Sensation and Perception

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University Human color vision is impressive. People with normal color vision can tell apart millions of individual hues. Most amazingly, this ability is achieved with fairly simple hardware. Part of the power of human color vision comes from a clever bit of engineering in the human brain. There, color perception relies on what is known as an 'opponent system.' This means that the presence of one kind of stimulus is treated as evidence for...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University In the back of everyone's eye is a small piece of neural tissue called the retina. The retina has photosensitive cells that respond to stimulation by light. The responses of these cells are sent into the brain through the optic nerve, a bundle of neural fibers. In each retina there is a place somewhere in the periphery where the outputs from retinal cells collect and the bundled optic nerve exits to the brain. At that location,...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University The study of sensation—how signals are transduced from sensory organs, like the eyes—and perception—how the brain interprets these messages—has a rich history dating back to the 19th century, when great strides were made in understanding the properties of light and how they relate to the visual system. Importantly, such sensory and perceptual processes determine what we see, feel, taste, and hear in our surroundings. However, many...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University One thing becomes very salient after basic exposure to the science of visual perception and sensation: what people see is a creation of the brain. As a result people may fail to see things, see things that are not there, or see things in a distorted way. To distinguish between physical reality and what people perceive, scientists use the term awareness to refer to what people perceive. To study awareness, vision scientists often...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University Reaching for objects, walking without hitting obstacles, landing on a chair as you sit (instead of falling to the floor), these and all our physical actions depend on an ability to perceive our own bodies in space, to know where our limbs are relative to one another and relative to the rest of the world. One way that the human brain encodes this information is called proprioception, the brain relies on its own control and feedback...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University The most difficult challenge of visual perception is often described as one of recovering information about three-dimensional space from two-dimensional retinas. The retina is the light-sensitive tissue inside the human eye. Light is reflected from objects in the world, casting projections on the retina that stimulate these light sensitive cells. Objects that are side-by-side in the world will produce side-by-side stimulations on...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University We generally think that we see things pretty well if they are close by and right in front of us. But do we? We know that visual attention is a property of the human brain that controls what parts of the visual world we process, and how effectively. Limited attention means that we can't process everything at once, it turns out, even things that might be right in front of us. In the 1960s, the renowned cognitive psychologist Ulrich...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University Attention refers to the limited human ability to select some information for processing at the expense of other stimuli in the environment. Attention operates in all sensory modalities: vision, hearing, touch, even taste and smell. It is most often studied in the visual domain though. A common way to study visual attention is with a spatial cueing paradigm. This paradigm allows researchers to measure the consequences of focusing...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University In order for recognition of a certain stimulus to take place, visual attention needs to be directed towards said stimulus. To the earliest parts of the visual system, objects are not objects, they are collections of visual features-lines, corners, changes in texture, color, and light. Attention is the resource that is necessary for later processing in order to recognize what a given bundle of features adds up to. This makes...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University Human vision depends on light-sensitive neurons that are arranged in the back of the eye on a tissue called the retina. The neurons, called the rods and cones because of their shapes, are not uniformly distributed on the retina. Instead, there is a region in the center of the retina called the macula where cones are densely packed, and especially so in a central sub-region of the macula called the fovea. Outside the fovea there...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University In perception, it is often the case that the ability to recognize and interpret complex stimuli feels effortless but actually demands complicated and intensive processing. This is because processing is specialized and automated for certain types of very important stimuli. Among the best examples of this phenomenon is face processing. People do not try to detect and recognize faces. It just seems to happen. However, detecting faces...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University Spoken language, a singular human achievement, relies heavily on specialized perceptual mechanisms. One important feature of language perception mechanisms is that they simultaneously rely on auditory and visual information. This makes sense, because until modern times, a person could expect that most language would be heard in face-to-face interactions. And because producing specific speech sounds requires precise articulation,...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University Psychophysics is a branch of psychology and neuroscience that tries to explain how physical quantities are translated into neural firing and mental representations of magnitude. One set of questions in this area pertains to just-noticeable differences (JND): How much does something need to change in order for the change to be perceivable? To pump intuitions about this, consider the fact that small children grow at an enormous...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University Psychophysics is the name for a set of methods in perceptual psychology designed in order to relate the actual intensity of stimuli to their perceptual intensity. One important aspect of psychophysics involves the measurement of perceptual thresholds: How bright does a light need to be for a person to be able to detect it? How little pressure applied to the skin is detectable? How soft can a sound be and still be heard? Put...

Source: Laboratory of Jonathan Flombaum—Johns Hopkins University Visual masking is a term used by perceptual scientists to refer to a wide range of phenomena in which in an image is presented but not perceived by an observer because of the presentation of a second image. There are several different kinds of masking, many of them relatively intuitive and unsurprising. But one surprising and important type of masking is called Object Substitution Masking. It has been a focus of research in vision...