SEMINARS PT. II
13. A Selective Impairment of Motion Perception
Following Lesions of the Middle Temporal Visual
Area (MT)
Physiological experiments indicate that the middle temporal visual area (MT) of primates plays a prominent role in the
cortical analysis of visual motion. We investigated the role of MT in visual perception by examining the effect of
chemical lesions of MT on psychophysical thresholds. We trained rhesus monkeys on psychophysical tasks that enabled
us to assess their sensitivity to motion and to contrast. For motion psychophysics, we employed a dynamic random dot
display that permitted us to vary the intensity of a motion signal in the midst of masking motion noise. We measured
the threshold intensity for which the monkey could successfully complete a direction discrimination. In the contrast
task, we measured the threshold contrast for which the monkeys could successfully discriminate the orientation of
stationary gratings. Injections of ibotenic acid (to damage) into MT caused striking elevations in motion thresholds but
had little or no effect on contrast thresholds. The results indicate that neural activity in MT contributes selectively to
the perception of motion.
14. Cortical microstimulation influences
perceptual judgements of motion direction
Neurons in the visual cortex respond selectively to perceptually salient features of the visual scene, such as the
direction and speed of moving objects, the orientation of local contours, or the colour or relative depth of a visual
pattern. It is commonly assumed that the brain constructs its percept of the visual scene from information encoded in
the selective responses of such neurons. We have now tested this hypothesis directly by measuring the effect on
psychophysical performance of modifying the firing rates of physiologically characterized neurons. We required rhesus
monkeys to report the direction of motion in a visual display while we electrically stimulated clusters of directionally
selective neurons in the middle temporal visual area (MT, or V5), an extrastriate area that plays a prominent role in the
analysis of visual motion information. Microstimulation biased the animals' judgements towards the direction of
motion encoded by the stimulated neurons. This result indicates that physiological properties measured at the
neuronal level can be causally related to a specific aspect of perceptual performance.
15. Optic flow is used to control human walking
How is human locomotion visually controlled? Fifty years ago, it was proposed that we steer to a goal using optic flow,
the pattern of motion at the eye that specifies the direction of locomotion. However, we might also simply walk in the
perceived direction of a goal. These two hypotheses normally predict the same behaviour, but we tested them in an
immersive virtual environment by displacing the optic flow from the direction of walking, violating the laws of optics.
We found that people walked in the visual direction of a lone target, but increasingly relied on optic flow as it was
added to the display. The visual control law for steering toward a goal is a linear combination of these two variables
weighted by the magnitude of flow, thereby allowing humans to have robust locomotor control under varying
environmental conditions.
13. A Selective Impairment of Motion Perception
Following Lesions of the Middle Temporal Visual
Area (MT)
Physiological experiments indicate that the middle temporal visual area (MT) of primates plays a prominent role in the
cortical analysis of visual motion. We investigated the role of MT in visual perception by examining the effect of
chemical lesions of MT on psychophysical thresholds. We trained rhesus monkeys on psychophysical tasks that enabled
us to assess their sensitivity to motion and to contrast. For motion psychophysics, we employed a dynamic random dot
display that permitted us to vary the intensity of a motion signal in the midst of masking motion noise. We measured
the threshold intensity for which the monkey could successfully complete a direction discrimination. In the contrast
task, we measured the threshold contrast for which the monkeys could successfully discriminate the orientation of
stationary gratings. Injections of ibotenic acid (to damage) into MT caused striking elevations in motion thresholds but
had little or no effect on contrast thresholds. The results indicate that neural activity in MT contributes selectively to
the perception of motion.
14. Cortical microstimulation influences
perceptual judgements of motion direction
Neurons in the visual cortex respond selectively to perceptually salient features of the visual scene, such as the
direction and speed of moving objects, the orientation of local contours, or the colour or relative depth of a visual
pattern. It is commonly assumed that the brain constructs its percept of the visual scene from information encoded in
the selective responses of such neurons. We have now tested this hypothesis directly by measuring the effect on
psychophysical performance of modifying the firing rates of physiologically characterized neurons. We required rhesus
monkeys to report the direction of motion in a visual display while we electrically stimulated clusters of directionally
selective neurons in the middle temporal visual area (MT, or V5), an extrastriate area that plays a prominent role in the
analysis of visual motion information. Microstimulation biased the animals' judgements towards the direction of
motion encoded by the stimulated neurons. This result indicates that physiological properties measured at the
neuronal level can be causally related to a specific aspect of perceptual performance.
15. Optic flow is used to control human walking
How is human locomotion visually controlled? Fifty years ago, it was proposed that we steer to a goal using optic flow,
the pattern of motion at the eye that specifies the direction of locomotion. However, we might also simply walk in the
perceived direction of a goal. These two hypotheses normally predict the same behaviour, but we tested them in an
immersive virtual environment by displacing the optic flow from the direction of walking, violating the laws of optics.
We found that people walked in the visual direction of a lone target, but increasingly relied on optic flow as it was
added to the display. The visual control law for steering toward a goal is a linear combination of these two variables
weighted by the magnitude of flow, thereby allowing humans to have robust locomotor control under varying
environmental conditions.
