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Chapter 3: Neurons and Perception

Chapter 3: Neurons and Perception. Overview of Questions. How do electrical signals represent objects? How does neural processing determine what we see? What is the effect of the environment on developing visual systems? What does it mean to say that perception is indirect?.

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Chapter 3: Neurons and Perception

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  1. Chapter 3: Neurons and Perception

  2. Overview of Questions • How do electrical signals represent objects? • How does neural processing determine what we see? • What is the effect of the environment on developing visual systems? • What does it mean to say that perception is indirect?

  3. Convergence in the Retina - continued • 126 million rods and cones converge to 1 million ganglion cells • Higher convergence of rods than cones • Average of 120 rods to one ganglion cell • Average of 6 cones to one ganglion cell • Cones in fovea have 1 to 1 relation to ganglion cells

  4. Convergence and Sensitivity • Rods are more sensitive to light than cones • Rods take less light to respond • Rods have greater convergence which results in summation of the inputs of many rods into ganglion cells increasing the likelihood of response • Trade-off is that rods cannot distinguish detail

  5. Convergence and Detail • All-cone foveal vision results in high visual acuity • One-to-one wiring leads to ability to discriminate details • Trade-off is that cones need more light to respond than rods

  6. Summation results in greater excitation in a single retinal ganglion cell and increases the likelihood of a response.

  7. Convergence results in poor acuity (the ability to detect details). For the rods, the RGC sends the same message to the brain in a and b. for the cones (especially those in the fovea) there is little convergence. RGCs send different messages to the brain for a and b.

  8. Lateral Inhibition of Neurons • Experiments with eye of Limulus (Hartline, 1956) • Ommatidia allow recordings from a single receptor • Light shown into a single receptor led to rapid firing rate of nerve fiber • Adding light into neighboring receptors led to reduced firing rate of initial nerve fiber

  9. Hartline’s Results

  10. Lateral Inhibition and Lightness Perception • Psychophysical results can be explained by lateral inhibition • The Hermann Grid: Seeing spots at an intersection • Mach Bands: Seeing borders more sharply • Simultaneous Contrast: Seeing areas of different brightness due to adjacent areas

  11. Hermann Grid • People see an illusion of gray images in intersections of white areas • Signals from bipolar cells cause effect • Receptors stimulated by dark areas inhibit the response of neighboring cells receiving input from white area • The lateral inhibition causes a reduced response which leads to the perception of gray

  12. Explanation of the Herman Grid

  13. Mach Bands

  14. Figure 3.10 Circuit to explain the Mach band effect based on lateral inhibition. The circuit works like the one for the Hermann grid in Figure 3.6, with each bipolar cell sending inhibition to its neighbors. If we know the initial output of each receptor and the amount of lateral inhibition, we can calculate the final output of the receptors. (See text for a description of the calculation.)

  15. Simultaneous Contrast

  16. Simultaneous Contrast

  17. Neural Circuits • Groups of neurons connected by excitatory and inhibitory synapses • A linear circuit has no convergence and only excitatory inputs • Input into each receptor has no effect on the output of neighboring circuits • Each circuit can only indicate single spot of stimulation

  18. Linear Circuit

  19. Neural Circuits - continued • Convergent circuit with only excitatory connections • Input from each receptor summates into the next neuron in the circuit • Output from convergent system varies based on input • Output of circuit can indicate single input & increases output as length of stimulus increases

  20. Circuit with convergence added.

  21. Neural Circuits - continued • Convergent circuit with excitatory and inhibitory connections • Inputs from receptors summate to determine output of circuit • Summation of inputs result in: • Weak response for single inputs & long stimuli • Maximum firing rate for medium length stimulus

  22. Receptive Fields • Area of retina that affects firing rate of a given neuron in the circuit • Receptive fields are determined by monitoring single cell responses • Stimulus is presented to retina and response of cell is measured by an electrode

  23. Center-Surround Receptive Fields • Excitatory and inhibitory effects are found in receptive fields • Center and surround areas of receptive fields result in: • Excitatory-center-inhibitory surround • Inhibitory-center-excitatory surround

  24. Response of a ganglion cell in the cat’s retina

  25. Center-Surround Antagonism • Output of center-surround receptive fields changes depending on area stimulated: • Highest response when only the excitatory area is stimulated • Lowest response when only the inhibitory area is stimulated • Intermediate responses when both areas are stimulated

  26. Response of an excitatory-center-inhibitory-surround receptive field

  27. Ganglion Cell Output • Small Spot Stimuli Spontaneous Activity Uniform Illumination Spike Record

  28. Lateral Inhibition • More natural Stimuli • How does GC output vary? 10 spikes/sec

  29. Lateral Inhibition • Enhances edges • De-emphasize broad unchanging surfaces • Early processing is already “modifying” the scene 10 spikes/sec

  30. Pathway Beyond the Retina • Pathway to visual processing area • Optic nerve • Lateral geniculate nucleus (LGN) • Superior colliculus • Striate cortex • Extrastriate cortex

  31. Striate Cortex • Simple cortical cells • Side-by-side receptive fields • Respond to spots of light • Respond best to bar of light oriented along the length of the receptive field • Orientation tuning curves • Shows response of simple cortical cell for orientations of stimuli

  32. The receptive field of a simple cortical cell.

  33. Striate Cortex - continued • Complex cells • Like simple cells • Respond to spots of light • Respond to bars of light • Unlike simple cells • Respond to movement of bars of light in specific direction • End-stopped cells • Respond to: • Moving lines of specific length • Moving corners or angles • No response to: • Stimuli that are too long

  34. Each stage marks an increase in stimulus complexity

  35. Feature Detectors • Neurons that fire to specific features of a stimulus • Pathway away from retina shows neurons that fire to more complex stimuli • Cells that are feature detectors: • Simple cortical cell • Complex cortical cell • End-stopped cortical cell • Face cells! (More in chapter 4)

  36. Response properties of a neuron in the monkey temporal lobe.

  37. Selective Adaptation • Neurons tuned to specific stimuli fatigue when exposure is long • Selective means that only those neurons that respond to the specific stimulus adapt • How to measure it • Measure sensitivity to range of one stimulus characteristic • Adapt to that characteristic by extended exposure • Re-measure the sensitivity to range of the stimulus characteristic

  38. Orientation sensitivity test (top0 and contrast sensitivity test (bottom)

  39. Method for Orientation Sensitivity • Use a high contrast grating • Measure sensitivity to different orientations • Adapt person to one orientation • Re-measure sensitivity to all orientations • Psychophysical curve should show selective adaptation for specific orientation if neurons are tuned to this characteristic

  40. Figure 3.27 (a) Results of a psychophysical selective adaptation experiment. This graph shows that the participant’s adaptation to the vertical grating causes a large decrease in her ability to detect the vertical grating when it is presented again, but has less effect on gratings that are tilted to either side of the vertical. (b) Orientation tuning curve of the simple cortical neuron from Figure 3.23b.

  41. Size Adaptation • Gratings have bars of different sizes • Similar method as for contrast and orientation • Size adaptation changes the perception of similarly sized bars

  42. Selective Rearing • Animals reared in specific environment • Limits type of stimuli present • Neural plasticity would result in lack of ability to see characteristics unavailable in environment • Shows that neurons need environment to develop fully

  43. Blakemore and Cooper’s (1970) selective rearing experiments.

  44. Perception is Indirect • Stimuli in environment impinge on receptors • Transduction takes place causing electro-chemical impulse in neurons • Provides accurate information about world • Feels direct but that is an illusion • This is true for all senses

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