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Sensation (psychophysics) Transduction, Thresholds, and Coding Do Now • Answer the following question: – If a tree falls in a forest and no one is around to hear it, does it make a sound? Sensation: the process by which physical energy is transduced into neural responses Psychophysics: the study of the relationship between physical stimuli and the mental or physical experiences the stimuli evoke Key ideas: transduction and coding Three steps of sensory processing: Physical stimulusPhysiological responseSensory or psychological experience • Physical stimulus: light energy, sound energy, pressure, chemical (taste), etc • Physiological response: receptor potential in the sensory receptor which causes a change in the release of NT which modifies the firing rate in neurons with which these cells form synapses and so on until the information reaches the brain • Sensory experience: see color, taste bitter, hear low tone Touch as Example • Receptor field – Activate/deactivate • Excited in central area • Inhibited in surrounding area • • • • Sensory neuron Brainstem Contralateral thalamus Primary sensory cortex (raw data) – Topographical parts relative to body – Complexity and integration on ascent • Posterior parietal cortex – Integration with other senses • EX: cats only respond to site and sound of birds Thresholds • What is a sensation? – Just the stimulus present? – Just the mind reaction? – Behavioral definition: absolute threshold where stimuli is identified 50% of the time • Attention key • Signal detection theory – Response bias » Need for multiple trials • Sensory adaptation • Just noticeable difference (JND) • 1/30 for weight, 1/100 for length • Weber’s law • Amounted added causing JND is dependent on the amount of a stimulus already present • the jnd for a stimulus magnitude is a constant proportion of the magnitude of the original stimulus Sensory Transduction •Transformation of one form of energy into another •Sensory events are transduced or transferred into changes in the cell’s membrane potential •Example: Light energy is transformed by an electrical chemical interaction in the sensory receptor into a receptor potential Quantitative Dimension – Amount or intensity of energy present • Dull or bright light, light or strong pressure, loud or soft tone, strong or weak bitter taste – Rate of firing and number of neurons – Different neurons for different intensities • Neuron 1: low threshold for small distinctions • Neuron 2: medium threshold • Neuron 3: accurate for strong stimulus Qualitative dimension – Precise kind of energy present • Red or yellow light, sweet or bitter taste, high or low pitched tone – Different receptor neurons are differentially sensitive to different kinds of energy • Quantitative: magnitude • Qualitative: relative firing Coding Processing •Sense-specific Processing center, sensors, and pathways •Convergence •As information proceeds up the brain, it is integrated and more complex Revisit Do Now • Did the tree make a sound? – What might psychologists say?