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

Highlights

• Locating stimuli and response manipulanda close together will produce more accurate performance and faster learning in brushtail possums.

• Performance peaked at the current training distance and generalized to distances close to the currently trained distance.

• Performance decreased in accuracy at distances further from the trained level.

Abstract

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.

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, Murphy and Miller, 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 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, White, 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 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.