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.
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 correct judgments
when the neuronal area is stimulated than when it is not.
Conclusion
These data establish a causal relationship between the activity of stimulated neurons
in area MT, and perceptual judgments of motion direction.