ניורוביולוגיה ומדעי המח חלק 2 – מערכת הראייה Introduction to Neurobiology Part 2 – The Visual System Shaul Hochstein

ניורוביולוגיה ומדעי המח Introduction to Neurobiology Introduction to Sensory Systems Retina & Lateral Geniculate Nucleus (LGN) “From Neuron to Brain” Chapters 19 & 20: pages & Transduction and signaling in the retina & Signaling in the LGN & primary visual cortex

Introduction to Sensory Systems- The retina 1- Structure of the eye - ✔ 2- Muscles of the eye – fixation; convergence; accommodation - ✔ 3- Structure of the retina - ✔ 4- The fovea: Comparing central and peripheral vision - ✔ 5-The blind spot - ✔ 6- Rods and Cones - ✔ 7- Structure-Function relationships - ✔ 8- Color vision: Young & Helmholtz - ✔ 9- Visual Pigments - ✔ 10- Photoreceptor sensitivity and signal amplification - ✔ 11- Light & dark adaptation - ✔ 12- lateral inhibition - ✔ 13- Ribbon synapses - ✔ 14- Action potentials versus passively conducted graded potentials - ✔ 15- Horizontal cells and gain control - ✔ 16- The Receptive Field and synaptic organization - ✔ 17- Bipolar cells; center-surround antagonism; On- & Off-center cells - ✔ 18- Retinal Ganglion Cells; X & Y in cat; M & P in primate

ניורוביולוגיה ומדעי המח Introduction to Neurobiology Introduction to Sensory Systems layers of neurons & connections in the retina.

The Retina and its layers Cell bodies connections Cell bodies

ניורוביולוגיה ומדעי המח Introduction to Neurobiology Introduction to Sensory Systems Hyperpolarizing receptor potential signal Depolarizing and Hyperpolarizing bipolar cells Depolarizing Amacrine & Retinal Ganglion Cell generator potentials Retinal Ganglion Cell action potentials

ניורוביולוגיה ומדעי המח Introduction to Neurobiology Introduction to Sensory Systems Transduction = the use of signal (optic) energy to trigger a neuronal response The absorption of light by photoreceptor cell Rhodopsin molecules is such a trigger, leading to a physiological response = an ion-specific change in cell conductance.

ניורוביולוגיה ומדעי המח Introduction to Neurobiology Introduction to Sensory Systems Adaptation: reduction in sensitivity of visual system in presence of high levels of ambient light. 1- in transduction process 2- Rhodopsin available to absorb light 3- Calcium level in cell 4- neural interactions

ניורוביולוגיה ומדעי המח Introduction to Sensory Systems Adaptation: 1- enable response over extremely large range of ambient light levels. 2- emphasize response to temporal change in light rather than steady state. 3- maintain same relative response irrespective of light (and response) level. 4- maintain constant appearance for same relative change in intensity.

Lateral Inhibition: 4 levels of analysis: 1- receptive field measurement – center/surround 2- physiological mechanism – lateral inhibition 3- computational equivalent – smooth and take second derivative 4- psychophysical phenomenon – Mach bands 5- perceptual advantage – edge enhancement

Center-surround receptive field in shape of a Mexican hat: looks like the difference of two Gaussians: DOG (green): one central, narrow and high, with a positive excitatory impact (in ON center cells), and the other broad and low with a negative inhibitory impact (in OFF surround cells).

Smoothing with a “boxcar” square-wave function or with a Gaussian function (to emphasize center of smoothing curve). In each case, smooth by taking the average of the multiples of each point of the smoothing curve and the parallel point on the stimulus curve. This gives the response of the cell at the center of the smoothing curve.

Use of the DOG Center-surround Receptive Field “smoothing curve” results in a smooth response of the layer of retinal ganglion cells with an emphasis of the corners where there is a transition from dark to light, as seen in Mach bands.

Thus, the DOG receptive field smoothes over a noisy signal, but still emphasizes the major transitions between dark and light areas.

According to David Marr, (father of the field of Artificial Intelligence), use of a DOG receptive field is the equivalent to smoothing and taking the second derivative of the illumination profile (because the DOG looks like the second derivative of a Gaussian smoothing curve).

Remember: each retinal ganglion cell applies the DOG function at a different point in the visual field and therefore responds according to the light profile around that point. Only the combined responses of many retinal ganglion cells reflects the full illumination profile.

4 principles of physiological research

Study of receptive fields is equivalent to study of computation; (from input/output can deduce computation).

Study of RF linearity/nonlinearity essential for deriving mechanisms.

Study of receptive fields is equivalent to study of computation; (from input/output can deduce computation). Study of RF linearity/nonlinearity essential for deriving mechanisms. Division into neuron classes basic for visual system, differentiating processing pathways, using one, the other, or a combination of streams.

Study of receptive fields is equivalent to study of computation; (from input/output can deduce computation). Study of RF linearity/nonlinearity essential for deriving mechanisms. Division into neuron classes basic for visual system, differentiating processing pathways, using one, the other, or a combination of streams. Analysis of response types may aid in understanding neuron role/function.

ניורוביולוגיה ומדעי המח Introduction to Neurobiology Introduction to Sensory Systems Linear and Nonlinear spatial summation What do neurons convey to each other? How do neurons sum their inputs?

Introduction to Sensory Systems Linear and Nonlinear spatial summation What do neurons convey to each other? How do neurons sum their inputs? We have already seen that the visual system emphasizes changes over space and time: lateral inhibition and adaptation. How can it sum over space and still notice local changes? Does it want to?

Stimulus spatial frequency Stimulus spatial phase (degrees) contrast: 55% temporal frequency 4 Hz

Stimulus spatial frequency Stimulus spatial phase (degrees) contrast: 55% temporal frequency 4 Hz X cell

Stimulus spatial frequency Stimulus spatial phase (degrees) contrast: 55% temporal frequency 4 Hz X cell

Stimulus X cell Receptive Field Time of response Position of Stimulus and Receptive field

Stimulus Y cell Receptive Field Time of response Position of Stimulus and Receptive field

Stimulus spatial frequency Stimulus spatial phase (degrees) contrast: 55% temporal frequency 4 Hz X cell Y cell

Cat X and Y cells (Hochstein & Shapley, 1976a) Contrast sensitivity of Fourier components to alternating grating as function of spatial phase. Contrast sensitivity determined by amplitude of harmonic. (A)X cell. 1 st harmonic (x) and negligible 2nd harmonic (D). (B)Y cell. 1 st harmonic: SF 0.14 & 0.7 c/deg (x, ☐ ); 2 nd harmonic 0.14 & 0.47 c/deg ( , ) Note Y cell 2 nd Harmonic presence with spatial-phase and relative (to Fundamental) SF insensitivity. ON-center X cell. SF 0.7 c/deg. ON-center Y cell.

Spatial model for the Y cell receptive field. Spatial extent of elements of a Y-type RGC, linear center and large linear surround, as well as, overlapping nonlinear surround sub-units, each smaller than the center. Linear center Linear surround Nonlinear subunits

Cone pedicle diagram (from DeVries et al., 2006). Diagram of invagination and surrounding region showing ribbon (Rb), horizontal cells (HCs), central invaginating contact (IC), and basal contacts (BCs). Ribbon synapse may be source of linear summation (without curtailment of response)

Contrast-response functions of cat and monkey RGCs. P cell (  ); X-type M cell (  ); cat X cell (). All data from RGC S-potentials recorded at LGN. P M X Monkey P & M cells are not the primate analogy of cat X and Y cells.

ניורוביולוגיה ומדעי המח Introduction to Neurobiology Introduction to Sensory Systems To where are retinal signals transmitted?