MK-801 reduces sensitivity to Müller-Lyer's illusion in capuchin monkeys

The Perception of the Müller-Lyer Visual Illusion in Schizophrenics and Non-human Primates: A Translational Approach

The Müller-Lyer Illusion (MLI) has been suggested as a potential marker for the perceptual impairments observed in schizophrenia patients. Along with some positive symptoms, these deficits are not easily modeled in rodent experiments, and novel animal models are warranted. Previously, MK-801 was shown to reduce susceptibility to MLI in monkeys, raising the prospects of an effective perception-based model. Here, we evaluate the translational feasibility of the MLI task under NMDA receptor blockage as a primate model for schizophrenia. In Experiment 1, eight capuchin monkeys (Sapajus spp.) were trained on a touchscreen MLI task. Upon reaching the learning criteria, the monkeys were given ketamine (0.3 mg/kg; i.m.) or saline on four consecutive days and then retested on the MLI task. In Experiment 2, eight chronic schizophrenia patients (and eight matching controls) were tested on the Brentano version of the MLI. Under saline treatment, monkeys were susceptible to MLI, similarly to healthy human participants. Repeated ketamine administrations, however, failed to improve their performance as previous results with MK-801 had shown. Schizophrenic patients, on the other hand, showed a higher susceptibility to MLI when compared to healthy controls. In light of the present and previous studies, the MLI task shows consistent results across monkeys and humans. In spite of potentially being an interesting translational model of schizophrenia, the MLI task warrants further refinement in non-human primates and a broader sample of schizophrenia subtypes.

Anisotropy of perceived space in non-primates? The horizontal-vertical illusion in bearded dragons (Pogona vitticeps) and red-footed tortoises (Chelonoidis carbonaria)

The horizontal-vertical illusion is a size illusion in which two same-sized objects appear to be different if presented on a horizontal or vertical plane, with the vertical one appearing longer. This illusion represents one of the main evidences of the anisotropy of the perceived space of humans, an asymmetrical perception of the object size presented in the vertical and horizontal space. Although this illusion has been widely investigated in humans, there is an almost complete lack of studies in non-human animals. Here we investigated whether reptiles perceive the horizontal-vertical illusion. We tested two reptile species: bearded dragons (Pogona vitticeps) and red-footed tortoises (Chelonoidis carbonaria). In control trials, two different-sized food strips were presented and animals were expected to choose the longer one. In test trials, animals received two same-sized strips, presented in a spatial arrangement eliciting the illusion. Only bearded dragons significantly preferred the longer strip in control trials; in test trials, bearded dragons selected the strip arranged vertically, suggesting a human-like perception of this pattern, while no clear choice for either array was observed in tortoises. Our results raise the interesting possibility that the anisotropy of perceived space can exists also in a reptile brain.