Download A brief version of: “Microstimulation in Visual Area MT: Effects of

A brief version of: “Microstimulation in Visual Area MT: Effects of Direction
Discrimination Performance” C.D. Salzman et al. 1992. The Journal of Neuroscience,
12, 2331 – 2355. Updated 11th October, 2007
Aim
To investigate the relationship between behavioural responses in psychophysical tasks
and neuronal responses by attempting to influence a monkey’s choice on a direction
discrimination task by directly stimulating specific MT neurons. The theory: The
direct activity of a specific cluster of MT neurons determines a direction judgment,
therefore, if the appropriate cluster of neurons are stimulated, judgments will be made
accordingly (even if these judgments are incorrect).
Method
The monkey was trained, using operant conditioning techniques (reward with juice)
over a period of several months to identify the direction of motion from a random dot
kinematogram (RDK). A RDK is a psychophysical stimulus which consists of a patch
of moving dots. Some of the dots move in a random direction (noise dots) while
others move coherently in one particular direction. The more dots that move
coherently together, the stronger the perception of motion is and the easier it is to
identify the direction of motion. The monkeys were trained to identify the direction of
motion from these stimuli, and to make their response by moving their eyes in the
direction of the coherent motion.
Surgical preparation
Area MT was identified on the basis of its characteristic location within the superior
temporal sulcus, its preponderance of responsive, directionally selective neurons and
its characteristic topography. Surgery was performed so that micro-electrodes could
be inserted into the cortex. These micro-electrodes emitted a stimulating pulse of 10
µA in amplitude and 200 Hz (this frequency was thought to match the responses
observed from single-cell recording). As the goal of the study was to stimulate
neurons with a preferred direction selectivity, initial measurements were made to
identify appropriate clusters of neurons.
Behavioural paradigm
Monkeys performed direction detection tasks using RDKs. On some trials, the
specific neurons were stimulated (by applying the stimulating pulse); other trials were
completed without stimulation. Performance (number of trials in which the direction
of motion is correctly identified) was then compared between trials where no microstimulation takes place and trials where micro-stimulation does take place.
Results
The predominant effect of micro-stimulation was to bias the monkey’s decisions in
favour of the direction preferred by the neurons at the stimulation site. The data
therefore established a causal relationship between the activity of the stimulated
neurons and perceptual judgments of motion direction. This result provides direct
support for the linking hypothesis associating direction selectivity with motion
perception and demonstrates a major functional role of the “motion pathway” within
visual cortex. The figure illustrates that the monkey makes more judgments that
match the preferred direction of the stimulated neurons when the cortex is stimulated
than when it is not.
Monkey J
Monkey E
Monkey E
Null Direction
0%
Preferred Direction
Monkey J
Null Direction
0%
Preferred Direction
Conclusion
These data establish a causal relationship between the activity of stimulated neurons
in area MT, and perceptual judgments of motion direction.
Further points to note:
 The paper reports a series of experiments carried out on 3 separate monkeys.
In total over 100 separate experiments were carried out. The results were
comparable, so this paper presents a selection of the data.
 An important goal of the methodology was to activate selectively neurons that
had a similar preferred direction (e.g. to the right). Therefore the
experimenters positioned the electrode in the middle of a cluster of neurons
that had previously been identified as having similar response properties such
as preferred directions, preferred speeds and receptive field locations (i.e. it is
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not a single neuron that is being stimulated, but a cluster of neurons with
similar turning curves).
The paradigm presented RDKs which varied in correlation strength. The
highest correlation was less than 100% and was set so that the monkeys made
about 80 – 90% correct responses in the un-stimulated conditions.
Importantly, in 50% of the trials the direction of motion in the RDK was the
same as the preferred direction of the cell (right). However in the other half of
the trials, the direction of motion in the RDK was in the opposite, or null,
direction (left). That is why the behavioural data in the plots above show less
than chance performance, and why the dependent variable is proportion of
judgments in the preferred direction rather than proportion of correct
responses. For example, in figure A below, when the stimulus is highly
correlated, but moving in the null direction, as depicted by the open circle in
the bottom left corner of the plot, the monkey is actually making 100% correct
responses as on every trial they are responding toward the direction of the
preferred cell (which, in our example, is right). This relates to the point that
Adam made and clarifies why the behavioural results are described as being
biased towards the preferred direction, rather than necessarily being correct.
The line fitted through the data points is the logistic regression function
describing the monkey’s performance.
The plots show different effect sizes of micro-stimulation. The site at which
the stimulation took place is not really the key factor here. On one hand the
plots show a range of results from the many different experiments that were
carried out (and note the results are from two of the monkeys, monkey J and
monkey E). Although not very clearly presented, the plots also show results
from experiments which ranged in difficulty of the motion task. Task
difficulty depended on several variables: eccentricity of motion display, the
size of the aperture it was presented through & the speed of the motion
stimulus.
When the monkey makes less correct responses the slope of the psychometric
function appears flatter. Therefore, plot D presents data from a hard condition.
For more details on the methodology I suggest you consult the primary source. The
key details summarised above are presented on pages 2332 to 2336.