Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityNeural circuitsPSY 310Greg FrancisLecture 05Why do you need bright light to read?Purdue UniversityRods and cones Photoreceptors are not evenly distributed across the retinaPurdue UniversityRods and cones Cones are most dense in the fovea Fovea, approximately 1.5 mm x 1.5 mm (about1 degree of visual space) 200,00 cones in the fovea 147,000 cones/mm^2 17,500 cones/degree^2 Periphery (outsidethe fovea), approximately 6,200,000 cones 125,000,000 rodsPurdue UniversityVisual acuity How much detail you see depends on the density ofphotoreceptorsPurdue UniversityVisual acuity The distribution of rods and codes partially explainswhy you have to move your eyes to see things Only all-cone vision (most central part of fovea) hasreally good visual acuity 20/20 vision is the ability to resolve a spatial patternseparated by a visual angle of one minute of visual arc One degree of visual angle at the fovea coversapproximately 120 cones So, a set of alternating black and white bars with morethan 120 elements in a degree will look grayPurdue UniversityVisual 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 thetarget Second number is how close a“normal” person has to stand toread the chart 20/40 vision means you can read at20 feet what a normal person canread at 40 feet Some hawks have approximately20/2 vision!Prof. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityVisual acuity Acuity varies acrossthe retina Larger eccentricitymeans further awayfrom fovea Snellen fraction isdoing the implieddivision of 20/20Purdue UniversityVisual acuity When we look at an image like thisPurdue UniversityVisual acuity The representation of detailed information wouldbe equivalent to something like thisPurdue UniversityNeural circuits Photoreceptor densityis not the only issue forvisual acuity Photoreceptors pass oninformation to neurons Can be two neurons foreach photoreceptor(fovea only), or manyphotoreceptors for eachneuron More photoreceptorsper neuron meanspoorer visual acuityPurdue UniversityNeural circuits VerycomplicatedneurophysiologyPurdue UniversityNeural circuits Schematichelps only alittle Note: spatialposition iscritical here Grounded to aposition onthe retinaProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityReading Reading requires distinguishing fineshapes Q versus O versus C Only the fovea can really do this, so youhave to move your eyes around Just watch someone read!Purdue UniversityReading and light As light gets fainter, the cones respond lessvigorously Eventually only rods responds The density of rods is muchless than for cones, sospatial acuity is muchworse in dim light Object edges becomefuzzy at nightPurdue UniversityWhy 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 shadowsPurdue UniversityThe world is not… …any of these things, because perception is not aduplication 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 caninfluence your perception of stimuli elsewhere The task we face is figuring out how this happens And relating it to neurophysiologyPurdue UniversityGanglion cells There are many cells that operate in theeye We will focuson the cellsthat sendoutputs out ofthe eye to thebrain 1,200,000neural axonsPurdue UniversityGanglion cells Anatomical structure Sample responses from other neurons Many different shapes and sizesProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityGanglion cells Anatomicalstructure Sample responsesfrom other neurons Many differentshapes and sizesPurdue UniversityAnatomy Spatial size and other propertiesdepend on position in the retinaPurdue UniversityAnatomy Tremendousspatialconvergenceof information Varies a lotfor type ofganglion celland positionon the retinaPurdue UniversityBehavior Present light fora few secondsand then turn itoff Different cellsresponddifferently Not always easyto categorizecells bybehaviorPurdue UniversityBehavior Ganglion cell behavior also varies with the spatiallayout of light on the retinaPurdue UniversityCenter surround Varying the shape of the stimulus leads to differentstrength of response (number of action potentials) Firing rate is important!Prof. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityReceptive field We talk about the “receptive field” of a cell This is an effort to describe the stimulus propertiesthat influence the cell’s behavior Excitation: increases the number of actionpotentials Inhibition: decreases the number of actionpotentials Can be complicatedPurdue UniversityReceptive field The receptive field of a ganglion cellincludes any place on the retina where lightexcites the cell and any place where lightinhibits the cell On-center, off-surround+-Purdue UniversityReceptive field There can be quite a bit of variability in the structureof receptive fields Size+-+-+-Purdue UniversityReceptive field There can be quite a bit of variability in the structureof receptive fields Arrangement of excitation and inhibition+-+-On-center, off-surroundOff-center, on-surroundPurdue UniversityCenter surround The properties of the receptive field are tied tothe spatial spread of the ganglion cell dendrites But not exclusively so Remember that the cells that a ganglion cellreceives inputs from may itself have a receptivefield that covers a region of retinal space The receptive field of a neuron depends on thereceptive fields of the neurons that feed in to itPurdue UniversityCenter surround This image shows theresponse of an off-center,on-surround ganglion cell The yellow bars indicate asmall slit of light lettinglight shine on the parts ofthe retina that this cellresponds to The picture of the neuronindicates the actual spatialposition of the dendrites ofthe neuron