Download The analysis of visual motion: A comparison of neuronal and

A brief version of: “The analysis of visual motion: A comparison of neuronal and
psychophysical performance.” K.H. Britten et al. 1992. The Journal of Neuroscience,
12, 4745 – 4765.
Aim
What is the relationship between psychophysical sensitivity, as measured by
behavioural tasks, and the firing rate of individual cortical neurons? The aim of this
paper was to investigate if such a relationship can be measured, and to further
understand the nature of the transformations between sensory signals and perceptual
responses. This was achieved by measuring electrical activity produced by single,
specific and identified cells within area MT in an awake monkey and simultaneously
measuring psychophysical behavioural responses. The behavioural task for the
monkey was to identify the direction of coherent motion in a random dot
kinematogram. The elegance of this experiment comes from the fact that neuronal
responses and behavioural responses were measured under the same conditions in the
same animal.
Method
Training stage
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 a particular direction. The more dots that move coherently
together, the stronger the perception of motion, 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 at 1mm intervals. The microelectrodes measured and
recorded the action potentials from single neurons. Before data collection began, the
preferred direction and speed of movement was established for particular neurons, as
well as their receptive field (this was to ensure that the neurons from which action
potentials were being measured were being optimally stimulated).
Data collection stage
During the data collection phase, the animal was shown random dot kinematograms
which varied in the strength (coherence) of the motion signal. The stimuli were
presented so that the dot stimuli were presented within the receptive field of the
specific neuron to be recorded from. The monkeys made behavioural responses
(moving their eyes in the direction of the motion) and the experimenters measured the
responses of single cells. The dependent variable of neuronal response was the total
number of action potentials that occurred in a 2 sec period while viewing the RDK.
The dependent variable of the behavioural part of the task was the number of correct
observations made.
Results
Behavioural data
The animals made more correct judgments of direction when more dots moved
coherently together. As can be seen on the psychometric function plotted below:
Electrophysiological data
At high levels of coherence (> 12.8%), individual cells showed a strong degree of
direction selectivity, at lower levels of coherence (< 12.8%) they became much less
directionally selective.
Comparison between electrophysiological and behavioural data
There was no statistical difference between the psychometric function of the
behavioural responses and of the neuronal responses.
Conclusion
The response of a typical MT neuron can provide an accurate account of a monkey’s
psychophysical performance. For most of the neurons measured, the neurometric
function derived from single unit data was statistically indistinguishable from the
psychometric function measured on the same set of trials. The overall finding from
this work is that information encoded by MT neurons is sufficient to account for
behavioural performance on the task. This was the first study to illustrate such a direct
correlation between neuronal activity and perception / behaviour.