Mind, Brain & Behavior Wednesday February 19, 2003

Extrastriate Pathways  Parallel processing of visual information from the striate cortex.  Three pathways: Color processing – P blob cells, goes from V1 to V2, then V4, then inferior temporal cortex. Shape processing, depth perception – P interblob cells, goes from V1 to interior temporal cortex. Motion & spatial relations – M cells, V1 to V2, then MT (V5), to parietal cortex.

Equiluminance  Brightness is held constant – permits study of the contribution of color to perception.  Results: Brightness, not color, is important to motion detection, perspective, relative sizes, depth perception, figure-ground relations, visual illusions. Motion is a cue for distinguishing among objects.  Things that move together belong together.

Visual Agnosias  Existence of distinct agnosias for aspects of perception suggests that these abilities are localized to areas selectively damaged.  Achromatopsia – good perception of form despite inability to distinguish hues.  Prosopagnosia – inability to recognize faces as particular people (identity). Can recognize that it is a face, and tell the parts.

Binding Mechanisms  How is information from the separate, parallel pathways brought together and associated?  Treisman & Julesz – combination requires attention. A pre-attentive process detects the major outline of an object. An attentive process notices, selects & highlights combinations of features. Maintained in separate global and detailed maps.

Edge Detection  Ganglion cells respond to contrast and change in visual input. Center-surround (on-off) receptive field.  Bipolar cells also have center-surround receptive fields.  Neurons in the visual cortex have rectilinear receptive fields with excitatory and inhibitory zones.  Complex cells provide positional invariance.

Complex Forms, Motion  Processing of form occurs outside the visual cortex – inferior temporal cortex. Not organized retinotopically. 10% selective for specific images (hands, faces).  Processing of motion occurs in middle temporal area (MT or V5), then parietal lobe. Used for seeing moving objects, pursuit eye movements, guidance of bodily movement

Color Chapter 24

Color Constancy  Wavelengths of light reflected by objects vary with ambient light (light in the environment). Perceptual processes compensate for illumination to produce color constancy. However, colors can appear different in daylight compared to artificial light.  Color is perceived in relation to its background. Colors appear different with different background

Types of Cones  Three types of cones respond preferentially to different wavelengths of light: Short wavelengths (419 nm) – blue Middle wavelengths (531 nm) – green Long wavelengths (559 nm) – red  All other colors can be produced by combining different proportions of blue, green and red.

Anomalous Vision  Monochromats – people with only rods or with only one type of cone. Unable to see color.  Dichromats – caused by genetic mutation on the X chromosome so occurs in men (1% red blind, 2% green blind). Hybrid gene causes red/green blindness.  Blue blindness rarer and not sex-linked.

Divariant Vision in the Fovea  Short wavelength cones (blue) are missing in the fovea. The lens focuses short wavelength light in front of the retina (chromatic aberration).  Because color vision is divariant in the area of greatest visual acuity, color vision is not used for fine spatial discrimination.

Color Opponency  Certain colors are never seen in combination: Reddish green, bluish yellow. Red and green mix to form yellow; yellow and blue mix to form white.  Hering’s opponent process theory – perceptual cancellation occurs because colors are processed as opponent pairs. Three color-opponent channels.

Color Processing  The brain compares responses of three types of cone cells.  Inputs from the three types of cones are combined in different ways. The brain computes responses of specific cones but also all cones in the retina (background) to compensate for ambient light (constancy). Area V4 responsible for color constancy – damage results in loss of color experience.