Fun Bird Fact

In Perception …

Processing of the Müller-Lyer illusion by a Grey parrot (Psittacus erithacus)

Irene M Pepperberg, Jennifer Vicinay, Patrick Cavanagh

Received 28 November 2006, in revised form 31 May 2007; published online 1 May 2008

Abstract. Alex, a Grey parrot (Psittacus erithacus)
who identifies the bigger or smaller of two objects by reporting its
color or matter using a vocal English label and who states “none” if
they do not differ in size, was presented with two-dimensional
Müller-Lyer figures (Brentano form) in which the central lines were of
contrasting colors. His responses to “What color bigger/smaller?”
demonstrated that he saw the standard length illusion in the
Müller-Lyer figures in 32 of 50 tests where human observers would also
see the illusion and reported the reverse direction only twice. He did
not report the illusion when (a) arrows on the shafts were
perpendicular to the shafts or closely approached perpendicularity,
(b) shafts were 6 times thicker than the arrows, or (c) after being
tested with multiple exposures—conditions that also lessen or eliminate
the illusion for human observers. These data suggest that parrot and
human visual systems process the Müller-Lyer figure in analogous ways
despite a 175-fold difference in the respective sizes of their brain
volumes. The similarity in results also indicates that parrots with
vocal abilities like Alex’s can be reliably tested on visual illusions
with paradigms similar to those used on human subjects.


  1. This finding suggests that the neuronal mechanisms for capturing the centroids of visual patterns and determining the distance/length between captured centroid “points” is an early evolutionary brain adaptation that is carried over to humans. See A. Trehub *The Cognitive Brain*, MIT Press 1991, Ch. 14 “Illusions and Ambiguous Shapes: Epiphenomena of Brain Mechanisms”, The Muller-Lyer Illusion, pp. 250-252.

  2. kenneth aizawa

    The nativist take on this is also philosophically interesting. 

    Back in the 1980’s, Paul Churchland and Jerry Fodor had an exchange regarding the modularity of mind.  Churchland thought the ML illusion was acquired, a la the carpentered world hypothesis, where Fodor thought it was in some sense innate.   I think Fodor had the better side of the exchange back then, but the nativist implications of the Alex results seem to support a line more congenial to Fodor.

    But, Pete may disagree ….

  3. Hi Ken,

    My recollection of anti-nativist accounts of ML have to do with exposure to environments with lots of right angles in them. The one line looks longer because if you encountered that as an edge of a box, the “arrow heads” would define edges of walls that would make the “arrow shaft” an inside corner. The line that looks shorter is one with “arrow heads” defining an outside corner of a box. “Arrow shafts” subtending equal amounts of the visual field indicate, with the one set of “arrow heads”, a large far edge, and with the other set of “arrow heads” a small close edge.

    Anyway, whether the Alex experiment tells us much about nativism vs. empiricism of ML depends on how many right angles he’d been exposed to prior to the experiment. I’m betting he spent a lot of time in non-natural environments starring at the walls. He wasn’t subjected to the ML tests after having spent a life only in the jungle.

  4. Saying that ML results from living in a carpentered world provides no direct causal explanation for the illusion. On the other hand, the dynamics of the innate neuronal mechanisms that enable us to parse and analyze the salient features of the visual world do provide a direct explanation of the Muller-Lyer illusion. A computer simulation of the ML “perceived” by a detailed cognitive brain model (Trehub, 1991) demonstrates that the illusion naturally arises because of systematic errors made in locating the ends of each line due to the shapes of the arrows at their termini. When the end angles point out, the terminating centroids of the attached line are closer together than they are when the end angles point in. Thus an innate brain mechanism can fully account for the ML illusion.

  5. Wow!, this was a real quality post. In theory I’d like to write like this too – taking time and real effort to make a good article… but what can I say… I keep putting it off and never seem to get something done.

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