PSYC 494N1 Lecture Notes - Magnocellular Cell, Receptive Field, Cytochrome C Oxidase

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5 Feb 2013
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PHYSIOLOGY OF THE STRIATE CORTEX
Receptive Fields
The receptive fields of neurons in layer IVC are similar to the magnocellular and parvocellular
LGN neurons providing their input. Outside layer IVC, new receptive field characteristics, not
observed in the retina or LGN, are found.
Binocularity
Neurons in layers IVCα and IVCβ receive afferents from a layer of the LGN representing either
eye. Monocular neurons from either eye are also clumped together in V1 rather than randomly
intermixed and this accounts for ocular dominance columns.
Axons leaving layer IVC diverge and innervate more superficial cortical layers. As a result, there
is a mixing of inputs from the two eyes (see Fig. 10.17). Most neurons superficial to IVC are
binocular and have binocular receptive fields.
Orientation Selectivity
Most receptive fields in the retina, LGN and layer IVC are circular and give greatest response to
a spot of light matched in size to the receptive field center. Outside layer IVC, cells do not follow
this pattern.
Hubel and Wiesel found that many neurons in V1 respond best to an elongated bar of light
moving across their receptive fields, but the orientation of the bar is critical.
A particular orientation triggers the greatest response whereas perpendicular bars
generally elicit much weaker responses (see Fig. 10.20). This is orientation selectivity.
Orientation selectivity of nearby neurons is related and the preferred orientation remains the
same for all the selective neurons encountered from layer II down through layer VI. Such a
radial column of cells are called orientation columns.
As an electrode passes tangentially (i.e. parallel to the surface) through the cortex in a single
layer, the preferred orientation progressively shifts. There is a mosaic-like pattern of optimal
orientations in striate cortex. If an electrode is passed at certain angles through this mosaic, the
preferred orientation rotates like the sweep of the minute hand of a clock (see Fig. 10.21).
A complete 180° shift in preferred orientation required a traverse of about 1mm within
layer III.
Orientation selective neurons are thought to be specialized for the analysis of object
shape.
Direction Selectivity
Many V1 receptive fields exhibit direction selectivity and respond when a bar of light at the
optimal orientation moves perpendicular to the orientation in one direction but not in the
opposite direction (see Fig. 10.22). Sensitivity to the direction of the stimulus motion is a
hallmark of neurons receiving input from the magnocellular layers of the LGN.
Direction-selective neurons are thought to be specialized for the analysis of object
motion.
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