Visual Perception Glossary Amodal perception The part of an object that is not visible because occlu- ded can be amodally perceived. Amodal perception is different and one step removed from modal percepti- on of real or illusory contours. Apparent motion When an object is presented at two different locati- ons after a brief time interval observers perceive mo- tion. The fi rst empirical investigation was carried out by Sigmund Exner . His aim, as well as the subsequent work by Max Wertheimer , was in establishing that motion was a basic sensation. Attention Refers to the selective processing of some aspect of information, while ignoring other information. The sudden onset of a stimulus can capture attention, but people can also exert some control on where they di- rect their attention. Bi-stable stimulus A particular stimulus that can produce two percepts over time, even though it is unchanged as a stimulus. An example is the Necker cube . Brightness Brightness refers to perception of how much light is coming from a given surface or object. A brighter objects refl ects more light than a less bright object. However perception of brightness is not fully deter- mined by luminance (see Illusory contours). 192 Visual Perception Glossary Cerebral lobe The cerebral cortex of the human brain is divided into four main lobes. Frontal (at the front), occipital (at the back), temporal (on the sides) and parietal (at the top). Consciousness Sorry this is too hard, your guess is as good as mine. Cortex The cortex is the outer layer of the brain . In most mammals the cortex is folded and this allows the surface to have a greater area given in the confi ned space available inside the skull. Figure-ground organisation Perceptual organisation separates regions of an image into fi gures, which belong to the foreground, and ground, which is shapeless. Figures are not necessarily familiar objects. Gestalt With the term Gestalt school we refer to a group of scientists who had a great infl u- ence on psychology. The best-known members are Max Wertheimer , Kurt Koffka , and Wolfgang Köhler . The word Gestalt comes from German and refers to a “whole form” (but it is hard to fi nd a good translation). A central principle is that the mind organises information into global wholes. Many illusions such as apparent motion provide examples of such process of perceptual organisation . Illusion It is hard to defi ne properly what is an illusion. In general it is an event in which ob- servers have a surprising percept, often at odds with what they know and what they expect. Other times the surprise is only possible after studying the physical stimulus and comparing the information in the stimulus (for example luminance) with the percept (for example brightness). Illusory contours Contours that are seen and therefore exist as percepts in locations where there is no change in luminance in the stimulus. They are sometimes also called subjective contours . Luminance Luminance is a measure of light intensity . More specifi cally it is the amount of light that is refl ected from a particular object within a particular angle. The unit of meas- ure is the candela per square metre (cd m–2). Neuroscience Neuroscience is a profoundly multidisciplinary fi eld of study. The scope is the study all aspects of the nervous system. It can be divided in a series of subfi elds such as clinical neuroscience, computational neuroscience, molecular neuroscience, visual neuroscience and many more. Visual Perception Glossary 193 Percept We use this term for a specifi c perceptual experience, such as one of the two inter- pretations of a Necker cube . It is an English word although you need a pretty good dictionary to fi nd it. Psychology It is the study of behaviour and mental processes , including perception , attention , emotion , intelligence , personality and more. This is therefore a very broad range of subjects, what all psychologists share is the use of empirical methodologies. Psychophysics A scientifi c fi eld that investigates the relationship between physical stimuli on the one hand, and sensations and perceptions on the other using a series of psychophysical methods. Psychophysics tries to describe the relationship quantitatively. The founder of Psychophysics was Gustav Theodor Fechner (1801–1887) who published a book titled Elements of Psychophysics in 1860. Sensory modalities This term is for the different senses. Traditionally we list fi ve sensory modalities: sight (vi- sion ) , hearing (audition ) , taste (gustation ) , smell (olfaction ) , and touch (somatosensation ) . Stimulus error Titchener used this term to refer to a pitfall. People often describe what they perceive in terms of what they know of the stimulus, instead of drawing on their perceptual experiences. Subjective contours See Illusory contours. V1 This term refers to the primary visual cortex . This is an area of the brain located in the occipital lobe and is the fi rst area of the cortex to receive visual information. It is also called striate (meaning stripy) cortex, and Broadband area 17. V2 This is an area in the visual cortex next to V1 . In general, neurons in both V1 and V2 are sensitive to edges , orientation , size and colour . V2 is part of a set of extra-striate areas that include V2, V3, V4 and V5. Visual illusion See Illusion for a defi nition. In the case of visual illusions we know of many famous examples involving shape , size , colour , motion and more. Visual system The human visual system is the part of the central nervous system that processes vis- ual information. This system includes the primary visual cortex but also a number of higher areas in both the temporal and the parietal lobes . In humans the visual areas in the broader sense extend to a majority of the cortex. Programming Glossary Algorithm A step-by-step procedure to arrive at a solution given a well-defi ned problem. API An acronym for application programming interface. It provides a set of clear defi nitions and methods of commu- nication between various software components. Well-writ- ten and documented the API and its documentation can serve as a manual for a specifi c programming environment. Argument A value provided to a function with parameters. There- fore the value of a parameter. BASIC Written with capitals it is an acronym (Beginner’s All- purpose Symbolic Instruction Code) for a general-pur- pose, high-level programming language. It was created in 1964 by students at Dartmouth College under the su- pervision of John G. Kemeny and Thomas E. Kurtz . Boolean A type of data that can only have two values: True or False. Named after the English mathematician George Boole (1815–1864) . Bug An unexpected fault, fl aw, or error in a program. Finding and eliminating bugs is called debugging . There are many stories about the origin of the term, often refer- ring to the early history of computing and a program- mer who found a beetle or a moth inside the computer. 196 Programming Glossary C++ C++ (pronounced cee plus plus) is a general-purpose programming language created in 1983 by Bjarne Stroustrup . Unlike Python® it is a compiled language. Class In object-oriented programming , objects are instances of classes, and classes defi ne their type. The API typically provides a large collection of predefi ned classes, but most object-oriented programming languages also offer to the user the possibility to create user-defi ned classes. Comment Within a program a comment is a text that is added to make the program easier for hu- mans to understand. It is therefore an annotation that is not part of the commands to the computer. Usually it is clearly marked as a comment (with special characters and with a different font/colour). In Python anything after # till the end of the line is a comment. Compiler A program that transforms commands written in a programming language into an- other language, often a lower-level machine language . The result of the relevant com- pilation process is an executable program . C, C++ and Pascal are examples of com- piled languages (see also Interpreter ). CPU This acronym (Central Processing Unit) refers to the processor inside a computer that carries out the instructions of a program. Data type A particular kind of data. Different data types can take different values, and different operations can be performed with them. In Python examples of built-in data types are integers , fl oat and strings . Dictionary (Python) In Python a dictionary is a type of variable with keys and values. The values inside a dictionary can be of any type. In a dictionary defi nition, each key is separated from its value by : and the whole thing is enclosed in curly brackets. Float In programming and in Python in particular, fl oat is short for “fl oating point real value”. This is a computer representation of a real number written with a decimal point that divides the integer and fractional parts. Example: 3.14159. Float is also a built-in data type in Python. Function A block of commands that perform a specifi c task. In Python functions are defi ned with def nameOfFunction():. They can have parameters inside the brackets and there- fore take arguments (i.e. values) for these parameters . Functions can also return values, for example total = sum(list). Programming Glossary 197 GPU This acronym (Graphics Processing Unit) refers to specialised circuitry inside a computer that carries out instructions relative to what is displayed on the monitor. Its function is to accelerate the creation of images in a frame buffer in a way that is more effi cient than the general-purpose CPU.