3/5/15 – Lecture 14 – Sensory Perception and Intro to the Visual System- Goal: to understand how our senses receive information about the world and to start learning about how the visual system works- Sensation: the registration of physical stimuli from the environment by sensory receptorso Our senses are how we receive information about the world (5 Senses) Sight/Vision – light energy is converted into action potentials Taste/Gustation – chemical signals are converted into action potentials Smell/Olfaction – chemical signals are converted into action potentials Hearing/Audition – mechanical signals (air pressure waves) are converted into action potentials Somatosensory – mechanical signals (touch, pressure, pain) are converted into action potentials- In all cases … light, chemical, or mechanical signals are converted into patternsof action potentials within the nervous system by specialized neurons called sensory receptorso 3 Main Types of Sensory Receptors Photoreceptors – convert light energy into action potentials (sense of vision)o Receptive Fields: the array of receptors for a given sensory modality Each sensory modality has thousands to millions of receptors that each receive slightlydifferent information- The differences allow us to locate event in time and spaceo Sensory Coding and Representation The presence of a sensory stimulus is encoded by a change in the number of action potentials fired by the sensory receptor per unit time, or the rate of discharge- Stimulus Intensity – greater intensity like a brighter light of louder noise is encoded by a greater discharge rate- Stimulus Quality – red vs green light can be coded for by the activity of different receptors or by different levels of discharge in the same receptoro Once the sensory signal has been converted into action potentials by receptor cells, it must be sent to the central nervous system Neural Relays- Sensory information is conveyed into the CNS through 3-4 neuron relayso Each neuron changes the firing of the next neuron in the relay, which then gets sent to the next neuron Convergence- There is some convergence of information in the relay neurons at each level receive input from more than one sensory receptor- Convergence decreases acuity (how precisely we can identify where a signal arose from)o BUT convergence increases sensitivity (how likely we are to detect a signal)o Sensory Processing and the Sensory Cortex Changes in action potential firing of sensory receptor neurons is ultimately relayed to specific brain regions specialized for each sense- These regions also send and receive input from other forebrain areas like the frontal cortex, the hippocampus, the limbic system, etc.- Perception: the subjective experience of sensationo Depends On – the nature of the sensation, the context in which sensory events take place, our emotional state, and past experiences and memorieso The Visual System Visible Light- Light is a wave of electromagnetic energy- EM waves have different wavelengths, which correspond to different colors- The amplitude of the wave tells how bright the light iso Our eyes can only detect a narrow range of wavelengths which we call the visible light spectrum The Human Eye- Cornea – the outer covering of the eye- Sclera – the eyeball or “white” of the eye- Iris – opens and closes to let more or less light in through the pupil- Pupil – the hold through which light enters the eye- Lens – focuses the light image on the retina- Retina – back of the eye containing the sensory receptors (the rods and cones)o Consists of sensory receptors (photoreceptors) and the sensory neurons that synapse with them Fovea – region containing the largest density of photoreceptors and where your vision is the sharpest- Makes up <1% of the visual field but eye movements called saccades driven by neurons in the midbrain move around your visual field to give a bigger more detailed picture Blind Spot – region containing no photoreceptors because the sensory neuron axons exit the eye here as the optic nerve but the other eye and photoreceptors around each blind spots “fillin” missing visual information Cones – responsive to bright light, responsible for color vision,and the ability to see fine detail (SMALLER)- 3 different types of light absorbing pigments Rods – more numerous than cones, more sensitive to dim light, used for night vision (LARGER)- Only 1 type of light absorbing pigmento Sensing Light with Photopigments Photoreceptors contain a special pigment molecule called rhodopsin- When light strikes the rhodopsin, it activates a G-protein- Activated g-protein indirectly closes a Na+ channel- Closing the Na+ channel meals less positive channels flowing into the cell, hyperpolarizing it- Hyperpolarization means decreased release of glutamate by the photoreceptor cello The MORE light hits the photoreceptor, the more the Na+ channels will close, the more hyperpolarized the cell will be, the LESS glutamate will be releasedo Retinal Neurons and Visual Transduction Photoreceptors – rods and cones Bipolar cells – link photoreceptors to retinal ganglion cells- Their firing of action potentials is increased when glutamate levels decrease Retinal Ganglion Cells – receive inputs from bipolar and amacrine cells and send information on into the brain Horizontal Cells – also collect information from several photoreceptors and sends it to bipolar cells Amacrine Cells – collect information from several bipolar cells and send them to retinal ganglion cellso Rod vs Cone Convergence More rods converge on each bipolar neuron, and more rod bipolar neurons converge on each rod ganglion cell Rods are important for sensing dim light, so more convergence makes sense since it means bipolar neurons will be activated if ANY of the rods get activated by light Cones are important for fine detain discrimination, so less convergence makes sense if we want a fine map of an object Flow of Visual Information Into the Brain- Retinal ganglion neurons send their axons to the lateral geniculate nucleus of the thalamuso The lateral geniculate nucleus neurons sends their axons to the striate cortex- The Striate Occipital Cortex is the Primary Visual Cortexo Visual information is roughly processed in the striate occipital cortex to start to identify color, form, and