The readings for this week will make it clear that the eye does not project an image upon the mind like a projector on a screen. Photoreceptor activation would not make sense to us. What we see is actively constructed.
First, there is lateral inhibition, which enhances contrast and helps us to see edges. The contrastiness of the image depends on sideways connections between cells in the retina, exerting lateral inhibition.
In lateral inhibition, the more active a cell is, the more it inhibits its neighbors. Image contrast is enhanced first when illuminated cells inhibit cells in the dark that were less active to begin with. It is enhanced further by the release from inhibition of illuminated cells that lie next to cells that are in the dark. Consequently, at a border between intense illumination and less-intense illumination there is extra-strong inhibition of cells on the dark side of the border and unusually weak inhibition of cells on the light side of the border. Both the light intensity and the distance of a cell from the light-dark border determine its activity level.
Lateral inhibition is the basis for the black spots in the Hermann grid.
Mach bands are a real-life illustration of lateral inhibition. For other examples you might try
In the second illustration down at this site, you’ll find arrows that point to where the Mach bands are. Mach bands should prove to you that what you experience as sight contains more contrast than the real world actually offers. You’ll find more about how lateral inhibition produces Mach bands at this site.
The organization of the image also depends on receptive fields. A receptive field can be defined for any neuron in the visual system. All of the photoreceptors whose activity can influence that neuron make up the neuron’s receptive field, as you can see in this animation or that one. In the visual system, the receptive field is always defined as a patch of retinal surface–i.e., a group of neighboring photoreceptors. For a bipolar cell, the receptive field is all of the rods or cones that can activate or inhibit the bipolar cell. For a ganglion cell (which receives input from bipolar cells), the receptive field is all of the photoreceptors that activate/inhibit all of the bipolar cells that can activate the ganglion cell. These are organized into on-center/off-surround or off-center/on-surround fields by the way that bipolar neurons are connected to ganglion cells and by lateral inhibition.
We assemble our mental pictures out of what our eyes tell us, but our eyes are in constant motion, called saccades, and vision is blacked out during each movement. Each eye is attached to six little muscles that are the fastest in the body. They’re called the extraocular muscles because they’re outside the eye; inside the eye there is the contractile ciliary body that pulls on the lens as you accommodate to near objects. But Prof. Rolfs explains it more completely in just a few minutes. You can explore more about eye movements at this site or drag the cursor circle with your mouse to observe the working of the muscles.
We’ll have to leave to other psychology courses our extraordinary sensitivity to gaze, as important in basketball as in flirting . You can see how noticeable it is by moving your cursor across this image. You might keep gazing in mind when we reach the topic of face recognition.
Finally, constancy appears in several visual processes, in size, shape, color, and lightness judgments. Constancy keeps our subjective impressions unchanged while an object undergoes familiar physical changes. In this illusion about lateral inhibition, the border where lateral inhibition is evident creates a false impression of a lightness difference as well.
You cannot turn a perceptual constancy off. The brain constructs our perceptions out of ambiguous data provided by the retina. The strategies we use are the ones that have kept us alive as a species, but they can be fooled. Take a look at Session 2 of these animations (with sound–click “start” on the first screen, then click on the upper right image square in the next window) to get an idea of our limits.
Questions (Answer one.)
1. How is lateral inhibition important for detecting edges?
2. How much can you tell about someone from their eyes? “I looked the man in the eye. I found him to be very straightforward and trustworthy….I was able to get a sense of his soul.” —George W. Bush, after meeting Russian President Vladimir Putin, June 16, 2001
3. Information about the motion, color, form, luminance, and size of objects arrives in separate channels at the visual cortex. This information is routed along two major paths called the Where pathway and the What pathway, or the dorsal and ventral streams, that are discussed in the embedded NOBA textbook chapter on vision that was assigned for week 3.
The dorsal stream provides us with “how” or “where” information in the parietal lobe of the cerebral cortex of each hemisphere; the ventral stream provides us with “what” information–that is, it identifies what we see as recognizable objects, using inferotemporal (IT) cortex on the bottom of the temporal lobe.
Interruption of the dorsal stream results in optic ataxia. Damage to inferotemporal cortex results in visual agnosia. You can also try out dorsal-stream-without-ventral-stream function for yourself with the odd experience of blindsightat this site.
Do you think blindsight illustrates more the operation of the what pathway or the where pathway? Do these pathways seem to operate with conscious awareness or without it. (If in doubt, check it out. You can watch patient T.N. in this video.)