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Lecture #6- Part I- Sensation & Perception.docx

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University of Toronto St. George
Ashley Waggoner Denton

Lecture #6: Part I: Sensation & Perception Sensation involves the detection of external stimuli (e.g. light, pressure or odours), the responses to the stimuli and the transmission of these responses to the brain. This is the first step, which senses the stimulus. Perception involves the processing, organization and interpretation of sensory signals in the brain, which results in an internal representation of the stimuli – and your conscious experience of it. This is the second step, which makes sense of the stimuli to experience it. Everything is experienced in your brain; even though it is felt in the periphery, it is experienced in the brain. The world we live in is constructed by us. Context and change is important to respond and adapt. A stimulus needs to be translated into chemical or electrical signals for the brain to understand them (neuronal impulses). Transduction is the process by which sensory receptors pass impulses to connecting neurons when they receive stimulation. These impulses travel to the thalamus, which then directs it to the appropriate brain location in the cortex (unless the case is smell, which directly bypasses the thalamus). It is in these regions that the information is interpreted as one of the senses. Sensory adaptation: sense initial change, but prolonged stimulation causes adaptation, which eventually cause the system to ignore the stimulus. Sensing Chemicals: Taste & Smell: When chemical substances dissolve in the saliva, the taste receptors found in the taste buds are activated. A taste bud can have more than one receptor. Every taste experience is composed of a mix of five basic qualities: sweet, salty, sour, bitter and umami (savoury). Contrary to popular belief, the tongue is not divided among flavour; all of the receptors are found dispersed all over the tongue. Taste is also dependent on smell and texture. Since the taste experience occurs in the brain, it is able to integrate all of these signals. The amount of taste receptors can vary between humans. Those with a greater amount are called supertasters. The stimuli for smell are chemical substances from the external environment that dissolve in fluid on the mucous membranes in the nose. The olfactory epithelium is a thin layer of tissue embedded with smell receptors, which transmit the neural signal to the olfactory bulb, which is the brain center for smell (since it bypasses the thalamus). A smell is determined by is pleasantness versus its intensity. The intensity of a smell is determined by the amygdala. The brain is good at distinguishing good smells from bad smells, however it is poor at naming a smell if the source is not seen. Touch: Temperature, Pressure and Pain: The sense of touch is otherwise referred to as the haptic sense. The skin contains three kinds of receptors: temperature, pressure and pain. The pressure receptors are located at the hair follicle and the pain receptors and thinner and more dendritic. People cannot tickle themselves because the coordinated movement required to do so comes from the cerebellum. Therefore, the cerebellum also predicts that something is about to happen, so it is not unexpected and not ticklish. When nerve signals enter the thalamus, they are projected to the primary somatosensory cortex in the parietal lobe (front part of the lobe). Areas that are most sensitive take up more space in the lobe. This is how phantom limb works. When a person gets a body part amputated, even though the limb is gone the part is the brain that corresponds to it is still present. It is possible for the person to experience feeling in the phantom limb due to this. However, over time due to brain plasticity, the brain can remap to remove the unnecessary region. Fast, myelinated fibres in the body are responsible for sharp, immediate pain. This allows for protection to remove that part of the body away from danger (i.e. touching a hot stove). Slow, non-myelinated fibres in the body are responsible for dull, steady pain. This acts as a recuperation mechanism because it tells the body that the area needs conscious attention over time to heal properly (i.e. the burn received afterwards). Pain is a perceptual experience, as it occurs in the brain, which means there is a partial emotional component. Gate Control Theory of Pain: For pain to be experienced, pain receptors must be activated, and the neural gate in the spinal cord must allow signals to get through to the brain. Only when the gate is open is pain experienced. This is often why soldiers don’t experience pain until after the war is over. This is also why rubbing a bite prevents pain. The pressure from rubbing activates the larger sensory pressure nerve fibres, which causes the neural gate to close, shutting off the pain signal. Children can fall down, but the context of the situation depends on whether the pain is felt or not. It is only felt if there is focus upon the pain. If their attention is distracted from the pain, their spinal cord gate closes, preventing them from feeling the pain since the sensory information isn’t being processed. Vision: Parts of the eye:  Cornea: clear window in the front of the pupil, and with the help lens focuses the image on the retina. It is the first place where the light is refracted (gathered and bent)  Pupil: the hole in the iris that controls the amount of light that enters the eye  Lens: focuses light onto the retina by bending at an appropriate angle  Retina: receives lights and records visual messages. This is where transduction occurs  Sclera: eye’s strong outer coat  Optic Nerve: carries visual messages from the retina to the brain Accommodation is the
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