Chapter 3:

Neural Processing and Perception

Neural Processing and Perception

• Neural processing is the interaction of signals in many neurons.

Figure 3-1 p54

Lateral Inhibition and Perception

• Experiments with eye of Limulus

– Ommatidia allow recordings from a single

receptor.

– Light shown into a single receptor leads to

rapid firing rate of nerve fiber.

– Adding light into neighboring receptors

leads to reduced firing rate of initial nerve fiber.

Figure 3-2 p54

Figure 3-3 p55

Lateral Inhibition and Lightness

Perception

• Three lightness perception phenomena

explained by lateral inhibition

– The Hermann Grid: Seeing spots at an

intersection

– Mach Bands: Seeing borders more sharply

– Simultaneous Contrast: Seeing areas of

different brightness due to adjacent areas

Hermann Grid

• People see an illusion of gray images in intersections of white areas.

• Signals from bipolar cells cause effect

– Receptors responding to white corridors send inhibiting signals to receptor at the intersection

– The lateral inhibition causes a reduced response which leads to the perception of gray.

Figure 3-4 p55

Figure 3-5 p55

Figure 3-6 p56

Figure 3-7 p56

Figure 3-8 p56

Mach Bands

• People see an illusion of enhanced lightness and darkness at borders of light and dark areas.

– Actual physical intensities indicate that this is not in the stimulus itself.

– Receptors responding to low intensity (dark) area have smallest output.

– Receptors responding to high intensity (light) area have largest output.

Figure 3-9 p57

Figure 3-10 p57

Mach Bands - continued

– All receptors are receiving lateral inhibition

from neighbors

– In low and high intensity areas amount of

inhibition is equal for all receptors

– Receptors on the border receive differential

inhibition

Mach Bands - continued

– On the low intensity side, there is

additional inhibition resulting in an

enhanced dark band.

– On the high intensity side, there is less

inhibition resulting in an enhanced light

band.

– The resulting perception gives a boost for

detecting contours of objects.

Figure 3-9 p57

Figure 3-12 p58

Figure 3-13 p58

Lateral Inhibition and Simultaneous Contrast

• People see an illusion of changed brightness

or color due to effect of adjacent area

– An area that is of the same physical

intensity appears:

• lighter when surrounded by a dark area.

• darker when surrounded by a light area.

Lateral Inhibition and Simultaneous

Contrast - continued

– Receptors stimulated by bright surrounding area send a large amount of inhibition to cells in center.

– Resulting perception is of a darker area than when this stimulus is viewed alone.

– Receptors stimulated by dark surrounding area send a small amount of inhibition to cells in center.

– Resulting perception is of a lighter area than when this stimulus viewed alone.

Figure 3-14 p58

Figure 3-15 p59

A Display That Can’t Be Explained by Lateral Inhibition

• White’s Illusion

– People see light and dark rectangles even

though lateral inhibition would result in the

opposite effect.

Explanation of White’s Illusion

• Belongingness

– An area’s appearance is affected by where

we perceive it belongs.

– Effect probably occurs in cortex rather than

retina.

– Exact physiological mechanism is unknown.

Figure 3-16 p59

Figure 3-17 p59

Figure 3-18 p60

Processing From Retina to Visual Cortex and Beyond

• Area of receptors that affects firing rate of a

given neuron in the circuit

• Receptive fields are determined by

monitoring single cell responses.

• Research example for vision

– Stimulus is presented to retina and

response of cell is measured by an

electrode.

Figure 3-19 p60

Figure 3-20 p61

Figure 3-21 p61

Center-Surround Antagonism

• Output of center-surround receptive fields

changes depending on area stimulated:

– Highest response when only the excitatory

area is stimulated

– Lowest response when only the inhibitory

area is stimulated

– Intermediate responses when both areas are stimulated

Figure 3-22 p62

Figure 3-23 p62

Hubel and Wiesel’s Rational for

Studying Receptive Fields

• Signals from the retina travel through the

optic nerve to the

– Lateral geniculate nucleus (LGN)

– Primary visual receiving area in the

occipital lobe (the striate cortex or area V1)

– And then through two pathways to the

temporal lobe and the parietal lobe

– Finally arriving at the frontal lobe

Figure 3-24 p63

Figure 3-25 p63

Hubel and Wiesel’s Rational for

Studying Receptive Fields - continued

• LGN cells have center-surround receptive fields.

• Major function of LGN is to regulate neural information from the retina to the visual cortex.

– Signals are received from the retina, the cortex, the brain stem, and the thalamus.

– Signals are organized by eye, receptor type, and type of environmental information.

Hubel and Wiesel’s Rational for

Studying Receptive Fields - continued

• Excitatory and inhibitory effects are found in

receptive fields.

• Center and surround areas of receptive fields

result in:

– Excitatory-center-inhibitory surround

– Inhibitory-center-excitatory surround

Figure 3-26 p64

Receptive Fields of Neurons in the Visual Cortex

• Neurons that fire to specific features of a

stimulus

• Pathway away from retina shows neurons

that fire to more complex stimuli

• Cells that are feature detectors:

– Simple cortical cell

– Orientation tuning curve

– Complex cortical cell

– End-stopped cortical cell

Figure 3-27 p65

Figure 3-28 p65

Figure 3-29 p66

Table 3-1 p66

Selective Adaptation

• Neurons tuned to specific stimuli fatigue

when exposure is long.

• Fatigue or adaptation to stimulus causes

– Neural firing rate to decrease

– Neuron to fire less when stimulus

immediately presented again

• Selective means that only those neurons that respond to the specific stimulus adapt.

Figure 3-30 p67

Selective Adaptation - continued

• Measure sensitivity to range of one stimulus

characteristic

• Adapt to that characteristic by extended

exposure

• Re-measure the sensitivity to range of the stimulus characteristic

Figure 3-31 p67

Selective Adaptation - continued

• Gratings are used as stimuli

– Made of alternating light and dark bars

– Angle relative to vertical can be changed to

test for sensitivity to orientation

– Difference in intensity can be changed to

test for sensitivity to contrast

Figure 3-32 p67

Selective Adaptation - continued

• Measure contrast threshold by decreasing

intensity of grating until person can just see it.

• Calculate the contrast sensitivity by taking

1/threshold.

• If threshold is low, person has high contrast

sensitivity.

Figure 3-33 p68

Selective Rearing

• Animals are reared in environments that contain only

certain types of stimuli

– Neurons that respond to these stimuli will become

more predominate due to neural plasticity.

– Blakemore and Cooper (1970) showed this by

rearing kittens in tubes with either horizontal for

vertical lines.

– Both behavioral and neural responses showed the

development of neurons for the environmental

stimuli.

Figure 3-34 p69

Higher Level Neurons

• Inferotemporal (IT) cortex

• Prosopagnosia

• Fusiform face area

Figure 3-35 p69

Figure 3-36 p70

The Sensory Code

• Sensory code - representation of perceived

objects through neural firing

– Specificity coding - specific neurons

responding to specific stimuli

• Leads to the “grandmother cell”hypothesis

• Recent research shows cells in the

hippocampus that respond to concepts

such as Halle Berry.

The Sensory Code continued

– Problems with specificity coding:

• Too many different stimuli to assign

specific neurons

• Most neurons respond to a number of

different stimuli.

• Distributed coding - pattern of firing across

many neurons codes specific objects

– Large number of stimuli can be coded by a

few neurons.

Figure 3-37 p70

Sensory Code The Sensory Code -

continued

• How many neurons are needed for an object

in distributed coding?

– Sparse coding - only a relatively small

number of neurons are necessary

• This theory can be viewed as a midpoint

between specificity and distributed

coding.

Figure 3-38 p71

Figure 3-39 p71

Figure 3-40 p72

The Mind-body Problem

• How do physiological processes become transformed into perceptual experience?

– Easy problem of consciousness

• Neural correlate of consciousness (NCC) - how physiological responses correlatewith experience

– Hard problem of consciousness

• How do physiological responses causeexperience?

Figure 3-41 p73