sessions at each distance, before moving to the next distance. A probing session consisted of blocks of 10 trials; five were at the training level and one trial at each distance above the lever. Contrary to the training procedure, the delivery of reinforcement on the training level trails only was specific to side only to increase the rate

K.E. Cameron et al. / Behavioural Processes 118 (2015) 28–33 1.4

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1.4 1.2

1.2

1.0

1.0

0.8

0.6

Log d’

log d’ d' Log

0.8

0.6

0.4 0.2

0.4 0.0

0.2

-0.2 -0.4

0.0 0

25

50

75

0

100

Distance from trained distance (mm) Fig. 2. Mean log d probe data during training at 0 mm and 100 mm from the response levers for four possums. A log d value of 0 is equivalent to 50% correct.

of reinforcement, for example, 50% of reinforcers were delivered on trials when the S+ was the left lever regardless of the color of the light. An abort feature ended a trial if the possum had not responded within two minutes and stimuli at this distance was not repeated within the block of 10 trials. Probing at each training distance was completed after 50 ± 4 accumulated probe trials at each distance over a minimum of four probing sessions at which training at the next distance began. 3. Results Six possums completed training at a distance of 25 mm above the lever, and four possums complete training at 100 mm above the lever. The results show two main effects in responding by possums to the yellow and blue LED stimuli in the MTS task. The first effect is that possums responded more accurately to the lever closest to the stimuli. The second effect was that after training at a particular distance, responding at the same distance or the next distance was better than distances further away. In the following analyses, the primary measure of performance was log d . Log d is an extension of the performance measure log d proposed by Hautus (1995) which is discriminability ‘free from bias’ (Davison and Tustin, 1978). This was used to correct for occurrences where there were ceiling effects of either no responses (resulting in a 0%) or no errors in responding (resulting in 100%); both incal-

25

50

75

100

Distance from trained distance (mm) Fig. 4. Mean log d probe data during training at 25 mm, 50 mm, 75 mm and 100 mm from the response levers for four possums. The filled circles denote performance at previously trained distances and unfilled circles represent performance at untrained distances. The dashed line indicates a log d value of 0 which equates to responding at 50% correct.

culable for log d . A value of 0.5 was added to the scores in the 2 × 2 array of correct and incorrect trials prior to calculating log d . Log d was then calculated as: 

logd = 0.5log

 BlueCorrects  BlueIncorrects

+ 0.5log

 YellowCorrects  YellowIncorrects

.

Values of 85%, 90% and 95% correct would result in log d values of 0.74, 0.94, 1.24, respectively. Fig. 2 shows log d across all probe trials for four possums training at 0 mm and 100 mm above the training level. Responding at 0 mm above the response levers was generally more accurate than responding at a distance of 100 mm. A paired samples t-test showed a significant difference in mean responding during probe levels between 0 mm above the lever and 100 mm above the lever [t (4) = 3.545, p = 0.038, Cohen’s d = 1.78]. The log d performance of one possum on each of the distances from the response levers is shown in Fig. 3. Performance peaked at the training distance for all distances except when trained at 100 mm above the response levers. Performance decreased as the distance increased from the current training distance the effect was larger for distances above the trained distance compared below the trained distance). The effect of increased performance at the trained distance is evident across possums in Fig. 4.

Fig. 3. Log d for probe trials at 0 mm, 25 mm, 50 mm, 75 mm and 100 mm from the training distance for one possum. The asterisk indicates the training level and the dashed line indicates a log d’ value of 0 which equates to responding at 50% correct.

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Performance generalization from the trained distances to surrounding distances may be affected by the amount of the distance between the trained and probed distance, as well as prior training of the probed distance. For instance, training of distances occurred first at 0 mm, then at each distance to 100 mm, which was trained last. This means that when trained at a distance of 50 mm, training had already occurred at 0 mm and 25 mm. Fig. 4 shows mean log d probe performance at distances between 0 mm and 100 mm from trained distances. For example, for the 25 mm distance on the graph the filled symbols denote training at 0 mm (a previously trained distance) and 50 mm (an untrained distance). The graph shows that performance at distances previously trained was higher than performance at distances previously untrained up until a distance of 100 mm where performance is similar for previously trained and untrained distances. A repeated measures 2-way ANOVA between log d , the distance from the lever and the occurrence of prior training showed significant main effects of distance on performance [F (3, 52) = 10.61, p < .001, p 2 = 0.76], and the training variable [F (1,52) = 57.71, p < .001, ␩p 2 = 1.82] on performance. There was a significant interaction effect between distance and training [F (3,52) = 3.38, p = .025, ␩p 2 = 2.26]. A post hoc analysis of log d and the distance variable produced a significantly larger log d between the distance of 25 mm above the lever and 50 mm, 75 mm and 100 mm above the lever [p ≤ .019]. This means that performance was significantly better at previously trained levels and at distances closer to the trained level compared to non-trained levels and distances further from the lever.

4. Discussion The purpose of this experiment was to vary the distance between stimuli and response manipulanda and observe the effect on performance on a MTS task. These findings suggest that locating the stimuli and response manipulanda as close together as possible will produce more accurate performance and faster learning of an MTS task than when the stimuli and response manipulanda are separated, at least for possums. In addition, the performance during probe trials was likely to peak at the current training distance and generalized to distances close to the currently trained distance and decreased in accuracy at distances further from the trained level. This experiment was successful in producing accurate performance with stimuli up to 100 mm away from the manipulanda in contrast to previous attempts of training DMTS in possums. Accurate performance at 100 mm was achieved by training at progressively greater distances, starting with 0 mm and working up to 100 mm, but was not as accurate at greater distances without prior training at that distance. Stollnitz and Schrier (1962) achieved accurate performance in monkeys by gradually working up to a 450 mm distance between the stimulus and response manipulanda. These processes was similar to reinforcement of successive approximations as a means of producing a particular behaviour. The closest analogy to these present findings can be found in the “errorless discrimination training” literature where the fading of an aspect of one stimulus, such as intensity or presentation time, results in the same effect on responding with different stimuli (e.g., Terrace, 1963a,b). In the current experiment, training at 0 mm produced accurate responding which was maintained as the distances between the stimuli and response levers were faded toward the 100 mm distance. Dimensions of physical distance and delay have similar properties. In this MTS distance experiment, possums demonstrated a lack of accuracy when the stimuli were further away from the response levers and when performance was tested at locations that differed from the trained level. These data, to a degree, mirror the results

using delay in place of distance (White and Cooney, 1996): Two different reinforcer ratios for correct responding produced independent responding at 0.1 s–4.0 s delay. The birds also had a higher proportion of correct responses at the short delay compared to the long delay when the proportions were averaged over all conditions. The same effect was found between 0 mm and 100 mm in the current experiment. These results confirm the statement by White and Cooney (1996) that there is little difference between temporally discriminative events and spatially discriminative events. That is, as the distance increases between the stimuli and the required response, by either seconds or millimeters, there is a reduction in the ability to respond accurately. In the current experiment, generalization occurred between the training level and the closest stimuli to that level and produced monotonic generalization gradients where responding fell away at distances that were further away from the training distance. The data suggests that possums were unable to discriminate short distances from the trained level, and performance generalized to these levels of ‘similar’ distance. Other researchers have found similar effects in MTS and generalization experiments: Sargisson and White (2001) and Guttman and Kalish (1956) trained pigeons to respond to different delays and wavelengths, respectively. For both types of stimuli, the proportion of correct responses peaked at the trained level of the stimulus and decreased at the non-trained level. Guttman and Kalish (1956) observed monotonic gradients, similar to the possums in the MTS task in that performance was similar to the trained delay when the probed delay was shorter than the trained delay resulting in flat generalization gradients. The results from these experiments indicate a comparable effect on performance across a range of stimuli; the closer in the time, distance or wavelength to the trained stimulus the more accurate the performance appears to be. In previous research at our laboratory we have found very poor overall DMTS performance compared to other species. All possums required training for up to 300 sessions to reach performance criteria, and there were many failures to learn the task which was attributed to the lack of combined stimuli and response mainipulanda. For the present experiment, more possums learned the MTS task than in previous experiments with the average number of training sessions required to reliably perform simultaneous MTS at 0 mm was 81 sessions across possums. There were, however, only six possums that performed the task to criterion at 25 mm above the lever and four possums, three of which were experimentally naïve, performed the task at 100 mm above the lever. This result indicates that DMTS is difficult to train with stimuli distances more than 25 mm above the lever and that the task is more readily learned if the stimuli were located immediately above the response lever. This suggestion is in keeping with the current methodology for experimental research with hens and pigeons and most other species where the stimuli also function as the response keys. The findings from this experiment provide evidence for placing stimuli and response manipulanda close together to improve acquisition of a complex task, and increase the responding accuracy in MTS experiments. This suggests that the relative location of stimuli and response manipulanda is critical to the design of the experimental apparatus and is likely to impact the performance of non-traditional animal subjects in experiments measuring psychophysical ability and other operant experiments.

Acknowledgements This research was completed as a Master’s thesis by the first author and the publication is dedicated to Bill Temple who passed away in December 2014. Bill contributed so much of his knowledge and time to helping young researchers find their feet in academia.

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Effect of stimulus and response separation in a matching-to-sample task in the brushtail possum (Trichosurus vulpecula).

This study seeks to investigate the impact of changing the proximity of stimulus and response manipulanda on matching-to-sample performance in possums...
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