Redwood Neuroscience
Title "Perceptual Organization of
Occluding Contours”
Jim
Muller
Department
of Neurobiology
Howard
Hughes Medical Institute
Abstract:
When attention is directed to a localized region
within the visual field, discrimination
of stimuli in that region is enhanced. At the same time, attention modulates
responses of visual neurons in specific ways. Recent evidence suggests these
effects may be mediated via feedback to visual cortex from oculomotor
structures, for example the SC (e.g. Ignashchenkova
et al., 2004). To test this, we examined the effects of microstimulation
of the SC (current < eye-movement threshold) on the direction
discrimination performance of monkeys, and on neural responses in the middle
temporal visual area (MT). Monkeys discriminated the direction of coherent
motion of a localized patch of random dots positioned at one of two locations
within a large noise field. Performance (i.e. coherence threshold) was greatly
improved by a spatial cue indicating the location of the patch of coherent
dots. To test the SC’s role in improving performance, we stimulated the SC
while measuring the monkeys’ coherence thresholds. If SC microstimulation
mediates attention, it should improve performance when the coherent patch is
within the movement field (MF) of the stimulated SC site, but not when the
patch is remote from the SC MF. Both predictions were confirmed by the
experimental results, consistent with SC playing a role in mediating visual
attention. At the physiological level, attention affects MT neurons in two ways
(Treue & Maunsell,
1999). When two competing stimuli, one preferred and one nonpreferred,
are presented simultaneously within a receptive field (RF), attention to the
preferred stimulus increases the response (effect #1).
But attention to a non-preferred stimulus decreases the response, whether the nonpreferred stimulus is within the RF (effect #2A)
or remote from it (effect #2B). To
test the SC’s role in causing these effects, we measured visual responses in MT
while microstimulating SC. A preferred stimulus was
always placed within the MT RF. Non-preferred stimuli were placed within the RF
in some experiments and remote from the RF in others. Consistent with the
effects of visual attention described by Treue and Maunsell, we found that electrical stimulation of the SC 1)
increased responses in MT when the SC MF contained the preferred stimulus (#1
above), and 2) decreased MT responses when the SC MF contained the
non-preferred stimulus, whether it fell within (#2A)
or outside (#2B) the MT RF. Thus both
lines of evidence – behavioral and electrophysiological – are consistent with
SC playing a role in mediating effects of attention on visual responses in MT.