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Kyriaki Mikellidou

Post-Doc in Department of Translational Research on New Technologies in Medicine and Surgery, University of Pisa


  • Email: kmikellidou (AT) gmail.com
  • Telephone:  +39 050 3153175
Research laboratories
  • CNR Institute of Neuroscience, Pisa
  • Department of Translational Research on New Technologies in Medicine and Surgery, University of Pisa


  • 2009 – 2012     University of York, United Kingdom
    PhD in Psychology
  • 2008 – 2009    University of York, United Kingdom
    MSc in Cognitive Neuroscience
  • 2005 – 2008    University of York, United Kingdom
    BSc (Hons) Psychology

Current research and interests

  • Visual illusions
  • Neuroimaging of vision



Mikellidou, K., Gouws, A. D., Clawson, H., Thompson, P., Morland, A. B. & Keefe, B. D. (2016). An Orientation Dependent Size Illusion Is Underpinned by Processing in the Extrastriate Visual Area, LO1, i-Perception, 5 (7), PDF

We use the simple, but prominent Helmholtz’s squares illusion in which a vertically striped square appears wider than a horizontally striped square of identical physical dimensions to determine whether functional magnetic resonance imaging (fMRI) BOLD responses in V1 underpin illusions of size. We report that these simple stimuli which differ in only one parameter, orientation, to which V1 neurons are highly selective elicited activity in V1 that followed their physical, not perceived size. To further probe the role of V1 in the illusion and investigate plausible extrastriate visual areas responsible for eliciting the Helmholtz squares illusion, we performed a follow-up transcranial magnetic stimulation (TMS) experiment in which we compared perceptual judgments about the aspect ratio of perceptually identical Helmholtz squares when no TMS was applied against selective stimulation of V1, LO1, or LO2. In agreement with fMRI results, we report that TMS of area V1 does not compromise the strength of the illusion. Only stimulation of area LO1, and not LO2, compromised significantly the strength of the illusion, consistent with previous research that LO1 plays a role in the processing of orientation information. These results demonstrate the involvement of a specific extrastriate area in an illusory percept of size.

Mikellidou, K., Turi, M. & Burr, D. C. (2016). Spatiotopic maps during dynamic head tilt, J Neurophysiol, jn 00508 02016. PDF

Humans maintain a stable representation of the visual world effortlessly, despite constant movements of the eyes, head and body, across multiple planes. Whereas visual stability in the face of saccadic eye-movements has been intensely researched, fewer studies have investigated retinal image transformations induced by head movements, especially in the frontal plane. Unlike head rotations in the horizontal and sagittal planes, tilting the head in the frontal plane is only partially counteracted by torsional eye-movements, and consequently induces a distortion of the retinal image to which we seem to be completely oblivious. One possible mechanism aiding perceptual stability is an active reconstruction of a spatiotopic map of the visual world, anchored in allocentric coordinates. To explore this possibility, we measured the positional Motion Aftereffect (PMAE: the apparent change in position after adaptation to motion) with head-tilts of ~42 degrees between adaptation and test (to dissociate retinal from allocentric coordinates). The aftereffect was shown to have both a retinotopic and spatiotopic component. When tested with unpatterned Gaussian blobs rather than sinusoidal grating stimuli, the retinotopic component was greatly reduced, while the spatiotopic component remained. The results suggest that perceptual stability may be maintained at least partially through mechanisms involving spatiotopic coding.


Greco, V., Frijia, F., Mikellidou, K., Montanaro, D., Farini, A., D'Uva, M., et al. (2015). A low-cost and versatile system for projecting wide-field visual stimuli within fMRI scanners,Behav Res Methods, PDF

We have constructed and tested a custom-made magnetic-imaging-compatible visual projection system designed to project on a very wide visual field (~80 degrees ). A standard projector was modified with a coupling lens, projecting images into the termination of an image fiber. The other termination of the fiber was placed in the 3-T scanner room with a projection lens, which projected the images relayed by the fiber onto a screen over the head coil, viewed by a participant wearing magnifying goggles. To validate the system, wide-field stimuli were presented in order to identify retinotopic visual areas. The results showed that this low-cost and versatile optical system may be a valuable tool to map visual areas in the brain that process peripheral receptive fields.

Mikellidou, K., Cicchini, G. M., Thompson, P. G. & Burr, D. C. (2015). The oblique effect is both allocentric and egocentric,Journal of Vision, 8 (15), 24-24. PDF

Despite continuous movements of the head, humans maintain a stable representation of the visual world, which seems to remain always upright. The mechanisms behind this stability are largely unknown. To gain some insight on how head tilt affects visual perception, we investigate whether a well-known orientation-dependent visual phenomenon, the oblique effect—superior performance for stimuli at cardinal orientations (0° and 90°) compared with oblique orientations (45°)—is anchored in egocentric or allocentric coordinates. To this aim, we measured orientation discrimination thresholds at various orientations for different head positions both in body upright and in supine positions. We report that, in the body upright position, the oblique effect remains anchored in allocentric coordinates irrespective of head position. When lying supine, gravitational effects in the plane orthogonal to gravity are discounted. Under these conditions, the oblique effect was less marked than when upright, and anchored in egocentric coordinates. The results are well explained by a simple “compulsory fusion” model in which the head-based and the gravity-based signals are combined with different weightings (30% and 70%, respectively), even when this leads to reduced sensitivity in orientation discrimination.


Mikellidou, K. & Thompson, P. (2014). Crossing the line: Estimations of line length in the Oppel-Kundt illusion,J Vis, 8 (14), PDF

In the Oppel-Kundt illusion, one of the oldest and least understood geometrical visual illusions, a line subdivided by a series of short orthogonal ticks appears longer than an identical line without these. Paradoxically, bisecting a long line with a single tick leads to perceived shortening of the line. We have systematically investigated the effects of adding 1 to 12 ticks on perceived line length and results suggest that at least three mechanisms must be at work: (a) bisection, which reduces perceived length; (b) a filled extent effect, which is also apparent in the von Helmholtz illusion, though no satisfactory explanation for it exists; and (c) a local contour repulsion effect of the penultimate tick upon the perceived position of the end tick, but this effect, though significant, is too small to explain the Oppel-Kundt illusion in its entirety.


Mikellidou, K. & Thompson, P. (2013). The vertical-horizontal illusion: assessing the contributions of anisotropy, abutting, and crossing to the misperception of simple line stimuli,J Vis, 8 (13), PDF

Mamassian and de Montalembert (2010) have proposed a simple model of the vertical-horizontal illusion. This model identified two components, an anisotropy which results in horizontal lines being perceived approximately 6% shorter than verticals and a bisection component which results in a bisected line being perceived approximately 16% shorter. We have shown that this bisection component confounds two effects: One when lines cross one another and a second effect when one line abuts another. We propose an extension to the Mamassian-de Montalembert model in which their bisection component is replaced by separate crossing and abutting components.


Thompson, P. & Mikellidou, K. (2011). Applying the Helmholtz illusion to fashion: horizontal stripes won't make you look fatter,Iperception, 1 (2), 69-76. PDF

A square composed of horizontal lines appears taller and narrower than an identical square made up of vertical lines. Reporting this illusion, Hermann von Helmholtz noted that such illusions, in which filled space seems to be larger than unfilled space, were common in everyday life, adding the observation that ladies' frocks with horizontal stripes make the figure look taller. As this assertion runs counter to modern popular belief, we have investigated whether vertical or horizontal stripes on clothing should make the wearer appear taller or fatter. We find that a rectangle of vertical stripes needs to be extended by 7.1% vertically to match the height of a square of horizontal stripes and that a rectangle of horizontal stripes must be made 4.5% wider than a square of vertical stripes to match its perceived width. This illusion holds when the horizontal or vertical lines are on the dress of a line drawing of a woman. We have examined the claim that these effects apply only for 2-dimensional figures in an experiment with 3-D cylinders and find no support for the notion that horizontal lines would be 'fattening' on clothes. Significantly, the illusion persists when the horizontal or vertical lines are on pictures of a real half-body mannequin viewed stereoscopically. All the evidence supports Helmholtz's original assertion.


  • Mikellidou, K., Thompson, P., & Burr, D. (2014). Two mechanisms subserve the oblique effect. Journal of Vision 14 (10), 1416-1416.
  • Burr, D., Mikellidou, K., Cicchini, G. M, & Thompson, P. (2014). Fine sensitivity of orientation discrimination around vertical and horizontal results from both gravity and variable neural sensitivity. 15th International Multisensory Research Forum, Amsterdam, Netherlands.
  • Morrone, M.C., Greco, V., Frijia, F., Mikellidou, K., Montanaro, D., D'Uva , M., Poggi, P., Pucci, M., Sordini, A., Farini, A., Burr, D. (2014). A system for projecting wide-field visual stimuli within fMRI scanners. Perception, 43, 166.
  • Mikellidou, K., Cicchini, G.M., Thompson, P., & Burr, D. (2014). The oblique effect: how do you know what’s vertical, tilted or horizontal? 14th European Workshop on Imagery and Cognition, Paphos, Cyprus.
  • Aghakhanyan, G., Tinelli, F., Mikellidou, K., Frijia, F., Arrighi, R., Greco,V., Canapicchi, R., Morrone, M.C., & Montanaro, D. (2014). Reorganisation of the visual pathways after early-age tumor removal assessed by functional magnetic resonance imaging and fiber tractography. XXth Symposium Neuroradiologicum, Istanbul, Turkey.
  • Mikellidou, K. & Thompson, P. (2012). Simple line-length estimation not so simple. Journal of Vision 12 (9), 320-320.
  • Mikellidou, K. & Thompson, P. (2011). Bisection and dissection of horizontal lines: the long and the short of the Oppel-Kundt illusion. Journal of Vision, 11, 1184.
  • Mikellidou, K. & Thompson, P. (2010). The vertical-horizontal and the Oppel-Kundt illusions: how our visual perception changes. Perception, 40, 116.
  • Thompson,P. & Mikellidou, K. (2009) The 3-D Helmholtz square illusion: more reasons to wear horizontal stripes. Journal of Vision, 9, 50.
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