The Visual Pathway

Parts of the Eye

PARTS OF THE EYE Aqueous humor - clear watery fluid found in the anterior chamber of the eye; maintains pressure and nourishes the cornea and lens Vitreous humor - clear, jelly-like fluid found in the back portion of the eye: maintains shape of the eye and attaches to the retina Blind spot - small area of the retina where the optic nerve leaves the eye: any image falling here will not be seen Ciliary muscles - involuntary muscles that change the lens shape to allow focusing images of objects at different distances Cornea - transparent tissue covering the front of the eye: does not have blood vessels; does have nerves Cones -photoreceptors responsive to color and in bright conditions; used for fine detail Rods - photoreceptors responsive in low light conditions; not useful for fine detail

Parts of the eye - continued Fovea - central part of the macula that provides sharpest vision; contains only cones Iris - circular band of muscles that controls the size of the pupil. The pigmentation of the iris gives "color" to the eye. Blue eyes have the least amount of pigment; brown eyes have the most Lens - transparent tissue that bends light passing through the eye: to focus light, the lens can change shape Macula - small central area of the retina that provides vision for fine work and reading Optic nerve - bundle of over one million axons from ganglion cells that carry visual signals from the eye to the brain Pupil - hole in the center of the eye where light passes through Choroid - Thin tissue layer containing blood vessels, sandwiched between the sclera and retina; also, because of the high melanocytes content, the choroid acts as a light-absorbing layer. Retina - layer of tissue on the back portion of the eye that contains cells responsive to light (photoreceptors) Sclera - tough, white outer covering of the eyeball; extraocular muscles attach here to move the eye

Visual Pathway Imagine that the colored bar (half red, half blue) is in front of your eyes. The red part of the bar will project to the nasal part of your left retina and the temporal (lateral) part of your right retina. The blue part of the bar will project to the nasal part of your right retina and the temportal (lateral) part of your left retina.

Visual pathway Like many pathways in the nervous system, right and left visual information cross to the other side of the brain. This occurs in the optic chiasm. After the optic chiasm, information about the right visual field (blue) is on the left side of the brain, and information about the left visual field (red) in on the right side. The pathways stay this way and all the way up to the visual cortex.

Visual pathway Follow the blue and red lines from the eyes to see the flow of information. From the retina, the first synapse is in the lateral geniculate nucleus of the thalamus. The next synapse is made in primary visual cortex in the occiptal lobe.

The Visual Pathway

The Magic Cube Look at the center cube. What side is the front? Is the front as shown on the cube on the right side or is the front as shown on the cube on the left side or is there no front at all?

Muller-Lyer Illusion Which of the lines shown below is longer? The lines are identical in length. We see them as different because we have been taught to use angles to tell us about size.

Black is the New White Stare at the middle of the picture with black squares 15-30 seconds. Are those really dots that appear at the corners of the squares? What happens if you focus on a dot? Now look at the middle of the picture with the white squares. Do you see dots again? What color are they?

Figure Ground Illusions Do you see a vase or a face in the figure below? This type of picture was first illustrated by psychologist Edgar Rubin in 1915. Notice that it is very difficult to see both the faces and the vase at the same time. This may happen because we tend to focus our attention on only one part of the image...either the faces or the vase.

Stare at the + for about 15 seconds, then shift your gaze to the right side of the image. Do the lines on the right look straight? Are they?

Titchener Illusion Is the center circle on the right the same size as the center circle on the left? For many people it appears that the circle that is surrounded by the small circles is larger that the circle that is surrounded by the larger circles. This illusion shows that our brains judge size by comparing objects to things in the surroundings

Which arc comes from the circle with the largest diameter?

It probably looks like arc C is part of the largest circle It probably looks like arc C is part of the largest circle. However, all the arcs are actually from the SAME circle. Look at the same figure again - however, this time I have blocked the right and left sides of the larger two arcs. Each arc comes from a circle of identical size.

Subjective Contours: Do you see a … Cube, triangle, square and rectangle?

Filling the gap: with Subjective Contours, your brain tries to fill in these pictures with images that really are not there.

Movement Illusions these two dimensional objects produce the illusion of movement. Look at the center of these images to see the circles of images rotate.

Context Illusions: we see what makes sense