Stevens' Handbook of Experimental Psychology and Cognitive Neuroscience, Sensation, Perception, and Attention

• Editore: Wiley
• Anno: 2018
• ISBN:
  9781119174073
• Edizione: 4
• Argomento: Psicología
• Categoria: Psicología experimental

II. Sensation, Perception & Attention: John Serences (Volume Editor)

(Topics covered include taste; visual object recognition; touch; depth perception; motor control; perceptual learning; the interface theory of perception; vestibular, proprioceptive, and haptic contributions to spatial orientation; olfaction; audition; time perception; attention; perception and interactive technology; music perception; multisensory integration; motion perception; vision; perceptual rhythms; perceptual organization; color vision; perception for action; visual search; visual cognition/working memory.)

CHAPTER 1
Foundations of Vision

FRANK TONG

THE PURPOSE OF VISION

For people with intact vision, it would be hard to imagine what life would be like without it. Vision is the sense that we rely on most to perform everyday tasks. Imagine if instead you had to accomplish all of your daily routines while blindfolded. We depend on vision whenever we navigate to work by foot or by car, search for our favorite snack in the grocery aisle, or scan the words on a printed page trying to extract their underlying meaning. For many mammals and especially for higher primates, vision is essential for survival, allowing us to reliably identify objects, food sources, conspecifics, and the layout of the surrounding environment.

Beyond its survival value, our visual sense provides us with an intrinsic source of beauty and pleasure, a tapestry of richly detailed experiences. We may find ourselves captivated by an expansive view from a seaside cliff, a swirl of colors in an abstract oil painting, or an endearing smile from a close friend.

The power of vision lies in the dense array of information that it provides about the surrounding environment, from distances near and far, registered by the geometry of light patterns projected onto the backs of the eyes. It is commonly said that a picture is worth a thousand words. Consider for a moment the chirping activity of the ganglion cells in your retinae right now, and their outgoing bundle of roughly 1 million axonal fibers through each optic tract. Following each glance or microsaccade, a new pattern of activity is registered by the photoreceptors, then processed by the bipolar neurons and the ganglion cells, after which these high‐bandwidth signals are relayed to the lateral geniculate nucleus and ultimately to the visual cortex for in‐depth analysis.

Psychologists and neuroscientists have made remarkable advances in understanding the functional organization of the visual system, uncovering important clues about its perceptual mechanisms and underlying neural codes. Computational neuroscientist David Marr (1982) once quipped that the function of vision is “to know what is where by looking.” As Marr well appreciated, the problem underlying vision is far easier to summarize than it is to solve. Our visual system does a remarkably good job of solving this problem, getting things pretty much right about 99.9% of the time. On those rare occasions where the visual system seems to come up with “the wrong answer,” as in the case of visual illusions, scientists can gain insight into the powerful computations that underlie the automatic inferences made by the visual system.

Perception, Introspection, and Psychophysics

Most fields of natural science rely exclusively on third‐person observation and experimentation. In contrast, vision scientists can learn a great deal from introspecting on their personal visual experiences and by directly testing their own eyes and brains. The seminal contributions of vision research to the emergence of psychology as a field can be explained by the fact that scientists could so readily test and analyze their own perceptions.

Some early discoveries were made by fortuitous observation, such as when Addams (1834) noticed after staring at a waterfall that his subsequent gaze at the neighboring rocky cliff led to an unexpected impression of upward motion. His description of the motion aftereffect, or waterfall illusion, helped set the path toward the eventual development of ideas of neuronal adaptation and opponent‐based coding to account for visual aftereffects. Other discoveries involved more purposeful observations and simple experiments to characterize a perceptual mechanism. Sir Charles Wheatstone devised an optical apparatus to present different pictures to the two eyes, and then drew simple pictures to capture how a 3D object would appear slightly differently from the vantage point of each eye. By presenting these image pairs in his stereoscope, he discovered that it was possible to re‐create an impression of stereo‐depth from flat pictures. He also found that distinct patterns presented to the two eyes could induce periodic alternations in perception, or form‐based binocular rivalry. His optical invention grew so popular (akin to the current‐day popularity of 3D TV and 3D movies) that the Wheatstone stereoscope could be found in many parlor rooms in England in the 1800s.

As the process of characterizing perception became more formalized, a scientific methodology evolved. Psychophysics refers to experimental methods for quantifying the relationship between the psychological world and the physical world, which usually involves systematic manipulations of a stimulus and measuring its perceptual consequences. For instance, Weber reported that the ability to detect a just noticeable difference (JND) between two stimuli depended on their relative difference (or ratio) rather than the absolute difference. Expanding upon this idea, Fechner (1860) proposed that the perceived intensity of a sensory stimulus should increase in a predictable manner proportional to the logarithm of its physical intensity. Specifically, S = log(I), where S refers to the intensity of the sensation and I refers to the intensity of the physical stimulus. By describing this simple lawful relationship between physical intensity and psychological experience, the field of visual psychophysics was born. A central tenet of visual psychophysics is that perceptual states can be quantified and formally characterized, to help reveal the underlying mechanisms.

Signal Detection Theory

A fundamental advance in visual psychophysics was the application of signal detection theory to quantify the sensitivity of the human visual system. This statistical theory was originally developed to address the problem of detecting faint radar signals reflected by a target in the presence of background noise (Marcum, 1947). In visual psychophysics, this same logic and approach can be applied to both visual detection and visual discrimination paradigms (Tanner & Swets, 1954). These concepts are central to vision research, so we will spend a good amount of time reviewing them, but if they are already very familiar to you, consider moving on to the section “Why Vision Is a Hard Computational Problem.”

A common design for a visual detection task is as follows. There is a 50/50 chance that a very faint target stimulus will be presented on each trial, and the observer's task is to make a binary decision regarding whether the target was present or absent. Let us assume that the stimulus is extremely weak and that the visual system has some inherent level of noise, so perfect performance is impossible. There are four possible stimulus‐response outcomes, as shown in Figure 1.1A. If the target stimulus is present and the observer correctly reports “target present” this would constitute a hit, but if the observer incorrectly reports “target absent” this would constitute a miss. Now, consider trials where the target is absent and the observer correctly reports “target absent”; this would be a correct rejection. But if the observer incorrectly reports “target present,” this would be considered a false alarm.

Figure 1.1 Overview of signal detection theory. (A) Table showing classification of an observer's responses to a target stimulus, regarding its presence or absence. (B) Signal detection theory proposes that the signal + noise distribution is separated from the noise only distribution by distance D. Assuming that both distributions share a common standard deviation, σ, then visual sensitivity or d′ in this task will be determined by D/σ. As the signal becomes stronger, the signal + noise distribution shifts rightward, leading to larger d′ and allowing for better detection performance. Examples of d′ = 1, 2, and 3 are shown. The vertical dashed line indicates the criterion (β) that the observer uses for deciding whether the target is present or absent. If the criterion lies midway between the two distributions, the observer is unbiased and the proportion of misses and false alarms will be equal (bottom panel). Relative to the midway point, leftward shifts lead to a more liberal criterion for reporting target present, while rightward shifts lead to a more conservative criterion. The middle panel depicts a conservative criterion, where the proportion of false alarm responses would be reduced, but at the cost of a greatly inflated proportion of miss responses. Color version of this figure is available at http://onlinelibrary.wiley.com/book/10.1002/9781119170174.

SOURCE: Figure created by Frank Tong; used with permission of the author.

Now, imagine that a set of neurons in the brain is selectively activated by the target, but these neurons exhibit some degree of intrinsic noise even when the target is absent. For example, the baseline firing rate of these neurons may...