Lecture 2b Readings: Kandell Schwartz et al Ch 27 Wolfe et al Chs 3 and 4.

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 preferred orientation is on the x axis, to a vertical line. Preferred orientation

Adaptation is often used as evidence for populations of cells coding certain features such 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.

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

Spatial-frequency adaptation

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

Problems for vision – surfaces and contours (segmentation) Low level circuitry in visual cortex (V1/V2) probably responsible for contours and surfaces Importance of perceptual learning

Cooperative interactions between V1 cells might also help grouping of line elements to form contours.

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

Local and global effects Classical and non-classical receptive fields

Perceptual Learning is task specific

The brain looks for changes across space – spatial outliers (similar to the retina) Detecting outliers depends on learning.

The constructive nature of perception: extracting regularities a process of guessing the state of the world from sometimes incomplete sensory data. Constructive in the sense that it relies on memory representations of past experience Goal of an organism to predict what’s likely to happen and choose the best actions (where’s the food? Is there a tiger nearby?). This means making good estimates of the state of the world.

The making of illusory contours Learning statistical patterns might also help the brain figure out what objects in the world might cause the visual stimulus.

Color vision – Wolfe Ch 5

Why is color vision useful?

Segmentation? Identification?

Figure 4.20 Examples of camouflage The flip side - camouflage

Color Constancy: surfaces appear the same color independently of changes in the illuminant. That is, we perceive reflectance, which is a property of the object. Not the color of the reflected light itself. This mean we must somehow “discount the illuminant”. How is this done? Basic mechanism: receptor adaptation. Higher level mechanisms: assumptions about the illuminant.

Assume lighting from above

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 illuminant In the left image or a yellow patch and blue illuminant in the right image.

Brightness and color depend on context