Chapter 3: Neural Processing and Perception
● Electrical signals dont travel through the nervous system in a nonstop route, they
travel through a complex network of interconnected neurons often meeting and being
affected by other signals
● Purpose of signals goes beyond signaling that a receptor is stimulated
● Neural processing: interaction of the signals in many neurons
Lateral Inhibition and Perception
● What happens when both convergence and inhibition are present?
○ Lateral inhibition: inhibition transmitted across the retina
○ Inhibition causes perceptual effects
○ The Limulus experiment summary (View Figure 3.3):
■ Its large eyes are made up of hundreds of ommatidia, each containing a
single receptor
■ This decrease in the firing of receptor A is caused by lateral inhibition
that is transmitted from B to A across the Limulus’s eye by the fibers of
the lateral plexus
○ The horizontal and amacrine cells transmit signals across the human retina
(like plexus in limulus) and lateral inhibition transmitted by them may influence
how humans perceive light and dark
Lateral Inhibition and Lightness Perception
● Perceptual phenomena explained by lateral inhibition and that involve perception of
lightness: the perception of shades ranging from white to gray to black:
The Hermann Grid: Seeing Spots at Intersections
● Noticing gray “ghost images” at the intersections of the white areas, which decrease or
vanish when we look directly at an intersection
● Fast rapid firing of a cell/generation of lots of action potentials → perception that
something is bright/white
● Slower responding from photoreceptor/generation of fewer action potentials (due to
lateral inhibition) → perception that something is dark/grayish
● Initial response – lateral inhibition = final response
● The more there is lateral inhibition (smaller response) the darker our perception
⇒ lateral inhibition explains seeing the dark blobs at the intersections of the grid
, Mach Bands: Seeing Borders More Sharply
● Mach bands: illusory light and dark bands near a light-dark border
● Light intensity distribution gives no hint of Mach bands (they are an illusion created by
our visual system)
● Same calculation style for the Hermann Grid can be done here to prove that lateral
inhibition affects our perception and how we see the bands (View Figures 3.11, 3.12,
3.13 )
Lateral Inhibition and Simultaneous Contrast
● Simultaneous contrast: occurs when our perception of the brightness/color of one
area is affected by the presence of an adjacent/surrounding area
● (View Figures 3.14 & 3.15):
○ The 2 center squares are the same shade of gray, the illusion that they are
different is because the square on the left receives more inhibition → slower
responding ⇒ looks darker
■ Light area around left square causes the receptors under it to respond
rapidly → send large amounts of inhibition to neurons under the center
square → these neurons response is decreased → center looks darker
■ Dark area around right square causes receptors under it to respond less
rapidly → send less inhibition to neurons under center square → center
square remains brighter than the one on the left because it has less
inhibition
● This explanation makes sense and is still accepted by some researchers, but lateral
inhibition is not considered to be the whole story behind simultaneous contrast
A Display That Can’t Be Explained by Lateral Inhibition
● White’s illusion: 2 rectangles appear to be different colors when they actually reflect
the same amount of light
● (View Figure 3.18):
○ The arrows indicate the amount of lateral inhibition received by parts of
rectangles A and B. Because the part of rectangle B is surrounded by more
white, it receives more lateral inhibition. This would predict that B should
appear darker than A (as in the simultaneous contrast display in Figure 3.14),
but the opposite happens. This means that lateral inhibition cannot explain our
perception of White’s illusion.
● Electrical signals dont travel through the nervous system in a nonstop route, they
travel through a complex network of interconnected neurons often meeting and being
affected by other signals
● Purpose of signals goes beyond signaling that a receptor is stimulated
● Neural processing: interaction of the signals in many neurons
Lateral Inhibition and Perception
● What happens when both convergence and inhibition are present?
○ Lateral inhibition: inhibition transmitted across the retina
○ Inhibition causes perceptual effects
○ The Limulus experiment summary (View Figure 3.3):
■ Its large eyes are made up of hundreds of ommatidia, each containing a
single receptor
■ This decrease in the firing of receptor A is caused by lateral inhibition
that is transmitted from B to A across the Limulus’s eye by the fibers of
the lateral plexus
○ The horizontal and amacrine cells transmit signals across the human retina
(like plexus in limulus) and lateral inhibition transmitted by them may influence
how humans perceive light and dark
Lateral Inhibition and Lightness Perception
● Perceptual phenomena explained by lateral inhibition and that involve perception of
lightness: the perception of shades ranging from white to gray to black:
The Hermann Grid: Seeing Spots at Intersections
● Noticing gray “ghost images” at the intersections of the white areas, which decrease or
vanish when we look directly at an intersection
● Fast rapid firing of a cell/generation of lots of action potentials → perception that
something is bright/white
● Slower responding from photoreceptor/generation of fewer action potentials (due to
lateral inhibition) → perception that something is dark/grayish
● Initial response – lateral inhibition = final response
● The more there is lateral inhibition (smaller response) the darker our perception
⇒ lateral inhibition explains seeing the dark blobs at the intersections of the grid
, Mach Bands: Seeing Borders More Sharply
● Mach bands: illusory light and dark bands near a light-dark border
● Light intensity distribution gives no hint of Mach bands (they are an illusion created by
our visual system)
● Same calculation style for the Hermann Grid can be done here to prove that lateral
inhibition affects our perception and how we see the bands (View Figures 3.11, 3.12,
3.13 )
Lateral Inhibition and Simultaneous Contrast
● Simultaneous contrast: occurs when our perception of the brightness/color of one
area is affected by the presence of an adjacent/surrounding area
● (View Figures 3.14 & 3.15):
○ The 2 center squares are the same shade of gray, the illusion that they are
different is because the square on the left receives more inhibition → slower
responding ⇒ looks darker
■ Light area around left square causes the receptors under it to respond
rapidly → send large amounts of inhibition to neurons under the center
square → these neurons response is decreased → center looks darker
■ Dark area around right square causes receptors under it to respond less
rapidly → send less inhibition to neurons under center square → center
square remains brighter than the one on the left because it has less
inhibition
● This explanation makes sense and is still accepted by some researchers, but lateral
inhibition is not considered to be the whole story behind simultaneous contrast
A Display That Can’t Be Explained by Lateral Inhibition
● White’s illusion: 2 rectangles appear to be different colors when they actually reflect
the same amount of light
● (View Figure 3.18):
○ The arrows indicate the amount of lateral inhibition received by parts of
rectangles A and B. Because the part of rectangle B is surrounded by more
white, it receives more lateral inhibition. This would predict that B should
appear darker than A (as in the simultaneous contrast display in Figure 3.14),
but the opposite happens. This means that lateral inhibition cannot explain our
perception of White’s illusion.
