Transcript Document

Chapter 3: Neurons and
Perception
Overview of Questions
• How do electrical signals represent objects?
• How does neural processing determine what
we see?
• What is the effect of the environment on
developing visual systems?
• What does it mean to say that perception is
indirect?
Convergence in the Retina - continued
• 126 million rods and cones converge to 1
million ganglion cells
• Higher convergence of rods than cones
– Average of 120 rods to one ganglion cell
– Average of 6 cones to one ganglion cell
– Cones in fovea have 1 to 1 relation to
ganglion cells
Convergence and Sensitivity
• Rods are more sensitive to light than cones
– Rods take less light to respond
– Rods have greater convergence which
results in summation of the inputs of many
rods into ganglion cells increasing the
likelihood of response
– Trade-off is that rods cannot distinguish
detail
Convergence and Detail
• All-cone foveal vision results in high visual
acuity
– One-to-one wiring leads to ability to
discriminate details
– Trade-off is that cones need more light to
respond than rods
Summation results in greater excitation in a single
retinal ganglion cell and increases the likelihood of a
response.
Convergence results in
poor acuity (the ability to
detect details). For the
rods, the RGC sends the
same message to the
brain in a and b. for the
cones (especially those in
the fovea) there is little
convergence. RGCs send
different messages to the
brain for a and b.
Lateral Inhibition of Neurons
• Experiments with eye of Limulus (Hartline, 1956)
– Ommatidia allow recordings from a single
receptor
– Light shown into a single receptor led to rapid
firing rate of nerve fiber
– Adding light into neighboring receptors led to
reduced firing rate of initial nerve fiber
Hartline’s Results
Lateral Inhibition and Lightness Perception
• Psychophysical results can be 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 stimulated by dark areas inhibit
the response of neighboring cells receiving
input from white area
– The lateral inhibition causes a reduced
response which leads to the perception of
gray
Explanation of the
Herman Grid
Mach Bands
Figure 3.10 Circuit to explain the Mach band effect based on lateral inhibition. The circuit works like the
one for the Hermann grid in Figure 3.6, with each bipolar cell sending inhibition to its neighbors. If we know
the initial output of each receptor and the amount of lateral inhibition, we can calculate the final output of the
receptors. (See text for a description of the calculation.)
Simultaneous Contrast
Simultaneous Contrast
Neural Circuits
• Groups of neurons connected by excitatory
and inhibitory synapses
• A linear circuit has no convergence and only
excitatory inputs
– Input into each receptor has no effect on
the output of neighboring circuits
– Each circuit can only indicate single spot of
stimulation
Linear Circuit
Neural Circuits - continued
• Convergent circuit with only excitatory
connections
– Input from each receptor summates into
the next neuron in the circuit
– Output from convergent system varies
based on input
– Output of circuit can indicate single input &
increases output as length of stimulus
increases
Circuit with convergence added.
Neural Circuits - continued
• Convergent circuit with excitatory and inhibitory
connections
– Inputs from receptors summate to determine
output of circuit
– Summation of inputs result in:
• Weak response for single inputs & long stimuli
• Maximum firing rate for medium length
stimulus
Receptive Fields
• Area of retina that
affects firing rate of a
given neuron in the
circuit
• Receptive fields are
determined by
monitoring single cell
responses
• Stimulus is presented
to retina and response
of cell is measured by
an electrode
Center-Surround Receptive Fields
• 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
Response of a ganglion cell in the cat’s retina
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
Response of an excitatory-center-inhibitorysurround receptive field
Ganglion Cell Output
• Small Spot Stimuli
Spontaneous Activity
Uniform Illumination
Spike Record
Lateral Inhibition
• More natural Stimuli
• How does GC output vary?
10 spikes/sec
Lateral Inhibition
• Enhances edges
• De-emphasize broad unchanging surfaces
• Early processing is already “modifying” the
scene
10 spikes/sec
Pathway Beyond the Retina
• Pathway to visual
processing area
– Optic nerve
– Lateral geniculate
nucleus (LGN)
– Superior colliculus
– Striate cortex
– Extrastriate cortex
Striate Cortex
• Simple cortical cells
– Side-by-side receptive fields
– Respond to spots of light
– Respond best to bar of light oriented along
the length of the receptive field
• Orientation tuning curves
– Shows response of simple cortical cell for
orientations of stimuli
The receptive field of a simple cortical cell.
Striate Cortex - continued
• Complex cells
– Like simple cells
• Respond to spots of light
• Respond to bars of light
– Unlike simple cells
• Respond to movement of bars of light in specific
direction
• End-stopped cells
– Respond to:
• Moving lines of specific length
• Moving corners or angles
– No response to:
• Stimuli that are too long
• Each stage
marks an
increase in
stimulus
complexity
Feature Detectors
• 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
– Complex cortical cell
– End-stopped cortical cell
– Face cells! (More in chapter 4)
Response properties of a neuron in the
monkey temporal lobe.
Selective Adaptation
• Neurons tuned to specific stimuli fatigue
when exposure is long
• Selective means that only those neurons that
respond to the specific stimulus adapt
• How to measure it
– 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
Orientation sensitivity test (top0 and contrast
sensitivity test (bottom)
Method for Orientation Sensitivity
•
•
•
•
•
Use a high contrast grating
Measure sensitivity to different orientations
Adapt person to one orientation
Re-measure sensitivity to all orientations
Psychophysical curve should show selective
adaptation for specific orientation if neurons
are tuned to this characteristic
Figure 3.27 (a) Results of a psychophysical selective adaptation experiment. This graph shows that the
participant’s adaptation to the vertical grating causes a large decrease in her ability to detect the vertical
grating when it is presented again, but has less effect on gratings that are tilted to either side of the vertical.
(b) Orientation tuning curve of the simple cortical neuron from Figure 3.23b.
Size Adaptation
• Gratings have bars of different sizes
• Similar method as for contrast and orientation
• Size adaptation changes the perception of
similarly sized bars
Selective Rearing
• Animals reared in specific environment
– Limits type of stimuli present
– Neural plasticity would result in lack of
ability to see characteristics unavailable in
environment
– Shows that neurons need environment to
develop fully
Blakemore and Cooper’s (1970) selective
rearing experiments.
Perception is Indirect
• Stimuli in environment impinge on receptors
• Transduction takes place causing electrochemical impulse in neurons
• Provides accurate information about world
• Feels direct but that is an illusion
• This is true for all senses