Vision II - Central Pathways & Processing

Retina has three main projections
1) Pretectal Area - pupillary reflex (imp. for midbrain damage)
2) Superior Colliculus - co-ord head and eye movement, saccades, visual map which registers with somato and auditory maps
3) Lateral Geniculate Nucleus

LATERAL GENICULATE NUCLEUS (LGN)
In the thalamus - remember you have two of them
'Genu' = bent or 'knee'
90% of ganglion cell axons go to LGN, 10% to superior colliculus/pretectal
Other inputs to LGN: from optic nerves, brain stem, visual ctx (lots of feedback) and other nuclei in thalamus

6 Layered Structure
Only 1 Eye input to each layer
2,3,5 from ipsilateral eye / 1,4,6 from contralateral eye

Layers 1-2: input from M-cells; MAGNOCELLULAR LAYERS
Evolved earlier, found in all mammals (some its all they have)
Lesion studies in primates found diff. in deficits btwn layers
1 & 2 analyzes form, depth, and movement

Layers 3-6: input from P-cells; PARVOCELLULAR LAYERS
Evolved later, most highly developed in primates
3-6 analyzes fine detail and color

Retinotopic Map: ganglion cells adjacent on retina go to areas adjacent in the LGN layers, hence the projection is organized and maintains orderly map of the retina (and visual space)
RF's - center / surround organization like ganglions w/ larger RFs

VISUAL CORTEX
1.5 million axons from cells in each LGN stream to the calcarine fissure of occipital lobe
Visual cortex also called Striate cortex ('striped', for axons) or V1
Also 6 layers - LGN axons come in layer 4C, then go to other layers
Magno > layer 4Ca and Parvo > layer 4Cb

Hubel & Wiesel - pioneer RF work in visual ctx in 1950s-60s (Nobel Prize in 1981) - story of projector slide response after no luck with spots
RF of Striate Cortex Neurons

Simple: side by side, elongated excitatory and inhibitory (not center-surround); responds to lines in RF center of a particular orientation; this property called orientation tuning
Complex: also responds line's orientation; responds best to moving stimuli anywhere in the RF, may prefer particular direction of motion
End-Stopped: responds best to moving lines of specific length or moving corners or angles anywhere in RF
As we progress up from retina to cortex, more complex stimuli required to make neurons respond

ORGANIZATION OF VISUAL CTX
Electrode recording from perpendicular and oblique penetrations
Location Columns: oblique - spatially close or adjacent RF (retinotopic)
perpendicular - RF on top of each other or very close together
Magnification Factor: more cortical space is allotted to parts of the retina that send more ganglon cells to cortex (e.g. fovea); foveal ganglions get about 4X more cortical tissue than peripheral ganglions
? This is in part due to the _____ nature of cone circuits ?
Magnification also assist in high visual acuity (more processing)
Orientation: perpendicular tracts revealed simple and complex cells with all the same orientation preference and RF
oblique tracts show organized progression of orientations
Confirmed with 2-DG experiments-inject, expose to stimuli, look at cortex
Ocular Dominance: most V1 neurons respond better to one eye or the other; oblique tracts reveal alternating left-right eye dominace
Processing modules of 1mm block or cortex
Hypercolumns: composed of 1 location, multiple orientation, and 2 ocular dominance columns; processes all features from one spot on retina

But this is not where vision 'stops'

BEYOND STRIATE CORTEX - EXTRASTRIATE AREAS
After V1 information is channeled to different areas; magno and parvo channels continue (V2, V3, V4, V5 / MT)
At least 60% of cortex involved in vision

Blindsight: damage to striate ctx (trauma, stroke); subjects are blind (at least in portion of visual field) but can point to a light in this 'blind' area, although they insist they do not see it (implications for consciousness)
Research in Monkeys: lesion studies, stimulation, electrophysiology
Ungerleider & Mishkin (1982)

MAIN EXTRASTRIATE AREAS
V5 (medial temporal or MT) - MOTION
Lesion studies-Newsome & Pare (1988) - used random dot correlation task; in normal monkeys = 1-2% detectable
in MT lesion monkeys = 10-20% to detect
Recording studies-90% neurons in MT react to motion, 5% in V4
Stimulation studies-improved detection ability (sensitivity) if neurons stimulated for the correct direction of random dot motion
V4 - COLOR
Recording-60% of neurons in V4 respond to color, < 5% in MT
Inferotemporal Cortex (IT) - FORM
We have seen that neurons get more selective in what they respond to; begin to show complex response properties
Tanaka (1993) - neurons in IT respond best to complex forms
Bruce, Desimone, & Gross (1981) - found neurons that respond most vigorously to face-like stimuli; may make sense that primates especially sensitive to faces

EXTRASTRIATE AREAS IN HUMANS
PET scans generally confirm the what and where distinctions
Clinical studies
Agnosia: "failure to know"
Damage to MT = motion agnosia
Damage to Temporal lobe = prosopagnosia (faces)
The Man Who Mistook His Wife for a Hat by Oliver Sacks; taking off the wife's head, patting the fire hydrant as a child

OTHER FACTORS IN VISUAL PROCESSING
Binding problem: if object attributes are processed seperately, how do they recombine to form a unified perception?
2 Hypotheses: rate of firing; correlated/synchronized firing

Attentional Effects
Moran & Desimone (1985) - monkey fixates on point, has been trained to attend on cue to one of two areas
2 colored bars in RF of V4 neuron, one is 'preferred' color
If monkey cued to attend to 'preferred' color, neuron fired more
SAME spot on the retina, but attention enhances response