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B. Frech, C. Neumeyer; Perception of Spatial Depth in Goldfish. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2546. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
The perception of spatial depth is crucial for most animals in a 3-dimensional surrounding. To get depth information from a 2-dimensional retinal image, it has to be analysed in many different ways. Some cues to perceive depth are binocular disparities, motion parallax and accommodation. Although the visual system of the goldfish is intensively studied, it is not known in what quality they can discriminate distances in spatial depth and which retinal image cues are crucial for the performance of depth perception. In a first approach we investigated how good goldfish are in discriminating different object distances in spatial depth.
Three goldfish were trained to discriminate between two equally sized objects. The stimuli consisted of two black disks with 4,5 cm diameter placed in a homogenous white surrounding. They were presented in different distances in a two alternative forced choice task. Goldfish were trained to choose the near disk (training stimulus) while the other (test stimulus) was first shown as far from the training stimulus as possible. Three experiments were done in which the training stimulus was situated at 5, 7 and 10 cm. One goldfish was trained on the distant stimulus (25, 30 and 40 cm) and tested against a near stimulus. The distance between training and test stimulus was decreased until the choice frequency for the training stimulus dropped under the threshold value of 75%. All stimuli combinations were at least shown in 15 trials containing 100 choices.
All goldfish were perfectly able to learn the discrimination task with every training stimulus position. In each experiment the results are similar for all fish: When the distance between the stimuli was diminished, the choice rate remains constant up to a certain point after which it declines rapidly. The smallest inter-stimuli distances with 75% choice frequency for the three goldfish trained on the near stimulus were 5; 5,5 and 7,5 cm and for the goldfish trained on the distant stimulus 12,5; 17,5 and 20 cm respectively. This shows clearly that the discrimination ability of the goldfish decreases with increasing training stimulus distance: the nearer the training stimulus is to the goldfish, the better.
The distance dependent decrease of the discrimination ability shows that goldfish do not discriminate the objects through size discrimination by use of the size difference of the retinal image, but that the distance has a great influence on their performance. We assume that accommodation plays an important role in depth discrimination, which we will investigate in further experiments.
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