Visual Brain Part I

Retinal Structure
1. Inner layer of eye- when light hits, initiates chemical and electrical events that trigger nerve impulses. Contains 10 layers, 3 of the most important being: (*also, astrocytes- Muller cells; interneurons- horizontal cells)
  1. PHOTORECEPTOR CELL LAYER electromagnetic energy = neural signal
    1. 100,000,000
    2. RODS= SCOTOPIC VISION (LOW-LIGHT LEVELS); CONES= COLOUR VISION & ACUITY
      1. FOVEAL VISION- CENTRAL 1 DEGREE OF VISUAL ANGLE; MACULAR VISION- CENTRAL 5 DEGREES.. Beyond this is peripheral.
    3. Rods and cones produce graded responses- light=hyperpolarisation and release of neurotransmitters
      1. Maximal stimulation of the receptors is determined by the spectral sensitivity of the photopigment, located on the outer segment of the receptor (Marks, 1965). There are more of these in cones suggesting they are more involved in processing wavelength/colour.
  2. BIPOLAR CELL LAYER
    1. 10,000,000
  3. GANGLION CELL LAYER
    1. 1,250,000
    2. Ganglion cells carry info along the optic nerve, which is made up of axons from all retinal ganglion cells & glial cells.
      1. RECEPTIVE FIELDS OF GANGLION CELLS: area of the retina from which it receives input. Their receptive fields have a centre-surround configuration- 2 TYPES: ON centre/OFF surround // OFF centre/ON surround i.e. photoreceptors-bipolar-ganglion is centre; the addition of a horizontal cell in this link means it is no longer direction and now considered the surround. Depolarisation of centre if on and hyperpolarisation of surround if off (ON centre: impulses sent during depolarisation; OFF surround: post-inhibitory rebound impulses sent after hyperpolarisation).
        RECEPTIVE FIELD SIZE: Increasing eccentricity (deviation from fovea) = increasing receptive field size. Closer to fovea means greater acuity (smaller receptive fields) 0.01mm to 0.5mm. Cells respond best to matching stimuli i.e. small stimuli - small receptive fields, which suggests object size analysis may start in the retina.
      2. Classification: as well as centre/surround & receptive field size: A) M cells (magnocellular) B) P cells (parvocellular) C) K cells (koniocellular)
        P CELLS: small; in fovea; high spatial resolution; low temporal resolution; low contrast sensitivity; responds to colour; conduction velocity slow; 80% of GC.
        M CELLS: large; periphery; all qualities opposite to P cells, 10% of GC.
2. CURVATURE OF THE EYE: Nasal retina registers temporal field & vice versa; lower retina registers upper visual field & vice versa. Thus, left hemi-field is observed by right temporal & left nasal.
Moving from Retina to LGN
1. THE OPTIC NERVE: each GC axon acquires myelin coating- insulates and increases speed.
  1. Following the Optic Nerve is the OPTIC CHIASM- here, decrossation occurs. The RETINA can be subdivided into 4 segments: NASAL, TEMPORAL, UPPER, LOWER. It can also be subdivided into PERIPHERAL and CENTRAL.
    1. Optic nerves for nasal retina of a specific eye cross over to the opposite hemisphere for processing in the LGN; contrarily, the optic nerves for the temporal retina of that eye do not cross over. Contralateral/ipsilateral.
      1. Visual pathway after this becomes known as the OPTIC TRACT, which informs the LGN in the cerebral hemisphere- 6 layered structure
        Inputs to the LGN are segregated: fibres coming from ipsilateral eye terminate in layers 5, 3 & 2; contralateral eye terminate 6, 4 & 1. Point to point projection means ^ acuity, but also each eye is represented twice.
        Points on layers correspond highly to points on retina and this is the same for each eye i.e. same point different layer, corresponds to eye.
        These 6 layers are made up of cells- the upper 4 being parvocellular cells and the lower 2 being magnocellular cells. They correspond to the matching GC type and are sourced from rods (m) and cones (p)
        *CRITICISM/HISTORY: Henschen erroneously believed p cells code only for colour & that m cells code only for light and that this was implicated in their size.
        LGN cell axons travel in optic radiation to terminate at the PVC.
      2. As well as going to the LGN, the retina connects to the superior colliculus & pulvinar. This is key to understanding certain types of blindness e.g. blindsight
CITATIONS: Parallel processing and GC receptive fields: Celesia & DeMarco (1994). Retina, LGN, V1 & V2: Zeki (1993), Prasad & Galetta (2011).
The Primary & Secondary Visual Cortex
1. "...parallel processing is a hallmark of sensory coding..." Erickson (1974)
  1. Radiation terminates in the occipital lobes- known as area V1 (each hemisphere has visual cortex).
    1. Cortex receives signals from contralateral hemifield; adjacent retinal points accounted for on topographical map (RETINOTOPIC).
      1. Cytoarchitectonic staining (RNA within cells stained and thus highlighted) reveals a stratified structure with 6 layers. Layer 4 is larger than usual.
        Layer 4 is comprised of 4 sublayers. The LGN projects to these layers. Cells in them are monocularly driven (other layers- binocularly driven)
        MAGNOCELLULAR ROUTE: from LGN to 4Cα to 4B to other cortical areas.
        PARVOCELLULAR ROUTE: from LGN to 4A to 4Cβ to 2 & 3 to other cortical areas
        Lower half of visual field is mapped onto upper calcarine cortex, and vice-versa (Zeki, 1993); central vision at occipital pole (*previous Henschen assumption that it had been anterior)
        CORTICAL MAGNIFICATION: a disproportionate number of cells in the V1 deal with information from the central retina (80% to 10 degrees) - receptive fields and eccentricity.
        AREA V2- prestriate cortex. Also indirect link between V1 and V3-5. Mapped as V1 is.
        Has a thin stripe (connections from blobs in V1, wavelength) and thick stripe (from 4B, orientation *direction), and interstripe (interblobs- orientation only)
        THIN STRIPES TO V4
        iNTERSTRIPES TO V4
        THICK STRIPES - ORIENTATION CELLS TO V3; ORIENTATION & DIRECTION CELLS TO V5
        MOTION PATHWAY- M GC CELLS IN RET -- M LAYERS IN LGN -- LAYER 4B IN V1 (ORIENTATION & MOTION) -- THICK STRIPES IN V2 -- AREA V5
        DYNAMIC FORM PATHWAY- M GC CELLS IN RET -- M LAYERS IN LGN -- LAYER 4B ORINETATION IN V1 -- THICK STRIPES IN V2 -- AREA V3.
        COLOUR PATHWAY- P GC CELLS IN RET -- P LAYERS IN LGN -- 2 & 3 IN V1 BLOB CELLS WAVELENGTH -- THIN STRIPES V2 -- AREA V4
        FORM & COLOUR PATHWAY- P GC CELLS IN RET -- P LAYERS IN LGN -- LAYERS 2 & 3 V1 INTERBLOB ORIENTATION -- INTERSTRIPES IN V2 -- AREA V4
    2. PROPERTY 1) ORIENTATION SELECTIVITY (Hubel & Wiesel,1981): most cells sensitive, unlike RET and LGN. Each has own preferred orientation- elicit optimal response. Electrode and oscilloscope record heightened activity when exposed to preferred ort.
      1. In line with receptive field shape, which is a column as opposed to circular- SIMPLE relationship between field layout and preferred orientation. Multiple LGN to 1 simple.
        COMPLEX cells - on and off ones are not as clear. Do NOT respond to static/stationary stimuli- moving & at optimal ort, direction.
        HYPERCOMPLEX cells: responds to luminance in particular part of receptive field, of specific length (end-stopping- inhibitory flanks).
    3. PROPERTY 2) SIZE SELECTIVITY: As with LGN & RET, cells receiving input from fovea = small RF, periphery = large RF. Unlike LGN & RET, also has orientation selectivity.
      1. The cytochrome oxidase method (Wong-Riley, 1979) reveals an even more complex anatomical architecture; staining sections for the metabolic enzyme cytochrome oxidase: POLKA DOT PATTERN layers 2 & 3- DARK BLOBS (wavelength) and LIGHT INTERBLOBS (orientation).
        N.B. Blobs with same wavelength selectivity connect
        OTHER ARCHITECTURAL METHODS: A) Cortical lesions: when destroyed fibres regenerate and as a result take up more silver (become more argyrophilic) - areas showing more silver denotes connection to the damaged area.
        B) Radioactive amino acids: injection- AAs taken up by cells in site, then transported to axon terminals, thus highlighting them.
        C) Enzyme horseradish peroxidase (HRP): injected at suspected termination site. HRP is transported retroactively to cell body- thus revealing layer location too. Can be combined with lectin wheatgerm glutinin (WGA) to show both prograde and retrograde projections (Zeki, 1993).
    4. PROPERTY 3) BINOCULARITY: Layer 4 has monocular segregation; unlike LGN, rest of V1 has BS. Individual cells respond to both eyes but usually to one better than the other - ocular dominance.
      1. Preferential response is one of two properties, other- relative positions of RF specify location in space that will excite cell.
        Functional Architecture- Each hypercolumn analyses orientation, motion direction, binocularity and size within a local region of the retina.
    5. PROPERTY 4) MOTION DETECTION: Shipp & Zeki (1989)- neurons in 4B sensitive to motion direction- and they also form connections.
DAMAGE
1. Retina + optic nerve = total blindness
2. Beyond chiasm = contralateral field of view blind i.e. homonymoushemianopia.
3. Occipital pole receives dual blood supply from posterior and middle cerebral arteries (rest only posterior) = macular sparing
4. Upper calcerine sulcus lip -> lower visual field and vice versa = quadrantanopia
5. Small area of damage in V1 = scotoma, determined by perimetry.

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Visual Brain Part I

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