Lecture 14: Vision 2 Flashcards
The Bionic Retina Steps
- Camera captures image and sends info to microprocessor
- Microprocessor converts data into electronic signal and transmits it to receiver
- Receiver sends signals through a tiny cable to an electrode panel implanted by doctors on back wall of eye (retina)
- Retinal implant emits pulses which travel through the optic nerve to the brain
- Brain perceives patterns of light and dark which correspond to the electrodes stimulated on the retinal implant
Blind Spot
- Retinal ganglion cells send axons through the optic nerve
- There can be photoreceptors where the nerve starts
- Hence we have a blind spot in our field of view
Two Halves of the Retina
- Each retina is divided into a nasal and temporal hemiretina
- Their projections diverge at the optic chasm
- Right visual field of each eye ends up in the left sides of the brain, and the left visual field of each eye ends up in the left side of the brain
From the LGN to the cortex
- Axons from LGN travel through the “optic radiation” to the primary visual cortex (V1)
- Maintains retinotopic organizations
- Each V1 neuron responds to a stimulus in a small area in the field of view, and neighboring V1 neurons respond to stimuli in nearby locations in the visual field
Lateral Geniculate Nucleus (LGN)
- RGC axons reach the LGN of the thalamus
- Center-surround like retinal ganglion cells
V1 neurons respond to __________
Bars of light, with different neurons responding to different angles
Lesions to V1
- Patients report partial or complete blindness
- When asked to detect objects they appear blind
- When forced to guess however, they do well
- This is called blindsight
Patient TN
- Lost V1 in both hemispheres due to stroke
- Ordinary vision tests indicated complete blindness
- Researchers asked him to walk down a corridor where they had placed obstacles without his cane
- He did not bump into any obstacles
V1 vs. V2
- Neurons in V2 have similar response properties to neurons in V1
- But V2 neurons respond to visual illusions (V1 does not)
- V2 neurons will respond to edges of the pac-man rectangle
From V1 and V2 to Higher Level Processing
- From V1: The ventral “what” stream and dorsal “where” stream
The Ventral Stream
- LGN –> V1 –> V2 –> V4 –> IT
- “What” stream
- Response to increasingly complex stimuli
- V4 responds to complex geometric shapes
- V4 is the first area in the ventral stream to show attention modulation
- IT (inferior temporal lobe) responds to visual objects in a position-invariant and size-invariant manner
Fusiform Face Area (FFA)
- In the ventral stream
- Responds to faces
Expertise Hypothesis
- FFA is selective to identifying objects of expertise (not necessarily faces)
- Support: care experts and bird experts show increased activity in FFA when shown their object of expertise
Lesions of IT
- Leads to visual agnosia
- If the lesion is in the FFA, there can be a specific difficulty in recognizing faces called prosopagnosia
Visual Agnosia
Severe and permanent impairment in learning and remembering to recognize visual stimuli
The dorsal “where” stream
- LGN –> V1 –> V2 –> V5 (MT) –> Parietal Cortex
- “Where” stream
- Spatial attention- guiding our view to points of interest
- Using vision for guidance of actions
- Detecting and analyzing movements
Change Blindness
Being unable to detect differences in two things, occurs because we cannot pay attention to full field of view simultaneously
Saccades
Fast eye movements which allow us to focus our fovea on a small area of interest at any given time. Occurs in the dorsal stream.
