Neural circuits PSY 310 Greg Francis. Lecture 05. Rods and cones

Transcription

1 Neural circuits PSY 310 Greg Francis Lecture 05 Why do you need bright light to read? Rods and cones Photoreceptors are not evenly distributed across the retina 1

2 Rods and cones Cones are most dense in the fovea Fovea, approximately 1.5 mm x 1.5 mm (about 1 degree of visual space) 200,00 cones in the fovea 147,000 cones/mm^2 17,500 cones/degree^2 Periphery (outside the fovea), approximately 6,200,000 cones 125,000,000 rods Visual acuity How much detail you see depends on the density of photoreceptors 2

3 Visual acuity The distribution of rods and codes partially explains why you have to move your eyes to see things Only all-cone vision (most central part of fovea) has really good visual acuity 20/20 vision is the ability to resolve a spatial pattern separated by a visual angle of one minute of visual arc One degree of visual angle at the fovea covers approximately 120 cones So, a set of alternating black and white bars with more than 120 elements in a degree will look gray Visual acuity Measured in several ways Snellen letters 20/20 vision (6/6 in metric form) First number is how close (in feet, meters for 6/6) you stand to the target Second number is how close a normal person has to stand to read the chart 20/40 vision means you can read at 20 feet what a normal person can read at 40 feet Some hawks have approximately 20/2 vision! 3

4 Visual acuity Acuity varies across the retina Larger eccentricity means further away from fovea Snellen fraction is doing the implied division of 20/20 Visual acuity When we look at an image like this 4

5 Visual acuity The representation of detailed information would be equivalent to something like this Neural circuits Photoreceptor density is not the only issue for visual acuity Photoreceptors pass on information to neurons Can be two neurons for each photoreceptor (fovea only), or many photoreceptors for each neuron More photoreceptors per neuron means poorer visual acuity 5

6 Neural circuits Very complicated neurophysiology Neural circuits Schematic helps only a little Note: spatial position is critical here Grounded to a position on the retina 6

7 Reading Reading requires distinguishing fine shapes Q versus O versus C Only the fovea can really do this, so you have to move your eyes around Just watch someone read! Reading and light As light gets fainter, the cones respond less vigorously Eventually only rods responds The density of rods is much less than for cones, so spatial acuity is much worse in dim light Object edges become fuzzy at night 7

8 Why is the world not? Upside down The retinal image is upside down Small The retinal image is smaller than the real world Red In some places there are more long-wave length cones Blurry Only the fovea has good acuity Filled with holes Blind spot Vein shadows The world is not any of these things, because perception is not a duplication of what happens at the retina Perception is a process It is based on the information at the retina Information about stimuli in one part of the scene can influence your perception of stimuli elsewhere The task we face is figuring out how this happens And relating it to neurophysiology 8

9 Ganglion cells There are many cells that operate in the eye We will focus on the cells that send outputs out of the eye to the brain 1,200,000 neural axons Ganglion cells Anatomical structure Sample responses from other neurons Many different shapes and sizes 9

10 Anatomical structure Sample responses from other neurons Many different shapes and sizes Ganglion cells Anatomy Spatial size and other properties depend on position in the retina 10

11 Tremendous spatial convergence of information Varies a lot for type of ganglion cell and position on the retina Anatomy Behavior Present light for a few seconds and then turn it off Different cells respond differently Not always easy to categorize cells by behavior 11

12 Behavior Ganglion cell behavior also varies with the spatial layout of light on the retina Center surround Varying the shape of the stimulus leads to different strength of response (number of action potentials) Firing rate is important! 12

13 Receptive field We talk about the receptive field of a cell This is an effort to describe the stimulus properties that influence the cell s behavior Excitation: increases the number of action potentials Inhibition: decreases the number of action potentials Can be complicated Receptive field The receptive field of a ganglion cell includes any place on the retina where light excites the cell and any place where light inhibits the cell On-center, off-surround

14 Receptive field There can be quite a bit of variability in the structure of receptive fields Size Receptive field There can be quite a bit of variability in the structure of receptive fields Arrangement of excitation and inhibition Off-center, on-surround - + On-center, off-surround

15 Center surround The properties of the receptive field are tied to the spatial spread of the ganglion cell dendrites But not exclusively so Remember that the cells that a ganglion cell receives inputs from may itself have a receptive field that covers a region of retinal space The receptive field of a neuron depends on the receptive fields of the neurons that feed in to it Center surround This image shows the response of an off-center, on-surround ganglion cell The yellow bars indicate a small slit of light letting light shine on the parts of the retina that this cell responds to The picture of the neuron indicates the actual spatial position of the dendrites of the neuron Notice that the cell responds best when the slit of light is at the very edge of the dendrites (bottom) 15

16 Modeling receptive fields There is a nice model of the spatial parts of ganglion cell receptive fields Difference of Gaussians (DOG) One dimensional Gaussian (also called a normal distribution) Modeling receptive fields Drops off smoothly as move away from center Sigma = 1 16

17 Modeling receptive fields Graph is centered on zero Drops off smoothly as move away from center Sigma = 5 Note the change in the y-axis scale! Difference of Gaussians Center region has a smaller standard deviation than the surround Take the difference at each position (X) 17

18 Difference of Gaussians Describes the center-surround effect of light Input on center is treated positively Input on surround is treated negatively Gaussian The receptive field covers both an x and y dimension Just expand the function 18

19 Difference of Gaussians In two dimensions Modeling We ll see later how describing these receptive fields mathematically helps us understand their purpose and behavior 19

20 Conclusions Phtoreceptor sensitivity sets a limit on visual perception Rods and cones Dim illumination Distribution Neurons set other properties Some to compensate for limits of photoreceptors Receptive fields Very important now and later Ganglion cells are center-surround Next time Some implications of ganglion cells on visual perception Where information goes after the eye Later geniculate nucleus Visual cortex 20