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Week 9 PSYCH online reading.docx

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Queen's University
PSYC 100

November 6, 12 PSYCH – Week 9 Online Readings Week 9: Perception Focus Question: How does our brain create meaning from what the body senses? We are not conscious of the processes involved in creating our perceptions. Our perception depends not just on sensation (sensory input) but also on our particular knowledge and experience. Scientific Study of Perception: How do we measure perception? Psychophysics: The study of how sensation relates to perception. Sensory thresholds: The point at which a stimulus triggers the start of an afferent nerve impulse. Difference threshold: The just-noticeable difference between two stimuli. Weber's Law: The size of the just-noticeable difference (JND) of a stimulus divided by its initial intensity is a constant. In general, across all sensory domains (loudness, weight, etc) the just-noticeable difference threshold for a stimulus goes up as its intensity goes up. Fechner's Law: In every sensory domain, each just-noticeable difference represents an equal step in the psychological magnitude of a sensation. This means that changes in stimulus can be compared across sensory domains, for example, between vision and touch, although each different domain could have a different JND. Stevens' Power Law: Proposed relationship between the magnitude of a stimulus and its perceived intensity or strength Absolute Threshold: Minimum value of a stimulus that can be detected. Ie: faintest sound, gentlest touch How can we detect faint signals? It depends on: Sensitivity:Ability to detect a given stimulus. Cognitive factors: Factors that involve thought processes, including changes in attention, expectation, and alertness. Influencing perceptual decisions: noise and bias November 6, 12 External noise (which researchers try to control for) and a participant’s response bias influence perception. Neurons, of course, have a minimum rate of firing called spontaneous firing, or internal/neural noise. Response bias:Aperson's tendency to say "yes" or "no" when he is not sure whether he detected the stimulus. Spontaneous rate: Neuron minimum rate of firing Signal Detection Theory (SDT): Mathematical theory of the detection of stimulation in which every stimulus event requires discrimination between signal (stimulus) and noise (consisting of both background stimuli and random activity of the nervous system). It allows independent assessment of sensitivity and bias, shown in receiver-operating characteristic (ROS) plots (Graph of hits and false alarms of participants under different motivational conditions; indicates people's ability to detect a particular stimulus.) Psychophysics of Sound Pitch: how high or low a sound is, depends on frequency of sound waves, or how rapidly they alternate between compression and rarefaction. Fundamental frequency: The lowest frequency of a periodic waveform. Think moving up and down the neck of a violin. How does the brain get from a vibrating string to perceiving a high or low pitch? The cochlea analyzes complex sounds, breaking them down into their component frequencies. The presence of each component and its relative intensity are relayed to the brain through the individual fibres of the auditory nerve. Each auditory nerve fibre is most sensitive to a particular frequency. How does the brain interpret frequencies? The fact that frequencies are anatomically separated and organized in the ear is called tonotopic organization. This tonotopic organization is maintained all the way to the primary auditory cortex. This can serve like a place code to signal which frequencies are present in the sound. Alternatively, a temporal code may be used, up to frequencies of 5000 Hz, at least. How so? Well, it turns out that auditory nerve fibres (the afferent axons from auditory sensory November 6, 12 neurons in the cochlea) work pretty fast – they can fire at up to 1000 times a second. Also, they tend to fire in synchrony with a stimulating waveform – every time there is a compression in the sound wave, the hair cell depolarizes, and an action potential is generated. Temporal codes are used for lower-frequency sounds. Any individual nerve axon can’t fire more than 1000 times a second (1000 Hz). The auditory nerve can comfortably follow – generate action potentials in synchrony with – auditory frequencies up to 5000 Hz, or five times the maximum firing rate. The Volley Principle Although any individual fibre can’t fire more than 1000 times a second (1000 Hz), fibres can take turns generating action principles in order to send temporal codes in synchrony with auditory frequencies up to 5000 Hz. This is known as the volley principle. For it to work, the auditory cells must create responses in a precise sequence. This figure illustrates the volley principle. The top graph labeled ‘Stimulus’shows a sound – we know this is a single frequency (a pure tone) because it is a sine wave. The numbers along the top label the stimulating sound waves. These trigger action potentials. Lines a – e are different auditory nerve fibres, and the vertical lines on the fibres indicate the action potentials. Any one fibre doesn’t fire on all the compressions. But the aggregate across the several fibres (‘Total response’) is in time with the stimulating waveform. Auditory Codes Although the neural code for pitch is still uncertain, the consensus view these days is that both codes are used: a place code is used particularly for higher frequencies, and a temporal code is used for lower frequencies. November 6, 12 Let’s see how these components combine to make up sound. For example, in this figure the complex sound wave is shown at the bottom. Let’s say it has a pitch of 100 Hz. The fact that it has a pitch tells us that all the components bear a mathematical relationship to each other. The lowest- frequency component, called the fundamental frequency, is shown at the top. Then all other components – the harmonics – are 2x, 3x, and 4x this frequency – “integer multiples” of the fundamental. The relationship among the frequency components – the fact that they are all 100 Hz apart – determines the pitch of the complex sound. A complex sound is made by adding a series of harmonics to a fundamental frequency. Pattern recognition: Identification of particular sound sources by the auditory system. We can tell a guitar from a violin by identifying the timbre of the sound. The timbre depends on the relative amplitudes of the components – the amount of each component in the mixture, just like the perceived colour of light depends on the wavelengths of the component lights in a mixture. Discrete parts of the basilar membrane respond to each of the components, detecting their frequencies and amplitudes. The brain can put this information together again to determine the timbre. Timbre perception is a kind of pattern recognition. Loudness Sound intensity (what we hear as loudness) is coded by the degree to which each auditory nerve fibre fires on every cycle of a stimulating waveform. If a sound is intense, then each nerve fibre attached to the responsive place on the basilar membrane will fire as much as it can. Look back to the figure on the volley principle on page 5 of this lesson. If the sound is intense, fibres a – e will be driven as hard as they can be and each individual fibre will look as much like the ‘total response’as it can. If the sound is very soft, individual fibres will still fire in synchrony with the stimulating waveform but sporadically, such that the ‘total response’might not be as faithful to the stimulating waveform. Loudness depends not just on the intensity of a sound, but also on its frequency. Humans are most sensitive to the middle frequencies (from 1,000 to 4,000 Hz), which overlap a lot with the range of frequencies used in speech. Where is that sound coming from? Suppose someone called your name from across campus – how do you know which direction to look?A: You probably heard the voice more loudly in one ear than the other. This intensity cue is helped by the shape of your pinna, or outer ears, which changes the timbre of a sound based on its location. In addition, each ear heard the sound at very slightly different times, telling you which side of your head was closer to the sound source. Pinna, Timbre, and Location November 6, 12 Spatial information is not directly preserved in the inner hair cell receptor array. Instead, locations of sound sources have to be inferred based on frequency and timing information in the sound. There are three ways we use our ears to locate sounds. First, our pinnae are unique to each of us. Their precise shape determines how sounds are transmitted into the ear canal, affecting the timbre of a sound systematically in ways that we can interpret as information about the elevation of a sound source – whether it’s coming from over our heads, below us, or in front or behind us. Timing and Intensity The timing cue works because sound is relatively slow. For example if we hear a sound from the left side of us, the sound reaches our left ear first before our right ear. If we hear it directly from the middle we hear the sound in both ears at the same time.Also to figure out location, intensity of a sound is measured by our ears and brain. Sounds are less intense in our far ear. Both timing and intensity cues are used by different structures early in the auditory neural pathway in the brainstem to compute sound locations. This information is then conveyed to the parietal lobe (which is important for our sense of space and where objects are in space). Midline vs. Side Hearing Humans are more sensitive to changes in sound location near the midline (straight ahead) compared to locations off to the side (opposite one ear). In other words, the JND in sound position is smaller near the midline than at the side. In general, though, our ability to determine the location of objects using hearing is poorer than our ability to locate them using touch or vision. If a small object is touching you, you can figure out where it is easily and with great confidence. Similarly if you see it, you know exactly where it is. If you hear it, you will probably estimate its location auditorily, and then turn your head in that direction so that you can see it and determine its location more precisely. The Psychophysics of Vision How does the brain create meaning from what the body senses? As you learned previously, the sensations of pitch, loudness, and timbre relate to the physical characteristics of sound, and can be measured using the different psychophysical methods. Similarly, your brain takes the wavelength, amplitude, and purity of light entering the eye, and ‘sees’these as colour, brightness, and hue, respectively. The location on the retina where light is detected tells us about the location of the object reflecting/emitting the light. Vision and Location Visual localization is more precise than auditory localization. When we see, the brain has to interpret light signals collected on a flat surface – the retina.Although we perceive the world in 3D, vision information passes through the retina, which processes in 2D. We use our knowledge and experience derived from moving around the world in three November 6, 12 dimensions to infer the third dimension from what we see. Perceptual systems are often working to interpret degraded or ambiguous sensory information, and the role of knowledge and experience in perception is relevant to all sensory domains. Motion Parallax We perceive the world in three dimensions through a number of cues, including monocular depth cues (Cues to distance that depend on input from only one eye). Think about traveling in a car and looking out the window. The world moves past, and objects that are further away (on the horizon) seem to be moving more slowly than things that are close to you. Furthermore, the direction of apparent motion of objects in the world depends on where you are looking – things farther away than the point you’re looking at will appear to be moving rather slowly, and in the same direction you are. Things closer than the point you’re looking at will appear to be moving faster and in the opposite direction. Binocular Cues Your eyes also let your brain determine the location of an object through binocular cues, such as the motions that each eye makes in order to focus on a specific location and the retinal disparity, which is the degree to which light from an object falls on a different location on your left retina compared to your right. (When you focus at a given distance, objects at that distance have no retinal disparity; light from objects closer to and further away from that that focus distance will fall on different locations on your two retinas). The accuracy of these binocular cues is determined by the distance between your eyes and is most accurate at relatively close distances. Most 3D illusions, including those using special glasses, use binocular cues to trick your brain into seeing distances within a flat image. Vision and the Brain The visual processing system of the brain is ridiculously complex. Information flows through this system, visual images are analyzed into their component features (e.g., form, colour, spatial location, and movement). Then these features are synthesized, organized into meaningful arrays of objects. This complex system takes us from sensation to perception. Ventral and Dorsal Researchers proposed in the early 1980s that the visual processing system could be thought of as two interconnected ‘streams’of processing. November 6, 12 The ventral stream (“the ‘what’stream”) involves areas at the bottom of the temporal lobe (shown here in blue). The processing of this stream enables us to see form, colour, and motion. It allows us to identify objects and to attach meaning and significance to them. The dorsal stream (“the ‘where’or ‘how’stream”) involves the parietal cortex (shown here in red) and enables us to perceive the location of objects such that we can direct appropriate actions (like reaching to grasp a Frisbee or to post a letter in a long narrow mail slot) to those objects. The two streams must work together in the production of useful, adaptive behaviour. Damage and Recognition Damage along the visual pathways, especially the ventral pathway, can result in visual agnosia – the inability of a person, in whom elementary sensory processing is intact (e.g., they aren’t blind) to recognize what they are seeing as a meaningful, recognizable object. An object tha
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