PERCEPTION OF COLORS AND LIGHT

When it comes to visual perception, color has a special status. Everything else that could be observed visually, i.e. sizes and shapes of objects, their spatial layout or motion, could also be assessed through other senses. For example, the shape of an object can easily be learned through touch. Color, however, can only be understood through vision. In order to study colors systematically, every shade is considered through three dimensions. Lightness is a variation from black to white and it represents all achromatic or gray shades. Hue refers to the different wavelengths leaving us with impressions of red, yellow or blue. And finally, there is saturation, referring to pronouncedness of a certain hue.
The color looks the same independent of the illumination. That is, a strawberry is equally red 1) in the bright sunlight, 2) in the artificial room-light, 3) in the deep shadow of the bushes. In psychophysical experiments, however, it is possible to measure subtle difference in color perception in the three described situations. How does the visual system allow us to see the color?

Todorović, D., Zdravković, S. (2014). The roles of image decomposition and edge curvature in the ‘snake’ lightness illusion. Vision Research. Vol 97, 1-15. PDF

Zdravković, S. (2008). Lightness constancy: object identity and temporal integration. Psihologija, Vol. 41, 1, 19-37. PDF
Zdravković, S. (2007). Efekat boje pozadine i osvetljenja na opaženu svetlinu. Psihologija, Vol. 40, 4, 543-565. PDF

LIGHTNESS AND MULTIPLE ILLUMINATION LEVELS

Although color does not change when it is under intense or dim illumination, our perception of that color is influenced by the amount of ambient light. Hence, our percept cannot be predicted only by the color (i.e. surface reflectance). Unfortunately, it cannot be completely predicted by luminance (product of color and illumination) either! Within lightness perception we study such problems, and a set of anchoring rules is offered to account for human perception of achromatic colors. As a rule lightness was studied on objects under a single illumination level. In such conditions some of the main issues were constancy, contrast and articulation. Novel research is concerned with more complex conditions: multiple illumination levels. The impact of differences in illumination levels was investigated both in spatial domain and temporal domain, also within a visual scene or on a single object. How does the visual system allow us to see color under different illumination?

Zdravković, S., Economou, E., Gilchrist, A., (2012). Grouping illumination frameworks. Journal of Experimental Psychology: Human Perception and PerformanceVol. 38(3), 776-784. PDF

Zdravković, S., Economou, E., Gilchrist, A. (2006). Lightness of an object under two illumination levels. Perception 35(9), 1185 – 1201. PDF

Zdravković, S. (2008). Lightness constancy: object identity and temporal integration. Psihologija. Vol. 41, 1, (19-37). PDF

Economou, E., Zdravković, S., Gilchrist, A. (2007). Anchoring versus spatial filtering accounts of simultaneous lightness contrast. Journal of Vision, 7(12):2, 1-15. PDF

MENTAL IMAGERY

It is very easy to imagine color of fresh grass or the intro to our favorite song. In such cases it feels like we can recreate colors and sounds in our head. In fact, it feels like those mental representations of percepts, coming from different sensory modalities, are themselves in different modalities. Is this really the case? What is the modality and code of our mental representations?

Ćirović, I., Zdravković, S. (2011). Verbal vs. visual coding in modified mental imagery map exploration task. Psihologija, 44, 1, 39-60. PDF

Stojanović R., Zdravković. S. (2007). Mentalna eksploracija distanci na mapama i u realnom prostoru. Psihologija, 40, 1, 93-111. PDF

FACE PERCEPTION

Among numerous visual objects that we can identify, discriminate and memorize human faces are particularly important. Apart from a persons identity, the human face can provide information about age, sex, family resemblance, emotional state and mood.
Behavioral studies demonstrate high recognition accuracy we have for human faces while studies from cognitive neuroscience have found a cortical area (FFA, fusiform face area) specialized for face processing. Finally, clinical studies describe prosopagnosia, a condition in which face perception is compromised although the processing of other complex visual objects might be completely intact.
Psychologists investigate face perception and the underlying cortical systems that support this extraordinary function.

Barzut, V., Zdravković, S. (2013). Discrimination of faces of the same and other race and gender modulated by familiarity. Psihologija, 46, 1, 45-59. PDF

Stevanov, Z., Zdravković. S. (2007). Prepoznavanje identiteta na osnovu pojedinih delova lica. Psihologija, 40, 1, 37-57. PDF

VISUAL SEARCH AND ATTENTION

Traditionally, visual search was described to consist of two independent phases, parallel and serial. Parallel is fast, stimulus driven and its cortical center is located in the visual cortex. Serial attention is based in the frontal cortex, and assumes guided search for a particular object. But is it possible to consciously control the attention during parallel visual search?

Popović, M., Zdravković, S. (2009). Can deployment of attention be strategically controlled? Psihologija. Vol. 42, 1, 67-79 PDF

Tracking more than a single object is an everyday routine for most of us. In traffic, drivers monitor the position and speeds of other cars; in team sports, players monitor the position and the motion direction of other players; a kindergarten teacher monitors the movements of the children she took to the park, etc. What is the foundation of the cognitive functions needed to perform such tasks?

Vidaković. V., Zdravković, S. (2010). Influence of depth cues on multiple objects tracking in 3D scene. Psihologija, 43,4, 389-409. PDF

Vidaković, V., Zdravković, S. (2009). Colour influences identification of the moving objects more than shape. Psihologija. Vol. 42, 1, 79-95. PDF

KINETIC DEPTH EFFECT

Vision enables us to function in 3D space, providing the information about our location in space as well as location of other objects around us. Out of the three space dimensions, depth perception traditionally posed a problem for researchers.
Motion perception provides some solutions to the problem because object motion can enhance depth perception. The kinetic depth effect clearly demonstrates this. Sets of dots or line drawings look 2D when presented statically. But as soon as they start to move they appear to form a 3D object. How does motion create perception of depth?

Zdravković, S. (2003). Opažanje dubine u pokretnim dvodimenzionalnim stimulusima. Psihologija, 36, 3, 289-313. PDF