The Eight Senses: Vision

The Eight Senses: Vision




Physical stimulus



Role of the Brain


Vision Light waves (the electromagnetic spectrum from red to violet visible to the human eyes) Cornea: protection, light-gathering


Light -> sclera (white), aqueous humor (outer eye liquid) -> pupil


Iris (the muscle controlling the dilation/constriction of the pupil size based on the light level and sympathetic nervous system activation);


Light -> lensĀ  accommodation (size and thickness change) -> vitreous humor (eyeball jellylike fluid) -> retina (neurons for light energy transduction)

Retina: photoreceptor using photopigments: rods (120-125 million thin neurons at the edges for dim light);

cones (6-7 million thick neurons in the center fovea for bright light, color, detail)

Optic chiasm: the crossover point that left visual field (retina) projections -> right hemisphere; right visual field (retina) projections -> left hemisphere


Visual information -> optic tract -> lateral geniculate nucleus (in the thalamus) -> primary visual cortex (in the occipital lobe), visual association areas (in the temporal lobe) -> finer processing


Feature detectors: neurons for processing visual stimulus types (size, curve, contour)


Lateral inhibition: dark backgrounds make things look brighter

Trichromatic theory: 3 types of cones in the retina (red-, green-, and blue-sensitive) corresponding to short, medium, and long light waves.

Pros: can explain color blindness with a lack of cone type.

Cons: cannot explain afterimages.

Middle layer: bipolar neurons, horizontal cells, amacrine cells Opponent process theory: cones work in pairs (e.g., red-green, blue-yellow) that work in opposition; combinations may also be stimulated.
Outer layer ganglion cells: optic nerve Integrated theory: the trichromatic and opponent process theories work at different stages; the former with additional processing in the ganglion cells, the later in the thalamus and occipital lobes (where neurons work in opponent ways).
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