Lecture 2b

Readings:Kandell Schwartz et al Ch 27Wolfe et al Chs 3 and 4.

I have one question about the residual "light" we see when we close our eyes. I noted that after closing my eyes I could see the outline of your silhouette in front of the light at the window, only in reverse. Essentially, the silhouette of you standing in front of the window was light, and the surrounding area of the window was dark. What causes the negative image?

Can you explain the key differences between the M (magnocellular) cells and the P (parvocellular) cells? I understand that the M cells are large and the P cells are small, but I do not seem to have the key differences noted other than size.

A) Is there a potential way to “sharpen” your visual senses through any kind of practice or exercise? For example, can peripheral vision be enhanced through specific activities? Would any increase in acuity, focus, or reaction times be tied to real changes in the inner-workings of the vision system or something within the visual cortex?

B) In the same way that color blindness is caused by a type of cone having difficulty discriminating different color wavelengths, or that myopia is caused by light focusing in front of the retina, are there other potentially deficiencies of the visual system that cause other variations, such as increased light sensitivity, poor light adjustment, poor peripheral vision, etc.? Are most of these other issues typically caused by nerve damage or issues such as glaucoma, or can they be genetic traits?

My question for the class is why exactly cones have input from rods through amacrine cells? How does this communication affect visual perception? Is it because change in light levels are important to the perception of visual stimulus in light condition and the functioning of the cones will be adjusted accordingly?

My question for class relates to slide 4 from your powerpoint (the one with the four squares). You said the reason we more clearly see the right image in the static is because it is a more familiar shape from our past experiences of the world. I would like to know if there is a deeper explanation for this. Also, does it have anything to do with Gestalt theory?

Phenomena that may be a consequence of processing in early visual cortex (V1, V2)

selective adaptation: orientation

Adaptation is a pervasive feature of perception

1. Cortical cells tuned to specific orientations.2. Cells in visual cortex fire less following repeated stimulation.3. Following adaptation, the balance of activity across the population of cells shifts.

Response of cells whose preferredorientation is on the x axis, toa vertical line.

Preferred orientation

Preferred orientation

Adaptation is often used as evidence for populations of cells coding certain featuressuch as orientation, spatial frequency etc

Cortical cells are tuned to spatial frequency eg one cell might fire most to the left patch,another to the middle patch, and so on.

Cortical cells are tuned to particular spatial frequencies.Demonstration of adaptation that is specific to spatial frequency

Adaptation is orientationspecific

A grating modulated by contrast (vertically) and by spatial frequency (horizontally)

Spatial-frequency adaptation

A compound grating pattern (right), made by the addition of a sine wave of frequency f (top left) to one of frequency 3f (bottom left)

Why all the fuss about gratings?

Idea is that any spatial pattern canbe made up of a sum of sinusoids.Called Basis FunctionsSimilarly and complex sound can be composed of a sum of pure tones.

Cortical cells respond best to Gaborpatches (like sine waves).

Therefore cortical cells can be thought of as a basis set for images.

A complete image (a) and simulations of the high-frequency (b) and low-frequency (c) components of that image.

The two right images add up to the left one.

Who is hidden behind the high-spatial-frequency mask in this image?

This demonstration suggestsThat identity is carried by fairlylow spatial frequency information

Problems for vision – surfaces and contours (segmentation)

Low level circuitry in visual cortex probably responsible for contoursand surfaces

The Gestalt principle of good continuation

Learning likely patterns in the world is probably the basis of “good continuation”

Measured probability distribution of natural images

Cooperative interactions betweenV1 cells might also help groupingof line elements to form contours.

The making of illusory contours

Learning statistical patterns might also help the brain figure out what objects in the worldmight cause the visual stimulus.

Local and global effects

Classical and non-classical receptive fields

Brightness and color depend on context

Local and global spatial interactions have a profound effect on appearance of surfaces

Gray light in that patch would be a consequence of a blue patch and a yellow illuminantIn the left image or a yellow patch and blue illuminant in the right image.

Brightness and color depend on context

Assume lighting from above

The brain looks for changesacross space – spatial outliers

(similar to the retina)

Detecting outliers depends on learning.

Figure 4.20 Examples of camouflage

The flip side - camouflage

Color vision – Wolfe Ch 5

1. Can you go into further detail regarding binocularity and how that is related to the Columnar organization we spoke about in class?

My question this week concerns the fact that we have two channels or pathways in which on cells measure intensity of light and off cells measure intensity of dark. I understand that this is to conserve energy. Is there any evolutionary evidence that might suggest our lineage having had only one pathway at some point in time for both intensity of light and dark? This question may be slightly off topic. Are these other species with only one type of pathway?

1.) I'd like to know more about the ventral stream slide from Nicolas Priebe's lecture (the tolerance vs. selectivity thing). Did they study the person's preferences or background to see if they had watched a lot of movies with Jennifer Aniston or had a girlfriend that looked like her? Or, for example, would an architectural historian be "selective" to architecture?2.) Why does eye color affect light sensitivity?

There are several brain regions that are specialized for certain stimulus. For example, parahippocampal area (PPA) is most responsive to places, fusiform face area (FFA) is activated by faces, and MT fires during motion. My question is about binding. In general, visual properties of objects bind together as they move toward upper level of visual pathway in the brains. Does simultaneous activation of those three areas give us the perception of seeing a moving face? Or does further area get input from those areas and generate the sensation?