The Müller-Lyer illusion in the teleost fish Xenotoca eiseni

Abstract

In the Müller-Lyer illusion, human subjects usually see a line with two inducers at its ends facing outwards as longer than an identical line with inducers at its ends facing inwards. We investigate the tendency for fish to perceive, in suitable conditions, line length according to the Müller-Lyer illusion. Redtail splitfins (Xenotoca eiseni, family Goodeidae) were trained to discriminate between two lines of different length. After reaching the learning criterion, the fish performed test trials, in which they faced two lines (black or red) of identical length, differing only in the context in terms of arrangement of the inducers, which were positioned at the ends of the line, either inward, outward, or perpendicular. Fish chose the stimulus that appear to humans as either longer or shorter, in accordance with the prediction of the Müller-Lyer illusion, consistently with the condition of the training. These results show that redtail splitfins tend to be subject to this particular illusion. The results of the study are discussed with reference to similar studies concerning the same illusion as recently observed in fish. Contrasting results are presented. The significance of the results in light of their possible evolutionary implications is also discussed.

Fish are Sensitive to Expansion-Contraction Color Effects

“…For the Ebbinghaus illusion, they were a circle of orange disks surrounding the target grey disk, which could be wider or smaller than the grey disk. The fish chose the figure that appeared either longer/wider or smaller/shorter, in accordance with the prediction of the illusions, and consistently with the condition of the training (Sovrano et al, 2015(Sovrano et al, , 2016. However, less attention has been paid to the role of color dimensions that might be responsible for similar perceptual effects.…”

“…The training and testing procedures and the experimental apparatus employed in the research corresponded to an established protocol employed in our laboratory, whose standardized description has been excerpted from our previous works (Sovrano et al, 2015(Sovrano et al, , 2016.…”

“…The apparatus was located in a darkened room and was identical to the apparatus (Figure 1) used in our previous studies (Sovrano et al, 2015(Sovrano et al, , 2016. It consisted of a square tank (15 cm long, 15 cm wide and 15 cm high), with uniform white walls (although this term 'white', as observed by Neumeyer (1992), is somewhat inadequate to refer to the white perceived by fish, also in absence of ultraviolet wavelengths, as in our case of incandescent source) and lit centrally with a 75 W tungsten light bulb.…”

“…Only males participated in the experiment because of their higher motivation to perform the task in order to rejoin female conspecifics. We decided to use Xenotoca e. as an animal model for comparative reasons, because this species and the procedures we used were also employed in all of our previous studies investigating the perception of visual illusions and of hierarchical configurations (Sovrano & Bisazza 2008Sovrano et al, 2015Sovrano et al, , 2016Truppa et al, 2010).…”

“…Fish have also shown the capacity to discriminate illusory shapes (Wyzisk, 2005;Wyzisk & Neumeyer, 2007; see also Rosa Salva et al, 2014). The tendency of redtail splitfins (Xenotoca eiseni, family Goodeidae) to perceive contextual phenomena affecting the metric size of the object in a similar manner to humans has been tested for the Ebbinghaus illusion (Sovrano, Albertazzi, & Rosa Salva, 2015) and the Müller-Lyer illusion (Sovrano, Da Pos, & Albertazzi, 2016). To investigate the perception of the Müller-Lyer and Ebbinghaus illusion, the fish were trained to discriminate between two stimuli of different sizes (two lines of different lengths for the Müller-Lyer illusion or two grey disks of different diameters for the Ebbinghaus illusion).…”