Visual discrimination and amodal completion in zebrafish

Abstract

While zebrafish represent an important model for the study of the visual system, visual perception in this species is still less investigated than in other teleost fish. In this work, we validated for zebrafish two versions of a visual discrimination learning task, which is based on the motivation to reach food and companions. Using this task, we investigated zebrafish ability to discriminate between two different shape pairs (i.e., disk vs. cross and full vs. amputated disk). Once zebrafish were successfully trained to discriminate a full from an amputated disk, we also tested their ability to visually complete partially occluded objects (amodal completion). After training, animals were presented with two amputated disks. In these test stimuli, another shape was either exactly juxtaposed or only placed close to the missing sectors of the disk. Only the former stimulus should elicit amodal completion. In human observers, this stimulus causes the impression that the other shape is occluding the missing sector of the disk, which is thus perceived as a complete, although partially hidden, disk. In line with our predictions, fish reinforced on the full disk chose the stimulus eliciting amodal completion, while fish reinforced on the amputated disk chose the other stimulus. This represents the first demonstration of amodal completion perception in zebrafish. Moreover, our results also indicated that a specific shape pair (disk vs. cross) might be particularly difficult to discriminate for this species, confirming previous reports obtained with different procedures.

“Classifying-together” phenomenon in fish (Xenotoca eiseni): Simultaneous exposure to visual stimuli impairs subsequent discrimination learning

“…This effect presumably emphasizes the great complexity of such visual discrimination for this teleost species ( X . eiseni ), unlike other teleost, the zebrafish, which easily learn to discriminate between a full red disk and an amputated red disk but not between a full red disk and a red cross [ 25 ]. On the other hand, the absence of differences in the amount of time needed to complete each trial within the learning session indicates that fish were equally adept at discriminating once they had reached the learning criterion, regardless of the experimental condition.…”

“…This difference may reflect a tendency to perceive a symmetric two-dimensional shape, such as a full disk or a cross, instead of an asymmetric one, such as an amputated disk (see, for instance, [ 29 ]). On the other hand, as noted above, this ability is species-specific since Danio rerio showed difficulties in discriminating full symmetric stimuli [ 25 ].…”

“…This unexpected evidence, in this animal species, may highlight an important difference in perceiving the similarity of the stimuli associated with the discrimination demand (the more similar they are–such as a disk and a disk missing only a small part—and the more difficult is to classify them apart; conversely, the less similar they are–such as a disk and a cross—and the easier is to classify them apart). On the other hand, there are also species-specific peculiarities that determine the discrimination complexity of the stimuli [ 25 ], as well as, as evidenced in other fish species, by modifying some physical attributes of the stimuli, such as color, size or density, other saliency-related effects could crop up and affect the discrimination learning performance [ 30 , 31 ].…”

“…42 X. eiseni took part in Experiment 1 (Visual discrimination learning between a full red disk and an amputated red disk), 42 fish took part in Experiment 2 (Visual discrimination learning between a full red disk and a red cross), and 28 fish took part in Experiment 3. Because of high motivation to rejoin female conspecifics, only males were engaged in these experiments (following the same procedure reported in previous studies [18][19][20][21][22][23][24][25]).…”

“…To this end, we performed two discrimination learning experiments after rearing different groups of fish with pairs of two-dimensional visual stimuli for 50 consecutive days. In the present study we decided to use simple geometric shapes (also considered for visual discrimination learning and amodal completion in zebrafish [ 25 ]): a full red disk and an amputated red disk in Experiment 1 ( Visual discrimination learning between a full red disk and an amputated red disk ), a full red disk and a red cross in Experiment 2 ( Visual discrimination learning between a full red disk and a red cross ). In both Experiments 1 and 2, for each pair of stimuli, three conditions were arranged: “congruence”, where fish were exposed and trained to choose the same stimulus; “narrow-incongruence”, where fish were exposed to both training the stimuli and trained to choose one of them (the same condition present in [ 1 ]); “wide-incongruence”, where fish were exposed to one stimulus and trained to choose the other one (condition not present in [ 1 ], investigated here for the first time).…”