Cognitive psy 53-77

physical processes and perceptual outcomes

1) Inhibition in the retina+ Optic nerve fiber firing -> Sharpness of borders and Perceiving light and dark

2) Cortex: Single neuron firing + Cortex: Firing of groups of neurons → Perceiving features of objects +Perceiving and recognizing objects

neural convergence of the rods and cones

rods: high convergence = high sensitivity and

cones: low convergence = high visual acuity

lateral inhibition

inhibition that is transmitted across the retina

lateral inhibition and limulus experiment

used the Limulus to demonstrate how lateral inhibition can affect the response of neurons in a circuit. Scientists shined light on a single receptor in the Limulus' eye and measured its response. When they stimulated neighboring receptors, the original receptor’s response decreased due to inhibition from surrounding cells. This confirmed that lateral inhibition sharpens edges and enhances contrast by reducing activity in neighboring neurons. This process helps explain how humans see clear edges and contrasts in vision

perception of lightness

the perception of shades ranging from white to gray to black.

the herman grid

• A visual illusion where gray "ghost-like" spots appear at the intersections of a black-and-white grid, but they disappear when you focus on them.

• Why does it happen?
  Due to lateral inhibition in the retina:

  • Retinal ganglion cells receiving strong light input inhibit neighboring cells.

  • More inhibition occurs at intersections, making them appear darker.

  • When you look directly at an intersection, foveal vision reduces the effect.

• Outcome:
  The illusion demonstrates how our visual system processes contrast and edges.

Mach Bands

• What is it?
  Mach Bands are an optical illusion where bands of different shades appear to have exaggerated contrast at their edges.

• Why does it happen?
  It occurs due to lateral inhibition, where neurons responding to light inhibit their neighbors, enhancing edge contrast.

• Outcome:

  • Darker areas near light regions appear even darker.

  • Lighter areas near dark regions appear even lighter.

  • This helps sharpen edges in vision, improving perception of shapes and depth.

Simultaneous contrast

Simultaneous contrast occurs when our perception of the brightness or color of one area is affected by the presence of an adjacent or surrounding area.

simultaneous contrast

• What is it?
  Simultaneous contrast is a visual effect where the perceived color or brightness of an object changes depending on the surrounding colors.

• Why does it happen?
  It occurs due to lateral inhibition, where nearby neurons influence each other’s activity, making colors and brightness appear more exaggerated.

• Example:

  • A gray square on a dark background looks lighter.

  • The same gray square on a light background looks darker.

• Outcome:

  • Enhances contrast in vision, making objects stand out.

whites illusion

• What is it?
  White’s Illusion is a brightness illusion where two identical gray bars appear lighter or darker depending on the background they are placed on.

• Why does it happen?
  Unlike Mach Bands (which rely on lateral inhibition), White’s Illusion is explained by contextual effects and perceptual grouping:

  • The gray bar on the white background appears darker.

  • The gray bar on the black background appears lighter.

  • This is due to brightness contrast and how the visual system groups objects based on patterns.

• Outcome:
  This illusion challenges traditional lateral inhibition models and supports the idea that higher-level visual processing (like grouping and context) plays a role in brightness perception.

belongingness (explanation for whites illusion)

states

that an area’s appearance is infl uenced by the part of the surroundings to which the area appears to belong

optic nerve

the nerve is made up of many individual nerve fibers traveling together. These fibers are the axons of the retinal ganglion cells

receptive field

Hartline called the area that caused the neuron to fire the nerve fiber’s receptive field which he defined as follows: The region of the retina that must receive illumination in order to obtain a response in any given fiber.

fiber’s receptive field

The fact that a fiber’s receptive field covers hundreds or even thousands of receptors means that the fiber is receiving converging signals from all of these receptors. Finally, Hartline noted that the receptive fields of many different fibers overlap. This means that shining light on a particular point on the retina activates many ganglion cell fibers.

optic nerve fiber

each optic nerve fiber monitors a small area of retina, and all of them together take in information about what is happening over the entire retina

center-surround organization

The cat receptive fields, it turns out, are arranged in a center-surround organization, in which the area in the “center” of the receptive field responds differently to light than the area in the “surround” receptive field

excitatory area

presenting a spot of light to the center increases firing, so it is called the excitatory area of the receptive field.

inhibitory area

stimulation of the surround causes a decrease in firing, so it is called the inhibitory area of the receptive field. . This receptive field is called anexcitatory-center, inhibitory-surround receptive field.

inhibitory-center, excitatory-surround receptive field

The receptive field which responds with inhibi-

tion when the center is stimulated and excitation when the surround is stimulated, is an inhibitory-center, excitatory-surround receptive field.