Abstract
The ability to recognize and distinguish between visual stimuli is fundamental for everyday survival of many species. While diverse aspects of cognition, including complex visual discrimination tasks were previously successfully assessed in fish, it remains unknown if fish can learn a matching-to-sample concept using geometrical shapes and discriminate between images and their mirror-image counterparts. For this purpose a total of nine Malawi cichlids (Pseudotropheus sp.) were trained in two matching-to-sample (MTS) and three two-choice discrimination tasks using geometrical, two-dimensional visual stimuli. Two out of the three discrimination experiments focused on the ability to discriminate between images and their mirror-images, the last was a general discrimination test. All fish showed quick associative learning but were unable to perform successfully in a simultaneous MTS procedure within a period of 40 sessions. Three out of eight fish learned to distinguish between an image and its mirror-image when reflected vertically; however none of the fish mastered the task when the stimulus was reflected horizontally. These results suggest a better discrimination ability of vertical compared to horizontal mirror-images, an observation that is widespread in literature on mirror-image discrimination in animals. All fish performed well in the general visual discrimination task, thereby supporting previous results obtained for this species.
Highlights
According to Shettleworth [1] ‘‘cognition refers to the mechanisms by which animals acquire, process, store, and act on information from the environment’’
Numerous experiments have been conducted on a variety of species, and simple to complex discrimination abilities have been found in both invertebrates [cephalopods [4]; crustaceans [5]; insects: ants [6], bees [7]] as well as vertebrates
Discrimination of color and line orientation was successfully tested in zebrafish (Danio rerio [8], [18]) and weakly electric elephantnose fish (Gnathonemus petersii) successfully learned to distinguish geometrical shapes [19]
Summary
According to Shettleworth [1] ‘‘cognition refers to the mechanisms by which animals acquire, process, store, and act on information from the environment’’. Cognitive abilities play an important role in foraging, mate-choice, predator avoidance, navigation and orientation [2]. For these behaviors, perception of environmental stimuli through the animal’s sensory systems is essential [3], which should be tuned to the animal’s specific needs and its respective habitat. Visual form discrimination abilities in fish The first experimental study that provided evidence of visual form discrimination in fish was conducted in 1926 by Schaller [16] who trained Eurasian minnows (Phoxinus phoxinus) to discriminate between different colors and geometrical shapes. Various form discrimination abilities in cichlids have already been tested by Herter [17], Behrend and Bitterman [25], Mark [26], Mark and Maxwell [27] and Schluessel et al [28]. The study by Schluessel et al [28] demonstrated that Pseudotropheus sp. is able to form object categories, i.e. ‘fish’ vs. ‘snail’
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