Unit 2, Week 1

Unit 2, Week 1 LGN/V1 & Exam 1 Debriefing

Some Good Stuffs From Last Chapter • Depth of field – clarify • Negative ganglion cell calculations – clarify • Hermann grid illusion – clarify • Brightness calculation – clarify

Depth of Field • The distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image • Controlled by lens aperature (commonly referred to as an “f-number”=focal length/aperature diameter if we’re talking cameras) • Reducing aperature size (increasing f number) INCREASES the depth of field (inverse proportion) decreases amount of light admitted • This means that smaller aperatures, greater diffraction, greater depth of field, greater distance between the nearest and farthest object still in focus by the lens

Depth of Field Visual f/22 f/2 F-number = focal length/aperature; Aperature = f/focal length Which of these pictures has a smaller aperature? Smaller DOF?

Negative Ganglion Cell Calculation • I said a negative ganglion cell calculation would not make much sense in one of my recitations. I misspoke what I meant by this. • I wanted to clarify that a calculation that is negative doesn’t mean the parallel pathway didn’t transduce light (which is what I meant to say but totally didn’t) – it definitely did! Otherwise there would have been no numbers to make a calculationto begin with. • A negative ganglion cell calculation just means it got more inhibition than it did excitation and is more likely to not fire.

For example, remember this?! 16 16 32 -1 1/8 1/8 What does the ganglion compute?

Hermann Grid Illusion – Explained The black dot fades when we focus with it with our fovea instead of our periphery. Why?

Hermann Grid – Explained • Because our foveal receptive fields are smaller & we have better acuity Ganglions in the fovea may be so small that they fit inside the intersection entirely, resulting in no difference in ganglion cell response between the intersections and the streets in central fovea, thus the grey disappears. For peripheral retinal ganglion cells – why the dark spot occurs at all. When in the intersection, more inhibition in surround = darker perception (less firing).

Brightness Calculations • An image consists of a white background with a gray rectangle in the foreground. When viewed outside, the white background reflects 1000 photons of light and the gray rectangle reflects 400 photons of light. When the same image is viewed inside, the white background reflects 500 photons of light. How much light does the gray rectangle reflect? ___________________

Lightness/Brightness Constancy • White paper looks white and black paper looks black regardless of the level of illumination. • Perceived lightness is not equal to the actual physical intensity of the stimulus. • Lightness depends on the surface reflectance, independent of the illumination conditions. Indoors, you know the light level is lower (i.e., perceived brightness is reduced), yet you attribute that to the illumination, not to the white and black surfaces, which are perceived to have constant lightness. • Psychologists have determined that an object will exhibit brightness constancy as long as both the object and its surroundings are in light of the same intensity

Lightness Constancy What they did here is establish a ratio reflectance/average illumination/lightness of all objects for each surface. When they go indoors, because of lightness constancy, this ratio remains. For black: 1000/(9000+1000/2) = 1/5

Simultaneous Brightness Contrast • Our perception of brightness percept depends on the context (mean/average light level) of the visual stimulus we are experiencing. • Simultaneous brightness contrast is an illusion beckons a mathematical relationship: • Brightness = Reflectance / Avg Intensity • Small avg intensity = brighter percept • High avg intensity = darker percept

Brightness Calculations • An image consists of a white background with a gray rectangle in the foreground. When viewed outside, the white background reflects 1000 photons of light and the gray rectangle reflects 400 photons of light. When the same image is viewed inside, the white background reflects 500 photons of light. How much light does the gray rectangle reflect? ____200 photons________ Show your work: Ratio = 400/(1000+400/2)=4/7 4/7 = (x/((500+x)/2)) X=200

Topics Covered • REVIEW: New “battleship” perspective –Retinal ganglion cells as the “samplers” of visual data, V1 as the interpreter • LGN – layers and connectivity • V1 – tonotopic mapping, cortical magnification • Receptive fields in V1 • Cell types: simple, complex, end-stopping cells • Hypercolumns – centers of singularity (color “blobs”) • Adaptation – tilt aftereffect

LGN • 6 Layers • 1-2: Magnocellular layers (receives input from WHICH type of retinal ganglion cell?!) • 3-6: Parvocellular layers (receives input from WHICH type of retinal ganglion cell?!) • Each layer only receives input from 1 eye • Magnocellular layer 1: contralateral (opposite eye) nasal retinal ganglions • Magnocellular layer 2: ipsilateral (same side eye) temporal retinal M-cell ganglions • Parvocellular layer 3: ipsilateral temporal retinal P-cell ganglions • Parvocellular layer 4: contralateral nasal retinal P-cell ganglion • Parvocellular layer 5: ipsilateral temporal retinal P-cell ganglions • Parvocellular layer 6: contralateral nasal retinal P-cell ganglions

LGN – Retinotopically Mapped Retinotopic mapping: The order of points in visual stimulus projected onto the retina is maintained in both LGN and V1. A will never be next to C. B is always in the middle & order is preserved.

Topographpic Mapping in V1 Look for numbers 1-7 as they are projected to V1 and note their order is preserved, along with cortical magnification for points transduced in a lower eccentric part of the VF.

Cell Count. • Number of photoreceptors? • Number of retinal ganglion cells? • Number of neurons in V1?

Oh Snap. • Photoreceptors: 100,000,000/eye • Retinal Ganglion Cells: 500,000/eye • LGN: 2,000,000 cells • Striate (V1) cortex: 200,000,000 • There are about 1.2 million axons sent to LGN, but way more neurons in cortex to process this information. What do the other neurons do?!

That’s RIGHT. They totally dominaaaate. • For every retinal ganglion cell, there are many types of V1 neurons monitoring that spatial point, looking for what they love to see. • Hubel & Wiesel theoretical hierarchy: LGN cells feed simple cells which feed complex cells, but we now know that simple and complex cells receive input from LGN in parallel • Neural interactions (lateral inhibition) within the cortex aids in visual processing • Orientation tuning • Receptive field

Cortical Magnification • A benefit of having so many neurons is the cortex has the power to define acuity past the retina. It honors the general trend of devoting more brainpower to processing the fovea, as the retina does with ganglion cell/pooling ratios. • This is called cortical magnification – the idea that 1 degree of visual angle is processed by 20 millimeters of V1 (that’s about .78 inches for ONE degree – wow!)

Cortical Magnification in V1 Does anybody else look at the way the cortical magnification of this woman’s face looks in V1 remind you of the homuculus of the premotor cortex? Both have to do with sensitivity!

Visual Crowding – Consequence of Cortical Magnification • Visual acuity declines with greater eccentricity • Eccentricity is relative scaled measuring the degree away from the fovea • The periphery is where we see the largest eccentricity, so we have poor visual acuity in our periphery. • But we already knew this! • Visual crowding is a phenomenon that illustrates this point

Visual crowding In a task investigating visual crowding you may be asked if the object right of the fixation point is included in the object left of the fixation point. Experiments show that people struggle when the objects are close together.

Receptive Fields in V1 • Bars instead of center-surround • Orientation tuned (they like some line swag more than others and fire more/less, correspondingly) Orientation tuning. This guy likes vertical lines. V1 receptive fields receive input from multiple center-surround LGN cells, which creates its bar-loving receptive field.

V1 Function – Fourier Analysis • The role of V1 is to break images down into sine wave gratings: they are little, “cute-sy” feature detectors in our brains! • V1 neuron sees the feature they like and they yell LISTEN TO ME, like a faithful neuron doctrine following daemon.

V1 – Cell Types • Simple • Complex • HYPERcomplex: End-Stoppersssss.

Simple Cells • Clearly defined ON (excitatory) and OFF (inhibitory) subregions in receptive field • Ocular preference • Edge, stripe detectors • Phase SENSITIVE (1 vs 2 below) • Orientation selective (2 vs 3) 1 2 3

Complex Cells • Motion detectors • Ocular preference (likes input from one eye more) • Phase INSENSITIVE (1 vs 2 below) • Orientation selective (2 vs 3 below) • Spatial frequency tuning 1 2 3

End-Stopping/Hypercomplex Cells • Like a certain LENGTH of a bar. Are inhibited if the visual stimulus is any better than the length they like. Will fire for lengths smaller than their “threshold liking length” (haha) • Both simple and complex cells can be end-stopped (or not)

V1 Cells – Overview • Often have ocular dominance – prefer to fire more for input from a given eye, but will fire for the other eye “if it has to” (think of mum asking you to take out the trash as a kid – can I just say that my mum NEVER DID THIS?) • Like certain orientations more than other ones (have orientation tuning curves) • All are looking for what they like, and fire most when they get that “perfect picture” aligned with their likes

Hypercolumns 1mm thick chunk of cortex – smallest unit containing all machinery for everything V1 is responsible for

Hypercolumns • Column: vertical arrangement of neurons (top of head to toes) • All neurons in hypercolumn has RELATIVELY the same receptive field location, but with all different orientation selectivities, direction selectivities, ocular dominances. • From left ear to right ear, orientation tuning in cortical V1 neurons changes • Ocular dominance columns switch from left to right, left to right, as you proceed from left ear to right ear across cortex

Adaptation • This idea that you if you bug something hard enough, it’ll get tired and say “forget it” and slug off. • If you fatigue a certain type of cell that likes to fire for a certain spatial frequency, it’ll change its sensitivity and become desensitized to that spatial frequency temporarily because you have worn it out! • Tilt aftereffect is the experiment we did in class, which strongly supports empirically the idea that our neurons are orientation selective.

Tilt Aftereffect Experiment Upon doing this experiment, you should find that adapting to the left tilted (upper left) stripes and right tilted (lower left) stripes causes the upper right to look like they are tilting RIGHT and lower right like they are tilting LEFT. Thus they are rotated in the opposite direction of the adapted tilt, illustrating weakened sensitivity to the adapted orientation.

Alright… Who Had Questions on the Exam 1?

Things I Learned… • I LOVE emails, but I WILL be posting the key to the next practice exam online next time. It will be the day before or the day before the day before the exam. • NOTE: I want you to TRY THEM OUT first. • Think of butterflies when I say: please don’t ask me what’s covered on the exam. I give you slides and page numbers and lectures. The reason I say this is because I think it’ll help you learn. • I’m on the fence about whether or not my practice questions helped you. I think some of you saw some of the exam questions and thought “I know what this is asking!”, without actually reading the question. You’ll see what I mean when we go over it. • Lastly.. I learned not to assume the exam will be proofread! We promise to be more diligent in the future.

Question 1** • Imagine we have measured how Sam and Jocelyn are able to detect a small flash of light in a dark room. If Sam has higher sensitivity than Jocelyn, then compared to Jocelyn he must have: • higher d’ • lower d’ • higher criterion • lower false alarm rate • (a) and (d)

Question 2 • Which of the following is NOT true? • Aperature size and level of diffraction is inversely related • Cameras also have built in lateral inhibition mechanisms • Age-related macular degeneration devastates foveal visual perception • Glacoma and retinitis pigmentosa end in blindness if left untreated • None of the above

Question 3** • Lateral inhibition is the • increase in activation caused by lateral connections. • measure of the finest detail that one can resolve. • process of inhibiting light from sideways moving objects. • excitatory neural interactions between adjacent regions of the retina. • none of the above.

Question 4 • Spatial frequency is measured in ____________ units. From the below pictures, the (left/right) sine wave grating has a higher spatial frequency. • Oscillations/second, right • Cycles/degree, left • Cycles/degree, right • Dioptres, left • Dioptres, right

Question 5 • Weber’s law describes a relationship between________ and __________. • absolute threshold; the smallest noticeable difference • a constant; a sensory modality • current stimulus level; difference threshold • stimulus intensity; neural firing rate

Question 6** • Refraction aides in facilitating our visual perception by • improving vision at lower ambient light levels. • compensating for the aftereffects of diffraction by the pupil. • creating sharp retinal images of objects at the accommodated distance. • controlling eye fixations. • All of the above.

Question 7 • Your task is to compare the length of two temporal durations. Suppose when the shorter duration is 500 milliseconds, you can detect the change only if the longer duration is no less than 550 milliseconds. According to Weber’s law, when the shorter duration is 2000 milliseconds, how much does the longer duration at least to be for you to detect a difference? • 2020 milliseconds • 2050 milliseconds • 2200 milliseconds • Cannot be predicted by Weber’s law

Unit 2, Week 1

Unit 2, Week 1

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