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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., Frijia, F., Montanaro, D., Greco, V., Burr, D. C. & Morrone, M. C. (2018). Cortical BOLD responses to moderate- and high-speed motion in the human visual cortex, Sci Rep, 1 (8), 8357. PDF

We investigated the BOLD response of visual cortical and sub-cortical regions to fast drifting motion presented over wide fields, including the far periphery. Stimuli were sinusoidal gratings of 50% contrast moving at moderate and very high speeds (38 and 570 °/s), projected to a large field of view (~60°). Both stimuli generated strong and balanced responses in the lateral geniculate nucleus and the superior colliculus. In visual cortical areas, responses were evaluated at three different eccentricities: central 0-15°; peripheral 20-30°; and extreme peripheral 30-60°. "Ventral stream" areas (V2, V3, V4) preferred moderate-speeds in the central visual field, while motion area MT+ responded equally well to both speeds at all eccentricities. In all other areas and eccentricities BOLD responses were significant and equally strong for both types of moving stimuli. Support vector machine showed that the direction of the fast-speed motion could be successfully decoded from the BOLD response in all visual areas, suggesting that responses are mediated by motion mechanisms rather than being an unspecific preference for fast rate of flicker. The results show that the visual cortex responds to very fast motion, at speeds generated when we move our eyes rapidly, or when moving objects pass by closely.

Cicchini, G. M., Mikellidou, K. & Burr, D. (2018). The functional role of serial dependence, Proceedings of the Royal Society of London B, PDF

The world tends to be stable from moment to moment, leading to strong serial correlations in natural scenes. As similar stimuli usually require similar behavioral responses, it is highly likely that the brain has developed strategies to leverage these regularities. A good deal of recent psychophysical evidence is beginning to show that the brain is sensitive to serial correlations, causing strong drifts in observer responses towards previously seen stimuli. However, it is still not clear that this tendency leads to a functional advantage. Here we test a formal model of optimal serial dependence and show that as predicted, serial dependence in an orientation reproduction task is dependent on current stimulus reliability, with less precise stimuli, such as low spatial frequency oblique Gabors, exhibiting the strongest effects. We also show that serial dependence depends on the similarity between two successive stimuli, again consistent with behavior of an ideal observer aiming at minimizing reproduction errors. Lastly, we show that serial dependence leads to faster response times, indicating that the benefits of serial integration go beyond reproduction error. Overall our data show that serial dependence has a beneficial role at various levels of perception, consistent with the idea that the brain exploits temporal redundancy of the visual scene as an optimization strategy


Mikellidou, K., Turi, M. & Burr, D. C. (2017). Spatiotopic coding during dynamic head tilt, J Neurophysiol, 2 (117), 808-817. 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 approximately 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, whereas the spatiotopic component remained. The results suggest that perceptual stability may be maintained at least partially through mechanisms involving spatiotopic coding.

Mikellidou, K., Kurzawski, J. W., Frijia, F., Montanaro, D., Greco, V., Burr, D. C., et al. (2017). Area Prostriata in the Human Brain, Curr Biol, 19 (27), 3056-3060 e3053. PDF

Area prostriata is a cortical area at the fundus of the calcarine sulcus, described anatomically in humans [ 1–5 ] and other primates [ 6–9 ]. It is lightly myelinated and lacks the clearly defined six-layer structure evident throughout the cerebral cortex, with a thinner layer 4 and thicker layer 2 [ 10 ], characteristic of limbic cortex [ 11 ]. In the marmoset and rhesus monkey, area prostriata has cortical connections with MT+ [ 12 ], the cingulate motor cortex [ 8 ], the auditory cortex [ 13 ], the orbitofrontal cortex, and the frontal polar cortices [ 14 ]. Here we use functional magnetic resonance together with a wide-field projection system to study its functional properties in humans. With population receptive field mapping [ 15 ], we show that area prostriata has a complete representation of the visual field, clearly distinct from the adjacent area V1. As in the marmoset, the caudal-dorsal border of human prostriata—abutting V1—represents the far peripheral visual field, with eccentricities decreasing toward its rostral boundary. Area prostriata responds strongly to very fast motion, greater than 500°/s. The functional properties of area prostriata suggest that it may serve to alert the brain quickly to fast visual events, particularly in the peripheral visual field.

Mikellidou, K., Arrighi, R., Aghakhanyan, G., Tinelli, F., Frijia, F., Crespi, S., et al. (2017). Plasticity of the human visual brain after an early cortical lesion, Neuropsychologia, PDF 

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In adults, partial damage to V1 or optic radiations abolishes perception in the corresponding part of the visual field, causing a scotoma. However, it is widely accepted that the developing cortex has superior capacities to reorganize following an early lesion to endorse adaptive plasticity. Here we report a single patient case (G.S.) with near normal central field vision despite a massive unilateral lesion to the optic radiations acquired early in life. The patient underwent surgical removal of a right hemisphere parieto-temporal-occipital atypical choroid plexus papilloma of the right lateral ventricle at four months of age, which presumably altered the visual pathways during in utero development. Both the tumor and surgery severely compromised the optic radiations. Residual vision of G.S. was tested psychophysically when the patient was 7 years old. We found a close-to-normal visual acuity and contrast sensitivity within the central 25 degrees and a great impairment in form and contrast vision in the far periphery (40-50 degrees ) of the left visual hemifield. BOLD response to full field luminance flicker was recorded from the primary visual cortex (V1) and in a region in the residual temporal-occipital region, presumably corresponding to the middle temporal complex (MT+), of the lesioned (right) hemisphere. A population receptive field analysis of the BOLD responses to contrast modulated stimuli revealed a retinotopic organization just for the MT+ region but not for the calcarine regions. Interestingly, consistent islands of ipsilateral activity were found in MT+ and in the parieto-occipital sulcus (POS) of the intact hemisphere. Probabilistic tractography revealed that optic radiations between LGN and V1 were very sparse in the lesioned hemisphere consistently with the post-surgery cerebral resection, while normal in the intact hemisphere. On the other hand, strong structural connections between MT+ and LGN were found in the lesioned hemisphere, while the equivalent tract in the spared hemisphere showed minimal structural connectivity. These results suggest that during development of the pathological brain, abnormal thalamic projections can lead to functional cortical changes, which may mediate functional recovery of vision.

Cicchini, G. M., Mikellidou, K. & Burr, D. (2017). Serial dependencies act directly on perception, J Vis, 14 (17), 6. PDF

There is good evidence that biological perceptual systems exploit the temporal continuity in the world: When asked to reproduce or rate sequentially presented stimuli (varying in almost any dimension), subjects typically err toward the previous stimulus, exhibiting so-called "serial dependence." At this stage it is unclear whether the serial dependence results from averaging within the perceptual system, or at later stages. Here we demonstrate that strong serial dependencies occur within both perceptual and decision processes, with very little contribution from the response. Using a technique to isolate pure perceptual effects (Fritsche, Mostert, & de Lange, 2017), we show strong serial dependence in orientation judgements, over the range of orientations where theoretical considerations predict the effects to be maximal. In a second experiment we dissociate responses from stimuli to show that serial dependence occurs only between stimuli, not responses. The results show that serial dependence is important for perception, exploiting temporal redundancies to enhance perceptual efficiency.


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.


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.



  • Cicchini, G. M., Mikellidou, K., & Burr, D.C. (2017). The perceptual consequences of serial dependencies. 40th European Conference on Visual Perception, Berlin, Germany.
  • Burr, D.C., Cicchini, G. M., Mikellidou, K. (2017). Serial-dependencies in the perception of orientation number, faces and bodies. 40th European Conference on Visual Perception, Berlin, Germany.
  • Mikellidou, K., Kurzawski, J.W., Frijia, F., Montanaro D., Greco V., Burr D., Morrone, M. C. (2017). Area prostriata in the human brain. Vision Sciences Society annual meeting.
  • Mikellidou, K., Cicchini G. M., & Burr D. (2016). Serial-dependencies in the perception of orientation. Applied Vision Association Christmas meeting, Queen Mary University, London, UK.
  • Mikellidou, K., Frijia, F., Montanaro D., Greco V., Burr D., Morrone M. C. (2016). Cortical responses to moderate- and high-speed gratings extending 60˚ in the peripheral visual field. Journal of Vision, 16 (12),1179.
  • Keefe, B., Mikellidou, K., Clawson, H., Gouws, A., Thompson, P., & Morland, A. B. (2015). The Helmholtz size illusion is processed by the extrastriate visual cortex, evidence from TMS. Society for Neuroscience, Chicago, USA.
  • Mikellidou, K., Frijia, F., Montanaro D., Greco V., Burr D., Morrone M. C. (2015). A specialised brain area for analysis of rapid motion in the peripheral visual field. 19th Conference of the European Society for Cognitive Psychology, Paphos, Cyprus.
  • Keefe, B., Mikellidou, K., Clawson, H., Gouws, A., Thompson, P., & Morland, A. B. (2015). The Helmholtz size illusion is processed by the extrastriate visual cortex. Perception, 44, 253-254.
  • Mikellidou K., Turi M., Cicchini G. M., Burr D. (2015). Spatiotopic maps during head tilt. 16th International Multisensory Research Forum, Pisa, Italy.
  • Mikellidou K., Frijia F., Montanaro D., Greco V., Burr D., Morrone M. C. (2015). A specialised area for analysis of rapid motion in the periphery. Images of the Mind: new frontiers in brain imaging, Milan, University of Milano-Bicocca, Italy.
  • Tinelli, F., Aghakhanyan, G., Mikellidou, K., Frijia, F., Arrighi, R., Greco, V., Morrone, M.C, & Montanaro, D. (2015). Reorganization of the visual pathways after early-age tumor surgery: the contribution of functional magnetic resonance imaging and fiber tractography. 35th European Brain and Behaviour Society Conference, Brides Les Bains, France.
  • 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., 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.
  • 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. 37th European Conference on Visual Perception, Belgrade, Serbia.
  • 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.
  • Mikellidou, K., Thompson, P., & Burr, D. (2014). Two mechanisms subserve the oblique effect. Journal of Vision, 14 (10), 1416.
  • Mikellidou, K. & Thompson, P. (2012). Simple line-length estimation not so simple. Journal of Vision, 12, 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.
  • Jiang, D., Kovanis, P., & Mikellidou, K. (2009). Using apparent motion to investigate the relationship between changes in BOLD in fMRI and gamma band frequency in MEG in human visual cortex. Proceedings of 4th International Congress on Brain & Behaviour & 17th Thessaloniki Conference, 35.


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