Class Notes (808,765)
Canada (493,382)
Psychology (385)
PSYO 2470 (27)
Lecture 3

PSYO 2470 Lecture 3: Vision

15 Pages
Unlock Document

Dalhousie University
PSYO 2470
Stamp Jennifer

Vision Chapter 9; 304-312, 319-326 Chapter 10; 331-367 The visual field - Extent of space seen by one eye - Points where you can no longer see an object with one eye marks the limits of visual field - Left visual field is imaged on the right side of the retina - Right visual field is imaged on the left side of the retina Visual acuity - Ability of the eye to distinguish two points near each other Anatomy of the retina - Begins the conversion of light energy into neural energy - Pathway for visual info to exit the eye o Photoreceptors  bipolar cells  ganglion cells ▪ Photoreceptors respond to light – influence membrane potential of connected bipolar cells • Ganglion cells fire action potentials in response to light – propagates along the optic nerve o Horizontal cells ▪ Receive input from photoreceptors and project laterally to influence surrounding bipolar cells and photoreceptors o Amacrine cells ▪ Receive input from bipolar cells, project laterally to influence surrounding ganglion cells o Ganglion cells are the output from the retina - Laminar organization of the retina o Laminar organization – cells organized in layers ▪ ▪ Inside-out formation – photoreceptors are closest to the back of the eye, while ganglion cells are further forward in the retina • Light passes through ganglion and bipolar cells before reaching photoreceptors o Ganglion and bipolar cells fairly transparent • Photoreceptors embedded in pigment epithelium o Involved in photoreceptor maintanence o Pigment epithelium also absorbs light that doesn’t reach photoreceptors ▪ Inner most layer is the ganglion cell layer • Cell bodies of ganglion cells ▪ Inner plexiform layer • Synaptic contacts between bipolar cells, amacrine cells, ganglion cells ▪ Inner nuclear layer • Cell bodies of bipolar cells, horizontal cells, and amacrine cells ▪ Outer plexiform layer • Photoreceptors make synaptic contact with bipolar and horizontal cells ▪ Outer nuclear layer • Cell bodies of photoreceptors ▪ Layer of photoreceptor outer segments • Light sensitive elements of the retina Photoreceptor structure - Conversion of electromagnetic radiation to neural signals - Four regions o Outer segment ▪ Contains membranous disks • Light sensitive photopigments in disk membranes o Absorb light and trigger changes in photoreceptor membrane potential o Inner segment o Cell body o Synaptic terminal - Rod photoreceptors o Long, cylindrical outer segment ▪ Contains many disks - Cone photoreceptors o Short, tapering outer segment ▪ Fewer disks Lecture recordings Vision I Visual system allows extensive info about objects - Movement, shape, orientation Retina - Responds to light - Light capturing - Over 100 million photorecetors o Only 1 million afferent axons ▪ Carry info to the brain ▪ Far more cells receiving info than neurons transmitting it Binocular visual system - Overlapping fields of view - Allows us to see depth Visual system manipulates external info - Not always accurate - Ex. Illusions Conscious visual perception - Retina – in the eye o Sends a project to the Lateral Geniculate Nucleus (LGN) in the thalamus ▪ Also have non thalamic projections but they’re not involved in visual perception ▪ Projections for primary visual cortex • In Area 17 in the brain o Also referred to as striate cortex o If damaged, you’d be blind o Projections from LGN – multiple parallel pathways that carry different info about the visual world ▪ Get integrated in the cortex • 2 dozen cortical areas o Occipital, temporal, and parietal Info flow in retina - From photoreceptors to the bipolar cells to the ganglion cells to the nerves - Horizontal/lateral communication o Horizontal cells ▪ Communication between photoreceptors to the bipolar cells • Modulate activity o Amacrine cells ▪ Modulation of vertical signal between bipolar and ganglion cells - Photoreceptors – only light sensitive cells o Receptors to photons - Ganglion cells – source of output from the retina o Other cells communication within retina ▪ Lots of processing before info leaves the eye - Ganglion cells fire action potentials Anatomy of the retina - Retina is a sheet of cells that lines the back of the eye - Outer segment of photoreceptor layer o Contains photopigments – respond to light o Way at the back of the eye ▪ Furthest from the source of light - Bipolar cell layer - Ganglion cell layer - Lamina organization - Know the arrangement of the cells in the retina o Don’t need to know all the layers Rods and cones - Photoreceptors – two types - Rod outer segment – rod shaped - Cone outer segment – cone shaped - 100 million rods compared to 5 million cones - Rods o Acromatic vision – scotopic vision ▪ Dim light ▪ Not responsive to differences in wavelengths of light - Cones o Colour vision – photopic vision o Different kinds of cones responsive to different wave lengths ▪ Due to different photopigments ▪ Different ranges of sensitivity to wavelengths o Responsible for high visual acuity - Mesopic vision o Between scotopic and photopic vision o Uses combination of rods and cones - Regional differences in retina in rod/cone distribution o Fovea ▪ Highly concentrated in cones ▪ Directly at the back centre of the eye ▪ No rods o Fewer cones in peripheral vision o Nasal periphery ▪ Periphery closer to nose o Move away from the centre ▪ Lots of rods, fewer cones ▪ Exception for blindspot • Where the optic nerve leaves the eye o Therefore, no photoreceptors o Where the axons of the ganglion cells congregate o Duplex system ▪ When you have rods and cones – one unit with two parallel systems o Central retina ▪ One cone per ganglion cell • No convergence o Good resolution o Peripheral retina ▪ Mostly rods ▪ Much more invergence • Many more photoreceptors per ganglion cells than in central retina o Convergence of info ▪ Rods have greater sensitivity due to being arranged in converging groups - The fovea o Less interference of cells (bipolar and ganglion) between light entry and photoreceptors ▪ Bipolar and ganglion “pushed” aside Receptive fields - How our visual info is put together o How light signals are changing into chemical signals - Area that a neuron is responsive to o Ex. Area of skin, area of photoreceptors o Neurons in the visual system have areas of visual space they are responsive to ▪ Donut shapes of outer receptive field and centre receptive field • Receptive fields can be centre ON or OFF, and surround ON or OFF o Disinhibition – sometimes by switching one portion off, it switches another off ▪ Taking something away, changing the activity of something else ▪ • Figure (left) – centre ON, and surround OFF • Light passes on to cells, can cause a hyperpolarization in surround – surround off o Depolarization in centre – centre on • Light in receptrive field centre (middle figure) o A lack of light can also switch things on o With light impacting, can also swtich things off – cause hyperpolarization – suppression of glutamate release • Figure (right) o Outside, light impact – hyperpolarized – switch off ▪ Specific organization ▪ Receptive fields can be on or off ▪ Photoreceptors organized in donut formation – centre and surround • On centre, off surround • Off centre, on surround • Glutamate receptors determine on or off - Ganglion cells and receptor cells have centre and surround arrangement o Photoreceptor hyperpolarized in response to light ▪ Glutamate release reduced ▪ Photoreceptors essentially respond to different amount of darkness • Hyperpolarized photoreceptor, depolarizes bipolar cell - Darkness in the centre, light in the surround o Photoreceptors in surround are hyperpolarized, horizontal cell is hyperpolarized, bipolar cell is also hyperpolarized ▪ Indicates there is a donut shape of light in that area - Ganglion cells also have centre surround arrangement as well - o Depending on where we shine light, can change electrical activity coming from ganglion axon ▪ Have off centre ganglion field – have a general buzz of AP activity • Dark spot covers centre, get an excitation of the area – more Aps – middle cells have been switched off, caused disinhibition of surrounding area o Generates hyperactivity due to disinhibition • Cover whole area, get underlying basal current – similar to light on both centre and surround ▪ Off centre, darkness covers part of receptive field – knocks out signal • Surround points important for signal • Cover both centre and surround – knocks out centre – high excitation generated by surround o Disinhibition o Nothing happen, a few action potentials fired (three) o Dark spot hits centre, ganglion cell fires lots of action potentials o Dark spot hits centre and surround o Light/dark edges ▪ Off centre ganglion cell – occasional AP firing • dark region moves over into off centre – get increased rate o covers entire field – get less AP firing - Area of space that a neuron is responsive to o Donut shaped in receptor and ganglion cells o Area of sensitivity for visual system Ganglion cells - M-type o Larger cells
More Less

Related notes for PSYO 2470

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.