Sensation, perception and attention

Transcript

RyanA new mother sleeps through a thunderstorm. She sleeps through the garbage truck at six a.m. But the faintest murmur from her baby down the hall, and she's awake.

SoheeSame ears, same night. So why does one quiet sound get through when the loud ones don't?

RyanWelcome to EPPP Prep, the podcast. I'm Ryan.

SoheeI'm Sohee. Today we start the knowledge statement on elements of cognition, with sensation, perception and attention. There's a lot of vocabulary here, so we'll keep tying it to everyday moments.

RyanStart with the distinction. Sensation happens when a sensory receptor detects sensory information. Converting stimulus energy into an action potential is called transduction.

SoheePerception is the way sensory information is organized, interpreted and consciously experienced. One way the OpenStax psychology textbook puts it: sensation is a physical process, perception is psychological.

RyanAnd it's more than five senses. Beyond vision, hearing, smell, taste and touch, we have balance, the vestibular sense, body position and movement, called proprioception and kinesthesia, plus pain, nociception, and temperature, thermoception.

SoheeNow thresholds. The absolute threshold is the minimum stimulus energy needed for a stimulus to be detected fifty percent of the time. Fifty percent, not every time.

RyanAnd our receptors are remarkably sensitive. It's been estimated that on a clear night, the most sensitive cells in the eye can detect a candle flame thirty miles away. In quiet conditions, the hair cells of the inner ear can detect a clock ticking twenty feet away.

SoheeBelow the absolute threshold are subliminal messages. In the lab, people can process and respond to information outside awareness. But hidden messages have little effect on behavior outside the laboratory.

RyanThen the difference threshold, also called the just noticeable difference. It's how much change is needed to detect a difference between stimuli. And unlike the absolute threshold, it changes depending on stimulus intensity.

SoheeThe textbook's example is a phone screen lighting up. In a dark movie theater, lots of people notice. In a brightly lit arena, almost nobody does. Same phone, same brightness.

RyanErnst Weber proposed this in the 1830s. Weber's law: the difference threshold is a constant fraction of the original stimulus.

SoheePause and think. You're holding a light envelope, and someone adds one sheet of paper. Now you're holding a heavy box of books, and they add the same sheet. When are you more likely to notice?

RyanWith the envelope. Under Weber's law, the change you need to notice is a constant fraction of what you started with. One sheet is a big fraction of an envelope and a tiny fraction of a box of books.

SoheeNow our new mother. Signal detection theory concerns the ability to identify a stimulus when it's embedded in a distracting background. And the textbook uses exactly this example: a mother awakened by a quiet murmur from her baby, but not by other sounds.

RyanIt's also tied to motivation. If you're waiting for an important call, you might think you hear the phone ringing in the shower. Motivation to detect a meaningful stimulus shifts how we separate a true signal from background noise.

SoheeAnd it's practical. The original signal detection work aimed to improve air traffic controllers' sensitivity to plane blips on radar.

RyanNext, sensory adaptation. We often don't perceive stimuli that stay relatively constant over long periods. A flashing light outside your hotel window is maddening at first, then you stop noticing, even though your eyes still sense it.

SoheeWhich is a neat demonstration that sensation and perception are different. The receptors still respond. The perception fades.

RyanThen the two directions of processing. Bottom-up processing is when sensory information from the environment drives the process. A server drops a tray of dishes in a noisy restaurant, and the crash captures your attention whether you like it or not.

SoheeTop-down processing is when knowledge and expectancy drive the process. It's generally goal-directed, slow, deliberate, effortful and under your control. Looking for your yellow key fob on the counter and the coffee table, but not on the ceiling fan.

RyanNow attention, which determines a lot of what's sensed versus what's perceived. The most famous demonstration is Simons and Chabris, 1999. People watched a video of players passing basketballs and counted the passes by the team in white.

SoheeMidway, a person in a black gorilla costume walks among the players, clearly visible for nine seconds. Nearly half the viewers didn't notice the gorilla at all.

RyanThat's inattentional blindness: failing to notice something completely visible because you're actively attending to something else. In a similar study, about a third of people didn't notice a red cross moving across a screen.

SoheeNow two classic theory pairs, where both theories turn out to be right at different levels. First, color vision. Trichromatic theory: normal-sighted people have three types of cones, each most sensitive to a different wavelength, and all colors can be produced by combining red, green and blue.

RyanOpponent-process theory: color is coded in opposing pairs, black-white, yellow-blue and green-red. Some cells are excited by one color in a pair and inhibited by the other. That's why we don't see greenish-reds, and why we get negative afterimages.

SoheePause and think. You stare at a green image for thirty seconds, then look at a white wall and see a red version. Which theory explains it?

RyanOpponent-process. A negative afterimage in the opposing color. And the resolution of the debate: trichromatic theory applies at the retina, in the cones. Once the signal moves past the retina, cells respond in a way consistent with opponent-process theory.

SoheeSecond pair, pitch. Temporal theory says frequency is coded by how fast a neuron fires. Intuitive, but we hear frequencies from twenty to twenty thousand hertz, and neurons can't fire fast enough to cover that whole range.

RyanPlace theory says different parts of the basilar membrane respond to different frequencies. The base responds best to high frequencies, the tip to low ones.

SoheeAnd the resolution. Up to about four thousand hertz, both firing rate and place contribute. Much higher frequencies can only be encoded by place.

RyanNow depth. Binocular cues rely on both eyes. Binocular disparity is the slightly different view each eye gets. It's the principle behind 3-D movie glasses.

SoheeMonocular cues need only one eye, and we actually have more of them. Linear perspective, where parallel lines seem to converge. Interposition, the partial overlap of objects. And the relative size and closeness of images to the horizon.

RyanFinally, Gestalt psychology. Max Wertheimer, with Wolfgang Köhler and Kurt Koffka, argued perception is more than combining sensations. Gestalt means form or pattern, and the idea is that the whole is different from the sum of its parts.

SoheeThe principles. Figure-ground: we split the visual world into a figure, the focus, and the ground, the background. The vase that's also two faces.

RyanProximity: things close together are grouped. Similarity: things that are alike are grouped, like players by uniform color.

SoheeContinuity, or good continuation: we see smooth, continuous lines rather than jagged, broken ones. And closure: we organize perceptions into complete objects rather than parts.

RyanAnd our perceptions rest on perceptual hypotheses, educated guesses informed by personality, experience and expectations. Those form our perceptual set. For example, verbal priming biases how people interpret ambiguous figures.

SoheeSo how might a question come at this? Often as a short scenario that needs a term. A threshold detected half the time? Absolute threshold. Change relative to starting intensity? Weber's law. Missing the obvious while focused? Inattentional blindness. Afterimage? Opponent-process. Grouping by nearness? Proximity.

RyanAnd watch the tempting swaps: sensory adaptation versus inattentional blindness. Adaptation is about a constant stimulus fading. Inattentional blindness is about attention being elsewhere.

SoheeLet's recap. Sensation is detection and transduction. Perception is organizing and interpreting. Absolute threshold is fifty percent detection. The difference threshold follows Weber's constant fraction.

RyanSignal detection is finding a signal in noise, shaped by motivation. Adaptation fades constant stimuli. Bottom-up is stimulus-driven, top-down is knowledge-driven. Attention determines what's perceived, as the gorilla showed.

SoheeTrichromatic at the retina, opponent-process beyond it. Temporal and place theories share pitch below about four thousand hertz. And Gestalt: figure-ground, proximity, similarity, continuity, closure.

RyanIf this felt like a lot of terms, that's because it is. Take them in small doses. Your brain, as we just learned, is very good at picking out the signal that matters to you.

SoheeTry a few practice questions, and we'll see you next time on EPPP Prep, with language, problem solving and executive function.

Loading the read-along timings…