Cortical microstimulation
influences perceptual
judgments of motion direction
Salzman et al., 1990
Laura Reich
Big Picture Question
Color
Speed
Direction
Neurons
Orientation
Big Picture Question
Does the brain construct a percept of a visual
scene through information that is encoded in
Neurons
neurons?
Visual Scene
Neurons
Perception
Narrowing the Question: Focusing on
Visual Motion
Brain Area of
Interest
Middle Temporal
Visual Area (MT/V5)
Narrowing the Question: MT Organization
The neurons in MT are organized in a
columnar fashion
Clusters of neurons in a
column preferentially
respond to stimuli that are
moving in certain directions
Hypothesis
If a cluster of neurons in the middle temporal (MT) area is stimulated,
there should be an increase in the intracortical signal related to the
particular direction of motion that is associated with that neuronal
cluster
Increased likelihood
to see mostly
rightward motion
from dots
Random Dot Stimulus
(Visual Scene)
MT Neurons with
Rightward preference
Experiment
1. Three rhesus monkeys were
trained to discriminate the
direction of motion in a random
dot display
2. The monkey moves its eyes to a
fixation point and the visual
stimulus is presented. A small
cluster of MT neurons (known to
prefer a certain stimulus direction)
is stimulated using a
microelectrode
3. Monkey moves eye to LED that
corresponded with the possible
directions of the motion stimulus
Rhesus Monkeys
Random Dot Stimulus
Experiment – Summary
What is being manipulated? – the stimulation site
What is being measured? – the monkey’s LED choice
How is it being measured? – LED choice and the % correlated dots
(stimulation-induced shift)
What comparison is being made? – monkey LED choice with and without
microstimulation
Experiment – Figure 1
Proportion preferred decisions
Proportion preferred decisions
Experiment – Figure 2
Correlation %
Correlation %
Experiment – Figure 3
Interpretation of Results
Do the results support the hypothesis?
Yes!
Increased likelihood
to see mostly
rightward motion
from dots
Random Dot Stimulus
(Visual Scene)
MT Neurons with
Rightward preference
Alternative Explanation
Alternative explanation
The microstimulation is
directly effecting the eye
movement of the monkey,
not its perception
Untested Hypotheses
Untested Hypothesis
Untested Hypothesis
Microstimulation transsynaptically activates an
extended circuit of neurons
Microstimulation transsynaptically activates an
extended circuit of neurons
Back to the Big Picture Question
Color
Speed
Direction
Neurons
Orientation